CN114503296A

CN114503296A - MTJ unit, VCMA driving method and MRAM

Info

Publication number: CN114503296A
Application number: CN201980100753.0A
Authority: CN
Inventors: 秦健鹰; 许俊豪
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2019-09-27
Filing date: 2019-09-27
Publication date: 2022-05-13
Also published as: WO2021056483A1

Abstract

An MTJ cell, a VCMA driving method and an MRAM, wherein an included angle exists between a stable magnetic moment direction of a free layer (101) and a magnetic moment direction of a reference layer (103) in the MTJ cell, and the included angle is larger than 0 degree, smaller than 180 degrees and not equal to 90 degrees. The MTJ cell may consume less power when writing data using the VCMA driving scheme, and thus the MTJ cell has lower power consumption.

Description

MTJ unit, VCMA driving method and MRAM

Technical Field

The present disclosure relates to the field of magnetic random access memory technologies, and in particular, to an MTJ unit, a VCMA driving method, and an MRAM.

Background

With the development of information technology, especially with the introduction of the concept of mass interconnection, the demand for high-speed, nonvolatile, and low-power memories is becoming stronger. Magnetic Random Access Memory (MRAM) has received much attention because of its excellent characteristics such as sub-nanosecond writing speed, theoretically unlimited erasing life, retention of permanent magnetization state, and low read/write power consumption.

The MRAM mainly includes an MTJ array formed of a plurality of Magnetic Tunneling Junction (MTJ) units, and the MTJ units can be switched between a high resistance state and a low resistance state by flipping a magnetic moment of a free layer in the MTJ units, thereby implementing the memory functions of "0" and "1". For example, the MTJ cell is in a low resistance state, which may indicate that the MTJ cell stores a number "0," or may be understood as not storing a number. The MTJ cell is in a high resistance state, which may indicate that the MTJ cell stores a number "1".

At present, the magnetic moment of the free layer may be switched using a magnetic field driving scheme, a Spin Transfer Torque (STT) effect driving scheme, a voltage-controlled magnetic anisotropy (VCMA) driving scheme, and the like. However, the magnetic field driving scheme is disadvantageous to increase the memory density of MRAM, and the STT effect driving scheme and VCMA driving scheme in turn cause high power consumption of the MTJ cell.

In summary, the current MRAM needs to be further improved.

Disclosure of Invention

In view of this, embodiments of the present disclosure provide a magnetic tunnel junction MTJ cell, a voltage-controlled magnetic anisotropy VCMA driving method, and a magnetic random access memory MRAM, so as to reduce power consumption of the MTJ cell and further reduce power consumption of the MRAM as a whole.

In a first aspect, an embodiment of the present application provides a magnetic tunnel junction MTJ cell, including: the free layer, the barrier layer and the reference layer are sequentially stacked; an included angle exists between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer, and the included angle is larger than 0 degree, smaller than 180 degrees and not equal to 90 degrees.

The MTJ cell provided by the embodiment of the application can be applied to a VCMA driving scheme. Specifically, in the current MTJ unit, the stable magnetic moment direction of the free layer is the intrinsic magnetic moment direction of the free layer, and the magnetic moment direction of the reference layer is the intrinsic magnetic moment direction of the reference layer, so that in the current VCMA driving scheme, a large amount of electric energy is used to drive the magnetic moment direction of the free layer to deviate from the intrinsic magnetic moment direction of the free layer by a certain angle, and then the magnetic moment of the free layer can be driven to continuously flip through the VCMA effect, so that the power consumption of the MTJ unit is high. In the MTJ unit provided in the embodiment of the present application, because an included angle exists between the stable magnetic moment direction of the MTJ unit and the magnetic moment direction of the reference layer, the magnetic moment of the free layer can be driven to flip by the VCMA effect at the initial stage of the VCMA driving scheme, so that more electric energy is saved, and the power consumption of the MTJ unit is favorably reduced.

In one possible design, both the free layer and the reference layer are perpendicular anisotropy.

In one possible design, a first spacer layer is further disposed between the free layer and the barrier layer, and the first spacer layer is used to increase the VCMA coefficient of the voltage-controlled magnetic anisotropy between the free layer and the barrier layer. The VCMA coefficient between the free layer and the barrier layer is increased through the first spacing layer, the VCMA effect between the free layer and the barrier layer is favorably enhanced, the threshold value of VCMA critical overturning voltage can be reduced, further, the magnetic moment overturning of the free layer can be driven by lower voltage, and the power consumption of the MTJ unit is favorably further reduced.

In one possible design, the MTJ cell further includes a pinned layer on a side of the reference layer remote from the barrier layer, and a second spacer layer between the pinned layer and the reference layer; wherein the pinned layer may be ferromagnetically (or antiferromagnetically) coupled to the reference layer through the second spacer layer such that the direction of the magnetic moment of the reference layer may be fixed such that the direction of the magnetic moment of the reference layer may remain fixed during the flipping of the magnetic moment of the free layer.

In one possible design, the MTJ cell further includes a first pinned layer on a side of the pinned layer away from the second spacer layer, the first pinned layer having in-plane anisotropy, and a magnetic moment of the first pinned layer coupled to a magnetic moment of the first pinned layer; the first standard layer can adjust the magnetic moment direction of the reference layer through the pinning layer, so that an included angle exists between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer. Specifically, the first normal layer may generate a stray field at the pinned layer, such that the magnetic moment direction of the pinned layer is offset from the intrinsic magnetic moment direction of the pinned layer, and due to the ferromagnetic coupling (or antiferromagnetic coupling) between the pinned layer and the reference layer, the magnetic moment direction of the reference layer may also be offset from the intrinsic magnetic moment direction of the reference layer by a certain angle, such that an included angle may exist between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer.

In one possible design, the MTJ cell further includes a third spacer layer between the pinned layer and the first gauge layer, which may reduce lattice mismatch between the pinned layer and the first gauge layer. Specifically, since the first normal layer is in-plane anisotropic and the pinned layer is perpendicular anisotropic, the growth directions of the first normal layer and the pinned layer are different, and the lattice mismatch between the first normal layer and the pinned layer is high. The third spacing layer is arranged between the first standard layer and the pinning layer, so that lattice mismatch between the pinning layer and the first standard layer is reduced, the defect density of the MTJ unit is favorably reduced, the service life of the MTJ unit is prolonged, and the stability of the MTJ unit is improved.

In one possible design, the MTJ cell further includes a second normal layer on a side of the free layer away from the barrier layer, the second normal layer being in-plane anisotropic, and a magnetic moment of the second normal layer being coupled to a magnetic moment of the free layer; the second normative layer can adjust the stable magnetic moment direction of the free layer, so that an included angle exists between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer. Specifically, the second normal layer may generate a stray field at the free layer, so that the stable magnetic moment direction of the free layer is offset from the intrinsic magnetic moment direction of the free layer, and thus an included angle may exist between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer.

In one possible design, the MTJ cell further includes a fourth spacer layer between the free layer and the second canonical layer, which may reduce lattice mismatch between the free layer and the second canonical layer. Specifically, since the second normal layer is in-plane anisotropic and the free layer is perpendicular anisotropic, the growth directions of the second normal layer and the free layer are different, and the lattice mismatch between the second normal layer and the free layer is high. The third spacing layer is arranged between the second standard layer and the free layer, so that lattice mismatch between the free layer and the second standard layer is reduced, the defect density of the MTJ unit is reduced, the service life of the MTJ unit is prolonged, and the stability of the MTJ unit is improved.

In one possible design, the free layer has a major axis length that is at least twice the length of a minor axis, both of which are parallel to an interface between the free layer and the barrier layer. Specifically, since the length of the long axis of the free layer is at least twice as long as the length of the short axis, a stray field parallel to the long axis can be generated inside the free layer. By adopting the MTJ unit with the structure, an included angle can be formed between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer without arranging an additional magnetic layer.

In a second aspect, an embodiment of the present application provides a voltage-controlled magnetic anisotropy VCMA driving method, including: applying a first voltage to the MTJ unit in the first state, wherein the voltage value of the first voltage is not less than the threshold value of the critical switching voltage of the MTJ unit, the MTJ unit in the first state comprises a free layer, a barrier layer and a reference layer which are sequentially stacked, the free layer and the reference layer are both vertical anisotropy, an included angle exists between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer, and the included angle is greater than 0 degree and less than 90 degrees; and stopping applying the first voltage after the duration of continuously applying the first voltage reaches a first duration, wherein the first duration is not less than the duration required by the magnetic moment precession of the free layer to flip by 90 degrees.

Specifically, since the voltage value of the first voltage is not less than the critical switching voltage threshold of the MTJ cell, the magnetic moment of the free layer starts to be inverted after the first voltage is applied to the MTJ cell. After the first duration, the magnetic moment of the free layer precedently flips by 90 degrees. Because the included angle in the MTJ unit in the first state is smaller than 90 degrees, after a first time length, the magnetic moment of the free layer is already turned from one side of the plane where the free layer is located to the other side, so that even if the first voltage is stopped being applied, the magnetic moment of the free layer can be continuously turned until the magnetic moment of the free layer is turned to the other stable magnetic moment direction, namely the magnetic moment of the free layer is turned by 180 degrees, and the MTJ unit is switched to the second state.

In a third aspect, an embodiment of the present application provides a voltage-controlled magnetic anisotropy VCMA driving method, including: applying a second voltage to the MTJ unit in the second state, wherein the voltage value of the second voltage is not less than the threshold switching voltage threshold of the MTJ unit, and the MTJ unit in the second state comprises a free layer, a barrier layer and a reference layer which are sequentially stacked, wherein the free layer and the reference layer are both in vertical anisotropy, and an included angle exists between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer, and the included angle is greater than 90 degrees and less than 180 degrees; after the duration of continuously applying the second voltage reaches a second duration, stopping applying the second voltage, and continuously applying a third voltage to the MTJ cell, wherein the voltage value of the third voltage is less than the threshold value of the critical switching voltage of the MTJ cell, and the second duration is not less than the duration required by the magnetic moment precession switching of the free layer by 90 degrees; and stopping applying the third voltage after the duration of continuously applying the third voltage reaches a third duration, wherein the third duration is not less than the duration of turning the magnetic moment direction of the free layer by a first angle under the STT effect, and the first angle is the complement angle of the included angle.

Specifically, since the voltage value of the second voltage is not less than the critical switching voltage threshold of the MTJ cell, the magnetic moment of the free layer starts to be inverted after the second voltage is applied to the MTJ cell. After the second duration, the magnetic moment of the free layer is precessionally flipped 90 degrees. At this time, the magnetic moment direction of the free layer is close to the plane of the free layer. In this case, a smaller current is injected into the MTJ cell by the third voltage, so that a more significant STT effect can be obtained. The magnetic moment of the free layer can be driven to continuously overturn through the STT effect, the third time is not less than the time length of the first angle of overturning of the magnetic moment of the free layer under the STT effect, therefore, after the third time, the magnetic moment of the free layer can be overturned to the other side of the free layer from one side of the plane where the free layer is located, and then, the magnetic moment of the free layer can automatically continuously overturn until reaching the other stable magnetic moment direction of the free layer, namely, the magnetic moment of the free layer finishes 180-degree overturning, and the MTJ unit is switched to the first state.

In a fourth aspect, embodiments of the present application further provide a magnetic random access memory MRAM, which mainly includes an MTJ array and a driving circuit of the MTJ array, where the MTJ array includes a plurality of MTJ cells, and the MTJ cells may be MTJ cells provided in any one of the first aspect. The driving circuit can execute the VCMA driving method provided by the second aspect or the third aspect for each MTJ cell in the MTJ array to write data into the MTJ cell.

The various possible implementations described above will be described in further detail in the following detailed description.

Drawings

FIG. 1 is a schematic diagram of an MRAM architecture;

FIG. 2 is a schematic diagram of a MTJ cell structure;

FIG. 3 is a schematic diagram of a MTJ cell structure;

FIG. 4 is a schematic diagram of a MTJ cell structure according to an embodiment of the present application;

FIG. 5 is a schematic diagram of a MTJ cell structure according to an embodiment of the present application;

FIG. 6 is a schematic diagram illustrating magnetic moment flipping according to an embodiment of the present disclosure;

FIG. 7 is a schematic diagram of a MTJ cell structure according to an embodiment of the present application;

FIG. 8 is a top view of a free layer provided in an embodiment of the present application;

fig. 9 is a schematic flowchart of a VCMA driving method according to an embodiment of the present disclosure;

FIG. 10 is a waveform diagram of a voltage signal provided by an embodiment of the present application;

fig. 11 is a schematic diagram of a hysteresis loop variation provided in the embodiment of the present application;

fig. 12 is a schematic flowchart of a VCMA driving method according to an embodiment of the present disclosure;

FIG. 13 is a waveform diagram of a voltage signal according to an embodiment of the present application;

fig. 14 is a schematic diagram of a hysteresis loop variation according to an embodiment of the present application.

Detailed Description

In order to make the objects, technical solutions and advantages of the present application more clear, the present application will be further described in detail with reference to the accompanying drawings. The particular methods of operation in the method embodiments may also be applied to apparatus embodiments or system embodiments. It is to be noted that "at least one" in the description of the present application means one or more, where a plurality means two or more. In view of this, the "plurality" may also be understood as "at least two" in the embodiments of the present invention. "and/or" describes the association relationship of the associated objects, meaning that there may be three relationships, e.g., a and/or B, which may mean: a exists alone, A and B exist simultaneously, and B exists alone. In addition, the character "/" generally indicates that the preceding and following related objects are in an "or" relationship, unless otherwise specified. In addition, it is to be understood that the terms first, second, etc. in the description of the present application are used for distinguishing between the descriptions and not necessarily for describing a sequential or chronological order.

For convenience, specific spatially relative terminology is used in the following description and is not intended to be limiting. The words "upper" and "lower" designate directions in the drawings to which reference is made. The terminology includes the words above specifically mentioned, derivatives thereof and words of similar import. "over ….," over … …, "on … … surface," "above," and the like are used to describe the spatial relationship of one device or feature to another device or feature as shown in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. For example, if a device in the figures is turned over, devices described as "above" or "on" other devices or configurations would then be oriented "below" or "under" the other devices or configurations. Thus, the exemplary term "above … …" can include both an orientation of "above … …" and "below … …". The device may be otherwise variously oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

In the field of memory, Magnetic Random Access Memory (MRAM) is a new generation memory with a wide application prospect. Fig. 1 schematically illustrates an MRAM structure, and as shown in fig. 1, the MRAM mainly includes an MTJ array formed by arranging a plurality of MTJ units, and a driving circuit capable of writing data to any MTJ unit in the MTJ array. The MTJ cell can then store the data written by the driver circuit.

Fig. 2 exemplarily shows an MTJ cell structure, and as shown in fig. 2, the MTJ cell mainly includes a free layer 101, a barrier layer 102, and a reference layer 103, which are sequentially stacked. Wherein the barrier layer 102 is located between the free layer 101 and the reference layer 103. Both the free layer 101 and the reference layer 103 may be composed of magnetic materials in the MTJ cell, and the free layer 101 and the reference layer 103 have perpendicular anisotropy.

Specifically, as shown in fig. 2, the magnetic moment direction of the free layer 101 is along the thickness direction of the free layer 101, perpendicular to the upper and lower surfaces of the free layer 101. The magnetic moment direction of the reference layer 103 is along the thickness direction of the reference layer 103, perpendicular to the upper and lower surfaces of the reference layer 103. As shown by the dotted line in fig. 2, for convenience of description, in the embodiment of the present application, a direction 1 refers to a direction in which the reference layer 103 vertically points to the free layer 101, a direction 2 refers to a direction in which the free layer 101 vertically points to the reference layer 103, and the direction 1 and the direction 2 are in a reverse relationship, which is not described in detail later.

In general, in the MTJ structure shown in fig. 2, the magnetic moment of the free layer 101 can be reversed, that is, the magnetic moment of the free layer 101 can be reversed from direction 1 to direction 2, or from direction 2 to direction 1. While the orientation of the reference layer 103 is generally fixed. Thus, depending on the direction of the magnetic moment of the free layer 101, the MTJ cell exists in two states: a high resistance state and a low resistance state. Illustratively, the MTJ cell shown in FIG. 2 is in the low resistance state, as shown in FIG. 2 where the magnetic moment of the free layer 101 is the same as the magnetic moment of the reference layer 103. As shown in FIG. 3, the magnetic moment of the free layer 101 is opposite to the magnetic moment of the reference layer 103, and the MTJ cell shown in FIG. 3 is in the high resistance state. When the MTJ unit is in the high resistance state, the MTJ unit has a higher resistance value (e.g., a first resistance value) to the outside, and when the MTJ unit is in the low resistance state, the MTJ unit has a lower resistance value (e.g., a second resistance value) to the outside, where a difference between the first resistance value and the second resistance value is mainly affected by parameters such as a material and a thickness of the barrier layer 102.

In reading the data stored in the MTJ cell, a bias voltage may be applied to two ends of the MTJ cell, where the two ends of the MTJ cell refer to the end where the free layer 101 is located and the end where the reference layer 103 is located. As shown in FIG. 2, a voltage V1 is applied to the free layer 101 and a voltage V2 is applied to the reference layer 103 to apply bias voltages V1-V2 to the MTJ cell. And calculating the resistance value of the current MTJ cell by detecting the current value flowing through the MTJ cell and the voltage value of the bias voltage applied to the two ends of the MTJ cell. Therefore, whether the current MTJ unit is in a low resistance state or a high resistance state can be determined according to the calculated resistance value, and the data stored in the MTJ unit can be read. For example, if the MTJ cell is in a low resistance state, the data stored in the read MTJ cell is "0", and if the MTJ cell is in a high resistance state, the data stored in the read MTJ cell is "1".

It should be noted that, in some other MTJ units, the direction of the magnetic moment of the free layer 101 in the high resistance state is the same as the direction of the magnetic moment of the reference layer 103, and the direction of the magnetic moment of the free layer 101 in the low resistance state is opposite to the direction of the magnetic moment of the reference layer 103, and these MTJ units are still suitable for the embodiments of the present application. For convenience of description, the present embodiment represents a state in which the magnetic moment direction of the free layer 101 and the magnetic moment direction of the reference layer 103 in the MTJ cell are the same in a first state, and represents a state in which the magnetic moment direction of the free layer 101 and the magnetic moment direction of the reference layer 103 in the MTJ cell are opposite in a second state. Specifically, there are two possibilities: the first state is a low resistance state and the second state is a high resistance state, or the first state is a high resistance state and the second state is a low resistance state.

In summary, the MTJ cell can be switched between the high resistance state and the low resistance state by flipping the magnetic moment of the free layer 101 in the MTJ cell, so as to store data "1" and "0", and it can also be understood that the process of writing data in the MTJ cell mainly includes the process of flipping the magnetic moment of the free layer 101. It should be noted that, in the absence of external driving factors (e.g., neither applied magnetic field nor applied electric field), the magnetic moment direction of the free layer 101 in the MTJ cell only has two cases, namely, the direction of the magnetic moment of the free layer 101 changes by 180 degrees before and after the magnetic moment of the free layer 101 flips, and it can be understood that the flipping of the magnetic moment of the free layer 101 mainly includes the flipping from the direction 1 to the direction 2 and the flipping from the direction 2 to the direction 1.

It is noted that the magnetic moment of the free layer 101 can be maintained in the post-flip orientation without interference from external driving factors. The first state and the second state of the MTJ cell can be stable when data is written into the MTJ cell. The stable state refers to a stable state in which the MTJ cell can be maintained for a long time without external driving factor interference, and is generally divided according to the magnetic moment direction in the free layer 101. For example, the stable state of the MTJ cell may be a first state in which the magnetic moment of the free layer 101 is in the direction 1 in fig. 2, or a second state in which the magnetic moment of the free layer 101 is in the direction 2 in fig. 3.

In the MTJ cell, at least two stable magnetic moment directions exist for the free layer, and the at least two stable magnetic moment directions are opposite. The stable magnetic moment direction can be understood as a magnetic moment direction in which the magnetic moment of the free layer can be maintained for a long time without being influenced by external driving factors. For example, in the MTJ cell shown in fig. 3, the direction 1 or direction 2 of the magnetic moment of the free layer can be maintained for a long time without external driving factor interference, so that the direction 1 or direction 2 can be used as the stable magnetic moment direction of the free layer 101 in fig. 3.

In short, during a data write operation, the MTJ cell switches from one stable state to another, and the magnetic moment of the free layer 101 is flipped from one stable magnetic moment direction to another stable magnetic moment direction. Currently, there are three main schemes that can switch the magnetic moment of the free layer 101:

scheme one, magnetic field driving scheme

Taking the most common switching (toggle) mode in the magnetic field driving scheme as an example, in the toggle mode, a current line may be buried near the MTJ cell, and generally, the current line may be fabricated in a Complementary Metal Oxide Semiconductor (CMOS) circuit in the MRAM. In data writing, a current may be applied to the current line, and an oersted magnetic field close to the free layer 101 is generated around the current line by a magnetic effect of the current, so that the magnetic moment of the free layer 101 of the MTJ cell is driven to be reversed, thereby completing data writing.

However, this solution requires an extra current line, which is not favorable for improving the integration of the CMOS circuit, and may also affect the normal functions of other operating regions in the CMOS circuit, and the larger the dispersion degree of the oersted magnetic field is, the larger the range of the affected operating region is. Therefore, the MTJ cells need to be spaced apart by a sufficient distance so that when data is written to a certain MTJ cell, the magnetic field of oersted for driving the MTJ cell does not affect the adjacent MTJ cell. In recent years, the MTJ cell arrangement density of the MRAM using the toggle mode is low, and the overall storage density of the MRAM is not high. In addition, when data is written, a large current is often applied to the current line to generate a strong enough oersted magnetic field to drive the magnetic moment of the free layer 101 to flip, so that the toggle mode is adopted to make the overall thermal effect of the MRAM more obvious, which is not beneficial to reducing the power consumption of the MRAM and also is not beneficial to improving the energy utilization rate of the MRAM.

Scheme two, Spin Transfer Torque (STT) effect driving scheme

In data writing, a current is injected vertically into the MTJ cell, for example, a current may be injected from the free layer 101, which may flow in the direction 2 toward the reference layer 103, or a current may be injected from the reference layer 103, which may flow in the direction 1 toward the free layer 101. When a current flows through the free layer 101, since the free layer 101 is a magnetic layer, a current polarization phenomenon occurs, which causes a magnetic moment in the free layer 101 to flip, and this phenomenon may also be referred to as STT effect. By the STT effect, data writing can be done.

However, since the stable magnetic moment direction of the free layer 101 is a strictly perpendicular direction, the STT effect is very weak at the initial stage of data writing. To obtain a more obvious STT effect, it is necessary to inject a current with a larger current density into the MTJ cell at the initial stage of data writing, e.g. the injection current density is 1MA/cm²So as to shift the direction of the magnetic moment of the free layer 101The magnetic moment direction is stabilized at a certain angle, a large amount of electric energy is consumed in the process, the whole power consumption of the MRAM is driven to be large by adopting the STT effect, and the energy utilization rate is not high.

Scheme three, voltage-controlled magnetic anisotropy (VCMA) drive scheme

When data is written, bias voltage is applied to two ends of the MTJ unit, and anisotropy of the interface of the free layer 101 and the barrier layer 102 is regulated and controlled through the bias voltage to realize the overturning of magnetic moment so as to write information. Specifically, under bias, the magnetic moment flipping process of the free layer 101 is precessional (precession) flipped. As shown in fig. 4, the stable magnetic moment of the free layer 101 is in direction 1, under the action of a bias, the magnetic moment of the free layer 101 rotates around direction 3 as a central axis, the rotation radius gradually decreases, and if the bias is continuously applied, the magnetic moment will be turned to direction 3 and then stopped, wherein direction 3 is perpendicular to either direction 1 or direction 2, and it can also be understood that direction 3 is parallel to either direction of the interface between the free layer 101 and the barrier layer 102, that is, after the magnetic moment is turned to direction 3, the free layer 101 changes from perpendicular anisotropy to in-plane anisotropy.

In the VCMA driving scheme, it is necessary to stop applying the bias when the magnetic moment of the free layer 101 rotates to any direction between the directions 3 and 2, and after stopping applying the bias, the magnetic moment of the free layer 101 is driven to flip to the direction 2 by the STT effect. It follows that the VCMA drive scheme requires precise control of the length of time that the bias voltage is applied, i.e., the pulse width of the bias voltage. Moreover, considering the uncertainty of the MTJ cell itself, such as the function degradation, the pulse width required in the VCMA driving scheme may also vary, which causes inconvenience for data writing.

In addition, similar to the STT effect driving scheme, in the beginning of the VCMA driving scheme, a high-density current is also required to drive the magnetic moment direction of the free layer 101 to shift from the stable magnetic moment direction by a certain angle, which consumes a large amount of power. The magnetic moment of the free layer 101 can be precessed by a bias voltage after the magnetic moment of the free layer 101 is driven to deviate from the stable magnetic moment by a certain angle. Therefore, the VCMA scheme also has a problem of high power consumption.

In summary, the MTJ cell and the driving scheme for the MTJ cell need to be further studied. In view of the above, embodiments of the present disclosure provide an MTJ cell that may include a free layer 101, a barrier layer 102, and a reference layer 103 stacked in sequence, where the free layer 101 and the reference layer 103 are both vertically anisotropic. In the embodiment of the present application, the magnetic moment direction of the free layer 101 or the reference layer 103 in the stable state is changed by the stray field, so that an included angle can be formed between the stable magnetic moment direction of the free layer 101 and the magnetic moment direction of the reference layer 103, where the included angle is greater than 0 degree, smaller than 180 degrees, and not equal to 90 degrees. Further, a process of injecting a high-density current at an initial stage of data writing can be omitted, and the magnetic moment of the free layer 101 can be driven to flip with less electric energy, which is beneficial to reducing the power consumption of the MTJ unit.

Next, the MTJ cell provided in the embodiments of the present application is further described by the following embodiments.

Example one

Fig. 5 schematically illustrates a structural diagram of an MTJ cell provided in an embodiment of the present application, where as shown in fig. 5, a free layer 101, a barrier layer 102, and a reference layer 103 in the MTJ cell are sequentially stacked, and both the free layer 101 and the reference layer 103 are vertically anisotropic.

Specifically, the free layer 101 may include a magnetic material having perpendicular anisotropy, for example, the free layer 101 may include one or more of cobalt (Co), iron (Fe), cobalt iron alloy (CoFe), cobalt iron boron alloy (CoFeB), iron boron alloy (FeB), and the like, and the thickness, shape, and material of the free layer 101 should be such that the free layer 101 has perpendicular anisotropy. The specific implementation of the reference layer 103 is similar to the free layer 101, and is not described in detail here.

Note that the free layer 101 is vertically anisotropic, which means that the intrinsic magnetic moment direction of the free layer 101 is perpendicular to the interface between the free layer 101 and the barrier layer 102, for example, the intrinsic magnetic moment direction of the free layer 101 may be direction 1 and direction 2 in fig. 5. The same reference to layer 103 is omitted for brevity.

The composition of barrier layer 102 may generally be magnesium oxide (MgO).

It should be noted that the "sequentially stacked" in the present application is merely used to describe the arrangement order (sequence) and arrangement manner (stacking) between layers, and when the present invention is specifically implemented, other layers may be interposed between the layers sequentially stacked. For example, in one possible implementation, as shown in fig. 5, a first spacer layer 104 may be further disposed between the free layer 101 and the barrier layer 102, and the first spacer layer 104 may increase the VCMA coefficient between the free layer 101 and the barrier layer 102. Specifically, the material of the first spacer layer 104 and the material of the barrier layer 102 have a larger work function, and the larger the work function is, the more beneficial the VCMA coefficient of the MTJ unit is to be increased, so as to enhance the VCMA effect in the data writing process, and the VCMA critical flip voltage threshold of the MTJ unit can be reduced, so that the power consumption of the MTJ unit can be further reduced.

Illustratively, the first spacer layer 104 may include one or more of hafnium (Hf), platinum (Pt), tantalum (Ta), iridium (Ir), tungsten (W), and other non-magnetic metal materials. In addition, the thickness of the first spacer layer 104 also affects the VCMA coefficient of the MTJ cell, and thus the VCMA effect of the MTJ cell can be enhanced by adjusting the thickness and material of the first spacer layer 104.

In the embodiment of the present application, the stable magnetic moment direction of the free layer 101 is shifted from the intrinsic magnetic moment direction of the free layer 101 by the stray field. For example, in fig. 5, the stable magnetic moment direction of the free layer 101 can be changed from direction 1 to direction 4, and the direction 1 and the direction 4 are separated by an included angle θ, which is greater than 0 degree and less than 90 degrees. In this case, the magnetic moment direction (direction 4) of the free layer 101 and the magnetic moment direction (direction 2) of the reference layer 103 are separated by an angle θ.

It is noted that the included angle in the MTJ cell shown in fig. 5 is less than 90 degrees, that is, the stable magnetic moment direction of the free layer 101 is close to the magnetic moment direction of the reference layer 103, in which case the MTJ cell shown in fig. 5 can be considered to be in the first state. Accordingly, in the second state, the magnetic moment of the free layer 101 in the MTJ cell is flipped 180 degrees from direction 4, that is, the stable magnetic moment direction of the free layer 101 in the MTJ cell in the second state is direction 5, where directions 4 and 5 are in an opposite relationship. In the second state, the angle between the stable magnetic moment direction of the free layer 101 and the magnetic moment direction of the reference layer 103 in the MTJ cell is 180- θ, which is greater than 90 degrees.

As described above, if the stable magnetic moment direction of the free layer 101 is the direction 1, the VCMA driving scheme needs to drive the magnetic moment direction of the free layer 101 to be deviated from the direction 1 by a certain angle by the STT effect at the beginning, and this process consumes a large amount of power. By adopting the technical scheme provided by the embodiment of the application, because an included angle exists between the stable magnetic moment direction of the free layer 101 and the magnetic moment direction of the reference layer 103 in a stable state, the process of driving the magnetic moment to deviate by utilizing the STT effect at the initial stage of the VCMA driving scheme can be omitted, and the magnetic moment of the free layer 101 is directly driven to overturn by bias voltage, so that a large amount of electric energy can be saved, and the reduction of the power consumption of an MTJ unit is facilitated.

For example, in the process of switching the MTJ cell from the first state shown in fig. 5 to the second state, the process of flipping the magnetic moment of the free layer 101 may be shown as a semi-circular dashed line in fig. 6, wherein the direction of the magnetic moment of the free layer 101 in the intermediate state is reached after the magnetic moment of the free layer 101 in the first state is flipped by 90 degrees. It is noted that the first state to intermediate state flip process is also implemented in a precession manner similar to that shown in fig. 4. The difference is that the conventional VCMA driving scheme stops applying the bias voltage to the MTJ cell before the magnetic moment of the free layer 101 reaches the direction 3 from the direction 1, so that the pulse width of the bias voltage needs to be precisely controlled. And this application can make the magnetic moment of free layer 101 totally overturn 90 degrees under the bias effect, because the magnetic moment of free layer 101 just can the automatic shutdown upset after overturning 90 degrees under the bias effect, therefore this application need not the pulse width of accurate control bias voltage.

In summary, the MTJ unit provided in the embodiments of the present application not only has low power consumption, but also is easy to control.

In one possible implementation, as shown in FIG. 5, the MTJ cell may also include a pinned layer 106. The pinning layer 106 is located on the side of the reference layer 103 remote from the barrier layer 102, and a second spacer layer 105 is also provided between the pinning layer 106 and the reference layer 103. The pinning layer 106 may fix the direction of the magnetic moment of the reference layer 103 by the second spacer layer 105 so that the magnetic moment of the reference layer 103 is not flipped along with the magnetic moment of the free layer 101 during data writing. In general, the pinned layer 106 has a magnetic moment in the same or opposite direction as the magnetic moment of the reference layer 103.

Illustratively, the pinning layer 106 is perpendicular anisotropy and may comprise cobalt platinum [ Co ]_x-Pt _y] _nThe multilayer film is characterized in that x represents the thickness of a single layer of Co, y represents the thickness of a single layer of platinum, Co layers and Pt layers are alternately and repeatedly arranged, and n represents the alternating period of the Co layers and the Pt, namely n can represent the total number of the Pt layers and the total number of the Co layers. Wherein, the values of x, y and n can make the multilayer film have vertical anisotropy. In addition, the pinning layer may also comprise other similar materials, such as may also include [ a ]_x-b _y] _nA multilayer film in which a is a ferromagnetic metal such as iron (Fe), nickel (Ni), etc., and b is a heavy metal such as palladium (Pd), iridium (Ir), hafnium (Hf), etc. It is to be noted that the values of x, y and n may be different for the same composition of the multilayer film, i.e., the same a and b, but the perpendicular anisotropy of the pinned layer 106 should be secured as a whole.

Illustratively, the second spacer layer 105 may include a non-magnetic conductive material, such as platinum manganese alloy (PtMn), iridium manganese alloy (IrMn), ruthenium (Ru), tantalum (Ta), palladium (Pd), etc., in combination with a particular thickness of the second spacer layer 105, to enable an antiferromagnetic or ferromagnetic coupling between the reference layer 103 and the pinning layer 106 via the second spacer layer 105. Also, the second spacer layer 105 has no effect on the VCMA characteristics of the MTJ cell.

Specifically, if an antiferromagnetic coupling is formed between the reference layer 103 and the pinned layer 106, the pinned layer 106 and the reference layer 103 have magnetic moments oriented in opposite directions, e.g., the pinned layer 106 has a magnetic moment oriented in direction 2, and the pinned layer 106 may fix the magnetic moment of the reference layer 103 in direction 1. If a ferromagnetic coupling is formed between the reference layer 103 and the pinned layer 106, the pinned layer 106 has the same magnetic moment direction as the reference layer 103, e.g., the magnetic moment direction of the pinned layer 106 is direction 2, the pinned layer 106 may fix the magnetic moment direction of the reference layer 103 in direction 2.

Example two

Next, a specific implementation manner of the stray field is provided in the second embodiment of the present application. As shown in fig. 4, the MTJ cell also includes a second pinned layer 107, the second pinned layer 107 being on a side of the free layer 101 away from the barrier layer 102, the second pinned layer 107 being in-plane anisotropic, and a magnetic moment of the second pinned layer 107 being coupled to a magnetic moment of the free layer 101.

Here, the second pinned layer 107 is in-plane anisotropic, which means that the intrinsic magnetic moment direction of the second pinned layer 107 is parallel to the plane of the second pinned layer 107. That is, the intrinsic magnetic moment direction of the second pinned layer 107 and the intrinsic magnetic moment direction of the free layer 101 are perpendicular to each other, and thus the second pinned layer 107 may generate a stray field at the free layer 101, thereby changing the magnetic moment direction of the free layer 101. The second pinned layer 107 may have a stable magnetic moment of the free layer 101 different from the intrinsic magnetic moment of the free layer 101, and it is understood that the stable magnetic moment of the free layer 101 and the magnetic moment of the reference layer 103 are not strictly in the same direction or in the opposite direction, i.e. there is an angle between the stable magnetic moment of the free layer 101 and the magnetic moment of the reference layer 103.

For example, in the embodiment of the present application, the second graded layer 107 may include a magnetic material having in-plane anisotropy, for example, a magnetic material such as Co, Fe, CoFe, CoFeB, FeB, nickel-iron-alloy (NiFe), nickel-iron-boron-alloy (NiFeB), etc., and the thickness, shape and composition of the second graded layer 107 should ensure that the second graded layer 107 has in-plane anisotropy.

In a possible implementation manner, a fourth spacer layer 108 is further disposed between the free layer 101 and the second regulation layer 107, and the fourth spacer layer 108 can reduce lattice mismatch between the free layer 101 and the second regulation layer 107. Specifically, since the free layer 101 is vertically anisotropic and the second regulation layer 107 is in-plane anisotropic, there tends to be a different material growth direction between the free layer 101 and the second regulation layer 107, indicating that the lattice mismatch between the free layer 101 and the second regulation layer 107 is high.

In view of this, a fourth spacer layer 108 may also be disposed between the free layer 101 and the second pinned layer 107 in the MTJ cell. Illustratively, the fourth spacer layer 108 may include one or more of Co, MgO, Mg, Ta, etc. On the side of the fourth spacer layer 108 near the free layer 101, the lattice of the fourth spacer layer 108 matches the lattice of the free layer 101. Along the thickness direction of fourth spacer layer 108, the lattice size of fourth spacer layer 108 gradually transitions until the lattice of fourth spacer layer 108 matches the lattice of second normalization layer 107 on the side of fourth spacer layer 108 near second normalization layer 107. Illustratively, the fourth spacer layer 108 may include one or more of Co, MgO, Mg, Ta, etc.

By arranging the fourth spacer layer 108 between the free layer 101 and the second normal layer 107, the defect problem caused by lattice mismatch is reduced, the service life of the MTJ unit is prolonged, and the stability of the MTJ unit is improved.

EXAMPLE III

It can be understood that the stable magnetic moment direction of the free layer 101 may also be an intrinsic magnetic moment direction of the free layer 101, and the magnetic moment direction of the reference layer 103 is shifted by a certain angle from the intrinsic magnetic moment direction of the reference layer 103 by a stray field, or an included angle may be formed between the stable magnetic moment direction of the free layer 101 and the magnetic moment direction of the reference layer 103.

Illustratively, as shown in fig. 7, the MTJ cell provided by the embodiment of the present application further includes a first pinned layer 109, the first pinned layer 106 is located on a side of the first pinned layer 109 away from the second spacer layer 105, the first pinned layer 109 has in-plane anisotropy, and a magnetic moment of the first pinned layer 109 is coupled to a magnetic moment of the first pinned layer 106. The specific implementation form of the first specification layer 109 is similar to that of the second specification layer 107, and is not described in detail here.

In a steady state MTJ cell, the first gauge layer 109 may generate a stray field at the pinned layer 106, offsetting the magnetic moment direction of the pinned layer 106 from the intrinsic magnetic moment direction of the pinned layer 106 by an angle. Due to the antiferromagnetic coupling (or ferromagnetic coupling) between the pinned layer 106 and the reference layer 103, the magnetic moment direction of the reference layer 103 is shifted, and the magnetic moment direction of the reference layer 103 is shifted from the intrinsic magnetic moment direction of the reference layer 103 by an angle, so that an included angle exists between the stable magnetic moment direction of the free layer 101 and the magnetic moment direction of the reference layer 103, and the included angle is greater than 0 degree, less than 180 degrees, and not equal to 90 degrees.

Similar to the second embodiment, a third spacer layer may also be provided between the first gauge layer 109 and the pinning layer 106, which may reduce the lattice mismatch between the pinning layer 106 and the first gauge layer 109. On the side of the third spacer layer adjacent to the pinned layer 106, the lattice of the third spacer layer matches the lattice of the pinned layer 106. Along the thickness direction of the third spacer layer, the lattice size of the third spacer layer gradually transitions until the lattice of the third spacer layer matches the lattice of the first canonical layer 109 on the side of the third spacer layer close to the first canonical layer 109.

Example four

In one possible implementation, stray fields may also be generated at the free layer 101 by adjusting the shape of the free layer 101. Specifically, the length of the long axis of the free layer 101 is at least twice the length of the short axis, wherein both the long axis and the short axis of the free layer 101 are parallel to the interface between the free layer 101 and the barrier layer 102. Due to the shape of the free layer 101, the magnetic moment of the free layer 101 has a component of in-plane shape anisotropy, which is equivalent to a stray field generated in the free layer 101, so that the stable magnetic moment direction of the free layer 101 is shifted from the intrinsic magnetic moment direction by a certain angle.

Illustratively, as shown in fig. 8, for a top view of the free layer 101 provided in the embodiments of the present application, the free layer 101 shown in fig. 8 has an elliptical cylindrical structure, the top view of which is elliptical, L represents a long axis of the free layer 101, D represents a short axis of the free layer 101, and a ratio between L and D is greater than 2. It should be noted that the major axis and the minor axis of the free layer 101 in the present embodiment are parallel to the interface between the free layer 101 and the barrier layer 102, that is, the major axis and the minor axis of the free layer 101 in the present embodiment refer to the major axis and the minor axis in the top view of the free layer 101 when the free layer 101 is viewed from the top along the direction 2.

It is understood that the free layer 101 may also be a cylindrical structure with a long axis much larger than a short axis, for example, the free layer 101 may also be a rectangular parallelepiped structure, a diamond-shaped cylindrical structure, and the like, which is not described in detail in this embodiment of the application.

EXAMPLE five

Based on the possible implementation manners of any one of the MTJ cells provided in embodiments one to four, the embodiments of the present application also provide a method that can be applied to the VCMA driving method, that is, data can be written into the MTJ cell by using the VCMA driving scheme. The method may be applied to a control circuit in an MRAM.

Specifically, two writing processes, mainly "0" to "1" and "1" to "0", are reflected on the stable state switching of the MTJ cell, and two switching processes, namely, a first state to a second state and a second state to a first state, are possible. Next, the two switching processes will be described with the MTJ cell shown in fig. 5 as an example. For convenience of description, it is assumed that one end of the MTJ cell is connected to a voltage signal Vi, and the other end is grounded, in which case the voltage signal Vi may be equivalent to a bias voltage applied across the MTJ cell.

First, first state to second state

Fig. 9 is a schematic flowchart of a VCMA driving method according to an embodiment of the present application, as shown in fig. 9, the method mainly includes the following steps:

s901: a first voltage is applied to the MTJ cell in the first state. The voltage value of the first voltage is not less than the critical flip-flop voltage threshold Vc of the MTJ cell. The threshold switching voltage Vc can be obtained by the following formula:

wherein k is₀The interfacial anisotropy factor μ between the free layer 101 and the barrier layer 102 when Vi is 0V₀The vacuum permeability of the free layer 101 is shown, Ms is the saturation magnetization of the free layer 101, Nx is the x-axis demagnetization factor, the x-axis is the magnetic moment direction of the standard layer 107, Nz is the z-axis demagnetization factor, the z-axis is the direction 1 or 2, and ξ is the voltage-controlled magnetic anisotropy coefficient.

After the first voltage is applied, a more pronounced VCMA effect occurs in the MTJ cell, driving the magnetic moment of the free layer 101 to flip.

S902: and stopping applying the first voltage after the duration of continuously applying the first voltage reaches a first duration. Wherein the first time length is not less than the time length required for the magnetic moment of the free layer to be precessionally flipped by 90 degrees. As shown in fig. 4, after a period of time, the magnetic moment of the free layer can be precedently flipped from direction 1 to direction 3 and remains in direction 3, wherein the time required for the precessional flipping from direction 1 to direction 3 can be understood as the time required for the magnetic moment of the free layer to precedently flip 90 degrees.

In the embodiment of the present application, referring to fig. 6, the magnetic moment of the free layer in the first state is flipped 90 degrees before reaching the magnetic moment direction of the free layer shown in the intermediate state, and at this time, the magnetic moment of the free layer is flipped from the side of the free layer away from the reference layer to the other side of the free layer close to the reference layer. In this case, even if the application of the first voltage is stopped, the magnetic moment direction of the free layer may automatically continue to be reversed until the magnetic moment direction of the free layer in the second state as illustrated in fig. 6 is reached.

For example, the driving method provided by the embodiment of the present application is further described by taking the voltage signal Vi as an example. Fig. 10 exemplarily shows a waveform diagram of the voltage signal Vi, wherein a region where the voltage signal Vi is higher than Vc may be understood as the above-mentioned first voltage. Specifically, before time t1, the voltage value of the voltage signal Vi is 0, less than Vc, and the MTJ cell remains in the first state. At this time, a variation curve of the resistance of the MTJ cell (MTJ resistance) with the magnetic field strength may be as shown in fig. 11(a) in fig. 11. The stable magnetic moment direction of the free layer 101 and the magnetic moment direction of the reference layer 103 tend to align in parallel, keeping the MTJ cell in the first state. By "tend", it is meant that the angle between the two is greater than 0 ° and less than 90 °. When the first state is a low resistance state, the MTJ resistance of the MTJ unit cell in the absence of an applied magnetic field (magnetic field strength) is 0 can be shown as a black dot in fig. 11(a), and the current MTJ resistance is low.

During the period from time t1 to time t2, the voltage value of the voltage signal Vi is greater than Vc, the magnetic moment of the free layer 101 is switched from direction 4 to direction 5, and the MTJ cell is switched from the first state to the second state. During this time, the variation curve of MTJ resistance with magnetic field strength may be as shown in fig. 11(b) in fig. 11. The Vi voltage causes the coercivity of the MTJ cell to gradually decrease and the coercivity sign to change on one side (e.g., the MTJ cell changes from the positive coercivity of fig. 11(a) to the negative coercivity of fig. 11 (b)), whereupon the magnetic moment of the free layer 101 flips from direction 4 to direction 5, and accordingly, the MTJ cell switches from the first state to the second state. As shown by the black dots in fig. 11(b), when the magnetic field strength is 0, the MTJ resistance of the MTJ cell is high.

After time point t2, the MTJ cell remains in the second state. The variation curve of the MTJ resistance with the magnetic field strength can be as shown in fig. 11(c) in fig. 11. That is, after time t2, Vi becomes 0V, and the coercivity returns to the state shown in fig. 11(a), but the magnetic moment of the free layer 101 is maintained in direction 5, and accordingly, the MTJ cell is also maintained in the second state. As shown by the black dots in fig. 11(c), the MTJ resistance of the MTJ cell remains high at a magnetic field strength of 0.

As is apparent from the above-described precessional flipping of the magnetic moment of the free layer 101 in VCMA driving, the magnetic moment of the free layer 101 can be flipped by 90 degrees after the precessional flipping is completed. In the embodiment of the present application, the first time period is not shorter than a time period required for the magnetic moment of the free layer 101 to precedently flip by 90 degrees. The stable magnetic moment of the free layer 101 in the first state is at an angle to direction 1, and after the magnetic moment of the free layer 101 is flipped by 90 degrees by the first voltage, the magnetic moment of the free layer 101 flips to the intermediate state shown in fig. 6. At this point, the first voltage is stopped and the magnetic moment of the free layer 101 will automatically flip to the direction 5.

Second, second to first state

Fig. 12 is a schematic flowchart of a VCMA driving method according to an embodiment of the present application, as shown in fig. 12, the method mainly includes the following steps:

s1201: a second voltage is applied to the MTJ cell in the second state. The voltage value of the second voltage is not less than the critical flip voltage threshold value Vc of the MTJ unit.

S1202: and stopping applying the second voltage after the duration of continuously applying the second voltage reaches a second duration, and continuing to apply the third voltage to the MTJ cell. Wherein the voltage value of the third voltage is smaller than the threshold value Vc of the critical switching voltage of the MTJ cell. The second time period is not less than a time period required for the magnetic moment of the free layer to precedently flip by 90 degrees.

S1203: and stopping applying the third voltage after the duration of continuously applying the third voltage reaches a third duration. The third time length is not less than the time length of the magnetic moment of the free layer turning over a first angle under the STT effect, and the first angle is a supplementary angle of an included angle between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer. That is, during the third time, the magnetic moment of the free layer continues to be flipped by the first angle by the STT effect. The first angle is a complement of the above-mentioned included angle, such as 180-theta, and after the second duration, the included angle between the magnetic moment direction of the free layer and the plane where the free layer is located is theta, as shown in fig. 6. And in a third time length, continuously overturning the magnetic moment of the free layer by adopting an STT effect, and overturning the magnetic moment of the free layer to the other side of the plane of the free layer. Thereafter, even if the application of the third voltage is stopped, the magnetic moment of the free layer can be freely flipped to the direction 4.

For example, the driving method provided by the embodiment of the present application is further described by taking the voltage signal Vi as an example. Fig. 13 exemplarily shows a waveform diagram of the voltage signal Vi, wherein a region of the voltage signal Vi higher than Vc may be understood as the second voltage, and a region lower than Vc and higher than 0 may be understood as the third voltage.

Specifically, before time t3, the voltage value of the voltage signal Vi is 0, less than Vc, and the MTJ cell remains in the second state. At this time, the MTJ cell is in the second state, and the magnetic moment direction of the free layer 101 is in the second state as shown in FIG. 6. As shown in fig. 14(a) of fig. 14, when the magnetic field strength is 0, the MTJ resistance is at a high resistance as shown by the black dot in fig. 14 (a).

During the period from time t3 to time t4, the voltage value of the voltage signal Vi is greater than Vc, the magnetic moment of the free layer 101 is flipped from direction 5 to direction 4, the flipping direction is opposite to the direction indicated by the semi-circle of the dotted line in fig. 6, and the MTJ cell is flipped from the second state to the intermediate state. As shown in fig. 14(b) in fig. 14, the MTJ resistance is an intermediate resistance value shown by a black dot in fig. 14 (b).

During the period from the time point t4 to the time point t5, the voltage value of the voltage signal Vi is smaller than Vc and larger than V0, at this time, the magnetic moment direction of the free layer 101 approaches to the plane of the free layer 101, the magnetic moment direction of the free layer 101 is approximately perpendicular to the flow direction of electrons in the MTJ unit, so that the efficiency of the STT effect is high, and only a small voltage (third voltage) needs to be applied, a relatively significant STT effect can be obtained by the current flowing through the MTJ unit, so that the magnetic moment of the free layer 101 is driven to continuously flip from the intermediate state to the first state. In the case where the first state is the low resistance state, as shown in fig. 14(c) in fig. 14, the MTJ resistance finally becomes the low resistance shown by the black dot in fig. 14(c), and the MTJ cell switches to the low resistance state.

After time point t5, the MTJ cell remains in the first state.

It will be apparent to those skilled in the art that various modifications and variations can be made in the embodiments of the present invention without departing from the spirit or scope of the embodiments of the invention. Thus, if such modifications and variations of the embodiments of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to encompass such modifications and variations.

Claims

A Magnetic Tunnel Junction (MTJ) cell, comprising: the free layer, the barrier layer and the reference layer are sequentially stacked;

an included angle exists between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer, and the included angle is larger than 0 degree, smaller than 180 degrees and not equal to 90 degrees.
The MTJ cell of claim 1, in which the free layer and the reference layer are both perpendicular anisotropy.
The MTJ cell of claim 1 or 2, further provided with a first spacer layer between the free layer and the barrier layer for increasing a VCMA coefficient of voltage-regulated magnetic anisotropy between the free layer and the barrier layer.
The MTJ cell of any of claims 1 to 3, further comprising a pinned layer and a second spacer layer;

the pinning layer is located on a side of the reference layer away from the barrier layer, and the second spacer layer is located between the pinning layer and the reference layer;

the pinning layer is used for fixing the magnetic moment direction of the reference layer through the second spacing layer.
The MTJ cell of claim 4, further comprising a first normal layer on a side of the pinned layer away from the second spacer layer, the first normal layer being in-plane anisotropic and having a magnetic moment coupled to a magnetic moment of the pinned layer;

the first standard layer is used for adjusting the magnetic moment direction of the reference layer through the pinning layer, so that the included angle exists between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer.
The MTJ cell of claim 5, further comprising a third spacer layer between the pinned layer and the first normalization layer, the third spacer layer to reduce lattice mismatch between the pinned layer and the first normalization layer.
The MTJ cell of any of claims 1 to 4, further comprising a second normal layer on a side of the free layer away from the barrier layer, the second normal layer being in-plane anisotropic, and a magnetic moment of the second normal layer being coupled with a magnetic moment of the free layer;

the second standard layer is used for adjusting the stable magnetic moment direction of the free layer, so that the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer form an included angle.
The MTJ cell of claim 7, further comprising a fourth spacer layer between the free layer and the second normalization layer, the fourth spacer layer to reduce lattice mismatch between the free layer and the second normalization layer.
The MTJ cell of any of claims 1 to 4, wherein a long axis length of the free layer is at least twice a short axis length, both the long and short axes of the free layer being parallel to an interface between the free layer and the barrier layer.
A voltage-controlled magnetic anisotropy VCMA driving method is characterized by comprising the following steps:

applying a first voltage to an MTJ (magnetic tunnel junction) unit in a first state, wherein the voltage value of the first voltage is not less than the threshold flip voltage threshold of the MTJ unit, the MTJ unit in the first state comprises a free layer, a barrier layer and a reference layer which are sequentially stacked, the free layer and the reference layer are perpendicular anisotropy, an included angle exists between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer, and the included angle is greater than 0 degree and less than 90 degrees;

and stopping applying the first voltage after the duration of continuously applying the first voltage reaches a first duration, wherein the first duration is not less than the duration of precessional overturning of the magnetic moment of the free layer by 90 degrees.
A voltage-controlled magnetic anisotropy VCMA driving method is characterized by comprising the following steps:

applying a second voltage to the MTJ unit in a second state, wherein the voltage value of the second voltage is not less than the threshold switching voltage threshold of the MTJ unit, the MTJ unit in the second state comprises a free layer, a barrier layer and a reference layer which are sequentially stacked, the free layer and the reference layer are both vertical anisotropy, an included angle exists between the stable magnetic moment direction of the free layer and the magnetic moment direction of the reference layer, and the included angle is greater than 90 degrees and less than 180 degrees;

after the duration of continuously applying the second voltage reaches a second duration, stopping applying the second voltage, and continuously applying a third voltage to the MTJ cell, wherein the voltage value of the third voltage is smaller than the threshold switching voltage threshold of the MTJ cell, and the second duration is not smaller than the duration of precessional switching of the magnetic moment direction of the free layer by 90 degrees;

and stopping applying the third voltage after the duration of continuously applying the third voltage reaches a third duration, wherein the third duration is not less than the duration of turning the magnetic moment direction of the free layer at a first angle under the STT effect, and the first angle is the supplementary angle of the included angle.
A Magnetic Random Access Memory (MRAM) comprises a MTJ array and a driving circuit of the MTJ array; wherein the MTJ array comprises a plurality of MTJ cells, the MTJ cells being the MTJ cells of any of claims 1 to 9;

the driving circuit for performing the VCMA driving method of claim 10 or 11 for each MTJ cell in the MTJ array.