# Patent

## Title
Spin-transfer torque memory self-reference read and write assist methods

## Abstract

A spin-transfer torque memory apparatus and self-reference read and write assist schemes are described. One method of self-reference reading a spin-transfer torque memory unit includes applying a first read current through a magnetic tunnel junction data cell and forming a first bit line read voltage and storing the first bit line read voltage. A magnetic field is applied through the magnetic tunnel junction data cell forming a magnetic field modified magnetic tunnel junction data cell. Then a second read current is applied thorough the magnetic field modified magnetic tunnel junction data cell forming a second bit line read voltage and the bit line read voltage is stored and compared with the first bit line read voltage to determine whether the first resistance state of the magnetic tunnel junction data cell was a high resistance state or low resistance state. Methods of applying a magnetic field to the MTJ and then writing the desired resistance state are also disclosed.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S., Provisional Application No. 61/108,798 filed Oct. 27, 2008, the contents of which is hereby incorporated by reference in its entirety.


BACKGROUND
Fast growth of the pervasive computing and handheld/communication industry has generated exploding demand for high capacity nonvolatile solid-state data storage devices. It is believed that nonvolatile memories, especially flash memory, will replace DRAM to occupy the biggest share of memory market. However, flash memory has several drawbacks such as slow access speed ( ̃ms write and  ̃50-100 ns read), limited endurance ( ̃103-104 programming cycles), and the integration difficulty in system-on-chip (SoC). Flash memory (NAND or NOR) also faces significant scaling problems at 32 nm node and beyond.
Magneto-resistive Random Access Memory (MRAM) is a promising candidate for future nonvolatile and universal memory. MRAM features non-volatility, fast writing/reading speed (<10 ns), almost unlimited programming endurance (>1015 cycles) and zero standby power. The basic component of MRAM is a magnetic tunneling junction (MTJ). Data storage is realized by switching the resistance of MTJ between a high-resistance state and a low-resistance state. MRAM switches the MTJ resistance by using a current induced magnetic field to switch the magnetization of MTJ. As the MTJ size shrinks, the switching magnetic field amplitude increases and the switching variation becomes more severe. Hence, the incurred high power consumption limits the scaling of conventional MRAM.
Recently, a new write mechanism, which is based upon spin polarization current induced magnetization switching, was introduced to the MRAM design. This new MRAM design, called Spin-Torque Transfer RAM (STRAM), uses a (bidirectional) current through the magnetic tunnel junction (i.e., “MTJ”) to realize the resistance switching. Therefore, the switching mechanism of STRAM is constrained locally and STRAM is believed to have a better scaling property than the conventional MRAM.
However, many yield-limiting factors must be overcome before STRAM enters the production stage. One challenge is the large MTJ resistance variation, which is exponentially dependent on the thickness of oxide barrier in it. For example, increasing the thickness of oxide barrier from 14 Angstroms to 14.1 Angstroms changes the MTJ resistance by 8%. This large MTJ resistance variation can create problems during a read operation of the MTJ.
BRIEF SUMMARY
The present disclosure relates to spin-transfer torque random access memory self-reference read and write assist operations. In particular, present disclosure relates to a spin-transfer torque random access memory self-reference read operation that utilizes an external magnetic field to overcome the large variation of MTJ resistance and provides a large dynamic reading current range, and a write operation that utilizes an external magnetic field to reduce a write current.
One illustrative method of reading a spin-transfer torque memory unit includes applying a first read current through a magnetic tunnel junction data cell and forming a first bit line read voltage, the magnetic tunnel junction data cell having a first resistance state, the magnetic tunnel junction includes a free magnetic layer having a free magnetization orientation and a reference magnetic layer having a reference magnetization orientation.
The magnetic tunnel junction data cell having a first energy barrier value required to switch the magnetic tunnel junction data cell between a high resistance state and a low resistance state. The first bit line read voltage is stored in a first voltage storage device. A magnetic field is applied through the magnetic tunnel junction data cell forming a magnetic field modified magnetic tunnel junction data cell. The magnetic field is parallel or anti-parallel with the magnetization orientation of the reference magnetic layer. The magnetic field modified magnetic tunnel junction data cell having a second energy barrier value being greater than the first energy barrier value. Then a second read current is applied thorough the magnetic field modified magnetic tunnel junction data cell having the first resistance state, forming a second bit line read voltage. The first read current is less than the second read current and the second bit line read voltage is stored in a second voltage storage device. Then the stored first bit line read voltage is compared with the stored second bit line read voltage to determine whether the first resistance state of the magnetic tunnel junction data cell was a high resistance state or low resistance state.
An illustrative method of magnetic field assisted writing of a spin-transfer torque memory unit includes applying a magnetic field through a magnetic tunnel junction data cell forming a magnetic field modified magnetic tunnel junction data cell. The magnetic tunnel junction data cell having a first energy barrier value required to switch the magnetic tunnel junction data cell between a high resistance state and a low resistance state. The magnetic field is parallel or anti-parallel with the magnetization orientation of the reference magnetic layer. The magnetic field decreases the energy barrier value required to switch the magnetic tunnel junction data cell between a high resistance state and a low resistance state. Then the method includes applying a write current through the magnetic field modified magnetic tunnel junction data cell to switch the magnetic field modified magnetic tunnel junction data cell between a high resistance state and low resistance state.



BRIEF DESCRIPTION OF THE DRAWINGS
The disclosure may be more completely understood in consideration of the following detailed description of various embodiments of the disclosure in connection with the accompanying drawings, in which:
FIG. 1 is a cross-sectional schematic diagram of an illustrative spin-transfer torque MTJ memory unit in the low resistance state;
FIG. 2 is a cross-sectional schematic diagram of another spin-transfer torque MTJ memory unit in the high resistance state;
FIG. 3 is a graph of a static R-V (resistance-voltage) curve of a spin-transfer torque MTJ memory unit;
FIG. 4 is a schematic circuit diagram of a spin-transfer torque MTJ memory unit;
FIG. 5A is a schematic perspective view of one embodiment of a magnetic memory unit having a magnetic tunnel junction cell with an adjacent magnetic field generator;
FIG. 5B is a schematic perspective view of another embodiment of a magnetic memory magnetic memory unit having a magnetic tunnel junction cell with an adjacent magnetic field generator;
FIG. 6 is a schematic circuit diagram of an illustrative spin-transfer torque MTJ memory apparatus;
FIG. 7 is a graph of I-R (current-resistance) curves of the MTJ high resistance state and low resistance state without a varying external magnetic field applied during the first and second read operations;
FIG. 8A is a graph of an I-R (current-resistance) curve of the MTJ high resistance state and low state;
FIG. 8B is a graph of an I-R (current-resistance) curve of the MTJ high resistance state and low state with an applied external magnetic field;
FIG. 9A is a graph of an I-R (current-resistance) curve of the MTJ high resistance state and low resistance state illustrating an applied magnetic field assisting in the writing of the high resistance state;
FIG. 9B is a graph of an I-R (current-resistance) curve of the MTJ high resistance state and low resistance state illustrating an applied magnetic field assisting in the writing of the low resistance state;
FIG. 10 is a flow diagram of an illustrative self-reference reading method; and
FIG. 11 is a flow diagram of an illustrative magnetic field assist write method.



The figures are not necessarily to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number.
DETAILED DESCRIPTION
In the following description, reference is made to the accompanying set of drawings that form a part hereof and in which are shown by way of illustration several specific embodiments. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense. The definitions provided herein are to facilitate understanding of certain terms used frequently herein and are not meant to limit the scope of the present disclosure.
Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein.
The recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.
As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” encompass embodiments having plural referents, unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
The present disclosure relates to spin-transfer torque memory apparatus and self-reference read and write assist schemes. In particular, the present disclosure relates to self-reference reading methods that determine whether a spin-transfer torque memory unit has a high resistance state or low resistance state data state, without disturbing the original data resistance state of the spin-transfer torque memory unit. The apparatus and methods described herein ensure that the value of the spin-transfer torque memory unit can be determined regardless of the resistance variation of the spin-transfer torque memory units within a memory array. The read voltage of the spin-transfer torque memory unit at a first read current and a second read current are stored sequentially and compared to detect the resistance state or data state of the spin-transfer torque memory unit. The first or second read voltage is read and stored after a magnetic field is applied through the spin-transfer torque memory unit. The magnetic field can stabilize the resistance data state of the magnetic tunnel junction so that a large read current can be used without switching the data state of the free layer, and the reading margin is increased. During writing, the magnetic field decreases the switching time and current required to switch the data state of the magnetic tunnel junction. Preserving the original resistance state eliminates the “standard-value-write” and “write-back” steps that are required in “destructive self-reference” read methods. Thus the disclosed read methods improve the read operation reading margin and improve the reliability of the non-volatile data, while the disclosed write method increases the writing speed and lowers the write current. While the present disclosure is not so limited, an appreciation of various aspects of the disclosure will be gained through a discussion of the examples provided below.
FIG. 1 is a cross-sectional schematic diagram of an illustrative spin-transfer torque MTJ memory unit 10 in the low resistance state and FIG. 2 is a cross-sectional schematic diagram of another spin-transfer torque MTJ memory unit 10 in the high resistance state. A magnetic tunnel junction (MTJ) memory unit 10 includes a magnetic (e.g., ferromagnetic) free layer 12 and a magnetic (e.g., ferromagnetic) reference (i.e., pinned) layer 14. The magnetic free layer 12 and a magnetic reference layer 14 are separated by an oxide barrier layer 13 or tunnel barrier. A first electrode 15 is in electrical contact with the magnetic free layer 12 and a second electrode 16 is in electrical contact with the magnetic reference layer 14. The magnetic layers 12, 14 may be made of any useful ferromagnetic (FM) alloys such as, for example, Fe, Co, Ni and the insulating barrier layer 13 may be made of an electrically insulating material such as, for example an oxide material (e.g., Al2O3 or MgO). Other suitable materials may also be used.
The electrodes 15, 16 electrically connect the magnetic layers 12, 14 to a control circuit providing read and write currents through the magnetic layers 12, 14. The resistance across the spin-transfer torque MTJ memory unit 10 is determined by the relative orientation of the magnetization vectors or magnetization orientations of the magnetic layers 12, 14. The magnetization direction of the magnetic reference layer 14 is pinned in a predetermined direction while the magnetization direction of the magnetic free layer 12 is free to rotate under the influence of a spin torque. Pinning of the magnetic reference layer 14 may be achieved through, e.g., the use of exchange bias with an antiferromagnetically ordered material such as PtMn, IrMn and others.
FIG. 1 illustrates the spin-transfer torque MTJ memory unit 10 in the low resistance state where the magnetization orientation of the magnetic free layer 12 is parallel and in the same direction of the magnetization orientation of the magnetic reference layer 14. This is termed the low resistance state or “0”data state. FIG. 2 illustrates the spin-transfer torque MTJ memory unit 10 in the high resistance state where the magnetization orientation of the magnetic free layer 12 is anti-parallel and in the opposite direction of the magnetization orientation of the magnetic reference layer 14. This is termed the high resistance state or “1”data state.
Switching the resistance state and hence the data state of the MTJ memory unit 10 via spin-transfer occurs when a current, passing through a magnetic layer of the MTJ memory unit 10, becomes spin polarized (i.e., polarized) and imparts a spin torque on the free layer 12 of the MTJ 10. When a sufficient spin torque is applied to the free layer 12, the magnetization orientation of the free layer 12 can be switched between two opposite directions and accordingly the MTJ 10 can be switched between the parallel state (i.e., low resistance state or “0” data state) and anti-parallel state (i.e., high resistance state or “1” data state) depending on the direction of the current.
The illustrative spin-transfer torque MTJ memory unit 10 may be used to construct a memory device that includes multiple MTJ memory units where a data bit is stored in spin-transfer torque MTJ memory unit by changing the relative magnetization state of the free magnetic layer 12 with respect to the pinned magnetic layer 14. The stored data bit can be read out by measuring the resistance of the cell which changes with the magnetization direction of the free layer relative to the pinned magnetic layer. In order for the spin-transfer torque MTJ memory unit 10 to have the characteristics of a non-volatile random access memory, the free layer exhibits thermal stability against random fluctuations so that the orientation of the free layer is changed only when it is controlled to make such a change. This thermal stability can be achieved via the magnetic anisotropy using different methods, e.g., varying the bit size, shape, and crystalline anisotropy. Generally, the anisotropy is associated with a soft and hard axis to form in thin magnetic layers. The hard and soft axes are defined by the magnitude of the energy, usually in the form of a magnetic field, needed to fully rotate (saturate) the direction of the magnetization in that direction, with the hard axis requiring a higher saturation magnetic field.
FIG. 3 is a graph of a static R-V sweep curve of a spin-transfer torque MTJ memory unit. When applying a positive voltage on the second electrode 16 in FIG. 1 or 2, the MTJ 10 enters the negative applied voltage region in FIG. 3 and switches from the low resistance state (FIG. 1) to the high resistance state (FIG. 2). When applying a positive voltage on the first electrode 15 in FIG. 1 or 2, the MTJ 10 enters the positive applied voltage region in FIG. 3. The resistance of the MTJ switches from the high resistance state (FIG. 2) to the low resistance state (FIG. 1).
Let RH and RL denote the high and low MTJ resistance, respectively. We define the Tunneling Magneto Resistance Ratio (TMR) as TMR=(RH−RL)/RL. Here RH, RL and TMR are determined by also the sensing current or voltage, as shown in FIG. 3. Generally, a large TMR makes it easier to distinguish the two resistance states of the MTJ.
FIG. 4 is a schematic diagram of a spin-transfer torque MTJ memory unit MTJ. The spin-transfer torque MTJ memory unit MTJ is electrically connected in series to a transistor such as, for example, a NMOS transistor. The opposing side of the spin-transfer torque MTJ memory unit MTJ is electrically connected to a bit line BL. The transistor is electrically coupled to a source line SL and a word line WL. The MTJ is usually modeled as a variable resistor in circuit schematic, as shown in FIG. 4.
A magnetic field generator MF is adjacent to the MTJ. The magnetic field generator MF provides a magnetic field through the magnetic tunnel junction data cell MTJ forming a magnetic field modified magnetic tunnel junction data cell. The magnetic field is parallel or anti-parallel with the magnetization orientation of the reference magnetic layer.
Knowing the direction of the reading current through the MTJ, an external magnetic field can assist (during writing) or hinder (during reading) free layer switching. For example, during the read operation, if the read current direction is from free layer to the pinned layer (from 12 to 14 in FIG. 1), it will stabilize the low resistance state (free layer parallel to pinned layer) and de-stabilize the high resistance state (free layer anti-parallel to pinned layer), because the current can cause the free layer magnetization direction switching from anti-parallel (high resistance state) to parallel (low resistance state). Application of the external magnetic field, whose direction is anti-parallel to pinned layer, such that the energy barrier is increased for the free layer to switch from high resistance state to low resistance state (of course, the energy barrier is decreased for free layer to switch from low resistance state to high resistance state), and a high read current can be used, increasing the reading margin. In other words, the external magnetic field will stabilize the high resistance state and de-stabilize low resistance state, which is opposite to the current effect, the external magnetic field effect will either fully or partially cancel the current effect, thus, a large read current can be used and the reading margin increases.
The reversed case happens during the write operation. For example, during write, to write “0” (low resistance state), application of current from free layer to the pinned lay (from 12 to 14 in FIG. 1), at the same time, the external magnetic field direction is parallel to pinned layer (opposite to the direction during reading), which decreases the energy barrier value for the free layer to switch from high resistance state to low resistance state, so a small write current is needed. In other words, the external magnetic field effect enhances the current effect during the write operation.
In both read and write cases, the initial free layer state does not matter. For example, during the read operation in above example, if the free layer is at the low resistance state, the read current will stabilize this state but the external magnetic field will de-stabilize this state, both effects will cancel each other; the reverse case happens if the free layer is at the high resistance state. During the write operation in above example, independent of the initial free layer state, both current and magnetic field stabilize the low resistance state, so the free layer will stay at a low resistance state if the initial data state is at the low resistance state and will switch to the low resistance state if the initial data state is at the high resistance state.
FIG. 5A is a schematic perspective view of one embodiment of a magnetic memory unit 1A having a magnetic tunnel junction cell with an adjacent magnetic field generator 25. Memory unit 1A includes magnetic tunnel junction cell 10. Magnetic tunnel junction cell 10 has a magnetic (e.g., ferromagnetic) free layer 12, a nonmagnetic tunnel barrier 13, a magnetic (e.g., ferromagnetic) pinned (i.e., reference) layer 14, and an antiferromagnetic pinning layer 18, arranged with tunnel barrier 13 positioned between free layer 12 and pinned layer 18, and pinning layer 18 proximate pinned layer 14 opposite tunnel barrier 13. In the illustrated orientation, free layer 12 is the top layer of the stack of layers. Tunnel barrier 13 spatially separates free layer 12 from pinned layer 14. Proximate pinning layer 18 is an electrode 20 that electrically connects magnetic tunnel junction cell 10 to control transistor 22.
The ferromagnetic layers for the structure, e.g., free layer 12 and pinned layer 14, can be, but not be limited to, transition metals such as Ni, Co, Fe and their alloys such as NiFe and CoFe, as described above. Ternary alloys, such as CoFeB, may be particularly useful because of their lower moment and high polarization ratio, which are desirable for the spin-current switch. Either or both of free layer 12 and pinned layer 14 may be either a single layer or an unbalanced synthetic antiferromagnetic (SAF) coupled structure, i.e., two ferromagnetic sublayers separated by a metallic spacer, such as Ru or Cu, with the magnetization orientations of the sublayers in opposite directions to provide a net magnetization.
Tunnel barrier layer 13 may be a nonmagnetic metallic material or a nonmagnetic metal oxide material; examples of suitable conductive metallic materials include Cu, Ag, and Au, and examples of insulating oxide and semiconductor barriers include AlO, Al2O3, TiO, and MgO. Tunneling barrier layer 13 could optionally be patterned with free layer 12 or with pinned layer 14, depending on process feasibility and device reliability.
Each of free layer 12 and pinned layer 14 has a magnetic orientation or magnetization orientation associated therewith. Pinned layer 14 is pinned by antiferromagnetic pinning layer 18, or in other embodiments, may be a fixed layer without pinning but with a high coercivity to stabilize itself.
In FIG. 5A, the magnetization orientation of free layer 12, due to spin polarization, is illustrated parallel to the magnetization orientation of pinned layer 14, due to a current being passed through magnetic tunnel junction cell 10 in the direction from free layer 12 to pinned layer 14 (illustrated by the dashed line).
Positioned orthogonal to the possible orientation directions of the magnetization of free layer 12 and pinned layer 14, a magnetic field generator 25 (e.g., conductive wire, or trace) is provided proximate magnetic tunnel junction cell 10. For memory unit 1A of FIG. 5A, magnetic field generator 25 is proximate free layer 12. Magnetic field generator 25 provides a magnetic field with current flow 25A in a direction that generates an ampere magnetic field that can either assist (during writing) or hinder (during reading) free layer switching, as described above.
FIG. 5B illustrates an alternate embodiment of a memory unit having magnetic field generator 25 (e.g., ampere magnetic field source) positioned orthogonal to the possible orientation directions of the magnetization of free layer 12 and pinned layer 14. In FIG. 5B memory unit 1B is similar to memory unit 1A in that it includes magnetic tunnel junction cell 10 having ferromagnetic free layer 12, nonmagnetic tunnel barrier 13, ferromagnetic pinned layer 14, antiferromagnetic pinning layer 18, and electrode 20. For memory unit 1B, however, magnetic field generator 25 is proximate electrode 20 and pinned layer 14. The operation of the memory unit 1B is similar to the operation of the memory unit 1A, described above. In other embodiments, the magnetic field generator can be an electrically conductive wire or trace that “wraps” around two or three sides of the magnetic tunnel junction cell 10.
FIG. 6 is a schematic circuit diagram of an illustrative spin-transfer torque MTJ memory apparatus. The apparatus includes a magnetic tunnel junction data cell MTJ including a ferromagnetic free layer and a ferromagnetic reference layer separated by an oxide barrier layer, as described above. The magnetic tunnel junction data cell MTJ is electrically between a bit line BL and a source line SL. The magnetic tunnel junction data cell MTJ is configured to switch between a high resistance state and a low resistance state by passing a polarized write current through the magnetic tunnel junction data cell MTJ.
A magnetic field generator MF is adjacent to the magnetic tunnel junction data cell MTJ. The magnetic field generator MF provides a magnetic field through the magnetic tunnel junction data cell MTJ forming a magnetic field modified magnetic tunnel junction data cell. Magnetic field generator MF provides a magnetic field with current flow in a direction that generates an ampere magnetic field that can either assist (during writing) or hinder (during reading) free layer switching of the magnetic tunnel junction data cell MTJ, as described above.
A cell transistor allows read and write current to flow through the magnetic tunnel junction data cell MTJ. A gate contact of the cell transistor is electrically coupled to a word line WL to allow selection of the particular cell transistor and associated magnetic tunnel junction data cell MTJ. An adjustable current driver ID is electrically coupled to the bit line, the adjustable current driver configured to provide a first read current IR1 and a second read current IR2 through the magnetic tunnel junction data cell MTJ. A first voltage storage device C1 is electrically coupled to the bit line BL and configured to store a first bit line voltage VBL1 formed by the first read current IR1. A second voltage storage device C2 is electrically coupled to the bit line BL and configured to store a second bit line voltage VBL2 formed by the second read current IR2. In many embodiments, the second voltage storage device C2 is a second voltage storage device C21 electrically coupled to the bit line BL and a third voltage storage device C22 electrically coupled to the bit line BL, and the second voltage storage device C21 and the third voltage storage device C22 are electrically connected in series, as shown in FIG. 6. In many embodiments, the first voltage storage device C1, the second voltage storage device C21, and the third voltage storage device C22 are capacitors. Useful capacitors include, for example, NMOS/PMOS capacitors, MIM capacitors, and vertical natural capacitors, among others.
A differential sense amplifier SA is electrically coupled to the first voltage storage device C1 and is electrically coupled to an intermediate node electrically between the second voltage storage device C21 and the third voltage storage device C22. The differential sense amplifier SA is configured to compare the first bit line voltage VBL1 with the second bit line voltage VBL2. A first switch transistor STL1 is electrically connected to the bit line BL and the first voltage storage device C1. A second switch transistor STL2 is electrically connected to the bit line BL and the second voltage storage device C21 and the third voltage storage device C22. In many embodiments, the voltage storage devices are capacitors. In many embodiments, the second voltage storage device C21 and the third voltage storage device C22 are capacitors that each has substantially the same capacitance value as each other.
A first read current IR1 is applied and incurs the corresponding BL voltage VBL1, which is stored in C1. Depending on the resistance state of the MTJ, VBL1 can be either VBL, L1 or VBL, H1, which are the BL voltage for low resistance state of MTJ or high resistance state of MTJ, at IR1. A second read current IR2 which is larger than IR1 is applied and incurs BL voltage VBL2, which is stored in C21 and C22.
By comparing VBL1 and VBL2 with the differential sense amplifier, the data resistance state of the MTJ can be readout. For example, if the first bit line read voltage VBL1 is not substantially the same as or is larger than or significantly larger than the second bit line read voltage VBL2 then the first resistance state is determined to be a high resistance state. Accordingly, if the first bit line read voltage VBL1 is substantially the same as or less than the second bit line read voltage VBL2 then the first resistance state is determined to be a low resistance state.
In many embodiments, if the stored first bit line read voltage VBL1 is 20% greater than, or 25% greater than, or 50% greater than, or 100% greater than, the stored second bit line read voltage VBL2 then the first resistance state is determined to be a high resistance state. Otherwise the resistance state is determined to be a low resistance state.
FIG. 7 is a graph of I-R curves of the MTJ high resistance state and low resistance state without providing an external magnetic field to the MTJ for either read current IR1 and IR2. The reference points RL1 and RL2 refer to the low resistance values (data state “0”) of the MTJ at the first read current IR1 and the second read current IR2, respectively. The reference points RH1 and RH2 refer to the high resistance values (data state “1”) of the MTJ at the first read current IR1 and the second read current IR2, respectively. The value ΔRLMAX refers to the change in the MTJ low resistance value from zero to the maximum allowable read current IRMAX. The value ΔRHMAX refers to the change in the MTJ high resistance value from zero to the maximum allowable read current IRMAX.
As illustrated in FIG. 7, the resistance of the magnetic tunnel junction data cell MTJ at the low resistance state is fairly insensitive to the change of read current/voltage. On the other hand, the resistance of the magnetic tunnel junction data cell MTJ at the high resistance state drops quickly when the read current/voltage increases. The change in the high state resistance value from the first read current IR1 to the second read current IR2 is shown as ΔRH. The change in the low state resistance value from the first read current IR1 to the second read current IR2 is shown as ΔRL. As illustrated, ΔARL is significantly less than ΔRH. Knowing that ΔRL is significantly less than ΔRH provides a means for comparing the voltage or resistance across the magnetic tunnel junction data cell MTJ to determine if it is in the high or low resistance state. The first read current IR1 is less than the second read current. In many embodiments, the first read current IR1 is 10% to 50% of the second read current IR2. In many embodiments, the first read current IR1 is 10% to 30% of the second read current IR2.
The magnetic field can hinder (during reading) free layer switching of the magnetic tunnel junction data cell, as described above, thus reading margin can be improved relative to the reading margin illustrated in FIG. 7. The magnetic field can assist (during writing) free layer switching of the magnetic tunnel junction data cell, as described above, thus decreasing the write current required to switch the resistance data state of the spin-transfer torque memory unit.
FIG. 8A is a graph of an I-R (current-resistance) curve of the MTJ high resistance state and low resistance state. FIG. 8B is a graph of an I-R (current-resistance) curve of the MTJ high resistance state and low resistance state with an applied external magnetic field. The change in the high state resistance value from the first read current IR1 to the second read current IR2 (with the applied external magnetic field, as described above) is shown as ΔRH. The change in the low state resistance value from the first read current IR1 to the second read current IR2 (with the applied external magnetic field, as described above) is shown as ΔRL. As similarly illustrated in FIG. 7, ΔRL is significantly less than ΔRH. Knowing that ΔRL is significantly less than ΔRH provides a means for comparing the voltage or resistance across the magnetic tunnel junction data cell MTJ to determine if it is in the high or low resistance state. The first read current IR1 is less than the second read current. In many embodiments, the first read current IR1 is 10% to 50% of the second read current IR2. In many embodiments, the first read current IR1 is 10% to 30% of the second read current IR2.
As illustrated in FIG. 8B, the external magnetic field increases the energy barrier value required to switch the magnetic tunnel junction data cell between the high resistance state and the low resistance state for the second read operation. This allows the second reading current to be a greater value than is possible in the graph of FIG. 8A (having a lower magnetic field or no magnetic field) without switching the resistance state of the MTJ.
Therefore, the reading margin for the high resistance state is increased as compared to the reading margin illustrated in FIG. 7. For example the second reading current IR2 FIG. 8B is greater than the switching current IR1 of FIG. 8A.
FIG. 9A is a graph of an I-R (current-resistance) curve of the MTJ high resistance state and low resistance state illustrating an applied magnetic field assisting in the writing of the high resistance state. FIG. 9B is a graph of an I-R (current-resistance) curve of the MTJ high resistance state and low resistance state illustrating an applied magnetic field assisting in the writing of the low resistance state. As compared to the FIG. 8A graph of the I-R (current-resistance) curve of the MTJ high resistance state and low resistance state, the applied magnetic field reduces the energy barrier, first switching current IS1, and time required to switch the MTJ to a high resistance state (FIG. 9A) or when applied in an opposing direction, the applied magnetic field reduces the energy barrier, second switching current IS2, and time required to switch the MTJ to a low resistance state (FIG. 9B).
FIG. 10 is a flow diagram of an illustrative self-reference reading method. The method includes applying a first read current through a magnetic tunnel junction data cell and forming a first bit line read voltage, the magnetic tunnel junction data cell having a first resistance state at block M1 and storing the first bit line read voltage in a first voltage storage device at block M2. A magnetic field is applied to or through the MTJ to increase the switching energy barrier of the MTJ, at block M3. Then second read current is applied thorough the first resistance state magnetic tunnel junction data cell to form a second bit line read voltage, where the first read current is less than the second read current at block M4 and storing the second bit line read voltage in a second voltage storage device and third voltage storage device at block M5. Then comparing the first bit line read voltage with the second bit line read voltage to determine whether the first resistance state of the magnetic tunnel junction data cell was a high resistance state or low resistance state at block M6.
The comparing block C1 compares the first bit line read voltage (VBL1) with the second bit line read voltage (VBL2). If the first bit line read voltage (VBL1) is greater than the second bit line read voltage (VBL2) then the first resistance state of the magnetic tunnel junction data cell is a high resistance state at block D1. If the first bit line read voltage (VBL1) is not greater than the second bit line read voltage (VBL2) then the first resistance state of the magnetic tunnel junction data cell is a low resistance state block D2.
The comparing step includes comparing the first bit line read voltage with the second bit line read voltage and if the first bit line read voltage is greater than or is significantly greater than the second bit line read voltage then the first resistance state is determined to be a high resistance state. Accordingly, if the first bit line read voltage is substantially the same as or less than the second bit line read voltage then the first resistance state is determined to be a low resistance state.
FIG. 11 is a flow diagram of an illustrative magnetic field assist write method. A magnetic field is applied though a MTJ to decrease the switching energy barrier of the MTJ, at block N1. Then, a write current is applied through the MTJ to switch the resistance data state of the MTJ, at block N2.
Thus, embodiments of the SPIN-TRANSFER TORQUE MEMORY SELF-REFERENCE READ AND WRITE ASSIST METHODS are disclosed. The implementations described above and other implementations are within the scope of the following claims. One skilled in the art will appreciate that the present disclosure can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.

## Claims

1. A method of self-reference reading a spin-transfer torque memory unit, comprising:
applying a first read current through a magnetic tunnel junction data cell and forming a first bit line read voltage, the magnetic tunnel junction data cell having a first resistance state, the magnetic tunnel junction comprising a free magnetic layer having a free magnetization orientation and a reference magnetic layer having a reference magnetization orientation, the magnetic tunnel junction data cell having a first energy barrier value required to switch the magnetic tunnel junction data cell between a high resistance state and a low resistance state;
storing the first bit line read voltage in a first voltage storage device;
applying a first magnetic field through the magnetic tunnel junction data cell forming a magnetic field modified magnetic tunnel junction data cell, the magnetic field being parallel or anti-parallel with the magnetization orientation of the reference magnetic layer, the magnetic field modified magnetic tunnel junction data cell having a second energy barrier value being greater than the first energy barrier value;
applying a second read current thorough the magnetic field modified magnetic tunnel junction data cell having the first resistance state and forming a second bit line read voltage, the first read current being less than the second read current;
storing the second bit line read voltage in a second voltage storage device; and
comparing the stored first bit line read voltage with the stored second bit line read voltage to determine whether the first resistance state of the magnetic tunnel junction data cell was a high resistance state or low resistance state.

2. A method according to claim 1, wherein the storing step comprises storing the second bit line read voltage in a second voltage storage device and a third voltage storage device, the second voltage storage device and the third voltage storage device are electrically connected in series.

3. A method according to claim 1, wherein the comparing step comprises comparing the stored first bit line read voltage with the stored second bit line read voltage and if the stored first bit line read voltage is substantially the same as or less than the stored second bit line read voltage, then the first resistance state is determined to be a low resistance state.

4. A method according to claim 1, wherein the comparing step comprises comparing the stored first bit line read voltage with the stored second bit line read voltage and if the stored first bit line read voltage is greater than or not substantially the same as the stored second bit line read voltage, then the first resistance state is determined to be a high resistance state.

5. A method according to claim 1, wherein the comparing step comprises comparing the stored first bit line read voltage with the stored second bit line read voltage and if the stored first bit line read voltage is 25% greater than the stored second bit line read voltage then the first resistance state is determined to be a high resistance state.

6. A method according to claim 1, wherein the applying a magnetic field through the magnetic tunnel junction data cell comprises generating a magnetic field by passing a current through a wire, the wire being adjacent to the magnetic tunnel junction data cell.

7. A method according to claim 1, wherein the first read current is 10% to 50% of the second read current.

8. A method according to claim 1, wherein the applying a first read current step occurs without applying a magnetic field through the magnetic tunnel junction data cell.

9. A method according to claim 1, wherein the applying a first read current step occurs while applying a second magnetic field through the magnetic tunnel junction data cell, the second magnetic field being parallel or anti-parallel with the magnetization orientation of the reference magnetic layer, and the second magnetic field having an equal magnitude to the first magnetic field.

10. A method according to claim 1, wherein the applying a first read current step occurs while applying a second magnetic field through the magnetic tunnel junction data cell, the second magnetic field being parallel or anti-parallel with the magnetization orientation of the reference magnetic layer, and the second magnetic field having a different magnitude than the first magnetic field.

11. A method of writing to a spin-transfer torque memory unit, comprising:
applying a magnetic field through magnetic tunnel junction data cell, forming a magnetic field modified magnetic tunnel junction data cell, the magnetic tunnel junction data cell having an energy barrier value required to switch the magnetic tunnel junction data cell between a high resistance state and a low resistance state, the magnetic field being parallel or anti-parallel with a magnetization orientation of a reference magnetic layer of the magnetic tunnel junction data cell, and the magnetic field decreases the energy barrier value required to switch the magnetic tunnel junction data cell between a high resistance state and a low resistance state; and
applying a write current through the magnetic field modified magnetic tunnel junction data cell to switch the magnetic field modified magnetic tunnel junction data cell between a high resistance state and low resistance state.

12. A method according to claim 11, wherein the applying a magnetic field step comprises generating a magnetic field by passing a current through a wire, the wire being adjacent to the magnetic tunnel junction data cell.

13. A method according to claim 11, wherein the applying a magnetic field step reduces a switching current required to switch the magnetic field modified magnetic tunnel junction data cell between a high resistance state and low resistance state as compared to a method without the applying a magnetic field step.

14. A method according to claim 11, wherein the applying a magnetic field step reduces a time duration required to switch the magnetic field modified magnetic tunnel junction data cell between a high resistance state and low resistance state as compared to a method without the applying a magnetic field step.

15. A method according to claim 11, wherein the write current is a polarized write current.

16. A method according to claim 11, wherein the magnetic field formed in a first direction through the magnetic tunnel junction data cell assists in switching the magnetic tunnel junction data cell from a low resistance state to a high resistance state and the magnetic field formed in a second direction, opposing the first direction, through the magnetic tunnel junction data cell assists in switching the magnetic tunnel junction data cell from a high resistance state to a low resistance state.

17. A spin-transfer torque memory apparatus comprising:
a magnetic tunnel junction data cell comprising a magnetic free layer and a magnetic reference layer separated by a oxide barrier layer, the magnetic tunnel junction data cell electrically between a bit line and a source line, the magnetic tunnel junction data cell is configured to switch between a high resistance state and a low resistance state by passing a polarized write current through the magnetic tunnel junction data cell;
an adjustable current driver electrically coupled to the bit line, the adjustable current driver configured to provide a provide a first read current and a second read current through the magnetic tunnel junction data cell;
a magnetic field generator adjacent to the magnetic tunnel junction data cell;
a first voltage storage device electrically coupled to the bit line and configured to store a first bit line voltage formed by the first read current;
a second voltage storage device electrically coupled to the bit line and configured to store a second bit line voltage formed by the second read current;
a third voltage storage device electrically coupled to the bit line and configured to store a second bit line voltage formed by the second read current, the second voltage storage device and the third voltage storage device are electrically connected in series; and
a differential sense amplifier electrically coupled to the first voltage storage device and electrically coupled to an intermediate node electrically between the second voltage storage device and the third voltage storage device, the differential sense amplifier configured to compare the first bit line voltage with the second bit line voltage.

18. A spin-transfer torque memory apparatus according to claim 17, the magnetic field generator generators a magnetic field through the magnetic tunnel junction data cell, the magnetic field being parallel or anti-parallel with a magnetization orientation of a reference magnetic layer of the magnetic tunnel junction data cell.

19. A spin-transfer torque memory apparatus according to claim 17, wherein the magnetic field generator comprises an electrically conductive wire.