Three-terminal MRAM with ac write-assist for low read disturb

Methods and structures useful for magnetoresistive random-access memory (MRAM) are disclosed. The MRAM device has a magnetic tunnel junction stack having a significantly improved performance of the free layer in the magnetic tunnel junction structure. The MRAM device also utilizes a three-terminal structure, thereby allowing efficient writing of the bit without a concomitant increase in read disturb.

FIELD

The present patent document relates generally to spin-transfer torque magnetic random access memory and, more particularly, to a magnetic tunnel junction stack having improved performance of the free layer in the magnetic tunnel junction structure.

BACKGROUND

Magnetoresistive random-access memory (“MRAM”) is a non-volatile memory technology that stores data through magnetic storage elements. These elements are two ferromagnetic plates or electrodes that can hold a magnetic field and are separated by a non-magnetic material, such as a non-magnetic metal or insulator. In general, one of the plates has its magnetization pinned (i.e., a “reference layer”), meaning that this layer has a higher coercivity than the other layer(s) and requires a larger magnetic field or spin-polarized current to change the orientation of its magnetization. The second plate is typically referred to as the free layer and its magnetization direction can be changed by a smaller magnetic field or spin-polarized current relative to the reference layer.

MRAM devices store information by changing the orientation of the magnetization of the free layer. In particular, based on whether the free layer is in a parallel or anti-parallel alignment relative to the reference layer, either a “1” or a “0” can be stored in each MRAM cell. Due to the spin-polarized electron tunneling effect, the electrical resistance of the cell changes due to the orientation of the magnetic fields of the two layers. The cell's resistance will be different for the parallel and anti-parallel states and thus the cell's resistance can be used to distinguish between a “1” and a “0”. One important feature of MRAM devices is that they are non-volatile memory devices, since they maintain the information even when the power is off. The two plates can be sub-micron in lateral size and the magnetization direction can still be stable with respect to thermal fluctuations.

Spin transfer torque or spin transfer switching, uses spin-aligned (“polarized”) electrons to change the magnetization orientation of the free layer in the magnetic tunnel junction (“MTJ”). In general, electrons possess a spin, a quantized number of angular momentum intrinsic to the electron. An electrical current is generally unpolarized, i.e., it consists of 50% spin up and 50% spin down electrons. Passing a current though a magnetic layer polarizes electrons with the spin orientation corresponding to the magnetization direction of the magnetic layer thus produces a spin-polarized current. If a spin-polarized current is passed to the magnetic region of a free layer in the MTJ device, the electrons will transfer a portion of their spin-angular momentum to the magnetization layer to produce a torque on the magnetization of the free layer. Thus, this spin transfer torque can switch the magnetization of the free layer, which, in effect, writes either a “1” or a “0” based on whether the free layer is in the parallel or anti-parallel states relative to the reference layer.

When a current is passed through a magnetic layer (e.g. a polarizer), the spin orientation of the electrons that flow out of the magnetic layer is generally aligned in the direction of the magnetization of the magnetic layer and will exert a spin-transfer torque in that direction (forming a transverse spin current) upon impinging on another magnetic layer. However, due to the conservation of angular moment for the system, the electrons on the opposite side of magnetic layer, those that do not go through the magnetic layer, generally have a spin orientation that is aligned in the direction that is anti-parallel to the magnetization direction of the magnetic layer. The net effect of this process is that the current applied to the magnetic layer undergoes spin filtering, which creates a spin current on one side of the magnetic layer, with spins that are aligned with magnetization direction of the magnetic layer, and a reflected spin current on the other side of the magnetic layer, with spins that are anti-parallel to the magnetization direction of the magnetic layer. This effect occurs upon application of a current to any magnetic layer, including an in-plane polarization layer or an out-of-plane reference magnetic layer. Thus, in a typical MTJ, when switching the magnetization direction of the free layer in one direction (e.g., from the parallel to anti-parallel state) is achieved using spin transfer torque from the transverse spin current, switching the free layer in the other direction (e.g., from the anti-parallel to parallel states) would be achieved using spin transfer torque from the reflected spin current. This is typically accomplished by running electrical current through the MTJ in one direction when switching from the anti-parallel to parallel state and running the electrical current through the MTJ in the other direction when switching from the parallel to anti-parallel state.

FIG. 1illustrates a MTJ stack100for an MRAM device including a magnetic tunnel junction MTJ130and a top polarizer layer150. As shown, stack100includes one or more seed layers110provided at the bottom of stack100to initiate a desired crystalline growth in the above-deposited layers. Furthermore, MTJ130is deposited on top of SAF layer120. MTJ130includes reference layer132, which is a magnetic layer, a non-magnetic tunneling barrier layer (i.e., the insulator)134, and the free layer136, which is also a magnetic layer. It should be understood that reference layer132is actually part of SAF layer120, but forms one of the ferromagnetic plates of MTJ130when the non-magnetic tunneling barrier layer134and free layer136are formed on reference layer132. As shown inFIG. 1, magnetic reference layer132has a magnetization direction perpendicular to its plane. As also seen inFIG. 1, free layer136also has a magnetization direction perpendicular to its plane, but its direction can vary by 180 degrees.

The first magnetic layer114in the SAF layer120is disposed over seed layer110. SAF layer120also has an antiferromagnetic coupling layer116disposed over the first magnetic layer114. Furthermore, a nonmagnetic spacer140is disposed on top of MTJ130and a polarizer150is disposed on top of the nonmagnetic spacer140. Polarizer150is a magnetic layer that has a magnetic direction in its plane, but is perpendicular to the magnetic direction of the reference layer132and free layer136. Polarizer150is provided to polarize a current of electrons (“spin-aligned electrons”) applied to MTJ structure100. Polarizer150polarizes the current in a direction perpendicular (orthogonal) to those of the magnetizations of the free magnetic layer136and reference magnetic layer132. Further, one or more capping layers160can be provided on top of polarizer150to protect the layers below on MTJ stack100. Finally, a hard mask170is deposited over capping layers160and is provided to pattern the underlying layers of the MTJ structure100, using a combination of reactive ion etch (RIE) and ion beam etching (IBE) processes.

Various mechanisms have been proposed to assist the free-layer magnetization switching in MTJ devices. One issue has been that to realize the orthogonal spin transfer effect for in-plane MTJ structures, large spin currents may be required for switching. The need for large switching currents may limit such device's commercial applicability. One way proposed to reduce switching current is to lower the magnetization of the free layer. However, if the effective magnetization of the free layer is lowered significantly, the orthogonal effect has to be limited so that the free-layer does not go into precessional mode that would make the end state of the free-layer magnetization un-deterministic. This defines the operation window for the in-plane OST structures. In an in-plane device, unlike that shown inFIG. 1, the magnetization direction of the reference layer and free layer are in the plane of the layer. Another aspect of in-plane devices is that the thermal stability requirements may limit the size of the MTJ devices to approximately sixty nanometers or higher.

In contrast to MTJ structures with an in-plane free layer and perpendicular polarizer perpendicular MTJ structures such as those shown inFIG. 1, are less prone to getting into a pure precessional regime. This is due to the fact that in perpendicular MTJ structures, the direction of the demagnetization field and perpendicular anisotropy contributions are the same. In this case the precession is generally not an issue and the end-state is more deterministic. However, precession may be an issue with regards to read disturb, particularly when stronger read currents are used. The orthogonal polarizer acts on the free layer magnetization at the initial state, but when the precession takes hold, the fixed orthogonal polarizer150helps only half the cycle of the free-layer magnetization rotation while it harms the other half of the cycle. This is demonstrated with reference toFIGS. 2-3.FIGS. 2A and 2Bshows switching of a free layer136of an MTJ. As is seen, free layer136has a magnetization direction200perpendicular to that of the polarizer150. The magnetization direction200of the free layer136can rotate by 180 degrees.FIGS. 2A and 2Bshow precession about the axis of the magnetization vector of free layer136. During precession, magnetic vector200begins to rotate about its axis in a cone-like manner such that its magnetization vector200′ deflects from the perpendicular axis202of free layer136. Whereas prior to initiating precession, no component of magnetic vector200is in the plane of free layer136, once precession starts, a component of magnetic vector200′ can be found both in-plane and orthogonal to free layer136. As magnetic vector200′ continues to precess (i.e., switch), the rotation of vector200′ extends further from the center of free layer136, as is seen inFIG. 2B.

In most prior MTJ devices using a polarizer such as polarizer150, the magnetization direction of polarizer150is fixed, which is shown inFIGS. 1 and 3. See also U.S. Pat. No. 6,532,164, which states that the direction of the magnetization of the polarizing layer cannot vary in the presence of current. Prior to current passing through the MTJ, the free layer136has a magnetization direction200perpendicular to that of the polarizer150. While the magnetization direction200of the free layer136can rotate by 180 degrees, such rotation is normally precluded by the free layer's inherent damping ability205, which is represented by a vector205pointing to axis202(shown as a dashed line inFIG. 2Aas well asFIG. 3). Axis202is perpendicular to the plane of free layer136. This damping205has value, defined by the damping constant, which maintains the magnetization direction of the free layer136.

The precession of the magnetization vector during switching of the free layer can be assisted by spin transfer torque exerted by the electrons of a spin-polarized current, which is generated in part by the orthogonal polarizer150. Applying a voltage across the MTJ device100produces a charge current through the device. This charge current, in turn, produces a spin-polarized current via spin filtering through the magnetic layers of the device (i.e., the orthogonal polarizer150and the reference layer132). The spin-polarized electrons of the spin-polarized current exerts a spin transfer torque on the magnetic vector200. This spin transfer torque has an in-plane component of the spin transfer torque210, which pushes magnetization vector200′ in the direction of the magnetic vector of polarizer150throughout precession of magnetic vector200′. In addition to the in-plane spin transfer torque210from the polarizer, the perpendicular spin transfer torque (not shown), generated by reference layer132, pulls the magnetic vector200′ towards the direction antiparallel to its initial direction200, thereby causing switching of the free layer136. In devices like those shown inFIG. 1, when the spin transfer torque210begins to help overcome the damping205inherent to the free layer136, the magnetic direction200′ begins to precess about its axis, as shown inFIG. 2A. As seen inFIG. 3, in-plane spin transfer torque210helps the magnetization direction of the free layer136to precess in a cone-like manner around an axis202perpendicular to the plane of the layers. When a spin polarized current traverses the stack100, the magnetization of the free layer136precesses in a continuous manner (i.e., it turns on itself in a continuous manner as shown inFIG. 3) with maintained oscillations until the magnetic direction of free layer136is opposite the magnetic direction prior to the spin torque causing precession, i.e., the magnetic direction of free layer136switches by 180 degrees.

FIG. 3illustrates precession of a free layer136of an MTJ assisted by orthogonal spin polarized current. The spin polarized electrons from polarizer150provide a spin transfer torque which has a component210in the plane of the precession (i.e., in-plane spin transfer torque) that helps overcome the damping205in the first half of the precession215because the in-plane spin transfer torque210provided by the spin polarized current is opposite that of the inherent damping205of the free layer136. This is shown on the right-hand side of the middle portion ofFIG. 3, which illustrates the projection of spin transfer torque210onto the precession plane (i.e., the plane defined by axis200and magnetization vector200′ as it steadily precesses around axis200). However, the in-plane spin transfer torque actually harms the switching process during the second half of the precession220. The reason for this is that the spin of the electrons in the spin polarized current only apply an in-plane spin transfer torque210in the direction of their polarization, which is aligned with the magnetic direction of the in-plane polarization layer150. Thus, when the magnetic vector is in the half of the precession cycle220that is opposite the spin of the polarized electrons, the in-plane spin transfer torque210actually works with the inherent damping205of free layer136to make rotation more difficult. This is shown in the left-hand side of the middle portion ofFIG. 3and can be seen in the projection of the spin transfer torque210onto the precessional plane of the free layer136, which is depicted on the bottom ofFIG. 3. Indeed, it is the perpendicular spin transfer torque created by the reference layer132(not shown inFIG. 3) that overcomes the damping205of free layer136as well as the in-plane spin transfer torque210during the half of a precession cycle where the spin of the electrons harms precession, and thus it is the reference layer132that allows for completion of precession. The precessional dynamics and the directionality of the spin transfer torque depicted inFIG. 3are merely approximations at small magnetization polar angles and do not necessarily reflect the phenomena occurring at larger magnetization polar angles. However, the precessional dynamics that occur when the magnetization vector of the free layer132is at small magnetization polar angles are, to a large extent, determinative of the efficiency of the switching process.

One solution that has been proposed to overcome this limitation is the use of a precessional spin current (“PSC”) magnetic layer having a magnetization vector that can freely rotate in any magnetic direction, shown inFIGS. 4A and 4B. The free layer336is similar to the free layer136previously discussed, in that it has an inherent damping characteristic205that can be overcome with the assistance of spin transfer torque. However, the polarizing layer150is replaced with a precessional magnetic layer350. As seen inFIG. 4A, which shows the projection onto the precessional plane of the direction of the spin transfer torque210created by spin current passing through free layer336, the direction of spin transfer torque210changes with the rotation of PSC magnetic layer350. As seen on the right side ofFIG. 4A, spin transfer torque210causes the magnetization direction200′ of the free layer336to precess in a cone-like manner around an axis202perpendicular to the plane of the layers. The PSC layer350and the free-layer336are magnetically and/or electronically coupled such that the magnetization direction of the magnetization vector270of the PSC magnetic layer350follows the precessional rotation of the magnetic vector of the free layer336.

As seen in on the right-hand side ofFIG. 4A, the spin polarized electrons provide torque211that helps to overcome the damping205in the first half of the precession215because the torque211provided by the spin polarized current is opposite that of the inherent damping205of the free layer336. In addition, torque211helps to overcome the damping205in the second half of the precession220by the same mechanism. Thus, unlike prior devices having a fixed polarization magnetic layer150, the spin of the electrons in the spin polarized current applies a torque211in both halves of the precession cycle, including the half of the precession cycle220where devices with fixed polarization magnetic layers150actually harmed precession. As is seen, the torque211continues to help overcome the inherent damping205of free layer136throughout the entire precession cycle. An MRAM device utilizing an MTJ structure with a PSC is depicted inFIG. 5.

However, because of the chirality of perpendicular MTJ structures that utilize a PSC, such as the structure shown inFIG. 5, the PSC only enhances switching of the free layer in one direction (i.e., from the parallel state to the anti-parallel state,FIG. 4A), but not the other (i.e., from the antiparallel state to the parallel state,FIG. 4B). As discussed above, when switching the free layer336from the first direction to the second direction, the spin current is generated by the electrons passing through the PSC layer and the in-plane spin transfer torque211is in line with the magnetic vector of the PSC layer (FIG. 4A). However, during switching free layer336from the second direction to the first direction, it is the reflected spin current from PSC layer that imparts the in-plane spin transfer torque211′ on the free layer336. As shown inFIG. 4B, the in-plane spin transfer torque211′ caused by the reflected spin current is in the direction anti-parallel to the magnetic vector270of the PSC layer350. When the magnetic vector270is aligned with the magnetic vector200, the in-plane spin transfer torque211′ might actually enhance the damping characteristic205of the free layer336. Therefore, when the precession of the magnetic vector270of the PSC layer350is synchronized with the precession of the magnetic vector200of the free layer336, the in-plane spin transfer torque211′ might enhance the damping characteristic205throughout the entire precession220′. Thus, the PSC layer can be highly effective at increasing the switching efficiency of the free layer in one direction, but may actually hamper switching in the other direction.

Thus, in prior devices that utilize in-plane polarization layers having a fixed magnetization direction and having a free magnetic layer150that is perpendicular to the plane of the device, once the precession holds, the in-plane spin transfer torque has no net positive effect on the switching mechanism for a full three hundred sixty degree precession. Moreover, in prior devices that utilize a PSC magnetic layer, the in-plane spin transfer torque enhances the switching of the free layer throughout the precession from the first direction to the second direction, but might not enhance the switching of the free layer from the second direction to the first direction.

Therefore, there is a need for a spin torque transfer device that reduces the amount of current needed for switching from both magnetization directions while also switching at high speeds and requiring reduced chip area.

SUMMARY

An MRAM device is disclosed that has a magnetic tunnel junction stack having a significantly improved performance of the free layer in the magnetic tunnel junction structure that requires significantly lower switching currents and which significantly reduces switching times for MRAM applications and maintains this characteristic for both switching directions (AP to P and P to AP)

In one embodiment, a magnetic device includes a reference magnetic layer in a first plane. The reference magnetic layer has a magnetization vector that is perpendicular to the first plane and has a fixed magnetization direction. The magnetic device also has a non-magnetic tunnel barrier layer in a second plane and disposed over the reference magnetic layer. The magnetic device also includes a free magnetic layer in a third plane and disposed over the non-magnetic tunnel barrier layer. The free magnetic layer has a magnetization vector that is perpendicular to the third plane and has a magnetization direction that can switch from a first magnetization direction to a second magnetization direction and from the second magnetization direction to the first magnetization direction. The switching process involves precessions of the magnetization vector around an axis perpendicular to the third plane at a precession radius. The magnetization vector of the free magnetic layer has a predetermined precession frequency. The magnetic device also includes a non-magnetic spacer in a fourth plane and disposed over the free magnetic layer. The magnetic device also includes an in-plane polarization magnetic layer in a fifth plane and disposed over the non-magnetic spacer. The in-plane polarization magnetic layer has a magnetization vector that is parallel to the fifth plane. The magnetic device also includes a non-magnetic insulator that is adjacent to the non-magnetic spacer and the in-plane polarization magnetic layer. The magnetic device also includes a metallic terminal coupled to the magnetic tunnel junction. The metallic terminal is adjacent to the non-magnetic insulator and is separated from the non-magnetic spacer and the in-plane polarization magnetic layer by the non-magnetic insulator. The magnetic device also includes a first current source that directs a read current through the metallic terminal and the magnetic tunnel junction. The read current measures the resistance across the magnetic tunnel junction. The magnetic device also includes a second current source that directs a programming current pulse through the magnetic tunnel junction, the non-magnetic spacer, and the in-plane polarization magnetic layer. The programming current pulse comprises a direct current pulse and an alternating perturbation pulse. The alternating perturbation pulse alternates between a maximum current value and a minimum current value at a first frequency. Application of the programming current pulse to the magnetic device produces a spin-polarized current having spin-polarized electrons. The spin-polarized current alternates between a maximum spin-current value and a minimum spin-current value at the first frequency. The spin-polarized electrons exert a spin transfer torque on the magnetization vector of the free magnetic layer, and the spin transfer torque alternates between a maximum magnitude and a minimum magnitude at the first frequency. The first frequency is synchronized with the predetermined precession frequency of the free magnetic layer, thereby causing the spin transfer torque to be at the maximum magnitude when the spin transfer torque increases the precession radius of the magnetization vector of the free magnetic layer, and at the minimum magnitude when the spin transfer torque decreases the precession radius of the magnetization vector of the free magnetic layer. In this way, the switching process of the free magnetic layer is accelerated from the first magnetization direction to the second magnetization direction and from the second magnetization direction to the first magnetization direction.

In another embodiment, a difference in frequency between the first frequency and the predetermined precession frequency of the free magnetic layer is less than twenty percent of the predetermined precession frequency of the free magnetic layer.

In another embodiment, a difference in frequency between the first frequency and the predetermined precession frequency of the free magnetic layer is less than ten percent of the predetermined precession frequency of the free magnetic layer.

In another embodiment, a difference in frequency between the first frequency and the predetermined precession frequency of the free magnetic layer is less than five percent of the predetermined precession frequency of the free magnetic layer.

In another embodiment, a difference in frequency between the first frequency and the predetermined precession frequency of the free magnetic layer is less than two percent of the predetermined precession frequency of the free magnetic layer.

In another embodiment, the direct current pulse has a current value different than zero.

In another embodiment, the direct current pulse has a value that is fixed throughout the duration of the pulse.

In another embodiment, the magnetization vector of the in-plane polarization magnetic layer is fixed.

In another embodiment, the metallic terminal comprises a metal, a metallic nanofiber, or a conductive carbon nanotube.

In another embodiment, the reference magnetic layer comprises CoFeB, the non-magnetic tunnel barrier layer comprises MgO, the free magnetic layer comprises CoFeB, the non-magnetic spacer comprises MgO, and the in-plane polarization magnetic layer comprises CoFeB.

In another embodiment, a magnetic device includes a reference magnetic layer in a first plane. The reference magnetic layer has a magnetization vector that is perpendicular to the first plane and has a fixed magnetization direction. The magnetic device also has a non-magnetic tunnel barrier layer in a second plane and disposed over the reference magnetic layer. The magnetic device also includes a free magnetic layer in a third plane and disposed over the non-magnetic tunnel barrier layer. The free magnetic layer has a magnetization vector that is perpendicular to the third plane and has a magnetization direction that can switch from a first magnetization direction to a second magnetization direction and from the second magnetization direction to the first magnetization direction. The switching process involves precessions of the magnetization vector around an axis perpendicular to the third plane at a precession radius. The magnetization vector of the free magnetic layer has a predetermined precession frequency. The reference magnetic layer, the non-magnetic tunnel barrier layer and the free magnetic layer form a magnetic tunnel junction (MTJ). The magnetic device also includes a non-magnetic spacer in a fourth plane and disposed over the free magnetic layer. The magnetic device also includes an in-plane polarization magnetic layer in a fifth plane and disposed over the non-magnetic spacer. The in-plane polarization magnetic layer has a magnetization vector that is parallel to the fifth plane. The magnetic device also includes a metallic spacer in a sixth plane and disposed over the in-plane polarization magnetic layer. The magnetic device also includes an in-plane spin torque oscillator layer in a seventh plane and disposed over the metallic spacer. The in-plane spin torque oscillator layer has a magnetization vector that precesses around an in-plane anisotropy axis or precesses in the seventh plane upon application of a programming voltage pulse. The magnetic device also includes a non-magnetic spin torque oscillator barrier layer in an eighth plane and disposed over the in-plane spin torque oscillator layer. The magnetic device also includes a perpendicular spin torque oscillator layer in a ninth plane and disposed over the non-magnetic spin torque oscillator barrier layer. The perpendicular spin torque oscillator layer has a magnetization vector that precesses around an out-of-plane anisotropy axis upon application of the programming voltage pulse. The non-magnetic spin torque oscillator barrier layer, the in-plane spin magnetized layer and the perpendicular spin torque oscillator layer form a spin torque nano oscillator (STNO). The magnetic device also includes a non-magnetic insulator that is adjacent to the non-magnetic spacer, the in-plane polarization magnetic layer, the metallic layer, the in-plane spin torque oscillator layer, the non-magnetic spin torque oscillator barrier layer, and the perpendicular spin torque oscillator layer. The magnetic device also includes a metallic terminal that is coupled to the MTJ. The metallic terminal is adjacent to the non-magnetic insulator and is separated from the non-magnetic spacer, the in-plane polarization magnetic layer, the metallic layer, and the STNO by the non-magnetic insulator. The magnetic device also includes a first voltage source that applies a read voltage across the metallic terminal and the MTJ. The read voltage measures the resistance across the MTJ. The magnetic device also includes a second voltage source that applies the programming voltage across the STNO, the metallic layer, the in-plane polarization magnetic layer, the non-magnetic spacer, and the MTJ. Application of the programming voltage pulse to the magnetic device results in a switching voltage across the in-plane polarization magnetic layer, the non-magnetic spacer and the MTJ. The switching voltage alternates between a maximum voltage value and a minimum voltage value at a first frequency. The first frequency is synchronized with the predetermined precession frequency of the free magnetic layer. In this way, the efficiency of the switching process of the free magnetic layer is enhanced during switching from the first magnetization direction to the second magnetization direction and from the second magnetization direction to the first magnetization direction.

In another embodiment, application of the switching voltage across the in-plane polarization magnetic layer, the non-magnetic spacer and the MTJ generates a spin-polarized current having spin-polarized electrons. The spin-polarized current alternates between a maximum spin-current value and a minimum spin-current value at the first frequency. The spin-polarized electrons exert a spin transfer torque on the magnetization vector of the free magnetic layer.

In another embodiment, the spin transfer torque alternates between a maximum magnitude and a minimum magnitude at the first frequency, thereby causing the spin transfer torque to be at the maximum magnitude when the spin transfer torque increases the precession radius of the magnetization vector of the free magnetic layer, and at the minimum magnitude when the spin transfer torque decreases the precession radius of the magnetization vector of the free magnetic layer.

In another embodiment, the metallic spacer comprises Ruthenium or Rhodium.

In another embodiment, the metallic spacer comprises a layer of Ruthenium, the layer of Ruthenium being between 2 and 10 angstroms thick.

In another embodiment, the magnetization vector of the in-plane spin torque oscillator layer and the magnetization vector of the in-plane polarization magnetic layer are magnetically coupled.

In another embodiment, a magnetic device includes a spin torque nano oscillator (STNO) in a first plane. The STNO comprises an in-plane spin torque oscillator layer, a non-magnetic spin torque oscillator barrier layer, and a perpendicular spin torque oscillator layer. The in-plane spin torque oscillator layer and the perpendicular spin torque oscillator layer are separated by the non-magnetic spin torque oscillator barrier layer. The in-plane spin torque oscillator layer has a magnetization vector that precesses around an in-plane anisotropy axis or precesses in the first plane upon application of a programming voltage pulse. The perpendicular spin torque oscillator layer has a magnetization vector that precesses around an out-of-plane anisotropy axis upon application of the programming voltage pulse. The magnetic device also includes a metallic layer in a second plane. The metallic layer separates the spin torque nano oscillator from an in-plane polarization magnetic layer. The magnetic device also includes the in-plane polarization magnetic layer in a third plane. The in-plane polarization magnetic layer has a magnetization vector that is parallel to the third plane. The magnetic device also includes a non-magnetic spacer in a fourth plane. The non-magnetic spacer separates the in-plane polarization magnetic layer from a magnetic tunnel junction (MTJ). The magnetic device also includes the MTJ in a fifth plane. The MTJ comprises a reference magnetic layer, a non-magnetic tunnel barrier layer, and a free magnetic layer. The free magnetic layer and the reference magnetic layer are separated by the non-magnetic tunnel barrier layer. The reference magnetic layer has a magnetization vector that is perpendicular to the fifth plane and has a fixed magnetization direction. The free magnetic layer has a magnetization vector that is perpendicular to the fifth plane and has a magnetization direction that can switch from a first magnetization direction to a second magnetization direction and from the second magnetization direction to the first magnetization direction. The switching process involves precessions of the magnetization vector around an axis perpendicular to the fifth plane at a precession radius. The magnetization vector of the free magnetic layer has a predetermined precession frequency. The magnetic device also includes a non-magnetic insulator that is adjacent to the non-magnetic spacer, the in-plane polarization magnetic layer, the metallic layer, and the STNO. The magnetic device also includes a metallic terminal coupled to the MTJ. The metallic terminal is adjacent to the non-magnetic insulator and the metallic terminal is separated from the non-magnetic spacer, the in-plane polarization magnetic layer, the metallic layer and the STNO by the non-magnetic insulator. The magnetic device also includes first voltage source that applies a read voltage across the metallic terminal and the MTJ, wherein the read voltage measures the resistance across the MTJ. The magnetic device also includes a second voltage source that applies the programming voltage across the STNO, the metallic layer, the in-plane polarization magnetic layer, the non-magnetic spacer, and the MTJ. Application of the programming voltage pulse to the magnetic device results in a switching current pulse that alternates between a maximum current value and a minimum current value at a first frequency. Application of this switching current pulse to the in-plane polarization magnetic layer, the non-magnetic spacer, and the MTJ results in a spin-polarized current having spin-polarized electrons. The spin-polarized current alternating between a maximum spin-current value and a minimum spin-current value at the first frequency. The first frequency is synchronized with the predetermined precession frequency of the free magnetic layer. In this way, the efficiency of the switching process of the free magnetic layer is enhanced during switching from the first magnetization direction to the second magnetization direction and from the second magnetization direction to the first magnetization direction.

In another embodiment, the spin transfer torque alternates between a maximum magnitude and a minimum magnitude at the first frequency, thereby causing the spin transfer torque to be at the maximum magnitude when the spin transfer torque increases the precession radius of the magnetization vector of the free magnetic layer and at the minimum magnitude when the spin transfer torque decreases the precession radius of the magnetization vector of the free magnetic layer.

In another embodiment, the in-plane polarization magnetic layer comprises CoFeB.

In another embodiment, the free magnetic layer comprises CoFeB.

In another embodiment, the non-magnetic tunnel barrier layer comprises MgO.

In another embodiment, the magnetization vector of the in-plane polarization magnetic layer is fixed.

In another embodiment, the in-plane spin torque oscillator layer comprises CoFeB and the perpendicular spin torque oscillator layer comprises CoFeB.

In another embodiment, the programming voltage pulse comprises a direct voltage.

In another embodiment, the magnetization vector of the in-plane polarization magnetic layer is fixed.

In another embodiment, the free magnetic layer comprises CoFeB.

In another embodiment, the non-magnetic tunnel barrier layer comprises MgO.

In another embodiment, the in-plane polarization magnetic layer comprises CoFeB, Fe, FeV, or FeB.

In another embodiment, the in-plane polarization magnetic layer comprises CoFeB.

In another embodiment, the non-magnetic spacer comprises MgO.

The figures are not necessarily drawn to scale and the elements of similar structures or functions are generally represented by like reference numerals for illustrative purposes throughout the figures. The figures are only intended to facilitate the description of the various embodiments described herein; the figures do not describe every aspect of the teachings disclosed herein and do not limit the scope of the claims.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled in the art to create and use methods and magnetic devices that utilize a programming current comprising an alternating perturbation current to assist in the switching of a magnetization vector for a magnetic semiconductor device such as an MRAM device. Each of the features and teachings disclosed herein can be utilized separately or in conjunction with other features to implement the disclosed system and method. Representative examples utilizing many of these additional features and teachings, both separately and in combination, are described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the claims. Therefore, combinations of features disclosed in the following detailed description may not be necessary to practice the teachings in the broadest sense, and are instead taught merely to describe particularly representative examples of the present teachings.

In the following description, for purposes of explanation only, specific nomenclature is set forth to provide a thorough understanding of the present teachings. However, it will be apparent to one skilled in the art that these specific details are not required to practice the present teachings.

The present patent document discloses a MRAM device that (1) comprises an in-plane polarization layer having a fixed magnetization direction and a perpendicular MTJ; (2) a first current source that directs a read current pulse through the MTJ, bypassing the in-plane polarization layer via a metallic terminal; and (3) a second current source that directs and programming current pulse through the in-plane polarization layer and the MTJ. This device is described with reference toFIGS. 6-17. The first current source applies a direct current to the MTJ to read the resistance across the MTJ. Instead of directing only a direct current through the in-plane polarization layer and the MTJ during the writing process, the second current source directs a programming current pulse that comprises an alternating perturbation current. In one embodiment, the programming current pulse comprises an alternating perturbation pulse and a direct current. Application of this programming current pulse to the magnetic device generates a spin-polarized current having spin polarized electrons that alternates between a maximum spin-current value and a minimum spin-current value at a first frequency. The spin-polarized electrons exert a spin transfer torque on the magnetization vector of the free magnetic layer, thereby assisting in the switching of the magnetization direction of the free layer. The first frequency is synchronized with the predetermined precession frequency of the magnetization vector of the free layer, such that the in-plane spin torque component provides a net benefit of assisting in the precessional motion of the magnetization vector, thereby enhancing the efficiency of the switching of the free layer. In one embodiment, by using the metallic terminal to bypass the in-plane polarizer, the possibility of accidently writing the bit during the read process is reduced.

The present patent document also discloses a MRAM device that (1) comprises a STNO, an in-plane polarization layer having a fixed magnetization direction, and a perpendicular MTJ; (2) a first voltage source that applies a read voltage pulse across the MTJ, bypassing the in-plane polarization layer and the STNO via a metallic terminal; and (3) a second voltage source that directs and programming voltage pulse across the STNO, the in-plane polarization layer and the MTJ. This device is described with reference toFIGS. 6-17. The first voltage source applies a direct current to the MTJ to read the resistance across the MTJ. In one embodiment, during the writing process, the second voltage source applies a direct voltage (i.e., programming voltage) across the pulse across the STNO, the in-plane polarization layer and the MTJ. This generates a switching current pulse that oscillates between a maximum current value and a minimum current value at a frequency determined by the structure and composition of the STNO (i.e., the first frequency). Application of the switching current pulse to the in-plane polarization layer and MTJ generates a spin-polarized current having spin polarized electrons that alternates between a maximum spin-current value and a minimum spin-current value at the first frequency. The spin-polarized electrons exert a spin transfer torque on the magnetization vector of the free magnetic layer, thereby assisting in the switching of the magnetization direction of the free layer. In another embodiment, during the writing process, the second voltage source applies the programming voltage pulse across the STNO, the in-plane polarization layer and the MTJ, generates a switching voltage across the in-plane polarization magnetic layer, the non-magnetic spacer, and the MTJ. The switching voltage oscillates between a maximum voltage value and a minimum voltage value at the first frequency. Application of the programming voltage to the in-plane polarization layer and MTJ generates a spin-polarized current having spin polarized electrons that alternates between a maximum spin-current value and a minimum spin-current value at a first frequency. The spin-polarized electrons exert a spin transfer torque on the magnetization vector of the free magnetic layer, thereby assisting in the switching of the magnetization direction of the free layer. In some embodiments, the first frequency is synchronized with the predetermined precession frequency of the magnetization vector of the free layer, such that the in-plane spin torque component provides a net benefit of assisting in the precessional motion of the magnetization vector, thereby enhancing the efficiency of the switching of the free layer. In one embodiment, by using the metallic terminal to bypass the STNO and the in-plane polarizer, the possibility of accidently writing the bit during the read process is reduced.

For ease of description, many of the phenomena occurring during the writing process will be described with respect to two-terminal MRAM devices that are analogous to the three-terminal MRAM devices described herein. In particular,FIG. 7depicts a two-terminal device500with an in-plane polarizer, a perpendicular MTJ, and a current source that directs an alternating perturbation current, andFIG. 11depicts a two-terminal device700with a STNO, in-plane polarizer, and a perpendicular MTJ.

As described above, when a programming current pulse comprising an alternating perturbation current is applied to the in-plane polarization layer550and a perpendicular MTJ530, a spin-polarized current is formed that alternates between a maximum spin-current value and a minimum spin-current value at the first frequency. As the programming current alternates between a maximum current value and a minimum current value, the spin-polarized current also alternates between a maximum spin-current value and a minimum spin-current value at the first frequency. Similarly, when the switching current (generated by application of a programming voltage pulse across device700) is applied to the in-plane polarization layer750and a perpendicular MTJ730, a spin-polarized current is formed. As the switching current alternates between a maximum current value and a minimum current value, the spin-polarized current also alternates between a maximum spin-current value and a minimum spin-current value at the first frequency.

In addition, the magnitude of the spin-transfer torque exerted on the free layers536or736is proportional to the spin-current value. Therefore, when the spin current is at the maximum spin-current value, the magnitude of the spin-transfer torque being exerted on the free layer is at the maximum magnitude. When the spin current is at the minimum spin-current value, the magnitude of the spin-transfer torque being exerted on the free layer is at the minimum magnitude. Therefore, in embodiments where the first frequency in synchronized with the predetermined precession frequency of the magnetization vector of the free layer, the magnitude of the spin transfer torque will oscillate between the maximum magnitude and minimum magnitude at a frequency that is synchronized with the precession of the free layer (i.e., the first frequency).

FIGS. 6A and 6Bshow the concepts behind the writing process of the various devices disclosed herein. For ease of explanation, descriptions of these concepts will first be made with respect to the two-terminal device500(the device inFIG. 7that comprises an in-plane polarization layer550, and a MTJ structure530). The magnetization dynamics depicted inFIGS. 6A and 6Bare approximations of the dynamics that occur during the initial stages of switching the magnetization vector of free layer536(i.e., when the polar angle between axis200and magnetic vector200′ is small). At larger magnetization polar angles (i.e., more than 10 degrees from equilibrium), the dynamics presented in these figures are no longer provide an accurate depiction of the magnetization dynamics of the free layer. Nevertheless, the precessional dynamics that occur at small magnetization polar angles are to a great extent determinative of the switching process and are therefore useful in understanding the disclosures herein. Moreover, the precessional dynamics at small magnetization polar angles are important to improved switching of the free layer, the various perturbation frequencies in the switching currents (i.e., the first frequency) are matched to the precession frequencies of the free layers at small magnetization polar angles (i.e., the predetermined precession frequency).

Like the in-plane polarizer150previously discussed, the in-plane polarization layer550in this embodiment has a magnetic vector with a fixed magnetization direction (top ofFIG. 6A). The free layer536in this embodiment is similar to the free layer136previously discussed, in that it has an inherent damping characteristic205that can be overcome with the assistance of spin transfer torque. As seen in the middle ofFIG. 6A, the in-plane spin transfer torque610causes the magnetization direction200′ of the free layer536to precess in a cone-like manner around an axis202perpendicular to the plane of the layers.FIG. 6Ashows a progression of rotation of the magnetic direction200′ about axis202. As discussed, when a spin polarized current traverses the device, the magnetization of the free layer536precesses in a continuous manner (i.e., it turns on itself in a continuous manner as shown in the middle ofFIG. 6A) with maintained oscillations until the magnetic direction of free layer536is opposite the magnetic direction prior to the spin torque causing precession, i.e., the magnetic direction of free layer136switches by 180 degrees.

The spin-polarized electrons of the spin-polarized current exert a spin transfer torque on the magnetization layer of the free layer. This spin transfer torque has both an in-plane spin torque component610and a perpendicular spin torque component (not shown inFIG. 6A). The perpendicular spin torque exerts a force on the magnetization vector of the free layer that pulls the magnetization vector from one perpendicular position to the other perpendicular position (e.g., from the parallel to the anti-parallel state). This perpendicular spin torque component is caused by spin-polarization of the electrons by the reference magnetic layer532(depicted inFIG. 7). The in-plane spin torque610assists in the switching of the free layer by providing a force that pushes the magnetization vector away from the perpendicular axis202, allowing the perpendicular spin transfer torque to act on the magnetization vector, thereby switching the free layer. This in-plane spin torque610is caused by spin-polarization of the electrons by the in-plane polarization magnetic layer550.

The in-plane spin torque610also enhances the precessional motion of the magnetization vector of the free layer. As seen on the right-hand side ofFIG. 6A, the spin-polarized electrons provide in-plane spin torque610that helps to overcome the damping205in the first half of the precession215because the in-plane spin torque610provided by the spin-polarized current is opposite that of the inherent damping205of the free layer536. During the first half of the precession, the spin-polarized current is at or near the maximum current value, thereby imparting in-plane spin transfer torque610at or near the maximum spin-torque magnitude (depicted by the longer arrow). As previously discussed, during the second half of the precession215, this in-plane spin torque610′ is actually aligned with the inherent damping of the free layer536. However, in one embodiment, during the second half of the precession, the spin-polarized current is at or near the minimum current value, thereby imparting in-plane spin transfer torque610′ at or near the minimum spin-torque magnitude (depicted by the shorter arrow). The frequency at which the switching current alternates between its maximum current value and its minimum current value is set at a first frequency. The first frequency is set to be synchronized with the predetermined precession frequency of the free layer. Therefore, the magnitude of the in-plane spin transfer torque is maximized (FIG. 6A, 610) when it enhances the precessional motion of the free layer and is minimized (FIG. 6A, 610′) when the in-plane spin transfer torque opposes the precessional motion (bottom ofFIG. 6A). Thus, the in-plane spin torque component provides a net benefit of assisting in the precessional motion of the magnetization vector, thereby enhancing the efficiency of the switching of the free layer.

Moreover, as shown inFIG. 6B, the use of a magnetic device500that comprises in-plane polarization magnetic layer550and MTJ structure530can provide a net enhancement to the precession throughout the switching process of the free layer536from both the first magnetization direction and the second magnetization direction (i.e., switching of the free layer from both the parallel direction to the antiparallel direction and from the antiparallel direction to the parallel direction). During switching of the free layer536from the second magnetic direction to the first magnetic direction, it is the reflected spin current generated by the in-plane polarizer that exerts the in-plane spin transfer torque611on the free layer536. As discussed above, the direction of the in-plane spin transfer torque611exerted by the reflected spin current is antiparallel to the magnetic direction of the in-plane polarizer550(top ofFIG. 7B). Nevertheless, the first frequency can be synchronized with the predetermined precession frequency of the magnetization vector200′ of the free magnetic layer in such a way as to maximize the in-plane spin transfer torque611when it opposes the damping force205and minimize the in-plane spin transfer torque611′ when it enhances the damping force205(middle ofFIG. 7B). Thus, synchronization of the first frequency with the predetermined precession frequency of the free layer can be accomplished regardless of the direction of the current, achieving a net enhancement of the precessional motion during switching of free layer536in both directions. In embodiments that utilize a STNO structure (such as STNO570) to generate the alternating perturbation in the switching current, the first frequency is tuned by varying the composition and dimensions of the various layers of the STNO.

FIG. 7shows a two-terminal memory cell500with an in-plane polarization layer550, and a MTJ structure530. MTJ structure530includes at least a reference layer532, a tunneling barrier layer540, a free layer536. As shown inFIG. 11, the reference layer532of MTJ530has a magnetization direction that is preferably perpendicular to its plane, although variations of several degrees are within the scope of what is considered perpendicular. As also seen inFIG. 7, free layer536also has a magnetization vector that is preferably perpendicular to its plane, but its direction can vary by 180 degrees. A nonmagnetic spacer540is disposed over free layer536. In-plane polarization magnetic layer550is disposed over nonmagnetic spacer540. In-plane polarization magnetic layer550has a magnetization vector having a magnetic direction parallel to its plane. In-plane polarization magnetic layer550has a magnetization direction that is preferably parallel to its plane, although variations of several degrees are within the scope of what is considered parallel.

The results of several simulations of a device having the structure described inFIG. 7are seen inFIG. 8. These results are representative of the writing process for three-terminal devices that utilize a programming current comprising an alternating perturbation current (such as those depicted byFIG. 14). The X axis of the graph is the voltage of the direct current component of the programming current that applied to the device. The Y axis is the write error rate (WER), which is indicative of the how easily the magnetization vector of the free layer switches from the first magnetization direction to the second magnetization direction or from the second magnetization direction to the first magnetization direction; the lower the WER, the more effective the applied current is at switching the free layer. The graph depicts two series of transitions: (1) switching the free layer from the parallel direction to the antiparallel direction (filled data points, indicated by “2AP” on the legend); and (2) switching the free layer from the antiparallel direction to the parallel direction (unfilled data points, indicated by “2P” on the legends.) Within each series, different sets of experiments were run, with the perturbation amplitude of the alternating current frequency set to 0%, 10%, 20%, and 30% of the direct current voltage (depicted on the legend as “polarized”; “A=0.1”; “A=0.2”; and “A=0.3”, respectively). In each of the simulations, the alternating perturbation frequency (ω) is synchronized with the predetermined precession frequency of the free layer (ωpi) and the alternating perturbation current is applied to the free layer for a pulse length of 5 ns.

For the first series of simulations (switching from the parallel to anti-parallel directions), both the constant non-polarized current (line8-A with filled squares) and the constant polarized current (line8-B with filled circles) had a similar effect on the switching of the free layer. The data show that application of an alternating perturbation current causes a reduction in switching Voltage values for the system, with the switching Voltage values decreasing as the perturbation amplitude increases. This effect can be seen by comparing the lines for the currents with 0% perturbation amplitude (line8-B with filled circles); 10% perturbation amplitude (line8-C with filled diamonds), 20% perturbation amplitude (line8-D with filled crosses); and 30% perturbation amplitude (line8-E with filled pentagons). The lines that correspond to higher values of the perturbation amplitude lie below the lines with lower values of the perturbation amplitude, suggesting that increasing the perturbation amplitudes reduces the WER, i.e., it reduces the probability that a bit is written incorrectly.

Similar results were obtained in the second series of experiments (switching from the anti-parallel to the parallel direction). No significant change in switching voltage was observed between the constant non-polarized current (line8-F with empty squares) and the constant polarized current (line8-G with empty circles). However, the switching Voltage values decrease as the perturbation amplitude increases, as can be observed by comparing the lines with 0% perturbation amplitude (line8-G with empty circles); 10% perturbation amplitude (line8-H with empty diamonds), 20% perturbation amplitude (line8-I with empty crosses); and 30% perturbation amplitude (line8-J with empty pentagons), respectively. The lines that correspond to higher perturbation amplitude lie below the lines with smaller perturbation amplitude.

FIGS. 9A and 9Bdemonstrate the effect of the frequency and amplitude of the alternating perturbation current on the WER value of selected simulations described inFIG. 10. The Y axis is the ratio between the frequency of the alternating perturbation current (ω) and the predetermined precession frequency of the free layer (ωp). The X axis is the perturbation amplitude, which ranges from 0% to 40% of the direct current voltage. For these simulations, the voltage of the alternating perturbation current was fixed at 0.6 volts and the system was pulsed for 5 ns. The colors represent the WER values at each of the corresponding amplitude and frequency ratios. Lower WER values indicate an increased efficiency in the switching of the free layer.

FIG. 9Adepicts the results from a set of simulations in which the magnetization vector of the free layer is switched from the parallel direction to the antiparallel direction. The results demonstrate that a reduction in WER occurs when the frequency of alternating perturbation current is roughly equal to the predetermined precession frequency of the free layer (with a ratio value of roughly 0.95 to 1.05) and the perturbation amplitude is at least roughly 15% of the direct current voltage. At higher perturbation amplitudes (30-40% of the direct voltage current), the impact on the WER values is significantly enhanced, indicating a greater assistance with the switching of the free layer. Moreover, the width of the resonance is greater at higher perturbation amplitudes, with appreciable reduction in WER occurring when the frequency ratio is between 0.9 and 1.1.

FIG. 9Bdemonstrates similar results during the switching of the magnetization vector of the free layer from the antiparallel direction to the parallel direction. AlthoughFIG. 11Bhas lower resolution thanFIG. 9A(due to fewer simulations), similar results are clearly discernable: substantial WER reduction when the ω/ωpratio is between 0.95 to 1.05; pronounced reduction in WER at higher perturbation amplitudes (particularly 0.3-0.4); and an increase in width of the resonance as the perturbation amplitude increases. Thus, the results of these simulations demonstrate the significant improvement provided by the various embodiments described herein.

Thus, the results inFIGS. 8-9demonstrate improved switching with the use of a programming current that comprises an alternating perturbation current with an oscillation frequency that is synchronized with the predetermined precession frequency of the free layer. Some of the various embodiments disclosed herein utilize the significant improvement provided by the alternating perturbation current. In general, the voltage required to switch the magnetization vector from the anti-parallel direction to the parallel direction was considerably less than that required to switch the magnetization vector from the parallel direction to the anti-parallel direction. Moreover, while the use of an alternating perturbation current effectively reduces the switching Voltage value for switching in both directions, this effect is more pronounced for switching from the parallel to the antiparallel direction because the spin-polarization efficiency from the in-plane polarizing layer to the free layer is higher in this case.

The three-terminal devices disclosed herein offer the additional advantage of bypassing the polarizer during the reading process. As described above, by passing a programming current comprising an alternating perturbation current through the in-plane polarizer and MTJ, efficient switching is obtained. However, during the reading phase, the objective is to measure the resistance across the MTJ without disturbing the free layer. In some embodiments, three-terminal structures such as those described inFIG. 14utilize a metallic terminal to bypass the in-plane polarizer, thereby allowing a read-current source (i.e., the first current source) to apply a direct current across only the MTJ. In this way, these three-terminal devices minimize the probability of errant switching while reading the bit. Thus, during the writing process, these three-terminal devices benefit from all of the advantages described above, but bypass these processes during the reading process, thereby improving the accuracy of the read.

In some embodiments, the three-terminal devices disclosed herein utilize a STNO to assist in switching the free layer. For ease of explanation, descriptions of the phenomena involved in switching the free layer of a device comprising an STNO will be made with reference to the two-terminal device depicted inFIG. 11. Three-terminal devices, such as those described herein utilize the various advantages of the STNO during the writing process, but allow bypass of the STNO during the reading process, thereby reducing errant switching when reading the bit.

FIG. 10Ashows a schematic for the magnetization dynamics of STNO770. Application of the programming voltage to the magnetic device causes the magnetization vectors of the in-plane spin torque oscillator layer772and the perpendicular spin torque oscillator layer776to precess around their respective anisotropy axes, as shown on the top ofFIG. 10A. The resistance across the STNO770is dependent on the relative orientation of the magnetization vectors of the in-plane spin torque oscillator layer772and the perpendicular spin torque oscillator layer776. For example, when the two magnetization vectors are arranged in opposite orientations (as depicted in the middle ofFIG. 10A), STNO770will possess a relatively high resistance value, compared to the relatively low resistance value possessed by the STNO when the two magnetization vectors are aligned (as depicted in the bottom ofFIG. 10A). Thus, as the magnetization vectors of the in-plane spin torque oscillator layer772and the perpendicular spin torque oscillator layer776precess around their respective axes, STNO770alternates between a high resistance state and a low resistance state at a first frequency.

FIG. 10Bdepicts the effect of these precessional dynamics on the resistance across STNO770and the voltage across the in-plane polarization magnetic layer, the non-magnetic spacer, and the MTJ. As described above, the precessional dynamics of the magnetization vectors of the in-plane spin torque oscillator layer772and the perpendicular spin torque oscillator layer776cause the resistance across STNO770to oscillate between a high resistance state and a low resistance state (shown in the top graph ofFIG. 10B) at the first frequency. As a result, the voltage across the magnetic spacer760, the in-plane polarization magnetic layer750, the non-magnetic spacer740, and the MTJ structure730(i.e., the switching voltage) oscillates between a high voltage value (corresponding to the time at which the resistance across the STNO770is at the minimum resistance value) and a low voltage value (corresponding to the time at which the resistance across the STNO770is at the maximum resistance value) at the same first frequency. The strength of the current leaving the STNO770and passing through the magnetic spacer760, the in-plane polarization magnetic layer750, the non-magnetic spacer740, and the MTJ structure730(i.e., the switching current) is directly proportional to the switching voltage. Therefore, the switching current alternates between a maximum current value and a minimum current value at the first frequency.

In one embodiment, the programming voltage pulse is a direct voltage with a fixed voltage value. Upon being subjected to the alternating resistance states of the STNO570, the resulting current leaving the STNO770and (i.e., the switching current) oscillates between a maximum current value and a minimum current value. In one embodiment, the frequency at which the oscillation occurs (i.e., the first frequency) is matched to the predetermined precession frequency of the magnetic vector of the free layer736while the precession frequency is near its maximum value. Thus, the switching current will oscillate between the maximum and minimum current values in a manner that is synchronized with the initial precessions of the magnetization vector of the free layer736(i.e., the predetermined precession frequency of the free layer536, which is the frequency at which the magnetization vector of free layer736precesses when the vector is nearly perpendicular to the plane). However, later in the switching process, as the precession frequency of the free layer decreases (when the magnetization vector of the free layer736precesses near or in the plane), the predetermined precession frequency of the free layer536may fall out of phase with the first frequency.

FIG. 11shows a two-terminal memory cell700with a STNO structure770, an in-plane polarization layer750, and a MTJ structure730. MTJ structure730includes at least a reference layer732, a tunneling barrier layer740, a free layer736. As shown inFIG. 11, the reference layer732of MTJ730has a magnetization direction that is preferably perpendicular to its plane, although variations of several degrees are within the scope of what is considered perpendicular. As also seen inFIG. 11, free layer736also has a magnetization vector that is preferably perpendicular to its plane, but its direction can vary by 180 degrees. A nonmagnetic spacer740is disposed over free layer736. In-plane polarization magnetic layer750is disposed over nonmagnetic spacer740. In-plane polarization magnetic layer750has a magnetization vector having a magnetic direction parallel to its plane. In-plane polarization magnetic layer750has a magnetization direction that is preferably parallel to its plane, although variations of several degrees are within the scope of what is considered parallel. Metallic spacer760is disposed over the in-plane polarization layer750. STNO structure770is deposited over metallic spacer760. STNO structure770includes in-plane spin torque oscillator layer772, a non-magnetic spin torque oscillator barrier layer774, and a perpendicular spin torque oscillator layer776. As shown inFIG. 11, the magnetization vector of in-plane spin torque oscillator layer772, has a magnetization direction that precesses around an anisotropy axis that is parallel to the plane, although variations of several degrees are within the scope of what is considered parallel. In some embodiments, the magnetic direction of the in-plane spin torque oscillator layer precesses in the plane if the in-plane anisotropy is weak. As also seen inFIG. 11, the perpendicular spin torque oscillator layer776has a magnetization vector magnetization direction that precesses around an anisotropy axis that is perpendicular to the plane, although variations of several degrees are within the scope of what is considered perpendicular. In one embodiment, voltage source795generates a direct voltage.

FIG. 12depicts the voltage across the two-terminal magnetic device700, which comprises STNO770, in-plane polarization layer750, and MTJ structure730. As the programming current pulse flows through device700, the programming current pulse initiates the precessional dynamics of the magnetic vectors of the STNO770. As discussed above, this causes the resistance across STNO770to oscillate between a high resistance state and a low resistance state at a first frequency. In one embodiment, the voltage across the entire device (labeled a V-Pulse) is fixed and constant. V-STNO oscillates between a maximum voltage value and a minimum voltage value at the first frequency due to the oscillating resistance value of STNO770. Metallic spacer760is comprised of a highly-conductive material, such as a metallic material. Therefore, the output voltage from STNO770is essentially equivalent to the input voltage for the V-pOST. Accordingly, V-pOST also oscillates between a maximum voltage value and a minimum voltage value at the first frequency, thereby causing the current flowing through the OST-MTJ structure (i.e., the switching current) to alternate between a maximum current value and a minimum current value at the first frequency.

The results of several simulations of a device having the structure described inFIG. 11are seen inFIG. 13. These results are representative of the writing process for three-terminal devices that utilize a programming voltage and an STNO (such as those depicted byFIG. 15). The X axis of the graph is the voltage of the programming pulse that was applied to the device. The Y axis is the write error rate (WER), which is indicative of the how easily the magnetization vector of the free layer switches from the first magnetization direction to the second magnetization direction or from the second magnetization direction to the first magnetization direction; the lower the WER, the more effective the applied current is at switching the free layer. The graph depicts two series of transitions: (1) switching the free layer from the parallel direction to the antiparallel direction (filled data points, indicated by “2AP” on the legend); and (2) switching the free layer from the antiparallel direction to the parallel direction (unfilled data points, indicated by “2P” on the legends.) To simulate a device comprising an STNO, an in-plane polarizer, and an MTJ, a switching current was used that comprised a direct current with a fixed value and an alternating perturbation current superimposed on the direct current. Within each series, different sets of simulations were run, with the perturbation amplitude of the alternating current frequency set to 0%, 10%, 20%, and 30% of the direct current voltage (depicted on the legend as “polarized”; “A=0.1”; “A=0.2”; and “A=0.3”, respectively). In each of the simulations, the alternating perturbation frequency (ω) is synchronized with the predetermined precession frequency of the free layer (ωpi) and the alternating perturbation current is applied to the free layer for a pulse length of 3 ns.

For the first series of simulations (switching from the parallel to anti-parallel directions), both the constant non-polarized current (line13-A with filled squares) and the constant polarized current (line13-B with filled circles) had a similar effect on the switching of the free layer. The data show that application of an alternating perturbation current causes a reduction in switching Voltage values for the system, with the switching Voltage values decreasing as the perturbation amplitude increases. This effect can be seen by comparing the lines for the currents with 0% perturbation amplitude (line13-B with filled circles); 10% perturbation amplitude (line13-C with filled diamonds), 20% perturbation amplitude (line13-D with filled crosses); and 30% perturbation amplitude (line13-E with filled pentagons). The lines that correspond to higher values of the perturbation amplitude lie below the lines with lower values of the perturbation amplitude, suggesting that increasing the perturbation amplitudes reduces the WER, i.e., it reduces the probability that a bit is written incorrectly.

Similar results were obtained in the second series of experiments (switching from the anti-parallel to the parallel direction). No significant change in switching voltage was observed between the constant non-polarized current (line13-F with empty squares) and the constant polarized current (line13-G with empty circles). However, the switching voltage values decrease as the perturbation amplitude increases, as can be observed by comparing the lines with 0% perturbation amplitude (line13-G with empty circles); 10% perturbation amplitude (line13-H with empty diamonds), 20% perturbation amplitude (line13-I with empty crosses); and 30% perturbation amplitude (line13-J with empty pentagons), respectively. The lines that correspond to higher perturbation amplitude lie below the lines with smaller perturbation amplitude.

Thus, the results inFIG. 13demonstrate improved switching with the use of a magnetic device comprising a STNO, an in-plane polarizer and a MTJ when the oscillating impedance frequency of the STNO is synchronized with the predetermined precession frequency of the free layer, thereby demonstrating the significant improvements to the writing process provided by some embodiments described herein. In general, the voltage required to switch the magnetization vector from the anti-parallel direction to the parallel direction was considerably less than that required to switch the magnetization vector from the parallel direction to the anti-parallel direction. Moreover, while the use of an alternating perturbation current effectively reduces the switching voltage value for switching in both directions, this effect is more pronounced for switching from the parallel to the antiparallel direction because the spin-polarization efficiency from the in-plane polarizing layer to the free layer is higher in this case.

The three-terminal devices disclosed herein offer the additional advantage of bypassing the SNTO and the polarizer during the reading process. As described above, by applying a programming voltage across the STNO, the in-plane polarizer and MTJ, efficient switching is obtained. However, during the reading phase, the objective is to measure the resistance across the MTJ without disturbing the free layer. In some embodiments, three-terminal structures such as those described inFIG. 15utilize a metallic terminal to bypass the in-plane polarizer, thereby allowing a read-current source (i.e., the first current source) to apply a direct current across only the MTJ. In this way, these three-terminal devices minimize the probability of errant switching while reading the bit. Thus, during the writing process, these three-terminal devices benefit from all of the advantages described above, but bypass these processes during the reading process, thereby improving the accuracy of the read.

A three-terminal memory cell800with an in-plane polarization layer850, an MTJ structure800, a programming current source875, a metallic terminal844, and a reading current source885is shown inFIG. 14. MTJ structure800includes one or more seed layers810provided at the bottom of stack800to initiate a desired crystalline growth in the above-deposited layers. Synthetic antiferromagnetic (SAF) layer820is disposed over seed layer810. SAF layer820is comprised of a first SAF layer832, anti-ferromagnetic coupling layer816and second SAF layer814. Second SAF layer814is deposited over seed layer810, while anti-ferromagnetic coupling layer816is placed over second SAF layer814. MTJ830is deposited over anti-ferromagnetic coupling layer816. MTJ830includes first SAF layer832, which acts as the reference layer of the MTJ, and is also part of SAF layer820. A tunneling barrier layer (i.e., the insulator)834is disposed over first SAF layer832while the free layer836is disposed over tunneling barrier layer834. As shown inFIG. 14, the magnetization vector of first SAF layer832has a magnetization direction that is preferably perpendicular to its plane, although variations of several degrees are within the scope of what is considered perpendicular. As also seen inFIG. 14, free layer836also has a magnetization vector that is preferably perpendicular to its plane, but its direction can vary by 180 degrees. In one embodiment, nonmagnetic spacer840is disposed over a first portion of MTJ830. In-plane polarization magnetic layer850is disposed over a portion of nonmagnetic spacer840. In one embodiment, in-plane polarization magnetic layer850has a magnetization vector having a magnetic direction parallel to its plane, and is perpendicular to the magnetic vector of the reference layer832and free layer836. In-plane polarization magnetic layer850has a magnetization direction that is preferably parallel to its plane, although variations of several degrees are within the scope of what is considered parallel. One or more capping layers880can be provided on top of in-plane polarization magnetic layer850to protect the layers below on MTJ stack800. Non-magnetic insulator842is disposed over a second portion of MTJ830. In one embodiment, non-magnetic insulator842is adjacent to non-magnetic spacer840, and in-plane polarization magnetic layer850. Metallic terminal844is disposed over a third portion of MTJ830and is adjacent to non-magnetic insulator842. In some embodiments, external non-magnetic insulator846is disposed over a fourth portion of MTJ830and is adjacent to metallic terminal844. In some embodiments, first current source885generates a direct current pulse. In one embodiment, the first current source directs the direct current pulse through metallic terminal844and MTJ830. In one embodiment, second current source875generates a programming current pulse that comprises an alternating perturbation current. In some embodiments, the alternating perturbation current has an oscillation frequency that is synchronized with the precession frequency of magnetization vector of free layer836. In one embodiment, current source875directs the alternating perturbation current through in-plane polarization layer850, non-magnetic spacer840, and MTJ830. In some embodiments, second current source875generates a programming current comprising an alternating perturbation current that is matched to the predetermined precession frequency of the free layer836and a direct current, which can be changed from a first direct current value to a second direct current value during switching of the free layer836.

Seed layer810in the MTJ structure shown inFIG. 14preferably comprises Ta, TaN, Cr, Cu, CuN, Ni, Fe or alloys thereof. Second SAF layer814preferably comprises either a Co/Ni, Co/Pt or Co/Pd multilayer structure. First SAF layer832preferably comprises either a Co/Ni or Co/Pt multilayer structure plus a thin non-magnetic layer comprised of tantalum having a thickness of two to five Angstroms and a thin CoFeB layer (0.5 to three nanometers). Anti-ferromagnetic coupling layer816is preferably made from Ru having thickness in the range of three to ten Angstroms. Tunneling barrier layer834is preferably made of an insulating material such as MgO, with a thickness of approximately ten Angstroms. Free layer836is preferably made with CoFeB deposited on top of tunneling barrier layer834. Free layer836can also have layers of Fe, Co, Ni or alloys thereof and W and Ta insertion layers to promote perpendicular anisotropy. Spacer layer840over MTJ830can be any non-magnetic material such as 2 to 20 Angstroms of ruthenium, 2-20 Angstroms of Ta, 2-20 Angstroms of TaN, 2-20 Angstroms of Cu, 2-20 Angstroms of CuN, or 2-20 Angstroms MgO layer, or or 2-20 Angstroms Al2O3layer.

Nonmagnetic spacer840has a number of properties. For example, nonmagnetic spacer840physically separates the free layer836and the in-plane polarization magnetic layer850. Nonmagnetic spacer840transmits spin current efficiently from the in-plane polarization magnetic layer850into the free layer836because it preferably has a long spin diffusion length if made metallic. Nonmagnetic spacer840also promotes good microstructure and high tunneling magnetoresistance (TMR) and helps keep the damping constant of the free layer536low. In one embodiment, nonmagnetic space840comprises MgO.

The in-plane polarization magnetic layer850is preferably made from CoFeB. It can also be made with Co, Fe, Ni magnetic layers or can be made out of their alloys. The magnetic alloys can also have boron, tantalum, copper or other materials.

Non-magnetic insulator842has a number of properties. For example, nonmagnetic insulator842physically separates the metallic terminal844from the in-plane polarization magnetic layer850and the non-magnetic spacer840. Nonmagnetic insulator842does not transfer electrical current or does so with very low efficiency, thereby preventing current from metallic terminal844from entering either the in-plane polarization magnetic layer850or the non-magnetic spacer840. Nonmagnetic insulator842preferably comprises a nonmagnetic, nonconductive material.

Metallic terminal844has a number of properties. Metallic terminal efficiently conducts electrical current from first current source885to MTJ structure830. Metallic terminal844preferably comprises a highly-conductive, non-magnetic material. Metallic terminal844preferably comprises a metal, a metallic nanofiber, or a conductive carbon nanotube.

Nonmagnetic external insulator846has a number of properties. For example, nonmagnetic external insulator842physically separates the metallic terminal844from anything external to the MTJ stack800. In addition, nonmagnetic external insulator846does not transfer electrical current or does so with very low efficiency. Nonmagnetic external insulator846preferably comprises a nonmagnetic, nonconductive material.

Finally capping layer880can be any material that provides good interface to the in-plane layer such as Ta, TaN, Ru, MgO, Cu, etc. In some embodiment, first current source885generates a direct current. In some embodiments, second current source875can generate a programming current pulse that comprises both an alternating perturbation current and a direct current component. In addition, second current source875can be configured to produce an alternating perturbation current having an oscillation frequency that is matched to the predetermined precession frequency of the free magnetic layer836. Current source875can also generate programming currents with various ratios of alternating perturbation current and direct current. In some embodiments, current source875can provide an alternating perturbation current pulse for a first time interval and a direct current pulse or a switching current pulse for a second time interval. In some embodiments, current source875can also generate a programming current comprising an alternating perturbation current and a direct current, and the direct current can be changed from a first direct current value to a second direct current value.

The manner in which a bit is written using a programming current pulse comprising an alternating perturbation current and three-terminal magnetic device800will now be described. In particular, an electrical current is supplied, for example, by second current source875, which passes electrical current through the in-plane polarization magnetic layer850, the non-magnetic spacer840, the free magnetic layer836, the non-magnetic tunneling barrier layer834, and the reference layer832. Application of the electrical current (i.e., programming current) to the magnetic device800creates a spin polarized current that passes through non-magnetic spacer layer840, free magnetic layer836, tunneling barrier layer834, and reference magnetic layer832. The spin polarized current exerts a spin transfer torque on free magnetic layer836, which helps overcome the inherent damping of the magnetic material making up the free layer836. The spin transfer torque is composed of an in-plane spin transfer torque and a perpendicular spin transfer torque. In one embodiment, when switching the free layer836in one direction (e.g., from the parallel direction to the anti-parallel direction), the in-plane spin transfer torque is caused by the transverse spin current generated by the in-plane polarization magnetic layer850and the perpendicular spin transfer torque is caused by the reflected spin current generated by the reference magnetic layer832. This causes the free magnetic layer836to precess about its axis, which is shown inFIG. 6A.

The programming current pulse comprises an alternating perturbation current generated, for example, by current source875. Therefore, the programming current to alternate between a maximum current value and a minimum current value at a first frequency. As a result, the spin-polarized current, which is generated by application of the programming current to magnetic device800, alternates between a maximum spin-current value and a minimum spin current value at the same first frequency. The magnitude of the spin transfer torque exerted on the magnetic vector of the free layer836is proportional to the spin-current value because at higher spin-current values, there are more spin-polarized electrons flowing through free layer836. Therefore, the magnitude of the spin transfer torque alternates between a maximum spin torque magnitude and a minimum spin torque magnitude at the same first frequency.

The in-plane spin transfer torque causes the magnetic vector of the free magnetic layer836to precess, as depicted inFIG. 6A. The precession of the magnetic vector of free magnetic layer836occurs at a predetermined precession frequency. In some embodiments, the alternating perturbation current is synchronized with the predetermined precession frequency. Because the magnitude of the in-plane spin transfer torque alternates between the maximum spin torque magnitude and the minimum spin torque magnitude at the first frequency, these magnitude oscillations are also synchronized with the predetermined precession frequency. As depicted inFIG. 6A(middle, right), the in-plane spin transfer torque is at or near the maximum spin torque magnitude when the in-plane spin transfer torque opposes the inherent damping of the free magnetic layer836. Also shown inFIG. 6A. (middle left), the in-plane spin transfer torque is at or near the minimum spin torque magnitude when the in-plane spin transfer torque enhances the inherent damping of the free magnetic layer836. Therefore, the in-plane spin transfer torque from in-plane polarizer850provides a net spin torque throughout the precession that opposes the damping characteristic of the free magnetic layer836. The in-plane spin transfer torque from the in-plane polarization layer850thereby assists the perpendicular spin transfer torque from the reference layer832in switching the magnetization direction of the free layer. Thus, devices using a programming current pulse comprising an alternating perturbation current can enhance the efficiency of the switching of the magnetic direction of the free magnetic layer836.

In particular, the structures and methods described herein utilize an alternating perturbation current that is synchronized with the predetermined precession frequency of the free magnetic layer836that provides a net in-plane spin transfer torque throughout the whole precession cycle and therefore significantly enhances the free layer switching process in both switching directions, which will result in faster write times and lower switching threshold currents.

Three-terminal devices such as magnetic device800offer additional advantages over the two-terminal devices that are described above (e.g., magnetic device500). In particular, these three-terminal devices offer all of the advantages of improved switching during the writing process, but allow a significantly reduced read disturb. Benefits from the lower switching threshold currents that are derived from resonant switching currents include reduction in write error rate, reduced damage to the device during each writing process, and an increase in device stability over time. However, lower switching threshold currents can increase the probability of inadvertently writing the bit during the read process. Typically, a weaker current is used during the read process, allowing the resistance across the MTJ to be measured without generating a spin-current that is strong enough to switch the magnetic vector of the free layer. However, as described above, when an in-plane polarizer and an alternating perturbation current resonant with the free layer is used, the strength of the current required to switch the free layer is greatly reduced. Therefore, unless these systems can be bypassed, there will be a substantial probability of inadvertently switching the free layer during the reading process, in spite of the use of a weaker read current. Three-terminal devices such as magnetic device800allow a direct current to be used that bypasses the in-plane polarization layer during the read process, thereby reducing the probability of read disturb.

A memory cell with a STNO structure970, an in-plane polarization layer950, a MTJ structure930, a programming current source975, a metallic terminal944, and a reading current source985is shown inFIG. 15. MTJ structure930includes one or more seed layers910provided at the bottom of stack900to initiate a desired crystalline growth in the above-deposited layers. Synthetic antiferromagnetic (SAF) layer920is disposed over seed layer910. SAF layer920is comprised of a first SAF layer932, anti-ferromagnetic coupling layer916and second SAF layer914. Second SAF layer914is deposited over seed layer910, while anti-ferromagnetic coupling layer916is placed over second SAF layer914. MTJ930is deposited over anti-ferromagnetic coupling layer916. MTJ930includes first SAF layer932, which acts as the reference layer of the MTJ, and is also part of SAF layer920. A tunneling barrier layer (i.e., the insulator)934is disposed over first SAF layer932while the free layer936is disposed over tunneling barrier layer934. As shown inFIG. 15, the magnetization vector of first SAF layer932has a magnetization direction that is preferably perpendicular to its plane, although variations of several degrees are within the scope of what is considered perpendicular. As also seen inFIG. 15, free layer936also has a magnetization vector that is preferably perpendicular to its plane, but its direction can vary by 180 degrees. A nonmagnetic spacer940is disposed over a first portion of MTJ930. In-plane polarization magnetic layer950is disposed over nonmagnetic spacer940. In one embodiment, in-plane polarization magnetic layer950has a magnetization vector having a magnetic direction parallel to its plane, and is perpendicular to the magnetic vector of the reference layer932and free layer936. In-plane polarization magnetic layer950has a magnetization direction that is preferably parallel to its plane, although variations of several degrees are within the scope of what is considered parallel. Metallic spacer960is disposed over in-plane polarization magnetic layer950. STNO970is deposited over metallic spacer960. STNO970includes in-plane spin torque oscillator layer972, is disposed over magnetic spacer960. A non-magnetic spin torque oscillator barrier layer974is disposed over in-plane spin torque oscillator layer972while a perpendicular spin torque oscillator layer976is disposed over non-magnetic spin torque oscillator barrier layer974. As shown inFIG. 15, the magnetization vector of in-plane spin torque oscillator layer972, has a magnetization direction that precesses around an anisotropy axis that is parallel to the plane, although variations of several degrees are within the scope of what is considered parallel. In some embodiments, the magnetic direction of the in-plane spin torque oscillator layer precesses in the plane if the in-plane anisotropy is weak. As also seen inFIG. 15, the perpendicular spin torque oscillator layer976has a magnetization vector magnetization direction that precesses around an anisotropy axis that is perpendicular to the plane, although variations of several degrees are within the scope of what is considered perpendicular. One or more capping layers980can be provided on top of perpendicular spin torque oscillator layer976to protect the layers below on MTJ stack900. Non-magnetic insulator942is disposed over a second portion of MTJ930. In one embodiment, non-magnetic insulator942is adjacent to non-magnetic spacer940, in-plane polarization magnetic layer950, and STNO structure970. Metallic terminal944is disposed over a third portion of MTJ930and is adjacent to non-magnetic insulator942. In some embodiments, external non-magnetic insulator946is disposed over a fourth portion of MTJ930and is adjacent to metallic terminal944. In some embodiments, first voltage source985generates a direct voltage pulse. In one embodiment, the first voltage source applies the direct current pulse across metallic terminal944and MTJ930. In one embodiment, second voltage source975generates a direct voltage (i.e., programming voltage). In some embodiments, second voltage source975applies the direct voltage across STNO structure970, metallic spacer960, in-plane polarization magnetic layer950, non-magnetic spacer940, and MTJ930. In some embodiments, application of the programming voltage to the device results in a switching current that alternates between a maximum current value and a minimum current value at a first frequency.

Seed layer910in the MTJ structure shown inFIG. 15preferably comprises Ta, TaN, Cr, Cu, CuN, Ni, Fe or alloys thereof. Second SAF layer914preferably comprises either a Co/Ni, Co/Pt or Co/Pd multilayer structure. First SAF layer932preferably comprises either a Co/Ni or Co/Pt multilayer structure plus a thin non-magnetic layer comprised of tantalum having a thickness of two to five Angstroms and a thin CoFeB layer (0.5 to three nanometers). Anti-ferromagnetic coupling layer916is preferably made from Ru having thickness in the range of three to ten Angstroms. Tunneling barrier layer934is preferably made of an insulating material such as MgO, with a thickness of approximately ten Angstroms. Free layer936is preferably made with CoFeB deposited on top of tunneling barrier layer934. Free layer936can also have layers of Fe, Co, Ni or alloys thereof and W and Ta insertion layers to promote perpendicular anisotropy. Spacer layer940over MTJ930can be any non-magnetic material such as 2 to 20 Angstroms of ruthenium, 2-20 Angstroms of Ta, 2-20 Angstroms of TaN, 2-20 Angstroms of Cu, 2-20 Angstroms of CuN, or 2-20 Angstroms MgO layer, or or 2-20 Angstroms Al2O3layer.

Nonmagnetic spacer940has a number of properties. For example, nonmagnetic spacer940physically separates the free layer936and the in-plane polarization magnetic layer950. Nonmagnetic spacer940transmits spin current efficiently from the in-plane polarization magnetic layer950into the free layer936because it preferably has a long spin diffusion length if made metallic. Nonmagnetic spacer940also promotes good microstructure and high tunneling magnetoresistance (TMR) and helps keep the damping constant of the free layer936low. In one embodiment, the nonmagnetic spacer940comprises MgO.

The in-plane polarization magnetic layer950is preferably made from CoFeB. The in-plane polarization magnetic layer950can also be made with CoFeB, Fe, FeV. or FeB. It can also be made with Co, Fe, Ni magnetic layers or can be made out of their alloys. The magnetic alloys can also have boron, tantalum, copper or other materials.

Metallic spacer960has a number of properties. For example, metallic spacer960physically separates the STNO970from the in-plane polarization magnetic layer950. Preferably, metallic spacer960is composed of ruthenium or rhodium. Metallic spacer960is preferably made from Ru having thickness in the range of two to ten Angstroms or Rh having thickness in the range of two to ten Angstroms. In one embodiment, when metallic spacer960is comprised of a thin layer of Ru (e.g., 7 Angstroms), the in-plane polarization magnetic layer950and the in-plane spin torque oscillator layer972can be AFM coupled via stray fields, thereby minimizing the stray field that in-plane polarization magnetic layer550imparts on free magnetic layer536. This AFM coupling can be strengthened via electronic oscillatory (RKKY) coupling.

The STNO970can be in either of the following configurations: (1) in-plane spin torque oscillator layer972disposed over spin torque oscillator barrier layer974, which is disposed over perpendicular spin torque oscillator layer976; or (2) perpendicular spin torque oscillator layer976disposed over spin torque oscillator barrier layer974, which is disposed over in-plane spin torque oscillator layer972. In-plane spin torque oscillator layer972can be comprised of a SAF, such as a composite ferromagnetic metal/metallic spacer (Ru or Rd)/ferromagnetic metal structure. The in-plane spin torque oscillator layer972can be pinned by a 5-10 nm layer of PtMn exchange bias layer. Preferably, in-plane spin torque oscillator layer972comprises CoFeB. Spin torque oscillator barrier layer974can be any non-magnetic material such as 2 to 20 Angstroms of ruthenium, 2-20 Angstroms of Ta, 2-20 Angstroms of TaN, 2-20 Angstroms of Cu, 2-20 Angstroms of CuN, or 2-20 Angstroms MgO layer, or or 2-20 Angstroms Al2O3layer. Spin torque oscillator barrier layer974is preferably made from MgO. The perpendicular spin torque oscillator layer976can comprise 6-16 Angstoms of Ta/CoFeB. Preferably, the perpendicular spin torque oscillator layer976comprises CoFeB.

Nonmagnetic insulator942has a number of properties. For example, nonmagnetic insulator942physically separates the metallic terminal944from the STNO structure970, the metallic spacer960, the in-plane polarization magnetic layer950and the non-magnetic spacer940. In addition, nonmagnetic insulator942does not transfer electrical current or does so with very low efficiency, thereby preventing current from metallic terminal944from entering either the STNO structure970, the metallic spacer960, the in-plane polarization magnetic layer950and the non-magnetic spacer940. Nonmagnetic insulator942preferably comprises a nonmagnetic, nonconductive material.

Metallic terminal944has a number of properties. Metallic terminal efficiently conducts electrical current from first current source985to MTJ structure930. Metallic terminal944preferably comprises a highly-conductive, non-magnetic material. Metallic terminal944preferably comprises a metal, a metallic nanofiber, or a conductive carbon nanotube.

Nonmagnetic external insulator946has a number of properties. For example, nonmagnetic external insulator946physically separates the metallic terminal944from anything external to the MTJ stack900. In addition, nonmagnetic external insulator946does not transfer electrical current or does so with very low efficiency. Nonmagnetic external insulator946preferably comprises a nonmagnetic, nonconductive material. Finally, capping layer980can be any material that provides good interface to the perpendicular spin torque oscillator layer such as Ta, TaN, Ru, MgO, Cu, etc.

In some embodiments, first voltage source985can generate a direct voltage. In some embodiments, second voltage source975can generate a programming voltage pulse that comprises direct voltage pulse. In other embodiments, the strength of the bias produced by voltage source975can vary over time.

FIG. 16depicts the voltage across the magnetic device900, which comprises STNO570, in-plane polarization layer550, MTJ structure530, and metallic terminal944. Voltage diagrams are depicted for writing process as well as the reading process. During the writing process, second voltage source975generates a direct voltage across the STNO970, in-plane polarization layer950, and MTJ930. This results in an electrical current pulse that initiates the precessional dynamics of the magnetic vectors of the STNO570. As discussed above, this causes the resistance across STNO970to oscillate between a high resistance state and a low resistance state at a first frequency. The first frequency is set by the structure (e.g., composition and dimensions) of the STNO structure570.

In one embodiment, during the writing process, magnetic device900operates as a voltage divider (when supplied with a constant voltage pulse), with a first voltage across STNO970(labeled as V-STNO inFIG. 16) and a second voltage across in-plane polarization magnetic layer950, non-magnetic spacer940and MTJ structure930(labeled as V-pOST inFIG. 16). In one embodiment, the voltage across the entire device (labeled a V-Write) is fixed and constant. V-STNO oscillates between a maximum voltage value and a minimum voltage value at the first frequency due to the oscillating resistance value of STNO970. Metallic spacer960is comprised of a highly-conductive material, such as a metallic material. Therefore, the output voltage from STNO970is essentially equivalent to the input voltage for the V-pOST. Accordingly, V-pOST also oscillates between a maximum voltage value and a minimum voltage value at the first frequency. Moreover, the strength of the current flowing out of STNO970and passing through in-plane polarizer950, non-magnetic spacer940, and MTJ structure930(i.e., the switching current) is directly proportional to the voltage value of V-pOST. Thus, the switching current alternates between a maximum current value and a minimum current value at the first frequency.

In contrast, during the reading process, the voltage divider of the STNO/MTJ system is bypassed, reading the resistance across only the MTJ. First voltage source985generates a voltage across the metallic terminal944and MTJ930. In one embodiment, this voltage (labeled as V-Read inFIG. 16) is a constant, direct voltage. By bypassing the STNO570, metallic spacer960and in-plane polarizer950, three-terminal device900allows the bit to be read without an oscillating resistance value. The use of a direct read voltage to measure the resistance across only the MTJ930allows for a more determinative read at potentially lower read voltage values.

The manner in which a bit is written using three-terminal magnetic device900that comprises STNO570, in-plane polarization magnetic layer550, MTJ structure530, and metallic terminal944will now be described. In particular, an electrical voltage is supplied, for example, by voltage source975, which results in an electrical current through the perpendicular spin torque oscillator layer976, the non-magnetic spin torque barrier layer974, the in-plane spin torque oscillator layer972, the magnetic spacer960, the in-plane polarization magnetic layer950, the non-magnetic spacer940, the free magnetic layer596, the non-magnetic tunneling barrier layer934, and the reference layer932. Application of the programming voltage to STNO970causes the magnetization vectors of the perpendicular spin torque oscillator layer976and the in-plane spin torque oscillator layer972to precess around their respective axes. The precession of the magnetic vectors perpendicular spin torque oscillator layer976and the in-plane spin torque oscillator layer972causes the resistance across the STNO to oscillate between a maximum resistance value and a minimum resistance value. The net effect is that the current leaving the STNO and passing through the in-plane polarization magnetic layer950(i.e., the switching current) alternates between a maximum current value and a minimum current value at a first frequency.

Application of the switching current to the in-plane polarization magnetic layer950and MTJ structure930creates a spin polarized current that passes through non-magnetic spacer layer940, free magnetic layer936, tunneling barrier layer934, and reference magnetic layer932. The spin polarized current exerts a spin transfer torque on free magnetic layer936, which helps overcome the inherent damping of the magnetic material making up the free layer936. The spin transfer torque is composed of an in-plane spin transfer torque and a perpendicular spin transfer torque. In one embodiment, when switching the free layer936in one direction (e.g., from the parallel direction to the anti-parallel direction), the in-plane spin transfer torque is caused by the transverse spin current generated by the in-plane polarization magnetic layer950and the perpendicular spin transfer torque is caused by the reflected spin current generated by the reference magnetic layer932. This causes the free magnetic layer936to precess about its axis, which is shown inFIG. 6A.

The spin-polarized current, which is generated by application of the switching current to in-plane polarization magnetic layer950and the MTJ structure930, alternates between a maximum spin-current value and a minimum spin current value at the same first frequency. The magnitude of the spin transfer torque exerted on the magnetic vector of the free layer936is proportional to the spin-current value because at higher spin-current values, there are more spin-polarized electrons flowing through free layer936. Therefore, the magnitude of the spin transfer torque alternates between a maximum spin torque magnitude and a minimum spin torque magnitude at the same first frequency.

The in-plane spin transfer torque causes the magnetic vector of the free magnetic layer936to precess, as depicted inFIG. 6A. The precession of the magnetic vector of free magnetic layer936occurs at a predetermined precession frequency. In some embodiments, the first frequency is synchronized with the predetermined precession frequency. Because the magnitude of the in-plane spin transfer torque alternates between the maximum spin torque magnitude and the minimum spin torque magnitude at the first frequency, these magnitude oscillations are also synchronized with the predetermined precession frequency. As depicted inFIG. 6A(middle, right), the in-plane spin transfer torque is at or near the maximum spin torque magnitude when the in-plane spin transfer torque opposes the inherent damping of the free magnetic layer936. Also shown inFIG. 6A. (middle left), the in-plane spin transfer torque is at or near the minimum spin torque magnitude when the in-plane spin transfer torque enhances the inherent damping of the free magnetic layer936. Therefore, the in-plane spin transfer torque from in-plane polarizer950provides a net spin torque throughout the precession that opposes the damping characteristic of the free magnetic layer936. The in-plane spin transfer torque from the in-plane polarization layer950thereby assists the perpendicular spin transfer torque from the reference layer932in switching the magnetization direction of the free layer. Thus, devices using a STNO970, an in-plane polarizer950, and a MTJ930can enhance the efficiency of the switching of the magnetic direction of the free magnetic layer936.

In particular, the structures described herein utilize a STNO that has been designed to produce oscillations in the current value of the switching current that are synchronized with the predetermined precession frequency of the free magnetic layer936. As described above, this system provides a net in-plane spin transfer torque throughout the whole precession cycle and therefore significantly enhances the free layer switching process in both switching directions, which will result in faster write times and lower switching threshold currents.

Three-terminal devices such as magnetic device900offer additional advantages over the two-terminal devices that are described above (e.g., magnetic device700). In particular, these three-terminal devices offer all of the advantages of improved switching during the writing process, but allow a significantly reduced read disturb. Benefits from the lower switching threshold currents that are derived from the use of a STNO resonant with the free layer and an in-plane polarizer include reduction in write error rate, reduced damage to the device during each writing process, and an increase in device stability over time. However, lower switching threshold currents can increase the probability of inadvertently writing the bit during the read process. Typically, a weaker current is used during the read process, allowing the resistance across the MTJ to be measured without generating a spin-current that is strong enough to switch the magnetization vector of the free layer. However, as described above, when a STNO resonant with the free layer and an in-plane polarizer are used, the strength of the current required to switch the free layer is greatly reduced. Therefore, unless these systems can be bypassed, there will be a substantial probability of inadvertently switching the free layer during the reading process, in spite of the use of a weaker read current. Three-terminal devices such as magnetic device900allow a direct current to be used that bypasses the in-plane polarization layer during the read process, thereby reducing the probability of read disturb. In addition, three terminal device900allows the direct read current to bypass the STNO, thereby detecting the resistance across the MTJ without detecting the oscillating resistance values generated by the STNO. Thus, three-terminal devices such as magnetic device900offer significant advantages during both the writing and reading processes.

An alternative embodiment is shown inFIG. 17. In this embodiment, magnetic device1100has had its MTJ stack1140inverted with respect to the embodiment shown inFIG. 12. In addition, MTJ1140is placed over in-plane polarization magnetic layer1120. The magnetic device1100also has had its STNO structure inverted with respect to the embodiment show in inFIG. 12. In particular, magnetic device1100includes a seed layer1110. In-plane polarization magnetic layer1120placed over seed layer1110. In-plane polarization magnetic layer1120has the same properties, construction and characteristics as in-plane polarization magnetic layer950, discussed above. Nonmagnetic spacer1130is placed over in-plane polarization magnetic layer1120. Nonmagnetic spacer1130has the same properties, construction and characteristics as nonmagnetic spacer940, discussed above. MTJ1140is placed over nonmagnetic spacer1130. MTJ1140is generally constructed of free layer1142(which is placed over nonmagnetic spacer1130) and reference layer1146. Free layer1142and reference layer1146are spatially separated from each other by tunneling barrier layer1144, which is made of an insulating material. Reference magnetic layer1146and free magnetic layer1142have the same properties, construction and characteristics as reference magnetic layer932and free magnetic layer936, respectively, discussed above. Metallic spacer1150is placed over a first portion of the reference magnetic layer1146. Metallic spacer1150has the same properties, construction and characteristics as metallic spacer960, discussed above. Perpendicular spin torque oscillator layer1162is placed over metallic spacer1150. Perpendicular spin torque oscillator layer1162has the same properties, construction and characteristics as perpendicular spin torque oscillator layer976, discussed above. Nonmagnetic spin torque oscillator barrier layer1164is placed over perpendicular spin torque oscillator layer1162and has the same properties, construction and characteristics as nonmagnetic spin torque oscillator barrier layer974. In-plane spin torque oscillator layer1166is placed over nonmagnetic spin torque oscillator barrier layer1164. In-plane spin torque oscillator layer1166has the same properties, construction and characteristics as in-plane spin torque oscillator layer972, discussed above. Nonmagnetic insulator1122is placed over a second portion of reference magnetic layer1146and is adjacent to metallic layer1150, perpendicular spin torque oscillator layer1162, nonmagnetic spin torque oscillator barrier layer1164, and in-plane spin torque oscillator layer1166. Nonmagnetic insulator1122has the same properties, construction and characteristics as nonmagnetic insulator942, discussed above. Metallic terminal1124is placed over a third portion of reference magnetic layer1146and is adjacent to nonmagnetic insulator1122. Metallic terminal1124has the same properties, construction and characteristics as metallic terminal944, discussed above. Nonmagnetic external insulator1126is placed over a fourth portion of reference magnetic layer1146and is adjacent to metallic terminal1124. Nonmagnetic external insulator1126has the same properties, construction and characteristics as nonmagnetic external insulator946, discussed above. The reading voltage can be provided by a first voltage source1185. The programming voltage can be provided by a second voltage source1175. Other than the ordering of the layers, magnetic device1100operates during the writing process in the same manner as described with respect to the process shown inFIG. 7A. Thus, just as shown inFIG. 7A, the in-plane spin transfer torque610provides a net benefit of enhancing the efficiency of the switching throughout the entire precession cycle of free layer1142. Finally, capping layer1180is placed over STNO1160.

All of the layers of devices800,900and1100illustrated inFIGS. 14, 15 and 17can be formed by a thin film sputter deposition system as would be appreciated by one skilled in the art. The thin film sputter deposition system can include the necessary physical vapor deposition (PVD) chambers, each having one or more targets, an oxidation chamber and a sputter etching chamber. Typically, the sputter deposition process involves a sputter gas (e.g., oxygen, argon, or the like) with an ultra-high vacuum and the targets can be made of the metal or metal alloys to be deposited on the substrate. Thus, when the present specification states that a layer is placed over another layer, such layer could have been deposited using such a system. Other methods can be used as well. It should be appreciated that the remaining steps necessary to manufacture MTJ stacks800,900and1100are well-known to those skilled in the art and will not be described in detail herein so as not to unnecessarily obscure aspects of the disclosure herein.

It should be appreciated to one skilled in the art that a plurality of MTJ structures800,900, and1100can be manufactured and provided as respective bit cells of an STT-MRAM device. In other words, each MTJ stack can be implemented as a bit cell for a memory array having a plurality of bit cells.

The above description and drawings are only to be considered illustrative of specific embodiments, which achieve the features and advantages described herein. Modifications and substitutions to specific process conditions can be made. Accordingly, the embodiments in this patent document are not considered as being limited by the foregoing description and drawings.