Patent Description:
The present disclosure relates generally to the field of magnetic memory devices and specifically to a spin-transfer torque (STT) magnetoresistive random access memory (MRAM) device with assist layers and a method of operating the same.

Spin-transfer torque (STT) refers to an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve is modified by a spin-polarized current. Generally, electric current is unpolarized with electrons having random spin orientations. A spin polarized current is one in which electrons have a net non-zero spin due to a preferential spin orientation distribution. A spin-polarized current can be generated by passing electrical current through a magnetic polarizer layer. When the spin-polarized current flows through a free layer of a magnetic tunnel junction or a spin valve, the electrons in the spin-polarized current can transfer at least some of their angular momentum to the free layer, thereby producing torque to magnetize the free layer. When a sufficient amount of spin-polarized current passes through the free layer, spin-transfer torque can be employed to flip the orientation of the spin (e.g., change the magnetization) in the free layer. A resistance differential of a magnetic tunnel junction between different magnetization states of the free layer can be employed to store data within the magnetoresistive random access memory (MRAM) cell depending if the magnetization of the free layer is parallel or antiparallel to the magnetization of a reference layer. <CIT> describes a precessional spin current (PSC) structure for MRAM. A similar MRAM device with PSC structure in conjunction with a perpendicular MTJ is known from <CIT>. The PSC structure comprises first and second PSC ferromagnetic layers separated by a nonmagnetic PSC insertion layer. <CIT> describes magnetic random access memory. <CIT> describes a spin polarized magnetic device. <CIT> describes a crossbar diode-switched magnetoresistive random access memory system. In the <NPL>), <NPL>". All said documents disclose MRAM devices according to the preamble of independent claim <NUM>.

According to an aspect of the present invention, an MRAM device includes a magnetic tunnel junction containing a reference layer having a fixed magnetization direction, a free layer, and a nonmagnetic tunnel barrier layer located between the reference layer and the free layer, a negative-magnetic-anisotropy assist layer having negative magnetic anisotropy that provides an in-plane magnetization within a plane that is perpendicular to the fixed magnetization direction, and a first nonmagnetic spacer layer located between the free layer and the negative-magnetic-anisotropy assist layer. The MRAM device includes a pinned magnetization layer having a positive uniaxial magnetic anisotropy which provides a magnetization direction that is parallel or antiparallel to the fixed magnetization direction of the reference layer, and a second nonmagnetic spacer layer located between the negative-magnetic-anisotropy assist layer and the pinned magnetization layer.

According to another aspect of the present invention, a method of operating said MRAM device is provided.

As discussed above, the present invention is directed to a spin-transfer torque (STT) MRAM device with assist layers and a method of operating the same, the various aspects of which are described below.

The drawings are not drawn to scale. Multiple instances of an element may be duplicated where a single instance of the element is illustrated, unless absence of duplication of elements is expressly described or clearly indicated otherwise. Same reference numerals refer to the same element or to a similar element. Elements having the same reference numerals are presumed to have the same material composition unless expressly stated otherwise. Ordinals such as "first," "second," and "third" are employed merely to identify similar elements, and different ordinals may be employed across the specification and the claims of the instant disclosure. As used herein, a first element located "on" a second element can be located on the exterior side of a surface of the second element or on the interior side of the second element. As used herein, a first element is located "directly on" a second element if there exist a physical contact between a surface of the first element and a surface of the second element. As used herein, an "in-process" structure or a "transient" structure refers to a structure that is subsequently modified.

As used herein, a "layer" refers to a material portion including a region having a thickness. A layer may extend over the entirety of an underlying or overlying structure, or may have an extent less than the extent of an underlying or overlying structure. Further, a layer may be a region of a homogeneous or inhomogeneous continuous structure that has a thickness less than the thickness of the continuous structure. For example, a layer may be located between any pair of horizontal planes between, or at, a top surface and a bottom surface of the continuous structure. A layer may extend horizontally, vertically, and/or along a tapered surface. A substrate may be a layer, may include one or more layers therein, and/or may have one or more layer thereupon, thereabove, and/or therebelow.

As used herein, a "layer stack" refers to a stack of layers. As used herein, a "line" or a "line structure" refers to a layer that has a predominant direction of extension, i.e., having a direction along which the layer extends the most.

Referring to <FIG>, a schematic diagram is shown for a magnetic memory device including memory cells <NUM> of an embodiment of the present invention in an array configuration. The magnetic memory device can be configured as an MRAM device <NUM> containing MRAM cells <NUM>. As used herein, an "MRAM device" refers to a memory device containing cells that allow random access, e.g., access to any selected memory cell upon a command for reading the contents of the selected memory cell.

The MRAM device <NUM> includes a memory array region <NUM> containing an array of the respective MRAM cells <NUM> located at the intersection of the respective word lines (which may comprise electrically conductive lines <NUM> as illustrated or as second electrically conductive lines <NUM> in an alternate configuration) and bit lines (which may comprise second electrically conductive lines <NUM> as illustrated or as first electrically conductive lines <NUM> in an alternate configuration). The MRAM device <NUM> may also contain a row decoder <NUM> connected to the word lines, a sense circuitry <NUM> (e.g., a sense amplifier and other bit line control circuitry) connected to the bit lines, a column decoder <NUM> connected to the bit lines, and a data buffer <NUM> connected to the sense circuitry. Multiple instances of the MRAM cells <NUM> are provided in an array configuration that forms the MRAM device <NUM>. As such, each of the MRAM cells <NUM> can be a two-terminal device including a respective first electrode and a respective second electrode. It should be noted that the location and interconnection of elements are schematic and the elements may be arranged in a different configuration. Further, an MRAM cell <NUM> may be manufactured as a discrete device, i.e., a single isolated device.

Each MRAM cell <NUM> includes a magnetic tunnel junction or a spin valve having at least two different resistive states depending on the alignment of magnetizations of different magnetic material layers. The magnetic tunnel junction or the spin valve is provided between a first electrode and a second electrode within each MRAM cell <NUM>. Configurations of the MRAM cells <NUM> are described in detail in subsequent sections.

Referring to <FIG>, a first configuration of an exemplary STT MRAM cell <NUM> of a first example, such as known from the prior art, is schematically illustrated. The STT MRAM cell <NUM> includes a magnetic tunnel junction (MTJ) <NUM>. The magnetic tunnel junction <NUM> includes a reference layer <NUM> having a fixed vertical magnetization, a nonmagnetic tunnel barrier layer <NUM> located between the reference layer <NUM> and the free layer <NUM>. In one configuration, the reference layer <NUM> is located below the nonmagnetic tunnel barrier layer <NUM>, while the free layer <NUM> is located above the nonmagnetic tunnel barrier layer <NUM>. However, in other configurations, the reference layer <NUM> is located above the nonmagnetic tunnel barrier layer <NUM>, while the free layer <NUM> is located below the nonmagnetic tunnel barrier layer <NUM>, or the reference layer <NUM> and the free layer <NUM> may be located on opposite lateral sides nonmagnetic tunnel barrier layer <NUM>. The reference layer <NUM> and the free layer <NUM> have respective positive uniaxial magnetic anisotropy.

Generally, a magnetic thin film has magnetic energy per unit volume that depends on the orientation of the magnetization of the magnetic material of the magnetic thin film. The magnetic energy per unit volume can be approximated by a polynomial of the angle θ (or of sin<NUM>θ) between the direction of the magnetization and the vertical axis that is perpendicular to the plane of the magnetic thin film (such as a top surface or a bottom surface of the magnetic thin film) and the azimuthal angle φ between the direction of magnetization and a fixed vertical plane that is perpendicular to the plane of the magnetic thin film. The first and second order terms for the magnetic energy per unit volume as a function of sin<NUM>θ includes K<NUM>sin<NUM>θ + K<NUM>sin<NUM>θ. When K<NUM> is negative and K<NUM> is less than -K<NUM>/<NUM>, the function K<NUM>sin<NUM>θ + K<NUM>sin<NUM>θ has a minimum when θ is at π/<NUM>. If the magnetic anisotropy energy as a function of θ has a minimum only when θ is at π/<NUM>, the magnetization of the magnetic film prefers to stay entirely within the plane of the film, and the film is said to have "negative magnetic anisotropy. " If the magnetic anisotropy energy as a function of θ has a minimum only when θ is at <NUM> or π, the magnetization of the magnetic film is perpendicular to the plane of the film, and the film is said to have "positive magnetic anisotropy. " A thin crystalline magnetic film having positive magnetic anisotropy has a tendency for magnetization to stay perpendicular to the plane of the thin crystalline magnetic film, i.e., perpendicular to the two directions along which the thin crystalline magnetic film laterally extends. A thin crystalline magnetic film having negative magnetic anisotropy has a magnetization within the plane of the thin crystalline magnetic film, although within the film plane magnetization doesn't have a preferred orientation.

The configuration in which the reference layer <NUM> and the free layer <NUM> have respective positive uniaxial magnetic anisotropy provides bistable magnetization states for the free layer <NUM>. The bistable magnetization states include a parallel state in which the free layer <NUM> has a magnetization (e.g., magnetization direction) that is parallel to the fixed vertical magnetization (e.g., magnetization direction) of the reference layer <NUM>, and an antiparallel state in which the free layer <NUM> has a magnetization (e.g., magnetization direction) that is antiparallel to the fixed vertical magnetization (e.g., magnetization direction) of the reference layer <NUM>.

The reference layer <NUM> can include either a Co/Ni or Co/Pt multilayer structure. The reference layer <NUM> can additionally include a thin non-magnetic layer comprised of tantalum having a thickness of <NUM> ~ <NUM> and a thin CoFeB layer (having a thickness in a range from <NUM> to <NUM>). The nonmagnetic tunnel barrier layer <NUM> can include any tunneling barrier material such as an electrically insulating material, for example magnesium oxide. The thickness of the nonmagnetic tunnel barrier layer <NUM> can be <NUM> to <NUM>, such as about <NUM>. The free layer <NUM> can includes alloys of one or more of Fe, Co, and/or Ni, such as CoFeB, at a composition that provides positive uniaxial magnetic anisotropy.

The reference layer <NUM> may be provided as a component within a synthetic antiferromagnetic structure (SAF structure) <NUM>. The SAF structure <NUM> can include the reference layer <NUM>, a fixed ferromagnetic layer <NUM> having a magnetization that is antiparallel to the fixed vertical magnetization, and an antiferromagnetic coupling layer <NUM> located between the reference layer <NUM> and the fixed ferromagnetic layer <NUM> facing the first side of the reference layer <NUM> opposite to the second side of the reference layer <NUM> which faces the nonmagnetic tunnel barrier layer <NUM>. The antiferromagnetic coupling layer <NUM> has a thickness that induces an antiferromagnetic coupling between the reference layer <NUM> and the fixed ferromagnetic layer <NUM>. In other words, the antiferromagnetic coupling layer <NUM> can lock in the antiferromagnetic alignment between the magnetization of the reference layer <NUM> and the magnetization of the fixed ferromagnetic layer <NUM> to lock in place the magnetizations of the reference layer <NUM> and the magnetization of the fixed ferromagnetic layer <NUM>. The antiferromagnetic coupling layer can include ruthenium and can have a thickness in a range from <NUM> to <NUM>.

A first nonmagnetic spacer layer <NUM> is provided over the second side of the free layer <NUM> opposite to the first side of the free layer <NUM> which faces the nonmagnetic tunnel barrier layer <NUM>. The first nonmagnetic spacer layer <NUM> includes a nonmagnetic material such as tantalum, ruthenium, tantalum nitride, copper, copper nitride, or magnesium oxide. The first nonmagnetic spacer layer <NUM> can include an electrically conductive metallic material. Alternatively, the first nonmagnetic spacer layer <NUM> can include a tunneling dielectric material such as magnesium oxide. The thickness of the first nonmagnetic spacer layer <NUM> can be in a range from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed.

A negative-magnetic-anisotropy assist layer <NUM> can be provided over the first nonmagnetic spacer layer <NUM> and over the second side of the free layer <NUM>. The negative-magnetic-anisotropy assist layer <NUM> can have negative magnetic anisotropy with a sufficiently negative K<NUM> value to provide an in-plane magnetization for the negative-magnetic-anisotropy assist layer <NUM>. The in-plane magnetization is a magnetization located within a horizontal plane in <FIG> that is perpendicular to the fixed vertical magnetization of the reference layer <NUM>.

The hard magnetization axis is parallel to the direction normal to a major surface of the negative-magnetic-anisotropy assist layer <NUM> (i.e., the axis is perpendicular to the plane of the layer <NUM> and parallel to fixed vertical magnetization of the reference layer <NUM>), whereas the easy magnetization plane is parallel to the plane of the negative-magnetic-anisotropy assist layer <NUM> (i.e., the easy magnetization plane is perpendicular to the fixed vertical magnetization of the reference layer <NUM> in <FIG>). Within the plane (i.e., the easy magnetization plane) of the negative-magnetic-anisotropy assist layer <NUM>, there is no easy axis direction. The negative-magnetic-anisotropy assist layer <NUM> is spin-coupled with the free layer <NUM> through the first nonmagnetic spacer layer <NUM>.

The azimuthally-dependent component of the magnetic anisotropy of the negative-magnetic-anisotropy assist layer <NUM> may be zero or insignificant compared to the thermal energy at room temperature, i.e., kBT in which kB is the Boltzmann constant and T is <NUM> Kelvin (which is the room temperature). For example, the maximum variation of the magnetic anisotropy per unit volume around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM> can be less than <NUM>/<NUM> times the thermal energy at room temperature. In such cases, the magnetization of the negative-magnetic-anisotropy assist layer <NUM> is free to precess within the horizontal plane that is parallel to the interface between the first nonmagnetic spacer layer <NUM> and the negative-magnetic-anisotropy assist layer <NUM> upon application of electrical current through the negative-magnetic-anisotropy assist layer <NUM>. The magnetic energy of the negative-magnetic-anisotropy assist layer <NUM> may be invariant under rotation of the magnetization of the negative-magnetic-anisotropy assist layer <NUM> within the horizontal plane.

In one configuration, the negative-magnetic-anisotropy assist layer <NUM> comprises a homogeneous negative magnetic anisotropy material. As used herein, a "homogeneous" material refers to a material having a uniform material composition throughout. In one embodiment, the negative-magnetic-anisotropy assist layer <NUM> comprises, and/or consists essentially of, a cobalt-iridium alloy. The material composition of the cobalt-iridium alloy can be selected to provide negative magnetic anisotropy. The cobalt-iridium alloy can include cobalt atoms at an atomic concentration in a range from <NUM> % to <NUM> %, such as from <NUM> % to <NUM> %, for example <NUM> %, and iridium atoms at the atomic concentration in a range from <NUM> % to <NUM> %, such as from <NUM> % to <NUM> %, for example <NUM> %. In one configuration, the cobalt-iridium alloy contains only cobalt, iridium and unavoidable impurities. In another configuration, up to <NUM> atomic percent of elements other than cobalt and iridium may be added to the alloy. In an illustrative example, a cobalt-iridium alloy having a composition of Co<NUM>Ir<NUM> has a K<NUM> value of about -<NUM> × <NUM><NUM> J/m<NUM>. In another configuration, the negative-magnetic-anisotropy assist layer <NUM> comprises, and/or consists essentially of, a cobalt-iron alloy. The material composition of the cobalt-iron alloy can be selected to provide negative magnetic anisotropy. The cobalt-iron alloy can include cobalt atoms at an atomic concentration in a range from <NUM> % to <NUM> %, such as from <NUM> % to <NUM> %, such as <NUM> %, and iron atoms at the atomic concentration in a range from <NUM> % to <NUM> %, such as from <NUM> % to <NUM> %, for example <NUM> %. In an illustrative example, a cobalt-iron alloy having a composition of Co<NUM>Fe<NUM> has a K<NUM> value of about -<NUM> × <NUM><NUM> J/m<NUM>. The thickness of the negative-magnetic-anisotropy assist layer <NUM> can be in a range from <NUM> to <NUM>, such as from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed.

A nonmagnetic capping layer <NUM> can be located over the negative-magnetic-anisotropy assist layer <NUM>. The nonmagnetic capping layer <NUM> can include a non-magnetic, electrically conductive material, such as W, Ti, Ta, WN, TiN, TaN, Ru, and Cu. The thickness of the nonmagnetic capping layer <NUM> can be in a range from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed.

The layer stack including the material layers from the SAF structure <NUM> to the nonmagnetic capping layer <NUM> can be deposited upward or downward, i.e., from the SAF structure <NUM> toward the nonmagnetic capping layer <NUM> or from the nonmagnetic capping layer <NUM> toward the SAF structure <NUM>. The layer stack can be formed as a stack of continuous layers, and can be subsequently patterned into discrete patterned layer stacks for each MRAM cell <NUM>.

MRAM cell <NUM> can include a first terminal <NUM> that is electrically connected to or comprises a portion of a bit line <NUM> (shown in <FIG>) and second terminal <NUM> that is electrically connected to or comprises a portion of a word line <NUM> (shown in <FIG>). The location of the first and second terminals may be switched such that the first terminal is electrically connected to the SAF structure <NUM> and the second terminal is electrically connected to the capping layer <NUM>.

Optionally, each MRAM cell <NUM> can include a dedicated steering device, such an access transistor or diode configured to activate a respective discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) upon application of a suitable voltage to the steering device. The steering device may be electrically connected between the patterned layer stack and one of the respective word lines <NUM> or bit lines <NUM> of the respective MRAM cell <NUM>.

The polarity of the voltage applied to the first terminal <NUM> can be changed depending on the polarity of the magnetization state to be programmed in the free layer <NUM>. For example, a voltage of a first polarity can be applied to the first terminal <NUM> (with respect to the second terminal <NUM>) during a transition from an antiparallel state to a parallel state, and a voltage of a second polarity (which is the opposite of the first polarity) can be applied to the first terminal <NUM> during a transition from a parallel state to an antiparallel state. Further, variations in the circuitry for activating the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) are also contemplated herein.

The magnetization direction of the free layer <NUM> can be flipped (i.e., from upward to downward or vice versa) by flowing electrical current through the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>). The magnetization of the free layer <NUM> can precess around the vertical direction (i.e., the direction of the flow of the electrical current) during the programming process until the direction of the magnetization flips by <NUM> degrees, at which point the flow of the electrical current stops. In one configuration, the magnetization of the negative-magnetic-anisotropy assist layer <NUM> can rotate freely around a vertical axis that is parallel to the fixed magnetization direction of the reference layer <NUM> while electrical current flows through the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>). This configuration allows the negative-magnetic-anisotropy assist layer <NUM> to provide an initial non-vertical torque to the magnetization of the free layer <NUM> during an initial phase of precession of the magnetization of the free layer <NUM> around the vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM> upon initiation of flow of electrical current through the MRAM cell <NUM>.

The MRAM cell <NUM> can be configured to provide coupling between the in-plane magnetization of the negative-magnetic-anisotropy assist layer <NUM> and the magnetization of the free layer <NUM> during precession of the magnetization of the free layer <NUM> around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM>, and to provide synchronized precession of the in-plane magnetization of the negative-magnetic-anisotropy assist layer <NUM> and the magnetization of the free layer <NUM> while electrical current flows through the MRAM cell <NUM>.

Due to the negative magnetic anisotropy of the negative-magnetic-anisotropy assist layer <NUM>, the in-plane magnetization of the negative-magnetic-anisotropy assist layer <NUM> can provide an initial torque to the free layer to facilitate the initiation of switching of the free layer <NUM>. Once the free layer <NUM> precession starts, the free layer <NUM> can provide a spin torque to the negative-magnetic-anisotropy assist layer <NUM> to cause the negative-magnetic-anisotropy assist layer <NUM> magnetization to precess as well. This negative-magnetic-anisotropy assist layer <NUM> precession can in turn further assist the switching of the free layer <NUM>. The negative-magnetic-anisotropy assist layer <NUM> which has an in-plane easy magnetization plane but which lacks a fixed easy axis direction, is more efficient than a prior art assist layer where the assist layer's magnetization direction (e.g., easy axis) is fixed.

Referring to <FIG>, a second configuration of the exemplary spin-transfer torque MRAM cell <NUM> can be derived from the first configuration of the exemplary spin-transfer torque MRAM cell <NUM> of <FIG> by replacing the negative-magnetic-anisotropy assist layer <NUM> having a homogeneous material composition with a negative-magnetic-anisotropy assist layer <NUM> including a multilayer stack (<NUM>, <NUM>). The multilayer stack (<NUM>, <NUM>) can include multiple repetitions of a first magnetic material layer <NUM> and a second magnetic material layer <NUM>. The first magnetic material layer <NUM> can include, and/or can consist essentially of, a first magnetic material. The second magnetic material layer <NUM> can include, and/or can consist essentially of, a second magnetic material.

The composition and the thickness of each first magnetic material layer <NUM> and the composition and the thickness of each second magnetic material layer <NUM> can be selected such that the multilayer stack (<NUM>, <NUM>) provides an in-plane magnetization, i.e., a magnetization that is perpendicular to the fixed magnetization direction of the reference layer <NUM> (i.e., an easy magnetization plane that is perpendicular to the fixed magnetization direction of the reference layer <NUM> without an easy magnetization axis). The negative-magnetic-anisotropy assist layer <NUM> can have negative magnetic anisotropy with a sufficiently negative K<NUM> value to provide the in-plane magnetization for the negative-magnetic-anisotropy assist layer <NUM>.

The azimuthally-dependent component of the magnetic anisotropy of the negative-magnetic-anisotropy assist layer <NUM> may be zero or insignificant compared to the thermal energy at room temperature. For example, the maximum variation of the magnetic anisotropy per unit volume around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM> can be less than <NUM>/<NUM> times the thermal energy at room temperature. In such cases, the magnetization of the negative-magnetic-anisotropy assist layer <NUM> is free to precess within the plane that is parallel to the interface between the first nonmagnetic spacer layer <NUM> and the negative-magnetic-anisotropy assist layer <NUM> upon application of electrical current through the negative-magnetic-anisotropy assist layer <NUM>. The magnetic energy of the negative-magnetic-anisotropy assist layer <NUM> may be invariant under rotation of the magnetization of the negative-magnetic-anisotropy assist layer <NUM> within the horizontal plane.

The first magnetic material layers <NUM> may comprise cobalt, and the second magnetic material layers <NUM> may comprise iron. In one configuration, the first magnetic material layers <NUM> consist essentially of cobalt, and the second magnetic material layers <NUM> consist essentially of iron. The thickness of each first magnetic material layer <NUM> can be in a range from <NUM> to <NUM>, and the thickness of each second magnetic material layer <NUM> can be in a range from <NUM> to <NUM>. The total number of repetitions (i.e., the total number of pairs of a first magnetic material layer <NUM> and a second magnetic material layer <NUM>) within the negative-magnetic-anisotropy assist layer <NUM> can be in a range from <NUM> to <NUM>, such as from <NUM> to <NUM>. The multilayer stack (<NUM>, <NUM>) may comprise a periodic repetition of a unit layer stack that includes a first magnetic material layer <NUM> and a second magnetic material layer <NUM>. In an illustrative example, a cobalt-iron multilayer stack including repetitions of a unit layer stack consisting of a cobalt layer and an iron layer having the same thickness can have a K<NUM> value of about -<NUM> × <NUM><NUM> J/m<NUM>.

Referring to <FIG>, an embodiment of the exemplary spin-transfer torque MRAM cell <NUM> according to the present invention can be derived from the first configuration of the exemplary spin-transfer torque MRAM cell <NUM> of <FIG> by inserting a second nonmagnetic spacer layer <NUM> and a pinned magnetization layer <NUM> between the negative-magnetic-anisotropy assist layer <NUM> and the nonmagnetic capping layer <NUM>.

The second nonmagnetic spacer layer <NUM> can be located on the negative-magnetic-anisotropy assist layer <NUM> on the opposite side from the first nonmagnetic spacer layer <NUM>. The second nonmagnetic spacer layer <NUM> includes a nonmagnetic material such as tantalum, ruthenium, tantalum nitride, copper, copper nitride, or magnesium oxide. In one embodiment, the second nonmagnetic spacer layer <NUM> can include an electrically conductive material. Alternatively, the second nonmagnetic spacer layer <NUM> can include a dielectric material such as magnesium oxide. The thickness of the second nonmagnetic spacer layer <NUM> can be in a range from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed. The second nonmagnetic spacer layer <NUM> can include the same material as, or can include a material different from, the material of the first nonmagnetic spacer layer <NUM>.

The pinned magnetization layer <NUM> is a magnetic layer which has a positive uniaxial magnetic anisotropy. In other words, the value of K<NUM> is positive and the term K<NUM>sin<NUM>θ dominates all other higher order terms and terms depending on sin(nφ) (or cos(nφ)) in the magnetic anisotropy energy per volume for the material of the pinned magnetization layer <NUM>. The positive uniaxial magnetic anisotropy of the pinned magnetization layer <NUM> provides a magnetization that is parallel or antiparallel to the fixed vertical magnetization of the reference layer <NUM>. In one embodiment, the value of K<NUM> for the pinned magnetization layer <NUM> can be greater than the value of K<NUM> for the free layer <NUM> such that the magnetization of the pinned magnetization layer <NUM> stays pinned along the vertical direction, i.e., perpendicular to the interfaces among the various layers of the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), during programming of the MRAM cell <NUM>. The magnetization of the pinned magnetization layer <NUM> may remain parallel to, or antiparallel to, the magnetization of the reference layer <NUM>.

In one embodiment, the pinned magnetization layer <NUM> can include either a Co/Ni or Co/Pt multilayer structure. The pinned magnetization layer <NUM> can additionally include a thin non-magnetic layer comprised of tantalum having a thickness of <NUM> ~ <NUM> and a thin CoFeB layer (having a thickness in a range from <NUM> to <NUM>). The pinned magnetization layer <NUM> can cause the in-plane magnetization of the negative-magnetic-anisotropy assist layer <NUM> to oscillate. The oscillation of the in-plane magnetization of the negative-magnetic-anisotropy assist layer <NUM> can produce a rotating spin torque on the magnetization of the free layer <NUM> during programming, and thus, can help the switching of the magnetization of the free layer <NUM> with a lower electrical current through the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>). In one embodiment, the combination of the magnetization of the pinned magnetization layer <NUM> and the negative-magnetic-anisotropy assist layer <NUM> applies a non-horizontal and non-vertical magnetic field (i.e., a field which is neither parallel to nor perpendicular to the direction of the magnetization of the reference layer <NUM>) on the magnetization of the free layer <NUM> to reduce the magnitude of the required electrical current through the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) during switching of the magnetization of the free layer <NUM>.

Referring to <FIG>, another embodiment of the exemplary spin-transfer torque MRAM cell <NUM> according to the present invention can be derived from the configuration of the exemplary spin-transfer torque MRAM cell <NUM> of <FIG> by replacing the negative-magnetic-anisotropy assist layer <NUM> having a homogeneous material composition with the negative-magnetic-anisotropy assist layer <NUM> containing the multilayer stack (<NUM>, <NUM>) that includes multiple repetitions of a first magnetic material layer <NUM> and a second magnetic material layer <NUM> that was described above with respect to <FIG>.

Referring to all configurations of the exemplary spin-transfer torque MRAM cell <NUM> illustrated in <FIG>, the exemplary spin-transfer torque MRAM cell <NUM> can be programmed and read individually. Reading, i.e., sensing, the magnetization state of the free layer <NUM> can be performed by applying a read bias voltage across the first terminal <NUM> and the second terminal <NUM> of a selected discrete patterned layer stack {<NUM>, <NUM>, <NUM>, (<NUM> or <NUM>), <NUM>} or {<NUM>, <NUM>, <NUM>, (<NUM> or <NUM>), (<NUM>, <NUM>), <NUM>}. The parallel or antiparallel alignment between the magnetization of the free layer <NUM> and the reference layer <NUM> determines the electrical resistance of the selected discrete patterned layer stack in each MRAM cell <NUM>, and thus, determines the magnitude of the electrical current that flows between the first terminal <NUM> and the second terminal <NUM>. The magnitude of the electrical current can be sensed to determine the magnetization state of the free layer <NUM> and the data encoded by the detected magnetization state.

Programming of the exemplary spin-transfer torque MRAM cell <NUM> to the opposite magnetization state for the free layer <NUM> can be performed by flowing electrical current through the selected discrete patterned layer stack {<NUM>, <NUM>, <NUM>, (<NUM> or <NUM>), <NUM>} or {<NUM>, <NUM>, <NUM>, (<NUM> or <NUM>), (<NUM>, <NUM>), <NUM>} and by inducing the flipping, i.e., the switching, of the direction of the magnetization of the free layer <NUM>. Specifically, electrical current can be flowed through a selected discrete patterned layer stack which includes a magnetic tunnel junction <NUM>, a first nonmagnetic spacer layer <NUM>, and a negative-magnetic-anisotropy assist layer (<NUM> or <NUM>). The in-plane magnetization of the negative-magnetic-anisotropy assist layer (<NUM> or <NUM>) provides an initial non-vertical torque to the magnetization of the free layer <NUM> during an initial phase of precession of the magnetization of the free layer <NUM> around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM> upon initiation of flow of the electrical current through the magnetic tunnel junction <NUM>, the first nonmagnetic spacer layer <NUM>, and the negative-magnetic-anisotropy assist layer (<NUM> or <NUM>).

In one embodiment, the in-plane magnetization of the negative-magnetic-anisotropy assist layer (<NUM> or <NUM>) couples with a magnetization of the free layer <NUM> during precession of the magnetization of the free layer <NUM> around the vertical axis (VA) that is parallel to the fixed vertical magnetization of the reference layer <NUM> to provide synchronized precession of the in-plane magnetization M2 of the negative-magnetic-anisotropy assist layer (<NUM> or <NUM>) and the magnetization M1 of the free layer <NUM> while electrical current flows through the MRAM cell <NUM> as illustrated in <FIG> and <FIG>. <FIG> illustrates a transition of the magnetization M1 of the free layer <NUM> from an "up" state to a "down" state, and <FIG> illustrates a transition of the magnetization M1 of the free layer <NUM> from a "down" state to an "up" state. In one embodiment, the in-plane magnetization M2 of the negative-magnetic-anisotropy assist layer (<NUM> or <NUM>) and the magnetization M1 of the free layer <NUM> may remain within a same rotating vertical plane during the switching of the magnetization of the free layer <NUM>. The coupling between the horizontal (in-plane) component of the magnetization M1 of the free layer <NUM> and the in-plane magnetization M2 of the negative-magnetic-anisotropy assist layer (<NUM> or <NUM>) may be antiferromagnetic or ferromagnetic. <FIG> and <FIG> illustrate an example in which the coupling between the horizontal (in-plane) component of the magnetization M1 of the free layer <NUM> and the in-plane magnetization M2 of the negative-magnetic-anisotropy assist layer (<NUM> or <NUM>) is antiferromagnetic during the switching of the magnetization of the free layer <NUM>.

<FIG> illustrates a STT MRAM cell <NUM> according to the second example. Layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> of the STT MRAM cell <NUM> of the second example may be the same as respective layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> of the STT MRAM cell <NUM> of the first example illustrated in <FIG>, and thus are described above with respect to the first embodiment. A first magnetic assist layer <NUM> can be provided on the first nonmagnetic spacer layer <NUM>. The first magnetic assist layer <NUM> includes a first magnetic material having a first magnetic anisotropy. In one embodiment, the first magnetic assist layer <NUM> can have a first negative magnetic anisotropy with a sufficiently negative K<NUM> value to provide a first in-plane magnetization for the first magnetic assist layer <NUM>. The in-plane magnetization is a magnetization located within a horizontal plane that is perpendicular to the fixed vertical magnetization direction of the reference layer <NUM>.

The azimuthally-dependent component of the first magnetic anisotropy of the first magnetic assist layer <NUM> may be zero or insignificant compared to the thermal energy at room temperature, i.e., kBT in which kB is the Boltzmann constant and T is <NUM> Kelvin (which is the room temperature). For example, the maximum variation of the magnetic anisotropy energy per unit volume around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM> can be less than <NUM>/<NUM> times the thermal energy at room temperature. In such cases, the magnetization of the first magnetic assist layer <NUM> is free to precess within the horizontal plane that is parallel to the interface between the first nonmagnetic spacer layer <NUM> and the first magnetic assist layer <NUM> upon application of electrical current through the first magnetic assist layer <NUM>. In one embodiment, the magnetic energy of the first magnetic assist layer <NUM> may be invariant under rotation of the magnetization of the first magnetic assist layer <NUM> within the horizontal plane.

A material having a negative magnetic anisotropy, such as the first magnetic assist layer <NUM>, may have a hard magnetization axis that parallel to the direction normal to a major surface of the layer (i.e., the axis is perpendicular to the plane of the layer and parallel to fixed vertical magnetization direction of the reference layer <NUM>), whereas the easy magnetization plane is parallel to the plane of the layer (i.e., the easy magnetization plane is perpendicular to the fixed vertical magnetization direction of the reference layer <NUM> in <FIG>). In one example, there is no easy axis direction within the easy magnetization plane.

The first magnetic assist layer <NUM> may comprise a homogeneous negative magnetic anisotropy material. As used herein, a "homogeneous" material refers to a material having a uniform material composition throughout. The first magnetic assist layer <NUM> may comprise, and/or consist essentially of, a cobalt-iridium alloy. The material composition of the cobalt-iridium alloy can be selected to provide negative magnetic anisotropy. The cobalt-iridium alloy can include cobalt atoms at an atomic concentration in a range from <NUM> % to <NUM> %, such as from <NUM> % to <NUM> %, for example <NUM> %, and iridium atoms at the atomic concentration in a range from <NUM> % to <NUM> %, such as from <NUM> % to <NUM> %, for example <NUM> %. The cobalt-iridium alloy may contain only cobalt, iridium and unavoidable impurities. In another configuration, up to <NUM> atomic percent of elements other than cobalt and iridium may be added to the alloy. In an illustrative example, a cobalt-iridium alloy having a composition of Co<NUM>Ir<NUM> has a K<NUM> value of about -<NUM> × <NUM><NUM> J/m<NUM>. The first magnetic assist layer <NUM> may comprise, and/or consist essentially of, a cobalt-iron alloy having a hexagonal crystal structure. The material composition of the cobalt-iron alloy can be selected to provide negative magnetic anisotropy. The cobalt-iron alloy can include cobalt atoms at an atomic concentration in a range from <NUM> % to <NUM> %, such as from <NUM> % to <NUM> %, such as <NUM> %, and iron atoms at the atomic concentration in a range from <NUM> % to <NUM> %, such as from <NUM> % to <NUM> %, for example <NUM> %. In an illustrative example, a cobalt-iron alloy having a composition of Co<NUM>Fe<NUM> has a K<NUM> value of about -<NUM> × <NUM><NUM> J/m<NUM>. The thickness of the first magnetic assist layer <NUM> can be in a range from <NUM> to <NUM>, such as from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed.

The first magnetic assist layer <NUM> may comprise a multilayer stack that includes multiple repetitions of a first magnetic material layer and a second magnetic material layer. The first magnetic material layer can include, and/or can consist essentially of, a first magnetic material. The second magnetic material layer can include, and/or can consist essentially of, a second magnetic material. The composition and the thickness of each first magnetic material layer and the composition and the thickness of each second magnetic material layer can be selected such that the multilayer stack provides an in-plane magnetization, i.e., a magnetization that is perpendicular to the fixed magnetization direction of the reference layer <NUM>. The first magnetic assist layer <NUM> can have negative magnetic anisotropy with a sufficiently negative K<NUM> value to provide the first in-plane magnetization for the first magnetic assist layer <NUM>.

The first magnetic material layers may comprise cobalt, and the second magnetic material layers may comprise iron. The first magnetic material layers may consist essentially of cobalt, and the second magnetic material layers may consist essentially of iron. The thickness of each first magnetic material layer can be in a range from <NUM> to <NUM>, and the thickness of each second magnetic material layer can be in a range from <NUM> to <NUM>. The total number of repetitions (i.e., the total number of pairs of a first magnetic material layer and a second magnetic material layer) within the first magnetic assist layer <NUM> can be in a range from <NUM> to <NUM>, such as from <NUM> to <NUM>. The multilayer stack may comprise a periodic repetition of a unit layer stack that includes a first magnetic material layer and a second magnetic material layer.

An antiferromagnetic coupling spacer layer <NUM> can be located between the first and second magnetic assist layers, such as on the first magnetic assist layer <NUM> on the opposite side of the first nonmagnetic spacer layer <NUM>, which is located between the free layer <NUM> and the first magnetic assist layer <NUM>. The antiferromagnetic coupling spacer layer <NUM> comprises a metallic material that induces Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling interaction between the first magnetic assist layer <NUM> and a second magnetic assist layer <NUM> that may be located on the antiferromagnetic coupling spacer layer <NUM>. In the RKKY coupling interaction, localized inner d- or f-shell electron spins that define the magnetization direction of a ferromagnetic metal layer interact through the conduction electrons in an intervening nonmagnetic material layer to define a direction of preferred magnetization direction in another ferromagnetic metal layer. The thickness of the antiferromagnetic coupling spacer layer <NUM> can be selected such that a second in-plane magnetization direction of a second magnetic assist layer <NUM> is antiparallel to the first in-plane magnetization direction of the first magnetic assist layer <NUM>. In other words, the antiferromagnetic coupling spacer layer <NUM> can have a thickness within a range that provides antiferromagnetic coupling between a first magnetization direction of the first magnetic assist layer <NUM> and a second magnetization direction of the second magnetic assist layer 166The antiferromagnetic coupling spacer layer <NUM> may comprise, or consist essentially of, ruthenium, and has a thickness within a range from <NUM> to <NUM>.

A second magnetic assist layer <NUM> can be provided on the antiferromagnetic coupling spacer layer <NUM>. The second magnetic assist layer <NUM> includes a second magnetic material having second magnetic anisotropy, which can be the same as or different from the material of the first magnetic assist layer <NUM>. The second magnetic assist layer <NUM> can have a second negative magnetic anisotropy with a sufficiently negative K<NUM> value to provide a second in-plane magnetization direction for the second magnetic assist layer <NUM>. The in-plane magnetization direction is a magnetization direction located within a horizontal plane that is perpendicular to the fixed vertical magnetization direction of the reference layer <NUM>.

The azimuthally-dependent component of the magnetic anisotropy of the second magnetic assist layer <NUM> may be zero or insignificant compared to the thermal energy at room temperature, i.e., kBT in which kB is the Boltzmann constant and T is <NUM> Kelvin (which is the room temperature). For example, the maximum variation of the magnetic anisotropy per unit volume around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM> can be less than <NUM>/<NUM> times the thermal energy at room temperature. In such cases, the magnetization of the second magnetic assist layer <NUM> is free to precess within the horizontal plane that is parallel to the interface between the antiferromagnetic coupling spacer layer <NUM> and the second magnetic assist layer <NUM> upon application of electrical current through the second magnetic assist layer <NUM>. The magnetic energy of the second magnetic assist layer <NUM> may be invariant under rotation of the magnetization of the second magnetic assist layer <NUM> within the horizontal plane.

The second magnetic assist layer <NUM> may comprise a homogeneous negative magnetic anisotropy material. The second magnetic assist layer <NUM> may comprise, and/or consist essentially of, a cobalt-iridium alloy or a cobalt-iron alloy described with respect to the first magnetic assist layer <NUM>. The material composition of the cobalt-iridium alloy can be selected to provide negative magnetic anisotropy. The cobalt-iridium alloy can include cobalt atoms at an atomic concentration in a range from <NUM> % to <NUM> % and iridium atoms at the atomic concentration of the balance. The thickness of the second magnetic assist layer <NUM> can be in a range from <NUM> to <NUM>, such as from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed.

The second magnetic assist layer <NUM> may comprise a multilayer stack that includes multiple repetitions of a first magnetic material layer and a second magnetic material layer. The first magnetic material layer can include, and/or can consist essentially of, a first magnetic material. The second magnetic material layer can include, and/or can consist essentially of, a second magnetic material. The composition and the thickness of each first magnetic material layer and the composition and the thickness of each second magnetic material layer can be selected such that the multilayer stack provides an in-plane magnetization, i.e., a magnetization that is perpendicular to the fixed magnetization direction of the reference layer <NUM>. The second magnetic assist layer <NUM> can have negative magnetic anisotropy with a sufficiently negative K<NUM> value to provide the second in-plane magnetization for the second magnetic assist layer <NUM>.

The first magnetic material layers may comprise cobalt, and the second magnetic material layers may comprise iron. The first magnetic material layers may consist essentially of cobalt, and the second magnetic material layers may consist essentially of iron. The thickness of each first magnetic material layer can be in a range from <NUM> to <NUM>, and the thickness of each second magnetic material layer can be in a range from <NUM> to <NUM>. The total number of repetitions (i.e., the total number of pairs of a first magnetic material layer and a second magnetic material layer) within the second magnetic assist layer <NUM> can be in a range from <NUM> to <NUM>, such as from <NUM> to <NUM>. The multilayer stack may comprise a periodic repetition of a unit layer stack that includes a first magnetic material layer and a second magnetic material layer.

Generally, each of the first magnetic assist layer <NUM> and the second magnetic assist layer <NUM> can be independently selected from a homogeneous negative magnetic anisotropy material, and a multilayer stack including multiple repetitions of a first magnetic material layer and a second magnetic material layer. Each of the first magnetic assist layer <NUM> and the second magnetic assist layer <NUM> can be independently selected from a cobalt-iridium alloy, a cobalt-iron alloy having a hexagonal crystal structure and low iron content, or a multilayer stack including multiple repetitions of a unit stack of a cobalt layer and an iron layer. At least one of the first magnetic assist layer <NUM> and the second magnetic assist layer <NUM> may comprise a multilayer stack including a periodic repetition of a unit layer stack, and the unit layer stack may include the first magnetic material layer and the second magnetic material.

A nonmagnetic capping layer <NUM> can be located over the second magnetic assist layer <NUM>. The nonmagnetic capping layer <NUM> can include a non-magnetic, electrically conductive material, such as W, Ti, Ta, WN, TiN, TaN, Ru, and Cu. The thickness of the nonmagnetic capping layer <NUM> can be in a range from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed.

Optionally, each MRAM cell <NUM> can include a dedicated steering device, such an access transistor or diode configured to activate a respective discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) upon application of a suitable voltage to the steering device. The steering device may be electrically connected between the patterned layer stack and one of the respective word lines <NUM> or bit lines <NUM> of the respective MRAM cell <NUM>.

The polarity of the voltage applied to the first terminal <NUM> can be changed depending on the polarity of the magnetization state to be programmed in the free layer <NUM>. For example, a voltage of a first polarity can be applied to the first terminal <NUM> (with respect to the second terminal <NUM>) during a transition from an antiparallel state to a parallel state, and a voltage of a second polarity (which is the opposite of the first polarity) can be applied to the first terminal <NUM> during a transition from a parallel state to an antiparallel state. Further, variations in the circuitry for activating the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) are also contemplated herein.

The magnetization direction of the free layer <NUM> can be flipped (i.e., from upward to downward or vice versa) by flowing electrical current through the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>). The magnetization direction of the free layer <NUM> can precess around the vertical direction (i.e., the direction of the flow of the electrical current) during the programming process until the direction of the magnetization flips by <NUM> degrees, at which point the flow of the electrical current stops.

The first magnetization direction of the first magnetic assist layer <NUM> and the second magnetization direction of the second magnetic assist layer <NUM> are free to precess around a vertical axis that is parallel to the fixed vertical magnetization direction of the reference layer <NUM> while maintaining an antiferromagnetic alignment therebetween upon application of electrical current through the first magnetic assist layer <NUM>, the antiferromagnetic coupling spacer layer <NUM>, and the second magnetic assist layer <NUM>, e.g., during programming. The fixed vertical magnetization direction of the reference layer <NUM> maintains a same orientation upon application of electrical current through the reference layer <NUM>.

During operation of the magnetic memory device, electrical current can be flowed through the magnetic tunnel junction <NUM>, the first nonmagnetic spacer layer <NUM>, the first magnetic assist layer <NUM>, the antiferromagnetic coupling spacer layer <NUM>, and the second magnetic assist layer <NUM>.

The first magnetic assist layer <NUM>, the antiferromagnetic coupling spacer layer <NUM>, and the second magnetic assist layer <NUM> may help keep the electron spin of the free layer more in plane to counteract the spin torque which tilts the electron spin out of the plane. Due to the anti-ferromagnetic coupling, the anti-ferromagnetic coupled assist film comprising the combination of the first magnetic assist layer <NUM>, the antiferromagnetic coupling spacer layer <NUM>, and the second magnetic assist layer <NUM> further favors single domains within each layer, thus maintaining a more coherent magnetization during the process of assisting the free layer <NUM> switching, which is more desired. One additional benefit of this example is that the flux closure within the tri-layer assist film may minimize the stray field from the anti-ferromagnetic coupled assist film on the free layer <NUM>, which will help improve the thermal stability and data retention of the MRAM cell <NUM>.

The combination of the first magnetic assist layer <NUM>, the antiferromagnetic coupling spacer layer <NUM>, and the second magnetic assist layer <NUM> may be configured to provide an initial non-vertical torque to a magnetization of the free layer <NUM> during an initial phase of precession of the magnetization of the free layer <NUM> around a vertical axis that is parallel to the fixed vertical magnetization direction of the reference layer <NUM> upon initiation of flow of electrical current through the MRAM cell <NUM>. The MRAM cell <NUM> is configured to provide magnetic coupling between the magnetization direction of the free layer <NUM> and the first magnetization direction of the first magnetic assist layer <NUM> during precession of the magnetization direction of the free layer <NUM> around a vertical axis that is parallel to the fixed vertical magnetization direction of the reference layer <NUM>, and to provide synchronized precession of the first magnetization direction of the first magnetic assist layer <NUM> and the magnetization direction of the free layer <NUM> while electrical current flows through the MRAM cell <NUM>.

Referring to <FIG>, a second configuration of the exemplary spin-transfer torque MRAM cell <NUM> can be derived from the first configuration of the exemplary spin-transfer torque magnetic memory device illustrated in <FIG> by replacing the first magnetic assist layer <NUM> having the first in-plane magnetization with a first magnetic assist layer <NUM> including a first ferromagnetic material having no uniaxial magnetic anisotropy, and by replacing the second magnetic assist layer <NUM> having the second in-plane magnetization with a second magnetic assist layer <NUM> including a second ferromagnetic material having no uniaxial magnetic anisotropy. The first and second magnetic assist layers (<NUM>, <NUM>) can have a non-uniaxial magnetic anisotropy. As used herein, a "non-uniaxial magnetic anisotropy" refers to a magnetic anisotropy in which the minimum of the magnetic anisotropy energy per volume does not occur at the direction of θ = <NUM>, θ = π, or θ = π/<NUM> for all values of φ. In other words, the orientation of the magnetization in a magnetic film having a non-uniaxial magnetic anisotropy is not a vertical direction that is perpendicular to the plane of a magnetic film or the set of all in-plane directions.

The thickness of the antiferromagnetic coupling spacer layer <NUM> is selected to provide an antiferromagnetic coupling between the first magnetization of the first magnetic assist layer <NUM> and the second magnetization with a second magnetic assist layer <NUM>. Thus, the first magnetization of the first magnetic assist layer <NUM> and the second magnetization of the second magnetic assist layer <NUM> can be antiferromagnetically coupled. Further, the variations in the magnetic anisotropy energy per volume as a function of spatial orientations of the first and second magnetizations (which remain antiparallel to each other) can be on par with, or less than, the thermal energy at room temperature, i.e., kBT in which T is <NUM> Kelvin.

Each of the first magnetic assist layer <NUM> and the second magnetic assist layer <NUM> comprises a respective soft magnetic material having no uniaxial magnetic anisotropy, which may be the same or different. Each of the first magnetic assist layer <NUM> and the second magnetic assist layer <NUM> may comprise, and/or consist essentially of, a respective material selected from a CoFe alloy having more than <NUM> atomic percent iron, such as <NUM> to <NUM> atomic percent iron and balance cobalt or an NiFe alloy.

The magnetization direction of the free layer <NUM> can be flipped (i.e., from upward to downward or vice versa) by flowing electrical current through the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>). The magnetization direction of the free layer <NUM> can precess around the vertical direction (i.e., the direction of the flow of the electrical current) during the programming process until the direction of the magnetization direction flips by <NUM> degrees, at which point the flow of the electrical current stops.

The magnetization direction of the free layer <NUM> can be programmed by flowing electrical current through the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), e.g., from a parallel state that is parallel to the fixed vertical magnetization direction of the reference layer <NUM> to an antiparallel state that is antiparallel to the fixed magnetization direction of the reference layer <NUM> or vice versa. The first magnetization direction of the first magnetic assist layer <NUM> and the second magnetization direction of the second magnetic assist layer <NUM> are free to precess around a vertical axis that is parallel to the fixed vertical magnetization direction of the reference layer <NUM> at an angle between <NUM> degree and <NUM> degrees with respect to the vertical axis while maintaining an antiferromagnetic alignment therebetween upon application of electrical current through the first magnetic assist layer <NUM>, the antiferromagnetic coupling spacer layer <NUM>, and the second magnetic assist layer <NUM>, e.g., during programming. The tilt angles of the first magnetization direction of the first magnetic assist layer <NUM> and the second magnetization direction of the second magnetic assist layer <NUM> during programming is synchronized with the tilt angle of the magnetization direction of the free layer <NUM> as the tilt angle changes from <NUM> degrees to <NUM> degrees or from <NUM> degrees to <NUM> degrees with respect to the vertical axis during programming of the MRAM cell <NUM>. The fixed vertical magnetization direction of the reference layer <NUM> maintains a same orientation upon application of electrical current through the reference layer <NUM>.

During operation of the magnetic memory device, electrical current can be flowed through the magnetic tunnel junction <NUM>, the first nonmagnetic spacer layer <NUM>, the first magnetic assist layer <NUM>, the antiferromagnetic coupling spacer layer <NUM>, and the second magnetic assist layer <NUM>. The combination of the first magnetic assist layer <NUM>, the antiferromagnetic coupling spacer layer <NUM>, and the second magnetic assist layer <NUM> is configured to provide an initial non-vertical torque to a magnetization direction of the free layer <NUM> during an initial phase of precession of the magnetization direction of the free layer <NUM> around a vertical axis that is parallel to the fixed vertical magnetization direction of the reference layer <NUM> upon initiation of flow of electrical current through the MRAM cell <NUM>. The MRAM cell <NUM> is configured to provide magnetic coupling between the magnetization direction of the free layer <NUM> and the first magnetization direction of the first magnetic assist layer <NUM> during precession of the magnetization direction of the free layer <NUM> around a vertical axis that is parallel to the fixed vertical magnetization direction of the reference layer <NUM>, and to provide synchronized precession of the first magnetization direction of the first magnetic assist layer <NUM> and the magnetization direction of the free layer <NUM> while electrical current flows through the MRAM cell <NUM>.

Referring to <FIG>, a third configuration of the exemplary spin-transfer torque MRAM cell <NUM> can be derived from the first configuration of the exemplary spin-transfer torque MRAM cell <NUM> of <FIG> by inserting a second nonmagnetic spacer layer <NUM> and a pinned magnetization layer <NUM> between the second magnetic assist layer <NUM> and the nonmagnetic capping layer <NUM>.

The second nonmagnetic spacer layer <NUM> can be located on the second magnetic assist layer <NUM> on the opposite side of the antiferromagnetic coupling spacer layer <NUM>. The second nonmagnetic spacer layer <NUM> includes a nonmagnetic material such as tantalum, ruthenium, tantalum nitride, copper, copper nitride, or magnesium oxide. The second nonmagnetic spacer layer <NUM> can include an electrically conductive material. Alternatively, the second nonmagnetic spacer layer <NUM> can include a tunneling dielectric material such as magnesium oxide. The thickness of the second nonmagnetic spacer layer <NUM> can be in a range from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed. The second nonmagnetic spacer layer <NUM> can include the same material as, or can include a material different from, the material of the first nonmagnetic spacer layer <NUM>.

The pinned magnetization layer <NUM> is a magnetic layer which has a positive uniaxial magnetic anisotropy. In other words, the value of K<NUM> is positive and the term K<NUM>sin<NUM>θ dominates all other higher order terms and terms depending on sin(nφ) (or cos(nφ)) in the magnetic anisotropy energy per volume for the material of the pinned magnetization layer <NUM>. The positive uniaxial magnetic anisotropy of the pinned magnetization layer <NUM> provides a magnetization that is parallel or antiparallel to the fixed vertical magnetization of the reference layer <NUM>. The value of K<NUM> for the pinned magnetization layer <NUM> can be greater than the value of K<NUM> for the free layer <NUM> such that the magnetization of the pinned magnetization layer <NUM> stays pinned along the vertical direction, i.e., perpendicular to the interfaces among the various layers of the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), during programming of the MRAM cell <NUM>. The magnetization of the pinned magnetization layer <NUM> may remain parallel to, or antiparallel to, the magnetization of the reference layer <NUM>.

The pinned magnetization layer <NUM> can include either a Co/Ni or Co/Pt multilayer structure. The pinned magnetization layer <NUM> can additionally include a thin non-magnetic layer comprised of tantalum having a thickness of <NUM> ~ <NUM> and a thin CoFeB layer (having a thickness in a range from <NUM> to <NUM>). The pinned magnetization layer <NUM> can cause the in-plane magnetization of the second magnetic assist layer <NUM> to oscillate. The out-of-plane oscillation of the magnetization of the second magnetic assist layer <NUM> can produce a rotating spin torque on the magnetization of the free layer <NUM> during programming, and thus, can help the switching of the magnetization of the free layer <NUM> with lesser electrical current through the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>). The combination of the magnetization of the pinned magnetization layer <NUM>, the first magnetic assist layer <NUM>, and second magnetic assist layer <NUM> may apply a non-horizontal non-vertical magnetic field (i.e., a field which is neither parallel to not perpendicular to the fixed magnetization direction of the reference layer <NUM>) on the magnetization of the free layer <NUM> to reduce the magnitude of the required electrical current through the discrete patterned layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) during switching of the magnetization of the free layer <NUM>.

Referring to <FIG>, a fourth configuration of the exemplary spin-transfer torque MRAM cell <NUM> can be derived from the third configuration of the exemplary spin-transfer torque magnetic memory device illustrated in <FIG> by replacing the first magnetic assist layer <NUM> having the first in-plane magnetization with a first magnetic assist layer <NUM> including a first ferromagnetic material having no uniaxial magnetic anisotropy, and by replacing the second magnetic assist layer <NUM> having the second in-plane magnetization with a second magnetic assist layer <NUM> including a second ferromagnetic material having no uniaxial magnetic anisotropy.

Referring to all configurations of the exemplary spin-transfer torque MRAM cell <NUM> illustrated in <FIG> and <FIG>, the exemplary spin-transfer torque MRAM cell <NUM> can be programmed and read individually. Reading, i.e., sensing, the magnetization state of the free layer <NUM> can be performed by applying a read bias voltage across the first terminal <NUM> and the second terminal <NUM> of a selected discrete patterned layer stack {<NUM>, <NUM>, <NUM>, (<NUM> or <NUM>), <NUM>, (<NUM> or <NUM>), <NUM>} or {<NUM>, <NUM>, <NUM>, (<NUM> or <NUM>), <NUM>, (<NUM> or <NUM>), (<NUM>, <NUM>), <NUM>}. The parallel or antiparallel alignment between the magnetization of the free layer <NUM> and the reference layer <NUM> determines the electrical resistance of the selected discrete patterned layer stack in each MRAM cell <NUM>, and thus, determines the magnitude of the electrical current that flows between the first terminal <NUM> and the second terminal <NUM>. The magnitude of the electrical current can be sensed to determine the magnetization state of the free layer <NUM> and the data encoded by the detected magnetization state.

<FIG> illustrates a comparative spin-transfer torque MRAM cell <NUM> that can be derived from the exemplary spin-transfer torque MRAM cell <NUM> by omitting all magnetic assist layer (<NUM>, <NUM>, <NUM>, <NUM>), the antiferromagnetic coupling spacer layer <NUM>, the second nonmagnetic spacer layer <NUM>, and the pinned magnetization layer <NUM>. Thus, the assist features during precessing of the magnetization of the free layer <NUM> are absent in the comparative spin-transfer torque MRAM cell <NUM>. The effects of the first and second magnetic assist layer (<NUM>, <NUM>) and the antiferromagnetic coupling spacer layer <NUM> in the first configuration of the exemplary spin-transfer torque MRAM cell <NUM> are illustrated in <FIG> and <FIG>.

<FIG> is a graph illustrating the transition probability as a function of a current density through the comparative spin-transfer torque magnetic memory device of <FIG>. <FIG> is a graph illustrating the transition probability as a function of a current density through the first configuration of the exemplary spin-transfer torque magnetic memory device illustrated in <FIG>. The areas of the devices of <FIG> and <FIG> are substantially the same.

<FIG> shows that the current density of about <NUM> × <NUM><NUM> A/m<NUM> is necessary to induce a transition within <NUM> nanoseconds from a parallel state to an antiparallel state for the magnetization of the free layer <NUM> and that the current density of about -<NUM> × <NUM><NUM> A/m<NUM> is necessary to induce a transition within <NUM> nanoseconds from an antiparallel state to a parallel state for the magnetization of the free layer <NUM> for the comparative spin-transfer torque MRAM cell <NUM> of <FIG>. <FIG> shows that the current density of about <NUM> × <NUM><NUM> A/m<NUM> is necessary to induce a transition within <NUM> nanoseconds from a parallel state to an antiparallel state for the magnetization of the free layer <NUM> and that the current density of about -<NUM> × <NUM><NUM> A/m<NUM> is necessary to induce a transition within <NUM> nanoseconds from an antiparallel state to a parallel state for the magnetization of the free layer <NUM> for the exemplary spin-transfer torque MRAM cell <NUM> of <FIG>. Thus, <FIG> and <FIG> illustrate that the presence of the first and second magnetic assist layer (<NUM>, <NUM>) and the antiferromagnetic coupling spacer layer <NUM> in the first configuration of the exemplary spin-transfer torque MRAM cell <NUM> reduces the required the current density (i.e., the magnitude of the switching current) for making the parallel to antiparallel transition and the current density for making the antiparallel to parallel transition for the first configuration of the exemplary spin-transfer torque MRAM cell <NUM> by <NUM> to <NUM>%. Thus, the magnitude of the switching current of the embodiment MRAM cell <NUM> is reduced by at least <NUM> % compared to the same MRAM cell <NUM> which lacks the first magnetic assist layer, the antiferromagnetic coupling spacer layer, and the second magnetic assist layer.

<FIG> illustrates a STT MRAM cell <NUM> according to a third example, which is not in accordance with the present invention. Layers <NUM>, <NUM>, <NUM>, <NUM> and <NUM> of the STT MRAM cell <NUM> of the second example may be the same as respective layers <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> of the STT MRAM cell <NUM> of the first embodiment illustrated in <FIG>, and thus are described above with respect to the first embodiment. A first nonmagnetic spacer layer <NUM> is provided over the second side of the free layer <NUM> opposite to the first side of the free layer <NUM> which faces the nonmagnetic tunnel barrier layer <NUM>. The first nonmagnetic spacer layer <NUM> includes a nonmagnetic material. The first nonmagnetic spacer layer <NUM> can include an electrically insulating (i.e., dielectric material) such as magnesium oxide. Alternatively, the first nonmagnetic spacer layer <NUM> can include an electrically conductive metallic material, such as tantalum, ruthenium, tantalum nitride, copper, or copper nitride. The thickness of the first nonmagnetic spacer layer <NUM> can be in a range from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed.

A spin torque layer <NUM> can be provided on the first nonmagnetic spacer layer <NUM>. In the example shown in <FIG>, the spin torque layer <NUM> includes a first magnetic material having a first conical magnetization (e.g., magnetization direction) with respect to a vertical direction that is parallel to the fixed vertical magnetization (e.g., magnetization direction) of the reference layer <NUM>. As used herein, a "conical magnetization" refers to a rotating magnetization (e.g., magnetization direction) having an angle greater than zero but less than <NUM> degrees, such as <NUM> to <NUM> degrees, for example <NUM> to <NUM> degrees with respect to an axis parallel to the fixed vertical magnetization (e.g., magnetization direction) of the reference layer <NUM>.

A conical magnetization can be provided for various symmetry types for the magnetic anisotropy energy per volume. For example, a ferromagnetic film having magnetic anisotropy energy per volume having an axis of sixfold rotational symmetry about a vertical axis perpendicular to the plane of the ferromagnetic film can have a functional dependence on the tilt angle θ from the vertical axis and the azimuthal angle φ in the form of: E/V = K<NUM>sin<NUM>θ + K<NUM>sin<NUM>θ+K<NUM>sin<NUM>θcos(6φ). If K<NUM> is negative and K<NUM> is greater than K<NUM>/<NUM>, the ferromagnetic film has a bidirectional cone of easy magnetization direction at two values of q. The cone angles θc1 and θc2 for the bidirectional cone of easy magnetization direction are related by θc2 = π - θc1.

Ferromagnetic films having different magnetic anisotropy symmetry can provide a conical magnetization is a similar manner. For example, a ferromagnetic film having magnetic anisotropy energy per volume having a tetrahedral symmetry can have a functional dependence on the tilt angle θ from the vertical axis and the azimuthal angle φ in the form of: E/V = K<NUM>sin<NUM>θ + K<NUM>sin<NUM>θ+K<NUM>sin<NUM>θsin(2φ). A ferromagnetic film having magnetic anisotropy energy per volume having a rhombohedral symmetry can have a functional dependence on the tilt angle θ from the vertical axis and the azimuthal angle φ in the form of: E/V = K<NUM>sin<NUM>θ + K<NUM>sin<NUM>θ+K<NUM>cosθsin<NUM>θcos(3φ). If the value of K<NUM> is zero or insignificant compared to <NUM>/<NUM>BT in which kB is the Boltzmann constant and T is room temperature in Kelvin, i.e., <NUM> in the magnetic anisotropy energy per volume, the conical magnetization is free to rotate (e.g., oscillate with a high frequency) around the vertical axis.

The azimuthally-dependent component of the magnetic anisotropy of the spin torque layer <NUM> may be zero or insignificant compared to the thermal energy at room temperature, i.e., kBT in which kB is the Boltzmann constant and T is <NUM> Kelvin (which is the room temperature). For example, the maximum variation of the magnetic anisotropy energy per unit volume around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM> can be less than <NUM>/<NUM> times the thermal energy at room temperature. In such cases, the conical magnetization of the spin torque layer <NUM> is free to precess within the horizontal plane that is parallel to the interface between the first nonmagnetic spacer layer <NUM> and the spin torque layer <NUM> upon application of electrical current through the spin torque layer <NUM>. The magnetic energy of the spin torque layer <NUM> may be invariant under rotation of the magnetization of the spin torque layer <NUM> within the horizontal plane.

The spin torque layer <NUM> can include any ferromagnetic film that provides a conical magnetization. For example, the spin torque layer <NUM> can include a conical magnetization material such as rare-earth elements such as neodymium, erbium, or alloys of at least one rare-earth magnetic element and non-rare-earth element such as iron, boron, cobalt, copper, and/or zirconium. The spin torque layer <NUM> can include a homogeneous conical magnetization material, i.e., a homogeneous material that provides a conical magnetization. As used herein, a "homogeneous" material refers to a material having a uniform material composition throughout. The thickness of the spin torque layer <NUM> can be in a range from <NUM> to <NUM>, such as from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed.

A second nonmagnetic spacer layer <NUM> can be located on the spin torque layer <NUM> on the opposite side of the first nonmagnetic spacer layer <NUM>. The second nonmagnetic spacer layer <NUM> may comprise an electrically insulating layer, such as magnesium oxide, having a thickness between <NUM> and <NUM>.

A spin polarization layer <NUM> can be provided on the second nonmagnetic spacer layer <NUM>. In another embodiment, the order of formation of the spin torque layer <NUM> and the spin polarization layer <NUM> can be reversed, such that the spin polarization layer <NUM> is located closer to the free layer <NUM> than the spin torque layer <NUM>. In general, a spin torque oscillator stack (e.g., an assist layer stack) <NUM> includes the second nonmagnetic spacer layer <NUM> located between the spin torque layer <NUM> and the spin polarization layer <NUM>.

The spin polarization layer <NUM> has a conical magnetization, which is herein referred to as a second conical magnetization. The spin polarization layer <NUM> can include a single magnetic material layer or a plurality of magnetic material layers. The second conical magnetization of the spin polarization layer <NUM> can be provided by the single magnetic material layer having the second conical magnetization, or can be provided by a set of ferromagnetic material layers having an in-plane magnetization and a perpendicular (i.e., vertical or axial) magnetization. The second magnetic material has an in-plane magnetization component that is perpendicular to the fixed vertical magnetization (e.g., magnetization direction) of the reference layer <NUM>. The in-plane magnetization component of the second magnetic material is antiferromagnetically coupled to the in-plane component of the first conical magnetization.

<FIG> illustrates an example in which the spin polarization layer <NUM> consists of a single ferromagnetic material layer having the second conical magnetization with respect to a vertical direction that is parallel to the fixed vertical magnetization of the reference layer <NUM>. The azimuthally-dependent component of the magnetic anisotropy of the spin polarization layer <NUM> may be zero or insignificant compared to the thermal energy at room temperature, i.e., kBT in which kB is the Boltzmann constant and T is <NUM> Kelvin (which is the room temperature). For example, the maximum variation of the magnetic anisotropy energy per unit volume around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM> can be less than <NUM>/<NUM> times the thermal energy at room temperature. In such cases, the conical magnetization of the spin polarization layer <NUM> is free to precess within the horizontal plane that is parallel to the interface between the first nonmagnetic spacer layer <NUM> and the spin torque layer <NUM> upon application of electrical current through the spin polarization layer <NUM>. The magnetic energy of the spin polarization layer <NUM> may be invariant under rotation of the magnetization of the spin polarization layer <NUM> within the horizontal plane.

The spin polarization layer <NUM> can include any ferromagnetic film that provides a conical magnetization. For example, the spin polarization layer <NUM> can include a conical magnetization material such as rare-earth elements such as neodymium, erbium, or alloys of at least one rare-earth magnetic element and non-rare-earth element such as iron, boron, cobalt, copper, and/or zirconium. The spin polarization layer <NUM> can include a homogeneous conical magnetization material, i.e., a homogeneous material that provides a conical magnetization. The ferromagnetic materials of the spin torque layer <NUM> and the spin polarization layer <NUM> may be the same or different. The thickness of the spin polarization layer <NUM> can be in a range from <NUM> to <NUM>, such as from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed.

In case the magnetization of the spin polarization layer <NUM> is a conical magnetization, i.e., a second conical magnetization, the second conical magnetization of the spin polarization layer <NUM> can couple with the first conical magnetization of the spin torque layer <NUM> in various modes.

<FIG> illustrates a first mode of the antiferromagnetic coupling between the first conical magnetization MT of the spin torque layer <NUM> and the second conical magnetization MP of the spin polarization layer <NUM> within the exemplary spin-transfer torque MRAM cell <NUM>. <FIG> illustrates the relative alignment between the first conical magnetization MT of the spin torque layer <NUM> and the second conical magnetization MP of the spin polarization layer <NUM> at an instant during precessing. In the first mode, the vertical component of the first conical magnetization MT of the spin torque layer <NUM> and the vertical component of the second conical magnetization MP of the spin polarization layer <NUM> can be antiparallel to each other.

In this mode, the in-plane component of the first conical magnetization of the spin torque layer <NUM> and the in-plane magnetization component of the magnetization (which can be the second conical magnetization) of the spin polarization layer <NUM> can be antiferromagnetically aligned. In this mode, the in-plane component of the first conical magnetization of the spin torque layer <NUM> and the in-plane magnetization component of the magnetization of the spin polarization layer <NUM> are free to precess around a vertical axis that is parallel to the vertical direction (i.e., a direction that is parallel to the fixed vertical magnetization (e.g., magnetization direction) of the reference layer <NUM> and that is perpendicular to the various interfaces of the layer stack (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>)) while maintaining antiferromagnetic alignment upon application of electrical current through the spin torque layer <NUM>, the second nonmagnetic spacer layer <NUM>, and the spin polarization layer <NUM>. The fixed vertical magnetization of the reference layer <NUM> maintains a same orientation upon application of electrical current through the reference layer <NUM>.

In <FIG>, the magnetization MT of the spin torque layer <NUM> can be in a horizontal plane (as will be described in more detail with respect to the example below which is illustrated in <FIG>) or the magnetization MT of the spin torque layer <NUM> can be conical and its cone angle (i.e., the angle between the vertical direction perpendicular to the interfaces between the various material layers and the vector representing the direction of the first conical magnetization MT) and the cone angle of the second conical magnetization MP of the spin polarization layer <NUM> can remain the same during precessing of the first conical magnetization MT of the spin torque layer <NUM> and the second conical magnetization MP of the spin polarization layer <NUM>, which occurs during programming of the magnetization of the free layer <NUM> (i.e., during flipping of the vertical magnetization of the free layer <NUM> from a parallel state to an antiparallel state or vice versa). The relative angle between the total magnetization (i.e., the second conical magnetization) of the spin polarization layer <NUM> and the first conical magnetization of the spin torque layer <NUM> may remain fixed at a value selected within a range from, and not including, <NUM> degrees to, and including, <NUM> degrees upon application of electrical current through the spin torque layer <NUM>, the second nonmagnetic spacer layer <NUM>, and the spin polarization layer <NUM>.

<FIG> illustrates a second mode of the antiferromagnetic coupling between the first conical magnetization MT of the spin torque layer <NUM> and the second conical magnetization MP of the spin polarization layer <NUM> within the exemplary spin-transfer torque MRAM cell <NUM>. <FIG> illustrates the relative alignment between the first conical magnetization MT of the spin torque layer <NUM> and the second conical magnetization MP of the spin polarization layer <NUM> at an instant during precessing. In the second mode, the vertical component of the first conical magnetization MT of the spin torque layer <NUM> and the vertical component of the second conical magnetization MP of the spin polarization layer <NUM> can be parallel to each other, i.e., both can point upward or both can point downward.

The cone angle of the first conical magnetization MT of the spin torque layer <NUM> (i.e., the angle between the vertical direction perpendicular to the interfaces between the various material layers and the vector representing the direction of the first conical magnetization MT) and the cone angle of the second conical magnetization MP of the spin polarization layer <NUM> can vary during precessing of the first conical magnetization MT of the spin torque layer <NUM> and the second conical magnetization MP of the spin polarization layer <NUM>. In one embodiment, a relative angle between the total magnetization (e.g., the second conical magnetization) of the spin polarization layer <NUM> and the first conical magnetization of the spin torque layer <NUM> varies within a range from, and not including, <NUM> degrees to, and including, <NUM> degrees upon application of electrical current through the spin torque layer <NUM>, the second nonmagnetic spacer layer <NUM>, and the spin polarization layer <NUM>.

<FIG> illustrates a third mode of the antiferromagnetic coupling between the first conical magnetization MT of the spin torque layer <NUM> and the second conical magnetization MP of the spin polarization layer <NUM> within the exemplary spin-transfer torque MRAM cell <NUM>. <FIG> illustrates the relative alignment between the first conical magnetization MT of the spin torque layer <NUM> and the second conical magnetization MP of the spin polarization layer <NUM> at an instant during precessing. In the third mode, the vertical component of the first conical magnetization MT of the spin torque layer <NUM> and the vertical component of the second conical magnetization MP of the spin polarization layer <NUM> can be antiparallel to each other, i.e., one points upward and the other points downward.

<FIG> illustrates a fourth mode of the antiferromagnetic coupling between the first conical magnetization MT of the spin torque layer <NUM> and the second conical magnetization MP of the spin polarization layer <NUM> within the exemplary spin-transfer torque MRAM cell <NUM>. <FIG> illustrates the relative alignment between the first conical magnetization MT of the spin torque layer <NUM> and the second conical magnetization MP of the spin polarization layer <NUM> at an instant during precessing. In the fourth mode, the vertical component of the first conical magnetization MT of the spin torque layer <NUM> and the vertical component of the second conical magnetization MP of the spin polarization layer <NUM> may antiparallel to each other and the frequency of the first and second magnetization is the same such that the first and second magnetization direction vectors point in opposite directions (e.g., when one points left, the other points right and vice-versa). Thus, the first and second magnetization directions are anti-parallel in both vertical and horizontal directions (e.g., both the vertical and horizontal components of the first and second conical magnetization directions are antiparallel) in the fourth mode.

This fourth mode results in a high amount of noise and is not preferred compared to the first, second and third modes. Thus, the first and second magnetization directions are preferably not antiparallel in both vertical and horizontal directions.

Referring back to <FIG>, the spin polarization layer <NUM> can be provided as a single spin polarization layer having a homogeneous composition and can have the second conical magnetization with respective to the vertical direction. Generally, the single spin polarization layer can have an axial magnetization component that is parallel or antiparallel to an axial magnetization component of the first conical magnetization of the spin torque layer <NUM>. In some embodiments, the spin polarization layer <NUM> can have an axial magnetization component (i.e., a vertical magnetization component) that is antiparallel to an axial component of the conical magnetization of the spin torque layer <NUM>. The spin polarization layer <NUM> can have an axial magnetization component (i.e., a vertical magnetization component) that is parallel to an axial component of the conical magnetization of the spin torque layer <NUM>.

A nonmagnetic capping layer <NUM> can be located over the spin polarization layer <NUM>. The nonmagnetic capping layer <NUM> can include a nonmagnetic, electrically conductive material such as W, Ti, Ta, WN, TiN, TaN, Ru, and Cu. The thickness of the nonmagnetic capping layer <NUM> can be in a range from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed.

The first conical magnetization of the spin torque layer <NUM> and the second conical magnetization of the spin polarization layer <NUM> are free to precess around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM> upon application of electrical current through the spin torque layer <NUM>, the second nonmagnetic spacer layer <NUM>, and the spin polarization layer <NUM>, e.g., during programming. The fixed vertical magnetization of the reference layer <NUM> maintains a same orientation upon application of electrical current through the reference layer <NUM>.

During operation of the MRAM cell, electrical current can be flowed through the magnetic tunnel junction <NUM>, the first nonmagnetic spacer layer <NUM>, the spin torque layer <NUM>, the second nonmagnetic spacer layer <NUM>, and the spin polarization layer <NUM>. The spin torque oscillator stack <NUM> comprising the combination of the spin torque layer <NUM>, the second nonmagnetic spacer layer <NUM>, and the spin polarization layer <NUM> is configured to provide an initial non-vertical torque to a magnetization of the free layer <NUM> during an initial phase of precession of the magnetization of the free layer <NUM> around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM> upon initiation of flow of electrical current through the MRAM cell <NUM>. The MRAM cell <NUM> is configured to provide magnetic coupling between the magnetization of the free layer <NUM> and the first magnetization of the spin torque layer <NUM> during precession of the magnetization of the free layer <NUM> around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM>, and to provide synchronized precession of the first magnetization of the spin torque layer <NUM> and the magnetization of the free layer <NUM> while electrical current flows through the MRAM cell <NUM>.

Referring to <FIG>, a second configuration of the exemplary spin-transfer torque MRAM cell <NUM> is illustrated. The spin polarization layer <NUM> includes a layer stack of multiple layers (<NUM>, <NUM>, <NUM>) having different material compositions. The spin polarization layer <NUM> includes a layer stack of a first spin polarization component layer <NUM> having a magnetization that is the same as the in-plane magnetization component of the second conical magnetization. The first spin polarization component layer <NUM> can have a zero magnetic anisotropy or negative uniaxial magnetic anisotropy so that the magnetization of the first spin polarization component layer <NUM> is parallel to the interfaces among the various layers of the MRAM cell <NUM>.

The first spin polarization component layer <NUM> may comprise, and/or consists essentially of, a cobalt-iridium alloy. The material composition of the cobalt-iridium alloy can be selected to provide negative uniaxial magnetic anisotropy. The cobalt-iridium alloy can include cobalt atoms at an atomic concentration in a range from <NUM> % to <NUM> %, such as from <NUM> % to <NUM> %, and iridium atoms at the atomic concentration of the balance. In an illustrative example, a cobalt-iridium alloy having a composition of Co<NUM>Ir<NUM> has a K<NUM> value of about -<NUM> × <NUM><NUM> J/m<NUM>. The first spin polarization component layer <NUM> may comprise, and/or consist essentially of, a cobalt-iron alloy. The material composition of the cobalt-iron alloy can be selected to provide negative uniaxial magnetic anisotropy. The cobalt-iron alloy can include cobalt atoms at an atomic concentration in a range from <NUM> % to <NUM> %, such as from <NUM> % to <NUM> %, and iron atoms at the atomic concentration of the balance. In an illustrative example, a cobalt-iron alloy having a composition of Co<NUM>Ir<NUM> has a K<NUM> value of about -<NUM> × <NUM><NUM> J/m<NUM>. The first spin polarization component layer <NUM> may comprise, and/or consist essentially of, a cobalt-iron-boron (CoFeB) alloy. The thickness of the first spin polarization component layer <NUM> can be in a range from <NUM> to <NUM>, such as from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed.

The first spin polarization component layer <NUM> includes a multilayer stack containing multiple repetitions of a first magnetic material layer and a second magnetic material layer. The first magnetic material layer can include, and/or can consist essentially of, a first magnetic material. The second magnetic material layer can include, and/or can consist essentially of, a second magnetic material.

The first magnetic material layers may comprise cobalt, and the second magnetic material layers may comprise iron. The first magnetic material layers may consist essentially of cobalt, and the second magnetic material layers may consist essentially of iron. The thickness of each first magnetic material layer can be in a range from <NUM> to <NUM>, and the thickness of each second magnetic material layer can be in a range from <NUM> to <NUM>. The total number of repetitions (i.e., the total number of pairs of a first magnetic material layer and a second magnetic material layer) within the first spin polarization component layer <NUM> can be in a range from <NUM> to <NUM>, such as from <NUM> to <NUM>. The multilayer stack may comprise a periodic repetition of a unit layer stack that includes a first magnetic material layer and a second magnetic material layer. In an illustrative example, an interlaced cobalt-iron multilayer stack including repetitions of a unit layer stack consisting of a cobalt layer and an iron layer having the same thickness can have a K<NUM> value of about -<NUM> × <NUM><NUM> J/m<NUM>.

The spin polarization layer <NUM> further includes a second spin polarization component layer <NUM> having an axial magnetization that is parallel or antiparallel to the vertical direction of the reference layer <NUM>. The second spin polarization component layer <NUM> may include a multilayer stack of cobalt layers and either platinum or palladium layers. The second spin polarization component layer <NUM> can have a positive uniaxial magnetic anisotropy so that the magnetization of the second spin polarization component layer <NUM> is axial, i.e., perpendicular to the interfaces among the various layers of the MRAM cell <NUM>. The axial magnetization of the second spin polarization component layer <NUM> can be parallel or antiparallel to the fixed vertical direction of magnetization of the reference layer <NUM>.

The second spin polarization component layer <NUM> can be vertically spaced from the first spin polarization component layer <NUM> by an optional third nonmagnetic spacer layer <NUM>. The third nonmagnetic spacer layer <NUM> can include a nonmagnetic material such as MgO, Cu, Ag, AgSn, Cr, or Ge. The first spin polarization component layer <NUM> can be in contact with the second nonmagnetic spacer layer <NUM>.

In this case, the combined magnetization of the first spin polarization component layer <NUM> and the second spin polarization component layer <NUM> provides the second conical magnetization, which is free to rotate (e.g., oscillate) around the vertical axis during programming of the MRAM cell <NUM>. In this case, the combined magnetization of the first spin polarization component layer <NUM> and the second spin polarization component layer <NUM> provides an additional conical magnetization (i.e., the second conical magnetization) that is coupled to the first conical magnetization of the spin torque layer <NUM>. During programming, the second conical magnetization and the first conical magnetization precess around a vertical axis that is parallel to the vertical direction of the magnetization of the reference layer <NUM> upon application of electrical current through the spin torque layer <NUM>, the second nonmagnetic spacer layer <NUM>, and the spin polarization layer <NUM>. The mode of coupling between the first conical magnetization and the second conical magnetization is preferably any of the first, second or third modes illustrated in <FIG>.

Referring to <FIG>, a third configuration of the exemplary spin-transfer torque MRAM cell <NUM> can be derived from the second configuration of the exemplary spin-transfer torque MRAM cell <NUM> by exchanging the positions of the first spin polarization component layer <NUM> and the second spin polarization component layer <NUM>. In this case, the second spin polarization component layer <NUM> can be in contact with the second nonmagnetic spacer layer <NUM>. The exemplary spin-transfer torque MRAM cell <NUM> in the third configuration can operate in the same manner as the exemplary spin-transfer torque MRAM cells <NUM> in the first and second configurations.

<FIG> illustrates a fourth configuration of an exemplary spin-transfer torque MRAM cell <NUM>. The fourth configuration of the exemplary spin-transfer torque MRAM cell <NUM> can be derived from the first, second, and third configuration of the exemplary spin-transfer torque MRAM cell <NUM> by substituting a spin torque layer <NUM> having an in-plane magnetization (i.e., having negative uniaxial magnetic anisotropy) in place of the spin torque layer <NUM> having a first conical magnetization. In other words, the axial component of the magnetization of the spin torque layer <NUM> can be zero, and the magnetization (e.g., magnetization direction) of the spin torque layer <NUM> can consist of an in-plane component. In this case, the total magnetization of the spin torque layer <NUM> is the same as the in-plane magnetization component of the spin torque layer <NUM>.

The spin torque layer <NUM> may comprise a homogeneous negative uniaxial magnetic anisotropy material. As used herein, a "homogeneous" material refers to a material having a uniform material composition throughout. The spin torque layer <NUM> may comprise, and/or consist essentially of, a cobalt-iridium alloy. The material composition of the cobalt-iridium alloy can be selected to provide negative uniaxial magnetic anisotropy. The cobalt-iridium alloy can include cobalt atoms at an atomic concentration in a range from <NUM> % to <NUM> %, such as from <NUM> % to <NUM> %, and iridium atoms at the atomic concentration of the balance. In an illustrative example, a cobalt-iridium alloy having a composition of Co<NUM>Ir<NUM> has a K<NUM> value of about -<NUM> × <NUM> J/m<NUM>. The spin torque layer <NUM> may comprise, and/or consist essentially of, a cobalt-iron alloy. The material composition of the cobalt-iron alloy can be selected to provide negative uniaxial magnetic anisotropy. The cobalt-iron alloy can include cobalt atoms at an atomic concentration in a range from <NUM> % to <NUM> %, such as from <NUM> % to <NUM> %, and iron atoms at the atomic concentration of the balance. In an illustrative example, a cobalt-iron alloy having a composition of Co<NUM>Ir<NUM> has a K<NUM> value of about -<NUM> × <NUM><NUM> J/m<NUM>. The spin torque layer <NUM> may comprise, and/or consist essentially of, a cobalt-iron-boron (CoFeB) alloy. The thickness of the spin torque layer <NUM> can be in a range from <NUM> to <NUM>, such as from <NUM> to <NUM>, although lesser and greater thicknesses can also be employed.

The spin torque layer <NUM> may include a multilayer stack containing multiple repetitions of a first magnetic material layer and a second magnetic material layer. The first magnetic material layer can include, and/or can consist essentially of, a first magnetic material. The second magnetic material layer can include, and/or can consist essentially of, a second magnetic material.

The first magnetic material layers may comprise cobalt, and the second magnetic material layers may comprise iron. The first magnetic material layers may consist essentially of cobalt, and the second magnetic material layers may consist essentially of iron. The thickness of each first magnetic material layer can be in a range from <NUM> to <NUM>, and the thickness of each second magnetic material layer can be in a range from <NUM> to <NUM>. The total number of repetitions (i.e., the total number of pairs of a first magnetic material layer and a second magnetic material layer) within the spin torque layer <NUM> can be in a range from <NUM> to <NUM>, such as from <NUM> to <NUM>. The multilayer stack may comprise a periodic repetition of a unit layer stack that includes a first magnetic material layer and a second magnetic material layer. In an illustrative example, an interlaced cobalt-iron multilayer stack including repetitions of a unit layer stack consisting of a cobalt layer and an iron layer having the same thickness can have a K1 value of about -<NUM> × <NUM><NUM> J/m<NUM>. The spin torque layer <NUM> may be used with any of the spin polarization layers <NUM> described above with respect to the first embodiment or the second or third examples.

The various example combinations of a spin torque layer (<NUM> or <NUM>) and a spin polarization layer <NUM> provide the benefit of reduction in the current density for making the parallel to antiparallel transition and in the current density for making the antiparallel to parallel transition by providing initial non-vertical torque to the magnetization of the free layer <NUM> during an initial phase of precession of the magnetization of the free layer <NUM> around a vertical axis that is parallel to the fixed vertical magnetization of the reference layer <NUM> upon initiation of flow of electrical current through the MRAM cell <NUM>.

Specifically, during the writing process with current flowing across the stack of the MRAM cell <NUM>, magnetization of both the spin torque layer and the spin polarization layer are oscillating at high frequency with a cone angle (the angle between the magnetization and the normal axis of the stack layer interfaces) between <NUM> and <NUM> degrees, as shown in <FIG>. The oscillating magnetization of spin torque layer can have one or more of the following non-limited benefits leading to lower switching current for the free layer <NUM>. First, an in-plane component of the spin torque layer <NUM> magnetization that is orthogonal to the initial magnetization of the free layer <NUM> may produce a large spin torque on the free layer <NUM> to help its initial precession. Second, the oscillating magnetization of the spin torque layer <NUM> may lead to rotation of the aforementioned torque, which helps to maximize the assist effect throughout the precessional switching process of the free layer <NUM>. Third, a direct field produced by the spin torque layer <NUM> magnetization in the free layer <NUM> may be largely an in-plane AC field, which is also orthogonal to the free layer's initial magnetization direction. Thus, this also helps provide the rotating torque to assist free layer <NUM> switching.

Claim 1:
A magnetoresistive random access memory, MRAM, device (<NUM>; <NUM>) comprising:
a magnetic tunnel junction (<NUM>) comprising a reference layer (<NUM>) having a fixed magnetization direction, a free layer (<NUM>), and a nonmagnetic tunnel barrier layer (<NUM>) located between the reference layer (<NUM>) and the free layer (<NUM>);
a negative-magnetic-anisotropy assist layer (<NUM>; <NUM>) having negative magnetic anisotropy that provides an in-plane magnetization within a plane that is perpendicular to the fixed magnetization direction; and
a first nonmagnetic spacer layer (<NUM>) located between the free layer (<NUM>) and the negative-magnetic-anisotropy assist layer (<NUM>; <NUM>); the MRAM device characterized by:
a pinned magnetization layer (<NUM>) having positive uniaxial magnetic anisotropy which provides a magnetization direction that is parallel or antiparallel to the fixed magnetization direction of the reference layer; and
a second nonmagnetic spacer layer (<NUM>) located between the negative-magnetic-anisotropy assist layer (<NUM>; <NUM>) and the pinned magnetization layer (<NUM>).