Spin Hall magnetoresistance at the altermagnetic insulator/Pt interface

TL;DR

This paper addresses SMR at the Pt/insulating altermagnet interface Ba2CoGe2O7 (BCGO), reporting a notably large and crystal-direction–dependent SMR signal. Using Hall-bar devices with 15 nm Pt on single-crystal BCGO(001), the authors observe a canonical in-plane SMR modulation with a ratio around $2\times10^{-4}$ and demonstrate a systematic current-direction dependence, with one orientation yielding roughly half the SMR of another. Through temperature and spacer-control measurements, they rule out proximity-induced Pt magnetism and connect the effect to the magnetic order of BCGO, arguing that anisotropy arises from spin-current channels whose behavior is governed by the altermagnetic symmetry; a comprehensive theoretical treatment of the individual channel anisotropies and their frequency dependence is proposed, including THz SMR experiments. Overall, the work establishes altermagnetic insulators as robust platforms for highly anisotropic SMR and points toward tunable, electrically controllable spintronic responses in such materials.

Abstract

The resistance of a heavy metal can be modulated by an adjacent magnetic material through the combined effects of the spin Hall effect, inverse spin Hall effect, and dissipation of the spin accumulation at the interface. This phenomenon is known as the spin Hall magnetoresistance. The dissipation of the spin accumulation can occur via various mechanisms, with spin-transfer torque being the most extensively studied. In this work, we report the observation of spin Hall magnetoresistance at the interface between platinum and an insulating altermagnetic candidate, Ba$_2$CoGe$_2$O$_7$. Our findings reveal that this heterostructure exhibits a relatively large spin Hall magnetoresistance signal, which is anisotropic with respect to the crystal orientation of the current channel. We explore and rule out several potential explanations for this anisotropy and propose that our results may be understood in the context of the anisotropies of the spin current channels across the Pt/altermagnetic Ba$_2$CoGe$_2$O$_7$ interface.

Figures (5)

• Figure 1: (Color Online) a) Spin Hall magnetoresistance in HM/MI heterostructures, where charge current ($\mathbf{j_c}$) generates a spin current (1) that can be either reflected ($\mathbf{j^{ref}_s}$) or absorbed ($\mathbf{j^{abs}_s}$) at the interface depending on the available spin current channels across the interface (2,3). When there are no available channels the spin current is reflected and converted back to charge current (2) and when the channels are open the spin current is absorbed by the MI (3). This modulates the resistivity of the HM. b) Three channels contributing to the SMR ratio in magnetic insulators in the dc-limit: i. STT and spin pumping, ii. excitation of longitudinal magnons, and iii. magnon capacitance. Channel i. is relevant when $\pmb{\mu}_s\perp\mathbf{m}$ and channels ii. and iii. are relevant when $\pmb{\mu}_s\parallel\mathbf{m}$. c) The crystal structure of BCGO. The spins of the magnetic sublattices are marked in red and blue and the spin canting is exaggerated for the sake of clarity.
• Figure 2: (Color Online) a) An atomic force microscopy image of the surface of the BCGO after the chemical-mechanical polishing. The inset shows a height profile along two 1 $\mu$m-long lines, marked in red and blue. b-d) Structural characterization of the BCGO single crystal at 300 K: b) the $\{201\}$ pole figure, c) a radial scan (the intensity is plotted on logarithmic scale), and d) a rocking curve around (002). Note that the shape of the rocking curve is not a consequence of detector saturation.
• Figure 3: (Color Online) a) Temperature-dependence of the magnetization along the external magnetic field measured under a 0.5 T field along different crystal axes. b) Field-dependence of the magnetization measured at 2 K with the field along different crystal axes. The inset is a close-up of the plot marked with the dashed rectangle (in the range 0.2 to -0.2 T and 0.05 to -0.05 $\mu_B$/f.u). See a) for the legend. c) The change in magnetization plotted as $\frac{M-\langle M\rangle}{\langle M\rangle}$, where $\langle M\rangle$ is the average magnetization, as a function of the in-plane field orientation at 2 K. d) The magnetic field-dependence of the polarization along the [001]-direction at 2.5 K when the field is applied along the [110]-direction.
• Figure 4: (Color Online) a) The SMR signal in BCGO/Pt heterostructure at 2 K for $\mathbf{j_c}\parallel[100]$. A 1.9 T field is rotated in three perpendicular planes (ip, oopj, oopt). The top left inset shows the definitions of the field rotation planes and the top right inset is an optical image of the Hall bar devices. b) The temperature dependence of the SMR for ip-rotation of 1.9 T field. The positive SMR vanishes above the critical temperature of 6.7 K of BCGO.
• Figure 5: (Color Online) a) The full ip angular scans of the SMR for different orientations of the current channel. The definitions for angles $\varphi$ and $\alpha$ are shown in the inset. The measurements were conducted at 2 K and under 1.9 T field. Note that the data for $\alpha =$ 0 and 180 deg is identical. b) The dependence of the SMR ratio (closed symbols) and the longitudinal resistivity (open symbols) at $\varphi=0$ deg on the current channel orientation $\alpha$. The orange closed symbols represent the data measured for the independently fabricated and measured reference sample at 4 K and 1 T.