United States

The processing of artificial neural networks (ANNs) is limited by the memory wall in von Neumann architectures, driving the need for hardware ANNs realized by neuromorphic devices [1]. Previously proposed multi-weight synaptic devices often suffer from a nonlinear and asymmetric update response, limiting their viability for supervised learning [2]. Domain wall-magnetic tunnel junction (DW-MTJ) devices driven by spin transfer torque (STT) and spin orbit torque (SOT) have shown promise in neuromorphic applications, realizing leaky integrate-and-fire neurons in simulation [3-8]. Simulations also show that DW-MTJs can implement synaptic spike-timing dependent plasticity [9]. A type of DW-MTJ with multiple MTJs was demonstrated experimentally [10], and we have recent results on experimental DW-MTJ synapses that are submitted separately for consideration in the conference.&lt;br/&gt;Most prior work on DW-MTJ synapses in ANNs do not consider thermal effects and process variation. We evaluate notched DW-MTJ designs shown in Fig. 1a and account for these effects using MuMax3 for device simulation and CrossSim for network simulation [11-13]. Figure 1b shows that excellent linearity, high symmetry, and low thermal noise are maintained at 300 K. High training accuracy was obtained on the Fashion-MNIST task, shown in Fig. 2 [14]. Notably, the greater stochasticity of SOT-driven synapses counters the discretizing effect of notches, boosting accuracy to near ideal. We also implemented short term potentiation for network pruning using shape anisotropy. These results propose a magnetic synapse with tunable plasticity, superior backpropagation performance, and fast updates, a foundational step toward ANNs with fully spintronic matrix operations.&lt;br/&gt;&lt;i&gt;SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525&lt;/i&gt;

![](https://assets.underline.io/uploads/markdown_image/1/image/a44eed1961abbd5be85c2b76d154f44f.jpg)

a) Device diagram b) Conductance G vs dG statistics of a DW-MTJ at 300 K for STT and SOT propagation. Heatmaps are generated from micromagnetic simulations. The CDF represents the probability that a given conductance update is less than the average dG.

![](https://assets.underline.io/uploads/markdown_image/1/image/8b8eb9cbd24d31e6dd1da9e5c0cf1410.jpg)

Validation accuracy of a two-layer perceptron of STT and SOT DW-MTJ synapses at 300 K assuming (a) continuous levels and (b) 32 notches and periodic carry.

2022 Joint MMM-INTERMAG

Multifunctional Design of Domain Wall-Magnetic Tunnel Junction Artificial Synapses for Accurate Training of Neural Networks

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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MMM 2020

The processing of artificial neural networks (ANNs) is limited by the memory wall in von Neumann architectures, driving the need for hardware ANNs realized by neuromorphic devices [1]. Previously proposed multi-weight synaptic devices often suffer from a nonlinear and asymmetric update response, limiting their viability for supervised learning [2]. Domain wall-magnetic tunnel junction (DW-MTJ) devices driven by spin transfer torque (STT) and spin orbit torque (SOT) have shown promise in neuromorphic applications, realizing leaky integrate-and-fire neurons in simulation [3-8]. Simulations also show that DW-MTJs can implement synaptic spike-timing dependent plasticity [9]. A type of DW-MTJ with multiple MTJs was demonstrated experimentally [10], and we have recent results on experimental DW-MTJ synapses that are submitted separately for consideration in the conference. Most prior work on DW-MTJ synapses in ANNs do not consider thermal effects and process variation. We evaluate notched DW-MTJ designs shown in Fig. 1a and account for these effects using MuMax3 for device simulation and CrossSim for network simulation [11-13]. Figure 1b shows that excellent linearity, high symmetry, and low thermal noise are maintained at 300 K. High training accuracy was obtained on the Fashion-MNIST task, shown in Fig. 2 [14]. Notably, the greater stochasticity of SOT-driven synapses counters the discretizing effect of notches, boosting accuracy to near ideal. We also implemented short term potentiation for network pruning using shape anisotropy. These results propose a magnetic synapse with tunable plasticity, superior backpropagation performance, and fast updates, a foundational step toward ANNs with fully spintronic matrix operations. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525

![](https://assets.underline.io/uploads/markdown_image/1/image/a44eed1961abbd5be85c2b76d154f44f.jpg)

a) Device diagram b) Conductance G vs dG statistics of a DW-MTJ at 300 K for STT and SOT propagation. Heatmaps are generated from micromagnetic simulations. The CDF represents the probability that a given conductance update is less than the average dG.

![](https://assets.underline.io/uploads/markdown_image/1/image/8b8eb9cbd24d31e6dd1da9e5c0cf1410.jpg)

Validation accuracy of a two-layer perceptron of STT and SOT DW-MTJ synapses at 300 K assuming (a) continuous levels and (b) 32 notches and periodic carry.

technical paper

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

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Please register for the Joint MMM-Intermag conference to join the event.

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

Recent discovery of “unexpected” spin-orbit torques (SOTs) within magnetic single layers has attracted attention. However, it has remained elusive as to how to understand and how to tune the SOTs. In this talk, we will first show a strong bulk dampinglike SOT and its characteristics in composition-uniform ferromagnetic CoPt that is polycrystalline and chemically disordered. We find that the bulk dampinglike SOT arises from the interplay of a strong bulk spin Hall effect of CoPt (an imbalanced internal spin current that is transversely polarized and independent of the magnetization orientation) with a “hidden” inversion symmetry-breaking (not a gradient in composition, magnetism, electron scattering, or grain shapes). In the second part, utilizing the single layers of chemically disordered FexPt1-x, we unveil the mechanism of the bulk SOTs by studying their dependence on the introduction of a controlled vertical composition gradient and on temperature. We find that the associated “hidden” inversion symmetry-breaking that allows for a non-zero net torque is most likely a strain non-uniformity that can arise from a composition gradient and/or the substrate. The findings point to the possibility of developing low-power single-layer SOT devices by strain engineering.

Bulk spin-orbit torques in centrosymmetric magnetic single layers

The spin Hall effect, and its inverse, are important spin-charge conversion mechanisms widely investigated due to their fundamental importance in the development of spintronics devices. Its measurement has been mostly related to electrical detection schemes involving an interface with another magnetic material and thus, a combination of the properties of both materials as well as the interface are measured. The large scattering of reported results from these methods for the same material and temperature call out for a more direct and interface-free approach. Optical detection schemes have been successfully used for the determination of SHE in semiconductor. However, this approach has proven challenging for metallic systems, due to their considerably shorter spin diffusion lengths. Only recently, optical measurements for Pt and W have been reported [1]. Considering that magnetic circular dichroism (XMCD) has become a reference tool for precision measurements of small or diluted magnetic signals, We propose the use of XMCD-PEEM microscopy for direct, interface-free determination of SHE in metals. In particular, we report the observation of spin separation due to SHE in a single layer of Bi-doped Cu (Cu95Bi5), a material in which giant SHE has been already reported [2,3]. We have performed interface free x-ray spectroscopy measurements at the Cu L3,2 absorption edges while applying electrical current to the sample. The sign of spin accumulation depends on the direction of the current and the amplitude of the X-ray magnetic circular dichroism (XMCD) signal scales with the density current and have different sign when measuring in the L2 or L3 absorption edges, as expected for SHE. We have measured an induced magnetic moment of (2.7 ± 0.5) x 10-12μB A-1 cm2 per Cu atom averaged over the probing depth, which is of the same order as for Pt measured by magneto-optics. Our results constitute the proof of concept for the direct, interface free and element-selective measurement of the SHE in a single material by means of X-ray spectromicroscopy and highlight the potential of CuBi for spin-charge conversion applications [4].

![](https://assets.underline.io/uploads/markdown_image/1/image/0f5bce09fb325fa1a3fcfbf281b8cd98.jpg)

(a) Visualization of the spin accumulation in a Cu95Bi5 electrode. (b) XMCD signal.

Direct X-ray detection of the spin Hall effect in CuBi

Precise control of magnetic dynamics can enable many novel computing applications, including magnetic random access memory and spin torque oscillator-based neuromorphic computing. Spin torques stand out as an excellent tool for efficiently manipulating magnetic states, and can be easily produced via spin-orbit effects in a spin source layer. Unfortunately, underlying crystal symmetries in the spin source material often restrict the geometry of the spin torques, making them poorly suited to practical applications. Magnetic ordering can break the symmetries of a spin source material and allows the generation of versatile spin torques with controllable geometries tailored to applications. Antiferromagnetic materials in particular have robust magnetic ordering that makes them ideal as spin sources. We present spin torque ferromagnetic resonance measurements (Fig. 1) and second harmonic Hall measurements characterizing the geometry and efficiency of spin torques generated in antiferromagnetic iron rhodium alloy. We find highly temperature-dependent spin torques with spin torque efficiencies above 90% at room temperature and above 300% at 170K, much higher than most competing materials. Further, these torques do not show the angular dependence expected for ordinary spin Hall torques. Instead, it appears that the torque derives from a spin current with spin polarization parallel to the FeRh magnetic order, which makes these torques useful for exciting dynamics in a variety of samples. This makes iron rhodium a compelling option for the development of spin torque technology. This work was supported as part of Quantum Materials for Energy Efficient Neuromorphic Computing, an Energy Frontier Research Center funded by the U.S. DOE, Office of Science.

![](https://assets.underline.io/uploads/markdown_image/1/image/2acaeb111f16139961cbc9f870795133.jpg)

Fig. 1 DC Biased Spin Torque Ferromagnetic Resonance. (a) Diagram of the measurement setup. (b) Characteristic resonance scans. (c) Linewidth over a range of DC currents. Increasing current increases the spin current, modifying the effective damping linearly. Shown at 300K, 275K, 260K, and 170K. (d) Spin torque efficiency is huge and grows by a factor of 3.5 between 300K and 170K.

Large Exotic Spin Torques in Antiferromagnetic Iron Rhodium

While classical electronics focuses on the control of the charge current, spintronics aims at using a spin current, e.g. a flow of angular momentum for logical and memory applications. One of the main use of such a spin current is magnetization switching used for writing in memory devices. Indeed, when a spin current enters into a ferromagnet, its angular momentum (or polarization) is transmitted to the magnetization, and possibly changes its orientation. The transfer of angular momentum from the spin current to the magnetization can be done in two ways depending on the relative orientation between the polarization and the magnetization direction. If they are collinear, the polarization is absorbed by the ferromagnet through diffusive processes originating from the spin switchings during scattering on defects. However, when the magnetization and polarization are transverse, ballistic processes originating from the conduction electron’s spin rotation around the exchange field of the ferromagnet drive the polarization absorption. Understanding the mechanisms of the spin current relaxation into a ferromagnet is therefore of crucial interest for spintronics applications, and remain a seldom explored fundamental question. One of the main difficulty in studying spin current relaxation in a ferromagnetic material is the fact that a charge current generates parasitic effects through the anisotropic magnetoresistance and planar Hall effect. We consequently used lateral spin valves, allowing the measurement of the spin relaxation in a ferromagnet with different relative magnetization direction using a pure spin current, e.g. a flow of angular momentum in the absence of charge current. This measurement allowed us to observe the spin-current relaxation anisotropy in a ferromagnet and to extract the spin mixing conductance, an elusive but fundamental parameter of the spin-dependent transport [1]. Finally, these results allows us to move toward the study of magnetization dependent Spin Hall Effect in ferromagnetic materials.

Spin absorption anisotropy in lateral spin valves

Materials with helical crystal structures have been suggested to be a condensed matter analogue of solenoids, where a current flow produces inductive magnetic fields [1]. At the microscopic scale, a finite spin accumulation can be generated in strong spin-orbit coupling and low symmetry systems via the Edelstein effect [2,3]. Recently, chiral non-magnetic materials have received a lot of attention due to their low symmetries and complex radial spin textures, that could be potentially exploited for spintronic device applications [4,5,6]. However, a straightforward magnetotransport study on the relationship between chirality, spin texture and current-induced spin polarization is still missing. Here, we link one-to-one the direction of the spin polarization, induced by a chirality-dependent Edelstein effect, to the handedness of chiral tellurium (Te) nanowires (NWs), through simple angle-dependent magnetotransport experiments. Transmission electron microscopy (TEM) analysis and theoretical calculations corroborate the transport data. Te NWs show a large, chirality-dependent non-reciprocal charge transport when the electrical current and the magnetic field applied are collinear to the chiral axis of the crystal. By applying a gate voltage, we can also manipulate the magnetotransport response, acting on the carrier density and modifying the spin texture at the Fermi surface. The efficiency of the charge-to-spin interconversion process is one order of magnitude larger than what has been reported so far. Our results are a step forward towards the development of chiral spintronics.

![](https://assets.underline.io/uploads/markdown_image/1/image/cb46e659c3ceca2ac8d3e113ec8dbb71.jpg)

Figure 1: Sketch of the chiral structure of right- and left-handed Te NWs where the current-induced spin polarization depends on the crystal handedness

![](https://assets.underline.io/uploads/markdown_image/1/image/6de41c2cd5dcdb4672e53a8c0641cd6e.jpg)

Figure 2: On the leftt, sketch representing the Edelstein effect mechanism. The shift in kz of the Fermi contours lead to the formation of spin accumulations oriented in opposite directions according to the chirality-dependent radial spin texture of Te. On the right, current-dependent angular dependences of the magnetoresistance Rdiff= [R(Iz) - R(-Iz)]/2 measured at 9 T and 10 K for two Te NWs with opposites handedness. The signals show specular features between left- and right-handed NWs.




Chirality-dependent Edelstein effect in elemental Tellurium nanowires

Electrically conductive metals are typically good transmitters or absorbers of pure spin currents. Here, we report evidence that an interface of two conductive elemental metals – namely, Cu and Cr – constitutes a good spin-current reflector. Our study was initially motivated by the question of whether the elemental antiferromagnet Cr acts as a spin transmitter (akin to electrically insulating NiO [1]) or a spin absorber (akin to metallic IrMn [2]). To this end, we have performed ferromagnetic resonance (FMR) spin pumping measurements on spin-valve-like stacks of NiFe(10)/Cu(5)/Cr(tCr)/(Co)Fe(4), where the FMR-driven NiFe layer pumps an AC pure spin current. Any spin current transmitted through the Cu/Cr spacer would be absorbed by the (Co)Fe sink, generating additional Gilbert damping, △α, for the NiFe source. Thus, △α is proportional to how much a spin current is transmitted or absorbed across the Cr interlayer. In Figure 1, the sharp decline of △α with increasing Cr thickness indicates a remarkable trend: The introduction of Cr suppresses spin transmission and absorption – i.e., the spin current is largely reflected by the Cu/Cr spacer. However, our complementary element-selective X-ray FMR measurements confirm that the spin current is transmitted through the Cr spacer in NiFe/Cr/CoFe. We therefore identify the Cu/Cr interface, rather than the Cr bulk, as the source of the strong spin reflection. Figure 2 shows that △α varies only weakly with decreasing temperature (increasing antiferromagnetic order in Cr), which suggests that antiferromagnetism in Cr is not responsible for the spin-reflective behavior. Our finding points to the possibility that other electrically conductive metals can form effective spin reflectors, thereby offering new avenues in the fundamental studies and engineering of pure spin currents across interfaces.


![](https://assets.underline.io/uploads/markdown_image/1/image/e7884bfa3e9da4b8fbfbd64553b2d860.jpg)


[Fig. 1] △α vs. Cr layer thickness tCr in NiFe/Cu/Cr(tCr)/(Co)Fe measured at room temperature. Both the epitaxial and polycrystalline series show decaying with tCr, due to spin reflection at the Cu/Cr interface.

![](https://assets.underline.io/uploads/markdown_image/1/image/dc5e03a64fe9cbcdaa9a866583d3cb89.jpg)


[Fig. 2] △α vs. temperature, showing that spin transport is affected only weakly even as Cr develops stronger antiferromagnetic order at lower temperatures.

Spin-Reflective Metallic Cu/Cr Interface

Asymmetric charge transport at the interface of two materials with dissimilar electrical properties, such as metal/semiconductor and p-n junctions, is the fundamental aspect behind modern diode and transistor technology. Therefore, it is interesting to study pure-spin-current transport at the interface of two materials with different spin transport properties. Here we demonstrate asymmetric transport of pure-spin-currents across an interface of dissimilar non-magnetic materials, Au/Pt. In this work we present a study of spin transport by spin-pumping from either Py (Ni80Fe20) or Co in Py/Au/Pt/Co and Py/Au/Pt structures. The thickness of Pt was varied from 0 to 3 nm and the magnetic damping was measured by ferromagnetic resonance. The magnetic damping due to spin-pumping of Py in Py/Au/Pt follows the characteristic thickness dependence of ferromagnet/normal-metal. The typical ferromagnet/normal-metal/spin-sink thickness dependence is reproduced by spin-pumping from Py in the Py/Au/Pt/Co structure. Here the Co layer acts as a spin-sink. However, pumping from Co while Py is the spin-sink in the Py/Au/Pt/Co structure is very uncharacteristic of a spin-sink structure. Instead, this data suggests that very little spin-current makes it to the Py layer, i.e. a reflection of spin-current at the Au/Pt interface. The data could not be self consistently fit with the continuity of spin-accumulation and continuity of spin-current boundary conditions at the Au/Pt interface [1,2]. Including spin-memory-loss also failed to fit the data [3]. We proposed a new boundary condition that assumes that the reflected and transmitted spin-currents at the Au/Pt interface are proportional to the established spin chemical potentials on either side of the interface. This model can self consistently fit all the measured data. It suggests that spin-pumping from Py results in a ~30% reflection and ~70 transmissions of the spin-current across the Au/Pt interface. However, spin pumping from Co results in ~70% reflection and 30% transmission of spin-current at the Au/Pt interface.

Nonreciprocal transport of pure spin current at the Au/Pt interface

Spin-related magnetoresistance (MR) from versatile condensed matter systems can be employed as the main mechanism to detect the orientation of magnetization in next generation magnetoresistive random-access memories (MRAM). Recently, novel MR effects such as spin Hall MR (SMR) [1,2] and spin-dependent unidirectional MR (SD-UMR) [3,4] are extensively studied due to the correlation between MR and spin-orbit interactions in both classical and emergent materials systems. In our work, we disentangle the underlying mechanisms of the UMR in W/CoFeB magnetic heterostructures by systematic current-dependent and field-dependent MR measurements at room temperature. As shown in Fig.1, the current-dependent UMR of W/CoFeB heterostructures can be roughly divided into three regimes. In the first and second regimes, UMR increases linearly and then nonlinearly to the applied current density J as J reaches ~5×1010 A/m2, which originates from the SD-UMR [Fig.2(a)] and spin-flip UMR (SF-UMR) [Fig.2 (b)] [4]. In the third regime, as J reaches ~ 1011 A/m2, the UMR ratio will further saturate at 0.36%, which is larger than the typical UMR magnitudes reported previously in metallic system. With the aid of numerical simulations, this large UMR can be attributed to the extra contribution from SMR, which stems from the tilting of magnetization M caused by the competition between the spin Hall effect induced spin torques (STs) from W layer and the external field Hy. This tilting behavior is illustratively shown in Fig.2 (c). Consequently, we define this UMR as the spin-transfer torque UMR (STT-UMR). The damping like-ST (DL-ST) efficiency of W/CoFeB heterostructures can be also roughly estimated by using both the experimental UMR data and the simulation results. Our finding therefore verifies the correlation between the STs and this large room-temperature UMR, thereby providing an alternative approach to determine DL-ST efficiency.

![](https://assets.underline.io/uploads/markdown_image/1/image/40f566960248469705c79873e22caf42.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/7ec5345f8d8c1cc8ce22f8a88abc6e0c.jpg)

Observation of Large Spin Torques-Related Unidirectional Magnetoresistance in Metallic Heterostructures

Observation of Large Spin Torques-Related Unidirectional Magnetoresistance in Metallic Heterostructures

Topological magnetic semimetals, like Co3Sn2S2 and Co2MnGa, display exotic transport properties, such as large intrinsic anomalous (AHE) due to uncompensated Berry curvature. The highly symmetric XPt3 compounds display anti-crossing gapped nodal lines, a driving mechanism in the intrinsic Berry curvature Hall effects. Uniquely, these compounds contain two sets of gapped nodal lines that harmoniously dominate the Berry curvature in this complex multi-band system. We calculate a maximum AHE of 1965 S/cm in the CrPt3 by a state-of-the-art first principle electronic structure. We have grown high-quality thin films by magnetron sputtering and measured a robust AHE of 1750 S/cm for different sputtering growth conditions. Additionally, the cubic films display a hard magnetic axis along [111] direction. The facile and scalable fabrication of these materials makes them ideal candidates for integration into topological devices [1].

![](https://assets.underline.io/uploads/markdown_image/1/image/4135773548df436a10a909902db4a303.jpg)

Hard magnet topological semimetals in XPt3 compounds with the harmony of Berry curvature

Despite having published no papers related to magnetism prior to 1996, In 1996 Prof. Buhrman initiated a new project in his group to explore phenomena relevant to proposed MRAM technologies. Over the next 25 years, his group would make several seminal contributions to this field, and in this talk, I will present a retrospective on the impact Bob Buhrman’s group had on MRAM during this timeframe. Twenty-five years ago magnetism played a key role in HDD and tape storage technologies, but magnetism was not exploited in memories that could be integrated with planar semiconductor processing. Today, embedded MRAM is offered at all of the leading foundries, and standalone MRAM memory chips are available at capacities up to 1 Gbit. STT-MRAM is the basis of these of these memories. Many research groups have played key roles in the commercialization of this technology, but Prof. Buhrman’s group was responsible for significant advances including the first demonstration of STT switching in lithographically patterned bits [1] and early demonstrations of STT switching in magnetic tunnel junctions [2]. There is considerable effort today on using STT-MRAM as a replacement for embedded SRAM, but it is difficult for STT-MRAM to meet the endurance and speed requirements for these applications. Three-terminal SOT-MRAM technology is a potential alternative to STT-MRAM for such applications. Here again, Prof. Buhrman’s group was at the forefront of this research [3], and has subsequently made many important contributions to the understanding of SOT physics.

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