Patent Application
20170372761
The present invention generally relates to electronic circuits and more specifically to the implementation of magnetoelectric junctions.
Devices that rely on electricity and magnetism underlie much of modern electronics. Particularly, researchers have begun to develop and implement devices that take advantage of both electricity and magnetism in spin-electronic (or so-called “spintronic”) devices. These devices utilize quantum-mechanical magnetoresistance effects, such as giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR). GMR and TMR principles regard how the resistance of a thin film structure that includes alternating layers of ferromagnetic and non-magnetic layers depends upon whether the magnetizations of ferromagnetic layers are in a parallel or antiparallel alignment. For example, magnetoresistive random-access memory (MRAM) is a technology that is being developed that typically utilizes TMR phenomena in providing for alternative random-access memory (RAM) devices. In a typical MRAM bit, data is stored in a magnetic structure that includes two ferromagnetic layers separated by an insulating layer—this structure is conventionally referred to as a magnetic tunnel junction (MTJ). The magnetization of one of the ferromagnetic layers (the fixed layer) is permanently set to a particular direction, while the other ferromagnetic layer (the free layer) can have its magnetization direction free to change. Generally, the MRAM bit can be written by manipulating the magnetization of the free layer such that it is either parallel or antiparallel with the magnetization of the fixed layer; and the bit can be read by measuring its resistance (since the resistance of the bit will depend on whether the magnetizations are in a parallel or antiparallel alignment).
Systems for performing source line sensing of magnetoelectric junctions in accordance with embodiments of the invention are disclosed. In one embodiment, a magnetoelectric random access memory circuit includes a plurality of voltage controlled magnetic tunnel junction bits each magnetoelectric junction includes at least one free magnetic layer, one fixed magnetic layer, and one dielectric interposed between the two magnetic layers, application of a voltage with a given polarity to the magnetoelectric junction reduces the perpendicular magnetic anisotropy and the magnetic coercivity of the free layer through the voltage controlled magnetic anisotropy (VCMA) effect, application of a voltage with opposite polarity increases the perpendicular magnetic anisotropy and magnetic coercivity of the free layer through the VCMA effect, each magnetoelectric junction is connected to the drain of an MOS transistor, the combination includes a MeRAM cell, each MeRAM cell includes three terminals, each connected respectively to a bit line, a source line, and at least one word line, in an array, a pulse generator and a write MOS transistor connected to the bit line and the source line, a sense amplifier and a sense MOS transistor connected to the source line and the bit line, and a current source circuit connected to the source line and the reference line.
In a further embodiment, the magnetoelectric junction bit free layer includes a combination of Co, Fe and B.
In another embodiment, the magnetoelectric junction bit dielectric barrier includes MgO.
In a still further embodiment, the magnetoelectric junction bit free layer is placed adjacent to a metal layer, includes one or a combination of the elements Ta, Ru, Mn, Pt, Mo, Ir, Hf, W, and Bi.
In a still another embodiment, the free layer magnetization changes direction in response to a voltage pulse across the magnetoelectric junction bit, which is timed to approximately half the ferromagnetic resonance period of the free layer.
In a yet further embodiment, the free layer magnetization has two stable states which are perpendicular to plane in the absence of voltage.
In yet another embodiment, the free layer magnetization has two stable states in plane in the absence of voltage.
In a further embodiment again, the magnetoelectric junction bit has a circular shape.
In another embodiment again, the magnetoelectric junction bit has an elliptical shape.
In a further additional embodiment, the pulse generator involves a bit line driver.
In another additional embodiment, where the source of a MOS transistor of each MeRAM cell is connected to the source line.
In a still yet further embodiment, at least one output of the current source circuit is connected to the source line and supplies a constant current during the read operation.
In still yet another embodiment, a second output of the current source circuit is connected to the reference line and supplies a constant current during the read operation.
In a still further embodiment again, at least one input of the sense amplifier is connected to the source line.
In still another embodiment again, a second input of the sense amplifier is connected to the reference line.
In a still further additional embodiment, the drain of a MOS transistor is connected to the reference line.
In still another additional embodiment, the source of a MOS transistor is connected to a reference resistor.
In a yet further embodiment again, the drain of the sense MOS transistor is connected to the bit line.
In a yet further embodiment again, the drain of the write MOS transistor is connected to the source line.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The description will be more fully understood with reference to the following figures, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention, wherein:
Turning now to the drawings, systems, and methods for source line sensing of magnetoelectric junctions are illustrated. In the field of random access memory, bits of data in the memory are read or “sensed” to determine the value of each bit of stored data. The current application describes a scheme for sensing memory bits on the source line instead of the bit line as in traditional applications. The source line sensing system can reduce read disturbance and increase the sensing margin over traditional applications. These improvements allow for the use of magnetoelectric random access memories (MeRAM) in an increased number of applications.
Some challenges currently prevent MeRAM from being implemented in certain applications including embedded system memory applications. One potential problem is read failure, which occurs when a sensing circuit cannot distinguish between two states of the memory cell without changing the memory state due to the small sensing margin. The sensing margin can be defined as the difference between the voltage from the sense line node and the reference node. This can be caused by the low tunneling magnetoresistance (TMR) ratio in material systems typically used in spin-transfer torque magnetic random-access memory (STT-RAM) and MeRAM. As the sensing margin decreases, the memory can become more susceptible to noise, potentially increasing the read failure and perhaps requiring a more sophisticated circuit to amplify signals.
Another possible issue in traditional MeRAM applications is read disturbance, which is understood as a chance flipping of the magnetoelectric junction (MEJ) state after applying an electric read pulse (i.e., the probability of a destructive read), which is not affected by TMR but in many cases by thermal stability. A read disturbance can happen during reading when the bit lines of an MeRAM as well as STT-RAM are charged to a certain voltage level (sensing voltage).
Conventional sensing schemes such as bit line sensing (BLS) in MeRAM applications apply a positive voltage across a device to sense the state of the device, which might cause a read disturbance. The reading of a MEJ, unlike typical STT devices, is strongly affected by the choice of voltage polarity during the read operation, since the voltage-controlled magnetic anisotropy (VCMA) effect results in a change of coercivity in the free layer under voltage application. The change in coercivity can vary the thermal stability of the free layer. In many instances this can be related to the VCMA effect modulating the coercivity under the electric bias condition, which in turn can change the thermal stability of the device. Source line sensing (SLS) applies a sensing voltage in an opposite polarity compared to that of the BLS for reducing read disturbance by enhancing coercivity.
Source line sensing on a typical MeRAM chip is applied over a plurality of cell grids or groupings of cells. These cells are typically made up of memory cells. Numerous applications have used magnetic tunnel junctions (MTJs) as memory cells in magnetoresistive random access memory (MRAM). However, the magnetoelectric tunnel junction (MEJ) is an emerging variant of the MTJ device used in MRAM, which exploits magnetoelectric interface effects to control its free layer magnetization, and tunneling magnetoresistance (TMR) to read its state. Generally, the coercivity of the free layer of a MEJ can be reduced using voltage-controlled magnetic anisotropy (VCMA) phenomena, thereby making the free layer more easily switched to the opposite direction (‘writeable’). It has been demonstrated that such devices employing VCMA principles result in marked performance improvements over conventional MTJs. The electric-field-controlled nano-magnets used in MEJs are being developed as basic building blocks for the next generation of memory and logic applications, since they have the potential for significant reductions in power dissipation, offer high endurance and density, and can be applied to high-speed operation systems.
The MEJ differs from a conventional magnetic tunnel junction in that an electric field is used to induce switching, in lieu of substantial current flow for utilizing spin transfer torque (STT) in a current-controlled MTJ. Compared to MTJs, MEJs have at least three noticeable advantages: i) extremely low dynamic switching energy due to significant reduction of Ohmic loss, ii) sub-nanosecond writing speed based on precessional switching (which for STT devices requires very large currents through the device to achieve the same speed), iii) high density in a memory array application due to the use of minimum sized access transistors or diodes in a cell.
However, as a result of coercivity dependence, using a traditional BLS scheme has a possibility of causing read disturbances in MeRAM cell arrays. This may be especially the case for embedded system memory applications, which may only require a relatively short retention time (<1 ms) since they have a relatively low thermal stability (Δ˜20-30) compared to storage applications (typically Δ>40). In certain embodiments, the BLS scheme sensing voltage (pre-charge voltage) on the bit line should be limited, which, however, can reduce the sensing margin. Utilizing a source line sensing system in accordance with embodiments of the invention can reduce read disturbances and allow for the use of MeRAM in an increased amount of applications such as embedded system memory applications.
Source-line sensing systems can utilize a number of MEJ variants depending on the specific application required. In broad terms, a fundamental MEJ structure includes a ferromagnetic (FM) fixed layer, a FM free layer that has a uniaxial anisotropy, and a dielectric layer separating the FM fixed layer and FM free layer. For simplicity, it should be noted that the terms “FM fixed layer” and “fixed layer” will be considered equivalent throughout this application, unless otherwise stated; similarly, the terms “FM free layer”, “ferromagnetic free layer,” “free layer that has a uniaxial anisotropy”, and “free layer” will also be considered equivalent throughout this application, unless otherwise stated.
Generally, the FM fixed layer in accordance with many embodiments of the invention may have a fixed magnetization direction, i.e. the direction of magnetization of the FM fixed layer does not typically change during the normal operation of the MEJ. Conversely, in certain embodiments, the FM free layer can adopt a magnetization direction that is either substantially parallel with or antiparallel with the FM fixed layer, i.e. during the normal operation of the MEJ, the direction of magnetization can be made to change. For example, the FM free layer may have a magnetic uniaxial anisotropy, whereby it has an “easy axis” that is substantially aligned with the direction of magnetization of the FM fixed layer. The “easy axis” refers to the axis, along which the magnetization direction of the layer prefers to align. In other words, an easy axis is an energetically favorable direction (axis) of spontaneous magnetization that is determined by various sources of magnetic anisotropy including, but not limited to, magnetocrystalline anisotropy, magnetoelastic anisotropy, geometric shape of the layer, etc. Relatedly, an “easy plane” is a plane whereby the direction of magnetization is favored to be within the plane, although there is no bias toward a particular axis within the plane.
Typically, VCMA phenomena can be relied on in switching the FM free layer's characteristic magnetization direction, i.e. the MEJ can be configured such that the application of a potential difference across the MEJ can reduce the coercivity of the free layer, which can allow the free layer's magnetization direction to be switched more easily. In other words, with a reduced coercivity, the FM free layer can be subject to magnetization that can make it substantially parallel with or substantially antiparallel with the direction of the magnetization for the FM fixed layer.
Source line sensing systems in accordance with embodiments of the invention use the VCMA effect to improve read disturbance by re-engineering the structure of the MeRAM and the control signals. Unlike traditional bit line sensing schemes, a sense amplifier and a current source are connected to the source line instead of the bit line. A plurality of MeRAM cells are attached to both the source line and the bit line. In certain embodiments, a pulse generator is connected to the bit line of the system. Selection of a MEJ within a MeRAM cell is accomplished by applying a voltage to the MeRAM word line during each operation mode. The source line sensing MeRAM system will utilize the sense amplifier to sense the potential difference in voltages from the sense line and the reference line to generate an amplified output representing either a parallel or antiparallel state of the MEJ in the MeRAM cell. Certain embodiments may utilize the pulse generator to provide a write pulse to the bit line to improve the sensing.
While MEJs demonstrate much promise in use as memory cells in source line sensing systems, their potential applications and variations continue to be explored. For example, U.S. Pat. No. 8,841,739 (the '739 patent) to Khalili Amiri et al. discloses DIOMEJ cells that utilize diodes (e.g. as opposed to transistors) as access devices to MEJs. As discussed in the '739 patent, using diodes as access devices for MEJs can confer a number of advantages and thereby make the implementation of MEJs much more practicable. The disclosure of the '739 patent is hereby incorporated by reference in its entirety, especially as it pertains to implementing diodes as access devices for MEJs. Furthermore, U.S. patent application Ser. No. 14/073,671 (“the '671 patent application”) to Khalili Amiri et al. discloses MEJ configurations that demonstrate improved writeability and readability, and further make the implementation of MEJs more practicable. The disclosure of the '671 patent application is hereby incorporated by reference in its entirety, especially as it pertains to MEJ configurations that demonstrate improved writeability and readability. A conceptual illustration of a source line sensing system is discussed in the following section.
Turning now to
A source line sensing system uses the VCMA effect to reversely improve read disturbance by engineering the structure of the MeRAM and the control signals. In several embodiments, selection of a MEJ within a MeRAM cell can be accomplished by applying a voltage to the MeRAM word line during each operation mode. In certain embodiments, the sense amplifier senses the potential difference in voltages from the sense line and the reference line to generate an output representing either a parallel or antiparallel state of the MEJ in the MeRAM cell.
In additional embodiments, the source line sensing system may also contain a pair of MOS transistors connected to the bit line and sense lines respectively. The bit line MOS transistor is typically labelled a Sense_G signal, while the MOS transistor attached to the sense line is labelled a Write_G signal. Additionally, a reference word line (RWL) transistor is typically attached to the sense amplifier and current source generator that allows for current to flow through a reference transistor (REF).
A source line sensing MeRAM system in accordance with several embodiments of the invention is disclosed in
Although specific conceptual embodiments are described above regarding source line sensing MeRAM systems with respect to
A graph depicting control signals for a source line sensing MeRAM system in the read mode is conceptually illustrated in
Generally, a source line sensing MeRAM system has two main modes: write and read. In many embodiments, to enable a write mode, the BL 240 may be disconnected to the ground by applying a ground to the Sense_G 210 while the potential of the source line discharges to the ground level by applying a voltage on the Write_G 220. Then, the pulse generator provides a write pulse to the BL 240.
Conversely, in numerous embodiments, a read mode in source line sensing MeRAM systems can be accomplished by having the bit line BL 240 grounded by applying a voltage on the Sense_G transistor 210 and then disconnecting the source line SL 250 to the ground by applying a ground to Write_G 220. Additionally, in certain embodiments, a voltage is also applied on the RWL 230 which may allow current to flow through the reference transistor REF 260. The current source of the MeRAM system supplies a current to the source line SL 250 and reference transistor REF 260, generating Vsen and Vref respectively. This potential difference is sensed by the sense amplifier which then generates a digital output. In a number of embodiments, the sense amplifier output can be a 0 for antiparallel states detected and 1 for parallel states detected.
Although specific conceptual embodiments are described above regarding control signals in source line sensing systems with respect to
Source line sensing MeRAM systems in accordance with embodiments of the invention utilize a series of MeRAM cells to store bits of data. The MeRAM cells contain a combination of MEJ cell and access transistor. The MEJ cells are discussed in more detail in the following sections and can be composed of many different embodiments. In many embodiments, the fixed layer side of the MEJ is connected to the bit line while the free layer is connected to the access transistor, which itself contains a word line transistor and connection to the source line.
A conceptual illustration of a MeRAM cell in accordance with embodiments of the invention is shown in
Although specific conceptual embodiments are described above regarding source line sensing MeRAM cells with respect to
Pluralities of MEJs can be implemented in any of a variety of configurations for use in MeRAM cells in accordance with embodiments of the invention. Source line sensing MeRAM systems typically utilize MEJs as the MeRAM memory storage element. These MEJs are often implemented as a plurality of MEJs in a contained system. In certain embodiments, the MEJs in contained systems may be implemented as a series of MeRAM cells in a MeRAM system. For example, the '671 patent application (incorporated by reference above) discloses MEJ configurations that include a second dielectric layer proximate the free layer and configured to enhance the VCMA effect. It should be clear that any suitable MEJ configuration may be incorporated in accordance with embodiments of the invention.
Note that while the subsequent discussions largely regard the operation of single MEJs, it should of course be understood that in many embodiments, a plurality of MEJs are implemented together. For example, the '671 patent application discloses MeRAM configurations that include a plurality of MEJs disposed in a cross-bar architecture. It should be clear that MEJ systems can include a plurality of MEJs in accordance with embodiments of the invention. In several embodiments where multiple MEJs are implemented, they can be separated by field insulation, and encapsulated by top and bottom layers. Thus, for example,
Although specific conceptual embodiments are described above regarding implementing a plurality of MEJs with respect to
Magnetoelectric junctions used in source line sensing MeRAM systems can be described conceptually as having a unique structure. As previously discussed, a typical MEJ contains a fixed layer with a magnetic direction that does not change, a free layer that has a magnetic direction that may change, and an insulating layer between the fixed and free layers.
The free layer may have a magnetic uniaxial anisotropy, whereby it has an “easy axis” that is substantially aligned with the direction of magnetization of the fixed layer. The “easy axis” refers to the axis, along which the magnetization direction of the layer prefers to align. In other words, an easy axis is an energetically favorable direction (axis) of spontaneous magnetization. In several embodiments, the free layer having its magnetic direction is parallel to the easy axis, the direction of the magnetization of the fixed layer can be considered to be ‘substantially aligned’, resulting in an information state that can have a single definition. Likewise, when the free layer has a magnetic direction that is antiparallel with the “easy axis”, a second information state can be derived. In a number of embodiments, these two information states can be determined by the difference in resistance of the MEJ in each state.
In many embodiments, the magnetization direction, and the related characteristics of magnetic anisotropy, can be established for the FM fixed and FM free layers using any suitable method. For instance, the shapes of the constituent FM fixed layer, FM free layer, and dielectric layer, can be selected based on desired magnetization direction orientations. For example, in certain embodiments, implementing FM fixed, FM free, and dielectric layers that have an elongated shape, e.g. have an elliptical cross-section, may tend to induce magnetic anisotropy that is in the direction of the length of the elongated axis—i.e. the FM fixed and FM free layers will possess a tendency to adopt a direction of magnetization along the length of the elongated axis. In other words, the direction of the magnetization is ‘in-plane’. Alternatively, in several embodiments of the invention, where it is desired that the magnetic anisotropy has a directional component that is perpendicular to the FM fixed and FM free layers (i.e., ‘out-of-plane’), the shape of the layers can be made to be symmetrical, e.g. circular, along with the FM layers being made thinner. In this case, while the tendency of the magnetization to remain in-plane may still exist, it may not have a preferred directionality within the plane of the layer. In other several embodiments, because the FM layers are relatively thinner, the anisotropic effects that result from interfaces between the FM layers and any adjacent layers, which tend to be out-of-plane, may tend to dominate the overall anisotropy of the FM layer. Alternatively, a material may be used for the FM fixed or free layers which have a bulk perpendicular anisotropy, i.e. an anisotropy originating from its bulk (volume) rather than from its interfaces with other adjacent layers. In yet many additional embodiments, the FM free or fixed layers may also consist of a number of sub-layers, with the interfacial anisotropy between individual sub-layers giving rise to an effective bulk anisotropy to the material as a whole. Additionally, in numerous embodiments, FM free or fixed layers may be constructed which combine these effects, and for example have both interfacial and bulk contributions to perpendicular anisotropy.
In many embodiments, by appropriately selecting adjacent materials, the MEJ can be configured such that the application of a potential difference across the FM fixed layer and the FM free layer can modify the magnetic anisotropy of the FM free layer. For example, whereas in
Although specific conceptual illustrations are described above for both in-plane and out-of-plane MEJ structures with reference to
In many embodiments, a MEJ includes additional adjunct layers that function to facilitate the operation of the MEJ. For example, in certain embodiments, the FM free layer includes a capping or seed layer, which can (1) help induce greater electron spin perpendicular to the surface of the layer, thereby increasing its perpendicular magnetic anisotropy, and/or (2) can further enhance the sensitivity to the application of an electrical potential difference.
Although specific conceptual embodiments are described above for adjunct layers on a MEJ with reference to
MEJ operating principles—as they are currently understood—are now discussed. Note that embodiments of the invention are not constrained to the particular realization of these phenomena. Rather, the presumed underlying physical phenomena are being presented to inform the reader as to how MEJs are believed to operate. MEJs generally function to achieve two distinct states using the principles of magnetoresistance. As mentioned above, magnetoresistance principles regard how the resistance of a thin film structure that includes alternating layers of ferromagnetic and non-magnetic layers depends upon whether the ferromagnetic layers are in a substantially parallel or antiparallel alignment. Thus, a MEJ can achieve a first state where its FM layers have magnetization directions that are substantially parallel, and a second state where its FM layers have magnetization directions that are substantially antiparallel.
MEJs further rely on voltage-controlled magnetic anisotropy (VCMA) phenomena. Generally, VCMA phenomena regard how the application of a voltage to a ferromagnetic material that is adjoined to an adjacent dielectric layer can impact the characteristics of the ferromagnetic material's magnetic anisotropy. For example, it has been demonstrated that the interface of oxides such as MgO with metallic ferromagnets such as Fe, CoFe, and CoFeB can exhibit a large perpendicular magnetic anisotropy which is furthermore sensitive to voltages applied across the dielectric layer. This effect has been attributed to spin-dependent charge screening, hybridization of atomic orbitals at the interface, and to the electric field induced modulation of the relative occupancy of atomic orbitals at the interface. In many embodiments, MEJs can exploit this phenomenon to achieve two distinct states. For example, MEJs can employ one of two mechanisms to do so.
First, in several embodiments of the invention, MEJs can be configured such that the application of a potential difference across the MEJ functions to reduce the coercivity of the FM free layer, such that it can be subject to magnetization in a desired magnetic direction. In certain embodiments, these directions may include being either substantially parallel with or antiparallel with the magnetization direction of the fixed layer. Second, in additional embodiments of the invention, MEJ operation can rely on precessional switching (or resonant switching), whereby by precisely subjecting the MEJ to voltage pulses of precise duration, the direction of magnetization of the FM free layer can be made to switch.
In a number of embodiments, MEJ operation is based on reducing the coercivity of the FM free layer such that it can adopt a desired magnetization direction. With a reduced coercivity, the FM free layer can adopt a magnetization direction in any suitable way. In multiple embodiments, the magnetization can result from an externally applied magnetic field, the magnetic field of the FM fixed layer, and/or the application of a spin-transfer torque (STT) current. In additional embodiments, the magnetization can further result from the magnetic field of a FM semi-fixed layer, the application of a current in an adjacent metal line inducing a spin-orbit torque (SOT), and/or any combination of these mechanisms. Indeed, such magnetization may occur from any suitable method of magnetizing the FM free layer with a reduced coercivity.
By way of example and not limitation, suitable ranges for the externally applied magnetic field are in the range of 0 to 100 Oe. However, in cases involving voltage induced precessional switching, to achieve a 1 nanosecond switching speed, the externally applied magnetic field should be approximately 200 Oe. The magnitude of the electric field applied across the device to reduce its coercivity or bring about resonant switching can be approximately in the range of 0.1-2.0 V/nm, with lower electric fields required for materials combinations that exhibit a larger VCMA effect. The magnitude of the STT current used to assist the switching may be in the range of approximately 0.1-1.0 MA/cm2.
It should of course be understood that the direction of the FM fixed layer's magnetization direction need not be in-plane—it can be in any suitable direction. For instance, in certain embodiments, the magnetization can be substantially out of plane. Additionally, in many embodiments, the FM free layer can include both in-plane and out-of-plane magnetic anisotropy directional components.
Although specific conceptual illustrations are described regarding MEJ operation with respect to
In a number of embodiments, MEJs can also include a semi-fixed layer that can have a magnetic anisotropy that is altered by the application of a potential difference. In many embodiments, the characteristic magnetic anisotropy of the semi-fixed layer is a function of the applied voltage. For example, the direction of the magnetization of the semi-fixed layer can be oriented in the plane of the layer in the absence of a potential difference across the MEJ. However, when a potential difference is applied in several embodiments of the invention, the magnetic anisotropy is altered such that the magnetization may include a strengthened out-of-plane component. Moreover, in several embodiments the magnetic anisotropy of the semi-fixed layer may be modified by an applied voltage. Furthermore, the amount of modification of the semi-fixed layer in the presence of the applied voltage may also be less than free layer magnetic anisotropy is modified as a function of the same applied voltage. In additional embodiments, the incorporation of a semi-fixed layer can facilitate a more nuanced operation of the MEJ (to be discussed below in the section entitled “MEJ Operating Principles”).
A particular configuration of a MEJ that includes a semi-fixed layer is depicted in
Although specific conceptual illustrations are described above for utilizing semi-fixed layers in a MEJ with reference to
Note of course that the application of an externally applied magnetic field is not the only way for the MEJ to take advantage of reduced coercivity upon application of a potential difference. In many embodiments, the magnetization of the FM fixed layer can be used to impose a magnetization direction on the free layer when the free layer has a reduced coercivity. Moreover, in several embodiments a MEJ can be configured to receive a spin-transfer torque (STT) current when application of a voltage causes a reduction in the coercivity of the FM free layer. Generally, certain embodiments include STT current as a spin-polarized current that can be used to facilitate the change of magnetization direction on a ferromagnetic layer. In a number of embodiments, this current can be passed directly through the MEJ device, such as due to leakage when a voltage is applied, or it can be created by other means. In several embodiments, these means can include spin-orbit-torques (e.g., Rashba or Spin-Hall Effects) or when a current is passed along a metal line placed adjacent to the FM free layer. Accordingly, a spin orbit torque current can then help cause the FM free layer to adopt a particular magnetization direction, where the direction of the spin orbit torque may determine the direction of magnetization. This configuration is advantageous over conventional STT-RAM configurations since the reduced coercivity of the FM free layer reduces the amount of current required to cause the FM free layer to adopt a particular magnetization direction, thereby making the device more energy efficient.
Additionally, in many embodiments, a MEJ cell can further take advantage of thermally assisted switching (TAS) principles. Generally, in numerous embodiments, in accordance with TAS principles, heating up the MEJ during a writing process may reduce the magnetic field required to induce switching. Thus, where STT is employed in accordance with several embodiments of the invention, even less current may be required to help impose a magnetization direction change on a free layer, particularly where VCMA principles have been utilized to reduce its coercivity.
Moreover, in numerous embodiments, the switching of MEJs to achieve two information states can also be achieved using voltage pulses. In particular, when voltage pulses are imposed on the MEJ for a time period that is one-half of the precession of the magnetization of the free layer, then the magnetization may invert its direction. Using this technique in certain embodiments of the invention, ultrafast switching times, e.g. below 1 ns, can be realized. Moreover, in additional embodiments using voltage pulses as opposed to a current makes this technique more energy efficient as compared to precessional switching induced by STT currents, as is often used in STT-RAM. However, this technique may be subject to the application of a precise pulse that is half the length of the precessional period of the magnetization layer. For instance, it has been observed that pulse durations in the range of 0.5 to 3 nanoseconds can reverse the magnetization direction. Additionally, the voltage pulse must be of suitable amplitude to cause the desired effect, e.g. reverse the direction of magnetization.
Based on this background, it can be seen that MEJs in accordance with embodiments of the invention can confer numerous advantages relative to conventional MTJs. For example, many embodiments can be controlled using voltages of a single polarity—indeed, the '739 patent, incorporated by reference above, discusses using diodes, in lieu of transistors, as access devices to the MEJ, and this configuration is enabled because MEJs can be controlled using voltage sources of a single polarity. In various embodiments, the charge current, spin current, and spin-polarization are all orthogonal to each other.
Although specific conceptual embodiments are described above regarding utilizing a metal line with MEJs with respect to
Although the present invention has been described in certain specific aspects, many additional modifications and variations would be apparent to those skilled in the art. It is therefore to be understood that the present invention may be practiced otherwise than specifically described, including various changes in the implementation, without departing from the scope and spirit of the present invention. Additionally, the figures and methods described herein can also be better understood through the attached documentation the disclosure of which is hereby incorporated by reference in its entirety. Thus, embodiments of the present invention should be considered in all respects as illustrative and not restrictive.
The current application claims priority to U.S. Provisional Application No. 62/355,705, filed Jun. 28, 2016, the disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62355705 | Jun 2016 | US |
