Unidirectional Magnetoresistance in Antiferromagnet/Heavy-Metal Bilayers

Open Access

Unidirectional Magnetoresistance in Antiferromagnet/Heavy-Metal Bilayers

Soho Shim, M. Mehraeen, Joseph Sklenar, Junseok Oh, Jonathan Gibbons, Hilal Saglam, Axel Hoffmann, Steven S.-L. Zhang, and Nadya Mason

Phys. Rev. X 12, 021069 – Published 30 June 2022

Abstract

The interplay between electronic transport and antiferromagnetic order has attracted a surge of interest as recent studies show that a moderate change in the spin orientation of a collinear antiferromagnet may have a significant effect on the electronic band structure. Among numerous electrical probes to read out such a 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 a UMR in the antiferromagnetic phase of a $FeRh / Pt$ bilayer, which undergoes a sign change and then increases strongly with an increasing external magnetic field, in contrast to UMRs in ferromagnetic and nonmagnetic systems. We show that Rashba spin-orbit coupling alone cannot explain the sizable UMR in the antiferromagnetic bilayer and that field-induced spin canting distorts the Fermi contours to greatly enhance the UMR by 2 orders of magnitude. Our results can motivate the growing field of antiferromagnetic spintronics and suggest a route to the development of tunable antiferromagnet-based spintronics devices.

Received 25 August 2021
Revised 29 April 2022
Accepted 26 May 2022

DOI:https://doi.org/10.1103/PhysRevX.12.021069

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Antiferromagnetism Magnetoresistance Spin accumulation Spin-orbit coupling

Antiferromagnets Magnetic multilayers

Tight-binding model Transport techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Soho Shim ^1,2, M. Mehraeen ³, Joseph Sklenar^4,1,2, Junseok Oh ^1,2, Jonathan Gibbons ^1,2,5,6, Hilal Saglam^5,7, Axel Hoffmann ^8,5,2,1, Steven S.-L. Zhang ³, and Nadya Mason ^1,2,*

¹Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
²Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
³Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, USA
⁴Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201, USA
⁵Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, USA
⁶Department of Physics, University of California San Diego, La Jolla, California 92093, USA
⁷Department of Applied Physics, Yale University, New Haven, Connecticut 06511, USA
⁸Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA

^*nadya@illinois.edu

Popular Summary

Antiferromagnets possess many appealing properties for future spintronics, which exploits electron spin for the storage and transfer of information. In such devices, information is encoded in the direction of the microscopic magnetic moments, so it is crucial to be able to monitor the antiferromagnetic order. In ferromagnetic and topological insulator-based systems, the direction of magnetization can be measured using unidirectional magnetoresistance (UMR), a nonlinear resistance that changes upon reversal of current. But the analogous effect in antiferromagnets has not yet been observed. We have now discovered a sizable UMR in a type of antiferromagnet.

The UMR is an additional resistance on top of the usual Ohmic resistance. Upon application of a magnetic field, one can say there is a UMR contribution when the total resistance value is different when the current flips direction. In the antiferromagnetic phase of a heavy metal bilayer ( $FeRh / Pt$ ), we observe a UMR that not only evolves strongly with an increasing magnetic field but also, more surprisingly, undergoes a sign change (the UMR changes direction) with respect to the field. This is largely different from the UMR seen in other ferromagnetic and nonmagnetic systems. We show that the mechanism responsible for the UMR in those systems cannot explain the UMR sign change in the antiferromagnetic bilayer. Instead, a strong effective magnetic field leads to the profoundly different antiferromagnetic UMR that we measure.

The observed UMR is exquisitely sensitive to the amount of modulation in the microscopic magnetic moments in the antiferromagnets. Since the UMR is enabled just by depositing a platinum layer on top of the FeRh layer, this effect represents a versatile approach to inferring antiferromagnetic spin textures through all-electrical means. Our work will help inspire the design of next-generation antiferromagnetic spintronics devices.

Key Image

Article Text

Click to Expand

Supplemental Material

Click to Expand

References

Click to Expand

Issue

Vol. 12, Iss. 2 — April - June 2022

Subject Areas

Reuse & Permissions

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
UMR effect in canted antiferromagnet and sample layout. (a),(b) Rashba-effect-induced nonlinear charge current $j_{e}^{(2)} (θ_{c})$ at the antiferromagnet/heavy-metal interface for $J ∥ \hat{x}$ $(J ∥ - \hat{x})$ at large field $B$ . The green and blue arrows in (a) and (b) indicate the direction of the spin polarizations. Resistance is low (high) when $j_{e}^{(2)} (θ_{c})$ is aligned (antialigned) with $J$ . (c) Optical microscope image of a $FeRh / Pt$ microwire device. (d) Schematic of the longitudinal resistance measurements. A large dc current of the order of $10^{7} A / {cm}^{2}$ is applied to induce a UMR, while a small ac current is applied to perform the lock-in amplifier measurement. The angle $φ$ is defined as the clockwise angle from the field $B$ to current $I$ , where $I = I \hat{x}$ .
Reuse & Permissions
Figure 2
Magnetoresistance characterization and UMR in the ferromagnetic phase of $FeRh / Pt$ . (a) Temperature sweep of the longitudinal resistance under an external in-plane field of $B = 4 T$ . Clear transition between the ferromagnetic (FM) phase and the antiferromagnetic (AFM) phase is observed. (b) Angular sweep at $T = 10 K$ (AFM phase) and $B = 4 T$ , under $J = 1.43 \times 10^{7} A / {cm}^{2}$ . $Δ R$ is defined as $Δ R = R - R_{av}$ . A magnetoresistance which is odd under the current polarity $R_{odd}$ manifests under high current density. (c) $R_{odd, \max}$ in the FM phase of FeRh $[110] / Pt$ as a function of $J$ at various field magnitudes. $R_{odd, \max}$ is normalized with the base resistance $R_{0}$ (measured at $J = 0$ and $B = 0$ ). Dashed lines show the fitting curves $α J + β J^{3}$ . (d) $R_{odd, \max}$ in the FM phase of FeRh $[110] / Pt$ as a function of $B$ at various current densities $J$ . Dashed lines show the fitting curves $α B^{- p} + β$ .
Reuse & Permissions
Figure 3
UMR in the antiferromagnetic phase of $FeRh / Pt$ . (a) UMR in the antiferromagnetic phase of FeRh $[110] / Pt$ as a function of $J$ for various field magnitudes. (b) UMR in the antiferromagnetic phase of FeRh $[110] / Pt$ as a function of field $B$ at various current densities $J$ . (c) Extracted linear UMR with respect to the field $B$ , with the purple dashed line indicating the observed field value at which the UMR undergoes a sign change. The observed sign-change field value is $7.19 \pm 0.17 T$ (see Supplemental Material Sec. S8 [52]). (d) Calculated UMR in the antiferromagnetic phase as a function of $B$ for different values of the applied current, with the purple dashed line indicating the theoretically predicted field value at which the UMR undergoes a sign change. Parameters used: $a = 3 Å$ , $τ = 10^{- 14} s$ , $g = 2$ , $ε_{0} = 10 eV$ , $ε_{F} = 0.617 ε_{0}$ , $t = 0.1 ε_{0}$ [55], $Δ_{ex} = 0.05 ε_{0}$ [55, 56], ${\tilde{α}}_{R} = α_{R} / a = 0.05 ε_{0}$ [55, 57], $H_{J} = 11.83 T$ (corresponding to $θ_{c} = 25 °$ at $B = 10 T$ ), and width of spectral function $= 0.002 ε_{0}$ .
Reuse & Permissions
Figure 4
Enhancement of UMR by spin canting. (a) Spin texture of the conduction bands in the absence of a magnetic field. Because of spin-momentum locking arising from the Rashba effect, when an electric field is applied, a pure spin current but no nonlinear charge current is produced, as depicted in the inset. (b) Spin-dependent distortion of the bands in the presence of a magnetic field at both zero canting (dashed curves) and finite canting (solid curves). The two bands are shown here in red and blue. (c) Generation of a nonlinear charge current transverse to the applied $B$ field (top left), which increases considerably in the presence of canting (bottom right). (d) Plots of the calculated UMR at finite and vanishing canting as functions of the magnetic field $B$ . In the presence of canting, the UMR is stronger by 2 orders of magnitude.
Reuse & Permissions
Figure 5
Mechanism of UMR sign change. Structure of the conduction bands at $k_{y} = 0$ and (a) 0 T, (b) 3 T, and (c) 9 T, indicating the breaking of time-reversal symmetry induced by the magnetic field. The two bands are shown here in red and blue, with the dashed line indicating the Fermi level. (d) Opposite contributions of the inner (red dots) and outer (blue triangles) conduction bands to the UMR, with the purple dashed line indicating the field at which the sign change in the overall UMR occurs.
Reuse & Permissions

Physical Review X