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

The interplay between electronic transport and the antiferromagnetic order has attracted a surge of interest. Recent studies have shown that a moderate change in the spin orientation of a collinear antiferromagnet may significantly affect the electronic band structure and subsequently manifest itself in transport properties [1]. Among numerous electrical probes to read out such magnetic order, unidirectional magnetoresistance (UMR), where the resistance changes under the reversal of the current direction, can provide rich insights into the transport properties of spin-orbit coupled systems. However, UMR has never been observed in antiferromagnets before, given the absence of intrinsic spin-dependent scattering. Here, we report that a sizable UMR can emerge in an antiferromagnetic system, specifically in a collinear-antiferromagnet/heavy-metal bilayer. The observed UMR in the antiferromagnetic phase of a FeRh/Pt bilayer evolves nonlinearly with increasing magnetic field, persists to large values at high magnetic field and, most notably, exhibits a sign change at a critical field, which is in stark contrast with the UMRs observed in ferromagnetic and non-magnetic systems [2-3]. We attribute this UMR to the strong effective magnetic field due to spin canting in FeRh. Our results can motivate the growing fields of non-centrosymmetric and topological systems, and suggest a route to the development of tunable antiferromagnet-based spintronics devices. This research was primarily supported by the NSF through the University of Illinois at Urbana-Champaign Materials Research Science and Engineering Center DMR-1720633 and was carried out in part in the Materials Research Laboratory Central Research Facilities, University of Illinois. Thin-film growth at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Materials Science and Engineering Division.

![](https://assets.underline.io/uploads/markdown_image/1/image/756eaf87bcd78eaae92bc5c59ee591d2.jpg)

Fig. 1: UMR in the antiferromagnetic phase of FeRh [110]/Pt (T = 10 K), as a function of field B at various current densities J.

![](https://assets.underline.io/uploads/markdown_image/1/image/71626e5a0aa513b3c7c5af64e05003bc.jpg)

Fig 2: Generation of a nonlinear charge current transverse to the applied B field (top left), which increases considerably in the presence of canting (bottom right).

Observation of unidirectional magnetoresistance in collinear-antiferromagnet/heavy-metal bilayers

**Session chair:** Mingzhong Wu, Colorado State University


GOJ - Metal Spintronics

technical paper

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

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Soho Shim

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Large Exotic Spin Torques in Antiferromagnetic Iron Rhodium

Observation of unidirectional magnetoresistance in collinear-antiferromagnet/heavy-metal bilayers

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