CN112864314B - Magneto-resistor device, method for changing resistance state of magneto-resistor device and synapse learning module - Google Patents
Magneto-resistor device, method for changing resistance state of magneto-resistor device and synapse learning module Download PDFInfo
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Abstract
The invention belongs to the technical field of memories, and mainly relates to a magneto-resistor device, a method for changing the resistance state of the magneto-resistor device and a synapse learning module; the magneto-resistor device comprises a top electrode, a ferromagnetic reference layer, a tunneling layer, a ferromagnetic free layer, a spin orbit coupling layer and a bottom electrode which are sequentially arranged along a preset direction; the spin orbit coupling layer comprises a first thickness area and a second thickness area which are alternately distributed, and the thicknesses of the first thickness area and the second thickness area are different; the ferromagnetic free layer comprises a pinning region, and the position of the pinning region corresponds to the position of the first thickness region one by one.
Description
Technical Field
The invention belongs to the technical field of memories, and mainly relates to a magneto-resistor device, a method for changing the resistance state of the magneto-resistor device and a synapse learning module.
Background
In recent decades, research on Magnetic Random Access Memories (MRAM) based on spintronics has been a major development in all countries around the world, and has become a hotspot. As one of the next generation of high performance and new memory, magnetic memory has many advantages such as non-volatility, high read/write speed, low power consumption, high density, and high durability.
Currently, the mainstream research includes Spin Transfer Torque-based magnetic random access memory (Spin Transfer Torque MRAM, abbreviated as STT-MRAM) and Spin Orbit Torque-based magnetic random access memory (Spin Orbit Torque MRAM, abbreviated as SOT-MRAM). STT-MRAM deflects the magnetization direction of the free layer by applying a torque to the free layer by a current polarized by the reference layer, and a change in resistance is characterized by a tunneling Magnetoresistance Effect (Tunnel Magnetoresistance Effect). However, a large current is required to generate enough torque to flip the free layer during writing. The SOT-MRAM induces Spin current to be injected in a vertical direction by a Spin Hall Effect (Spin Hall Effect) of a Spin-orbit coupling layer, that is, a current flowing in the Spin-orbit coupling layer having strong Spin-orbit coupling is converted into Spin current, the Spin current diffuses into a ferromagnetic layer, and a torque is applied to a magnetic moment of the ferromagnetic layer to flip it. And meanwhile, the SOT-MRAM realizes the separation of the read/write paths and has higher reliability. However, conventional SOT-MRAM requires the application of a magnetic field to break the symmetry and achieve deterministic magnetization switching. However, the main research is currently based on binary storage and corresponding integrative application of storage. In order to further improve the storage density of the device, simplify the circuit complexity and realize high-efficiency storage and calculation integration and brain-like intelligent chip application, research and development of a multi-resistance-state tunneling magneto-resistance device with high performance and low power consumption is imperative.
The existing SOT-MTJ technology is improved and perfected on the aspects of no external field inversion, differential multi-resistance state and the like aiming at the energy consumption of the SOT-MTJ. In order to realize the SOT-MTJ with multi-resistance state and no external field inversion, researchers have taken measures such as controlling the magnetic domain wall motion, anti-ferromagnetic coupling, stacking the SAF layer, etc.
In the related art, there is provided a three-terminal MTJ including a first ferromagnetic layer (reference layer), a barrier layer, a second ferromagnetic layer (storage layer), a buffer layer, a third ferromagnetic layer (switching layer), and a spin-orbit coupling layer. The storage layer and the switching layer form antiferromagnetic coupling, and after the switching layer is switched by the vertical spin polarization current, the storage layer is switched under the action of exchange bias.
In another related art, an MTJ is described based on an SOT layer of Ir-Mn material with antiferromagnetic properties that forms an in-plane exchange bias field (100-500Oe) at the interface with the free layer to assist SOT switching and a SAF layer stacked over the reference layer to eliminate the effects of stray fields.
In another related art, MTJ field-free switching is described as being achieved by etching the free layer of the MTJ to form a trench and a portion of the region that extends outward and to have in-plane magnetic anisotropy.
In another related art, pinning of a domain wall based on a boundary periodic notch is described, and motion of the domain wall and switching of a resistance state are realized through spin transfer torque. However, it needs a large current for writing, and the notch of the boundary also causes the deformation of the domain wall, which weakens the reliability of the device.
In another related art, a tetrapolar synapse device based on SOT domain wall motion is described, which has the following disadvantages: (1) the domain wall needs to move under the common drive of a magnetic field excited by current and spin-polarized current, which is not beneficial to the integration of devices; (2) the domain wall moves in the width direction of the device, the reference layer is only in a local area at one end of the device, and certain size waste exists in the length direction of the device, so that the integration of the device is not facilitated; (3) the lack of local pinning regions in the device is detrimental to the precise control and stabilization of the domain wall and difficult to differentiate resistance states.
Disclosure of Invention
In view of the above, the present invention provides a magnetoresistive device and a method for changing a resistance state thereof, which can partially solve the above-mentioned problems in the prior art.
A magneto-resistance device comprises a top electrode, a ferromagnetic reference layer, a tunneling layer, a ferromagnetic free layer, a spin orbit coupling layer and a bottom electrode which are sequentially arranged along a preset direction; the spin orbit coupling layer comprises a first thickness region and a second thickness region which are alternately distributed, and the thicknesses of the first thickness region and the second thickness region are different; the ferromagnetic free layer comprises a pinning region, and the position of the pinning region corresponds to the position of the first thickness region one by one.
According to an embodiment of the invention, the thickness of the first thickness region is greater than the thickness of the second thickness region.
According to an embodiment of the present invention, a DMI enhancement layer is further included, wherein the DMI enhancement layer is located between the tunneling layer and the ferromagnetic free layer.
According to an embodiment of the invention, the material of the DMI enhancement layer comprises at least one of: ti, Cu, W, Ta, Al.
According to an embodiment of the invention, the ferromagnetic free layer includes N pinned regions and the magnetoresistive device includes N +2 resistance states.
According to an embodiment of the invention, the material of the ferromagnetic reference layer comprises at least one of: CoFeB, CoFe, Co/Pt composite; the ferromagnetic free layer comprises a material comprising at least one of: CoFeB, CoFe, Co/Pt composite; the material of the tunneling layer includes at least one of: MgO and Al 2 O 3 。
According to an embodiment of the invention, the material of the spin-orbit coupling layer comprises at least one of: w, Ta, Pt.
A method of changing the resistance state of the magnetoresistive device, comprising: introducing a modulation driving current to the magneto-resistance device, wherein the modulation driving current acts on the spin orbit coupling layer; the resistance state of the magnetoresistive device is changed by changing the modulated drive current characteristic.
According to an embodiment of the present invention, changing the modulated drive current characteristic comprises: varying at least one of the following of the modulated drive current: pulse width, pulse amplitude, number of pulses, pulse direction.
A synapse learning module comprising the magnetoresistive device, comprising:
a training unit for generating a write pulse according to an arrival order of a front neuron signal and a rear neuron signal and a time interval between the front neuron signal and the rear neuron signal;
a synaptic device comprising a magnetoresistive device; the top electrode and the bottom electrode of the magneto-resistance device are connected with the training unit.
The magneto-resistor device provided by the embodiment of the invention comprises a spin orbit coupling layer with uneven thickness distribution, wherein a spin orbit torque is used for efficiently driving the movement of a magnetic domain wall in a ferromagnetic free layer under the condition of no external magnetic field, the anti-symmetric exchange effect (DMI) with different strengths is formed between the spin orbit coupling layer and the ferromagnetic free layer due to uneven thickness of the spin orbit coupling layer, a plurality of pinning areas corresponding to the first thickness areas one by one are formed in the ferromagnetic free layer under the anti-symmetric exchange effect with uneven thickness distribution, the movement and pinning of the magnetic domain wall are realized under the driving of modulating driving current, the stable resistance state is conveniently formed, and the performance of the magneto-resistor device is more stable; in addition, the magneto-resistor device provided by the embodiment of the invention utilizes spin orbit torque to efficiently drive the movement of the magnetic domain wall in the ferromagnetic free layer under the condition of no external magnetic field, the domain wall is pinned in the set pinning region through the interface anti-symmetric exchange effect, DMI is favorable for improving the movement rate of the domain wall, and compared with the prior art, a large current is not needed to generate enough torque to turn over the free layer in the writing process, so that the writing energy consumption is reduced; in addition, the magneto-resistor device provided by the embodiment of the invention has the advantages that under the condition of no external magnetic field, the movement of the domain wall is only driven by the modulation driving current, and the movement is not required to be driven by the common driving of the magnetic field excited by the current and the spin polarization current, so that the integration and compatibility of the device are facilitated. In summary, the magnetoresistive device provided by the embodiment of the invention realizes multi-resistance state difference, and has lower power consumption, higher device speed, higher reliability and circuit compatibility.
The magnetoresistive device provided by the embodiment of the invention can be combined with a peripheral memory circuit to realize a multi-valued memory array AND improve the storage density of data, AND can also be used for synapse devices in nerve morphology AND logic operation devices (half adders, full adders, AND, NAND, OR, NOR, XOR AND XNOR) to further realize the integration of existing operations.
Drawings
FIG. 1a is a schematic structural diagram of a magnetoresistive device according to an embodiment of the invention;
FIG. 1b is a schematic diagram of the structure of the spin-orbit coupling layer and ferromagnetic free layer in the magnetoresistive device shown in FIG. 1 a;
FIG. 2a is a schematic structural diagram of a magnetoresistive device according to another embodiment of the invention;
FIG. 2b is a schematic diagram of the structure of the spin-orbit coupling layer and ferromagnetic free layer in the magnetoresistive device shown in FIG. 2 a;
FIG. 3 is a schematic diagram of domain wall motion velocity versus pulse amplitude of a modulated drive current, and DMI in a magnetoresistive device according to an embodiment of the invention;
FIG. 4a is a schematic diagram of the relationship between domain wall position and pulse width of a modulated driving current in a magnetoresistive device and a corresponding domain state diagram according to an embodiment of the invention;
FIG. 4b is a phase diagram of domain wall motion versus pulse amplitude and pulse width of a modulated drive current in a magnetoresistive device according to an embodiment of the invention;
FIG. 5 is a graph of the spike-time dependent plasticity (STDP) characteristic of the magnetoresistive device provided by an embodiment of the invention;
FIG. 6 is a layout of a synapse learning module constructed based on the STDP characteristics of the magnetoresistive device provided by the embodiments of the invention;
FIG. 7 is a circuit diagram of a synapse learning module constructed based on the STDP characteristics of the magnetoresistive device according to an embodiment of the invention;
fig. 8 is a timing diagram of various pulses in a synapse learning module constructed based on the STDP characteristic of the magnetoresistive device according to an embodiment of the invention.
Description of the reference numerals
101. A top electrode; 102. a ferromagnetic reference layer; 103. a tunneling layer; 104. a ferromagnetic free layer; 105. a spin-orbit coupling layer; 1051. a first thickness region; 1052. a second thickness region; 106. a bottom electrode; 107. a DMI enhancement layer; 201. a domain wall; 202. a pinning region; 2021. a first pinning region; 2022. a second pinning region.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the accompanying drawings in combination with the embodiments.
Fig. 1a is a schematic structural diagram of a magnetoresistive device according to an embodiment of the invention.
As shown in fig. 1a, according to an embodiment of the present invention, there is provided a magnetoresistive device, including a top electrode 101, a ferromagnetic reference layer 102, a tunneling layer 103, a ferromagnetic free layer 104, a spin-orbit coupling layer 105, and a bottom electrode 106, which are sequentially arranged along a preset direction; wherein the spin-orbit coupling layer 105 comprises alternating first thickness regions 1051 and second thickness regions 1052, the first thickness regions 1051 and the second thickness regions 1052 having different thicknesses; the ferromagnetic free layer 104 includes pinning regions 202 therein, and the pinning regions 202 are located in one-to-one correspondence with the locations of the first thickness regions 1051.
Fig. 1b is a schematic diagram of the structure of the spin-orbit coupling layer 105 and the ferromagnetic free layer 104 in the magnetoresistive device shown in fig. 1 a. Optionally, according to an embodiment of the present invention, the thickness of the first thickness region 1051 is greater than the thickness of the second thickness region 1052. In general, the larger the DMI where the thickness of the spin-orbit coupling layer 105 is, the larger the DMI is, and accordingly, the pinning region 202 is easily formed at a corresponding position in the spin-orbit coupling layer 105 so that the domain wall 201 can be stabilized therein, and therefore, the position of the pinning region 202 corresponds to the position of the first thickness region 1051 having the larger thickness one by one.
In the magnetoresistive device provided by the embodiment of the invention, in the ferromagnetic free layer 104, the domain wall 201 is injected through a gap at the starting position, a diamond region or a low perpendicular magnetic anisotropy region. The magnetoresistive device provided by the embodiment of the invention comprises the spin-orbit coupling layer 105 with the non-uniform thickness distribution, and specifically, the thickness of the spin-orbit coupling layer 105 can be modulated through deposition and/or etching processes.
Under the condition of no external magnetic field, the motion of the magnetic domain wall 201 in the ferromagnetic free layer 104 is efficiently driven by using spin-orbit torque, the anti-symmetric exchange effects (namely DMI) with different strengths are formed between the spin-orbit coupling layer 105 and the ferromagnetic free layer 104 due to the uneven thickness of the spin-orbit coupling layer 105, under the condition of driving by modulating driving current, the DMI has a promoting effect on the motion of the magnetic domain wall 201, namely, the potential barrier of the magnetic domain wall 201 is reduced, under the anti-symmetric exchange effect of uneven distribution, the situation that the potential barrier and the magnetic well are alternately distributed according to the difference of the strengths of the DMI is formed, the place where the magnetic well is located is the pinning region 202, and the magnetic domain wall 201 can be effectively stabilized in the pinning region; that is, a plurality of pinned regions 202 corresponding to the positions of the first thickness regions 1051 one to one are formed in the ferromagnetic free layer 104.
Under the drive of the modulation driving current, the domain wall 201 in the ferromagnetic free layer 104 can be respectively stabilized when moving, when the modulation driving current drives the magnetic domain wall 201 to move from the initial state to the next pinning region 202 or the device boundary, the evolution of the area proportion corresponding to the magnetic domain with different magnetization orientation in the ferromagnetic free layer 104 and the ferromagnetic reference layer 102 is caused, namely different resistance states are formed in the magnetoresistive device, because the plurality of stabilized pinning regions 202 exist in the magnetoresistive device, the accurate control of the domain wall 201 is facilitated, the stable resistance states are formed, and the performance of the magnetoresistive device is more stable.
Fig. 3 is a schematic diagram of the relationship between the domain wall 201 motion speed and the pulse amplitude of the modulated driving current, and the DMI in the magnetoresistive device provided by the embodiment of the invention.
From the illustration in figure 3, in addition to increasing the domain wall 201 motion velocity by increasing the pulse amplitude of the modulated drive current, it can be seen that DMI has a significant contribution to the domain wall 201 motion velocity. Furthermore, by modulating the DMI, not only can the corresponding pinning region 202 be formed, but also the motion speed of the domain wall 201 can be greatly increased, thereby realizing high-speed writing and low operation power consumption of the device.
Therefore, the magnetoresistive device provided by the embodiment of the invention utilizes the spin orbit torque to efficiently drive the movement of the magnetic domain wall 201 in the ferromagnetic free layer 104 under the condition of no external magnetic field, and further improves the movement rate of the domain wall 201 through DMI (digital domain interface), compared with the prior art, a large current is not required to generate enough torque to turn over the free layer in the writing process, so that the writing energy consumption is reduced, and the working rate of the device is improved. The power consumption of the device can be reduced by modulating DMI, the power consumption of dozens of flying focus levels can be realized by matching the pulse width and the pulse amplitude of some common current, the amplitude of the current pulse can be reduced, the width of the current pulse can be reduced by modulating the size of the device and the like by combining the use of a heavy metal material with a larger spin Hall angle as the spin orbit coupling layer 105, the current pulse with smaller amplitude or shorter pulse width can be applied, or the power consumption equivalent to that of human brain neurons can be realized.
In addition, the magnetoresistive device provided by the embodiment of the invention has the advantages that under the condition of no external magnetic field, the movement of the domain wall 201 is driven only by the modulation driving current, and the movement is not required to be driven under the common driving of the magnetic field excited by the current and the spin polarization current, so that the integration of the device is facilitated.
Alternatively, the magnetic resistance device and the dimensions of the respective layers therein may be scaled down according to the process, and the shape of the magnetic resistance device may be simply replaced by a cube or the like.
In the magnetoresistive device provided by the embodiment of the invention, a plurality of groove-shaped (or protrusion-shaped) structures are formed on one side surface of the spin orbit coupling layer 105 by depositing and/or etching a plurality of first thickness regions 1051 and second thickness regions 1052 which are alternately distributed on the spin orbit coupling layer 105, and optionally, the shape of the groove-shaped (or protrusion-shaped) structures includes but is not limited to rectangular grooves, arcs, triangles and the like.
FIG. 2a is a schematic structural diagram of a magnetoresistive device according to another embodiment of the invention; FIG. 2b is a schematic diagram of the structure of the spin-orbit coupling layer 105 and the ferromagnetic free layer 104 in the magnetoresistive device shown in FIG. 2 a; as shown in fig. 2a, the magnetoresistive device differs from the magnetoresistive device provided in the embodiment of fig. 1a in that it further includes a DMI enhancement layer 107, wherein the DMI enhancement layer 107 is located between the tunneling layer 103 and the ferromagnetic free layer 104; also, the thickness of the first thickness region 1051 is greater than or less than the thickness of the second thickness region 1052 (fig. 2a and 2b show the case where the thickness of the first thickness region 1051 is less than the thickness of the second thickness region 1052, and the case where the thickness of the first thickness region 1051 is greater than the thickness of the second thickness region 1052 is not shown). According to an embodiment of the present invention, the material of the DMI enhancement layer 107 is selected from heavy metal materials, which may include at least one of the following: ti, Cu, W, Ta, Al.
In this embodiment, considering that the DMI modulation range between the spin-orbit coupling layer 105 and the interface of the ferromagnetic free layer 104 is small, a DMI enhancement layer 107 (i.e., a heavy metal layer) capable of generating a large DMI with the interface of the ferromagnetic free layer 104 is inserted between the ferromagnetic free layer 104 and the tunneling layer 103, and the DMI enhancement layer 107 is added, so that the velocity of the domain wall 201 can be increased by a large amplitude, and the requirement for modulation driving current is reduced. As shown in fig. 3, since DMI significantly enhances the motion speed of the domain wall 201, the writing power consumption can be further reduced and the device operation rate can be increased by enhancing DMI by adding the DMI enhancement layer 107.
In addition, in this embodiment, because the sign of the DMI varies according to the interface and stacking sequence of different materials, the thickness distribution of the spin-orbit coupling layer 105 may also be adjusted after the DMI enhancement layer 107 is added, which is specifically embodied that the thickness of the first thickness region 1051 is greater than or less than the thickness of the second thickness region 1052, the positions of the pinning regions 202 correspond to the positions of the first thickness regions 1051 one-to-one, and the process parameters need to be adjusted to form corresponding structures when the spin-orbit coupling layer 105 is deposited and/or etched.
According to an embodiment of the invention, N (N ≧ 1) pinned regions 202 are formed in the ferromagnetic free layer 104, and the magnetoresistive device can achieve N +2 resistance states. Specifically, the thicknesses of the spin-orbit coupling layer 105 are modulated through deposition and/or etching processes to form first thickness regions 1051 and second thickness regions 1052 which are alternately distributed, further, periodic alternate distribution of the DMI intensity in the ferromagnetic free layer 104 is achieved, N (N is larger than or equal to 1) pinning regions 202 with the number equal to that of the first thickness regions 1051 are formed in the ferromagnetic free layer 104, the achievable resistance states of the magnetoresistive device are related to the number of the pinning regions 202, and specifically, the number of the resistance states is N + 2. The following description refers to the embodiments described with reference to fig. 4a and 4 b.
Fig. 4a is a schematic diagram of the relationship between the position of the domain wall 201 and the pulse width of the modulated driving current in a magnetoresistive device according to an embodiment of the present invention and the corresponding magnetic domain state diagram.
The magnetoresistive device is 600nm long and 120nm wide, and comprises two pinning regions 202 in the ferromagnetic free layer 104, so that 2+2 resistance states can be realized, and the corresponding magnetic domain distribution and domain wall 201 positions are shown in the figure. The range of the first pinning region 2021 is 170-220nm, the range of the second pinning region 2022 is 380-430nm, and the pulse density of the applied modulation driving current is 1 × 10 8 A/cm 2 Different domain wall 201 motion and stable states can be achieved by adjusting the pulse width of the modulated drive current pulse.
When no modulated drive current pulse is applied, the domain wall 201 stays in the initial position, corresponding to the first resistance state; when the pulse width of the applied modulation driving current pulse is increased to 0.2-1.2 ns, the modulation driving current can drive the domain wall 201 to move to the first pinning region 2021 and keep stable, corresponding to the second resistance state; when the pulse width of the applied modulation driving current pulse is increased to 1.5-2.5 ns, the modulation driving current pulse driving domain wall 201 is separated from the first pinning region 2021, moves to the second pinning region 2022 and is kept stable, and corresponds to a third resistance state; when the pulse width of the applied modulated drive current pulse is increased to 2.7-3.0 ns, the domain wall 201 detaches from the second pinning region 2022 again, moving to the edge of the device, and achieving a fourth resistance state. It should be noted that for the intermediate pulse widths not mentioned above, the domain wall may still switch to the next resistance state at a slightly longer relaxation time, e.g., at a pulse width of 1.3ns, the domain wall may still move to the second pinning region 2022 at a slightly longer relaxation time.
Fig. 4b is a phase diagram of the domain wall 201 motion versus pulse amplitude and pulse width of the modulated drive current in the magnetoresistive device provided by the embodiment described in fig. 4 a. The amplitude of the pulse of the modulated drive current is 0.6-1.4 multiplied by 10 8 A/cm 2 When the modulation driving current pulse is not applied, the domain wall 201 stays at the initial position (the darkest region of the illustrated color), and when the pulse width is 0.2 to 1.2ns, the modulation driving current pulse can cause the domain wall 201 to move to the first pinning region 2021 and remain stable (the darker region of the illustrated color); when the pulse width is 1.2-2.5 ns, modulating the driving current pulse can cause the domain wall 201 to move to the second pinning region 2022 and remain stable (lighter colored regions are shown); when the pulse width is 2.5-3.0 ns, the domain wall 201 can be moved away from the second pinning region 2022 (the lightest color region is shown) by modulating the driving current pulse. It can also be seen from figure 4b that the domain wall 201 motion and pinned region 202 switching are less related to the modulation drive current pulse amplitude, but more closely related to the pulse width, with a strong time dependence.
According to an embodiment of the invention, the material of the ferromagnetic reference layer 102 comprises at least one of: CoFeB, CoFe, Co/Pt composite; the material of the ferromagnetic free layer 104 includes at least one of: CoFeB, CoFe, Co/Pt composite; the material of the tunneling layer 103 includes at least one of: MgO and Al 2 O 3 。
According to an embodiment of the present invention, the material of the spin-orbit coupling layer 105 includes at least one of: w, Ta, Pt.
For the magnetoresistive device provided by the embodiment of the invention, the embodiment of the invention also provides a method for changing the resistance state of the magnetoresistive device, which specifically comprises the following steps: introducing a modulation driving current to the magneto-resistance device, wherein the modulation driving current acts on the spin orbit coupling layer 105; the resistance state of the magnetoresistive device is changed by changing the modulated drive current characteristic. Specifically, according to an embodiment of the present invention, changing the modulated drive current characteristic includes: varying at least one of the following of the modulated drive current: pulse width, pulse amplitude, pulse number, pulse direction.
Specifically, as can be seen from the above analysis of the embodiments shown in fig. 4a and 4b, the domain wall 201 can be stabilized or cross different pinning regions 202 by changing the pulse width of the modulated driving current pulse, so as to achieve a multi-resistance state (under the driving of the modulated driving current, the domain wall 201 in the ferromagnetic free layer 104 can be stabilized during the movement, respectively, wherein when the modulated driving current drives the domain wall 201 to move from the initial state to the next pinning region 202 or the device boundary, the evolution of the area ratio corresponding to the different magnetization orientation magnetic domains in the ferromagnetic free layer 104 and the ferromagnetic reference layer 102 is caused, that is, different resistance states are formed in the tunnel junction device).
The resistance states may also be switched by varying the number of pulses of modulated drive current, e.g. one pulse switches one resistance state and two pulses switch two resistance states.
As can be seen from figure 3, the domain wall 201 motion speed can be increased by increasing the pulse amplitude of the modulated drive current, thereby switching the resistance state by the domain wall 201 motion.
Fig. 5 is a graph of Spike Timing-Dependent Plasticity (STDP) characteristics of the magnetoresistive device according to the embodiment of the invention. The curves consisting of point-line in the figure represent the resistance change curves for the Set and Reset processes fitted to the simulated data for the magnetoresistive device. Modeling a magnetoresistive device as a synaptic deviceThe two input spike signals represent the front neuron signal and the back neuron signal, respectively, and the amplitude of the modulated driving current pulse is 1 × 10 in this embodiment 8 A/cm 2 As can be seen from the figure, the current neuron signal is earlier than the post neuron signal, i.e., Δ t>When 0, a positive writing pulse is generated, and the weight of synapses is increased, namely the resistance value of the device is reduced; conversely, the current neuron signal is later than the later neuron signal, i.e., Δ t<At 0, a write pulse in the opposite direction will be generated, decreasing the synaptic weight, i.e., increasing the resistance of the device. As can be seen from fig. 5, the resistance state of the magnetoresistive device can be changed by changing the direction of the pulse that modulates the drive current.
In addition, the change of the resistance state of the device and the optimized modulation of the writing speed and the power consumption of the device can be realized by adjusting the size of the device and the magnetic characteristic parameters of the ferromagnetic free layer 104.
Meanwhile, it can be seen from fig. 5 that the resistance state of the magnetoresistive device changes with strong time dependence. Specifically, when Δ t >0, a positive write pulse will be generated, increasing the synaptic weight, decreasing the resistance of the device, increasing the on-current; conversely, when Δ t <0, a reverse write pulse will be generated, decreasing the synaptic weight, i.e., increasing the resistance of the device, decreasing the on-current. In addition, the shorter the time interval between the pre-neuron signal and the post-neuron signal, the greater the change in synaptic weight (i.e., resistance), and when the time interval between the pre-neuron signal and the post-neuron signal is greater than 3ns or more, the synaptic weight is not rewritten, which shows that the change in magnetoresistive device resistance state has a strong time dependence. According to the time dependence, the magneto-resistance device can be applied to high-density storage and storage integration and brain-like intelligent nerve synapse devices.
Accordingly, the embodiment of the invention also provides a synapse learning module comprising the magnetoresistive device.
FIG. 6 is a layout of a synapse learning module constructed based on the STDP characteristics of the magnetoresistive device provided by the embodiments of the invention; FIG. 7 is a circuit diagram of a synapse learning module constructed based on the STDP characteristics of the magnetoresistive device according to an embodiment of the invention; FIG. 8 isThe timing diagram of each pulse in the synapse learning module constructed based on the STDP characteristics of the magnetoresistive device provided by the embodiment of the invention. In FIGS. 6-8, MTJ represents a magnetoresistive device, S, provided by an embodiment of the invention pre Representing the pre-neuron signal, S post Representing the post-neuronal signal, T pre Is represented by S pre A section of square wave signal triggered, herein designated as the pre-neuron trigger signal; t is post Is represented by S post A section of square wave signal triggered, here named post-neuron trigger signal; i is write The write current to the magnetoresistive device is shown, and in fig. 7, NMOS transistors are used for N1 to N10.
As shown in fig. 6 to 7, the synapse learning module provided in the embodiments of the invention includes a training unit and a synapse device, wherein the training unit is configured to generate a write pulse (write current to the synapse device) according to an arrival sequence of pre-neuron signals and post-neuron signals and a time interval between the pre-neuron signals and the post-neuron signals; a synapse device comprising a magnetoresistive device provided by embodiments of the invention; wherein the top electrode and the bottom electrode of the magneto-resistance device are connected with the training unit.
By the synapse learning module provided by the embodiment of the invention, based on the STDP characteristic curve of the magnetoresistive device in FIG. 5, the weight of the synapse device is rewritten according to the arrival sequence and the interval time of the front neuron signal and the rear neuron signal, so that the high-energy-efficiency learning function of the neural network is realized.
The following describes the simulation learning process of the synapse device with reference to fig. 7 and 8.
The simulation in the embodiment of the invention is carried out based on 55nm process nodes, and the pulse amplitude of the modulation driving current is 1 multiplied by 10 8 A/cm 2 When S is pre When the first arrival, N9 is conducted, and T is not existed at the time post Signal, lower branch latch holds left "1" right "0" state, N3 is off, while S is off pre Will trigger a 3ns pulse T pre N1 and N7 are turned on; after a certain time interval, at T pre Duration of existence of pulse, S post At this point, N6 is on, and since N7 is now in the on state,the upper branch latch stores the state of left "0" and right "1", N2 is turned on, and S is turned on post Starting a 3ns pulse T post N4 and N10 are turned on, and there is no S pre Signal N3 is still off, the upper branch is on, the lower branch is off, and the forward write current I is off write Flowing through the magnetoresistive device, increasing the weight of the synapse.
Current posterior neuron signal S post On arrival first, N6 is on since there is no T pre The signal, the up-branch latch holds the left "1" right "0" state, N2 is off, and S is on post Will turn on a 3ns pulse T post N4 and N10 are turned on; after a certain time interval, at T post Pre-neuron spike during the pulse existence period S pre When N9 is turned on, since N10 is in the on state, the lower branch latch stores the state of left 0 and right 1, N3 is turned on, and S is turned on pre Starting a 3ns pulse T pre N1 and N7 are turned on, and there is no S post Signal N2 is still off, when the lower branch is on, the upper branch is off, and the reverse write current I write Flowing through the magnetoresistive device, the weight of the synapse is reduced. In addition, when T is removed pre 、T post When the signal is transmitted, N5 and N8 are conducted, the latch stores left 1 and right 0, N2 and N3 are turned off, and the circuit is reset. According to T pre 、T post The signal sequence and interval duration can generate write current I with different directions and pulse widths write (the timing diagram is shown in FIG. 8), so that the synapse devices are trained to learn their ideal states.
In addition, in the actual use process, the device parameters of the synapse learning module provided by the embodiment of the invention can be adjusted according to actual needs, so that write currents with different pulse widths and amplitudes can be generated conveniently to adapt to different synapse devices.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. A magnetoresistive device comprises a top electrode (101), a ferromagnetic reference layer (102), a tunneling layer (103), a ferromagnetic free layer (104), a spin orbit coupling layer (105) and a bottom electrode (106) which are sequentially arranged along a preset direction; wherein
The spin-orbit coupling layer (105) comprises alternating first thickness regions (1051) and second thickness regions (1052), the first thickness regions (1051) and second thickness regions (1052) having different thicknesses;
the ferromagnetic free layer (104) comprises a pinning region (202), and the pinning region (202) corresponds to the first thickness region (1051) in position one by one;
wherein antisymmetrical exchange effects with different strengths are formed between the spin orbit coupling layer (105) and the ferromagnetic free layer (104), and the antisymmetrical exchange effect is larger at places with larger thickness in the spin orbit coupling layer (105).
2. The magnetoresistive device of claim 1, wherein the first thickness region (1051) has a thickness greater than a thickness of the second thickness region (1052).
3. The magnetoresistive device according to claim 1, further comprising a DMI enhancement layer (107), wherein the DMI enhancement layer (107) is located between the tunneling layer (103) and the ferromagnetic free layer (104).
4. The magnetoresistive device according to claim 3, wherein the material of the DMI enhancement layer (107) comprises at least one of: ti, Cu, W, Ta, Al.
5. The magnetoresistive device according to claim 1, wherein the ferromagnetic free layer (104) includes N pinned regions (202) therein, the magnetoresistive device including N +2 resistance states.
6. The magnetoresistive device of claim 1 wherein:
the material of the ferromagnetic reference layer (102) comprises at least one of: CoFeB, CoFe, Co/Pt composite;
the ferromagnetic free layer (104) material includes at least one of: CoFeB, CoFe, Co/Pt composite;
the material of the tunneling layer (103) comprises at least one of: MgO and Al 2 O 3 。
7. The magnetoresistive device according to claim 1, wherein the material of the spin-orbit coupling layer (105) comprises at least one of: w, Ta, Pt.
8. A method of changing the resistance state of the magnetoresistive device of any of claims 1-7 comprising:
passing a modulated drive current to the magnetoresistive device, wherein the modulated drive current acts on the spin-orbit coupling layer (105);
the resistance state of the magnetoresistive device is changed by changing the modulated drive current characteristic.
9. The method of claim 8, the changing the modulated drive current characteristic comprising:
varying at least one of the following of the modulated drive current: pulse width, pulse amplitude, number of pulses, pulse direction.
10. A synapse learning module comprising the magnetoresistive device of any of claims 1-7, comprising:
a training unit for generating a write pulse according to an arrival order of a pre-neuron signal and a post-neuron signal and a time interval between the pre-neuron signal and the post-neuron signal;
a synapse device comprising said magnetoresistive device; wherein the top electrode (101) and the bottom electrode (106) of the magnetoresistive device are connected with the training unit.
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