Field-free perpendicular magnetization switching by altermagnet with collinear spin current

TL;DR

This work addresses the challenge of achieving deterministic, field-free switching of perpendicular magnetization by generating collinear spin currents in altermagnets. Through symmetry analysis and first-principles transport calculations (DFT and Boltzmann equation), the authors show that CSC can be produced for specific current directions in metallic and insulating altermagnets (RuO$_2$, Mn$_5$Si$_3$, KRu$_4$O$_8$, CuF$_2$), with spin-splitting angles $\alpha$ reaching up to about $0.57$ and remaining robust against spin-orbit coupling. The results indicate a significantly higher CSC efficiency than the anomalous spin-Hall effect, providing a promising route toward low-power SST-MRAM devices with field-free perpendicular switching. The findings highlight the potential of altermagnetism to enable efficient spin-current manipulation in spintronic architectures.

Abstract

The generation of collinear spin current (CSC), where both the propagation direction and spin-polarized direction aligned perpendicularly to the applied charge current, is crucial for efficiently manipulating systems with perpendicular magnetic anisotropy used in high-density magnetic recording. However, the efficient generation of CSC remains a challenge. In this work, based on the symmetry analysis, we propose that CSC can be effectively generated using altermagnets when the charge current is aligned along specific directions, due to spin-dependent symmetry breaking. This proposal is supported by density functional theory (DFT) and Boltzmann transport equation (BTE) calculations on a series of altermagnetic materials, including RuO2, Mn5Si3, KRu4O8 and CuF2, where unusually large CSC is produced by the charge current along certain orientations. Furthermore, we introduce a physical quantity, the spin-splitting angle, to quantify the efficiency of CSC generated by the charge current. We find that the spin-splitting angle ranges from 0.24 to 0.57 in these altermagnets, which is significantly larger than the spin-Hall angle typically observed in the anomalous spin-Hall effect, where the spin-Hall angle is generally less than 0.1. Our findings provide an effective method for manipulating spin currents, which is advantageous for the exploration of altermagnetic spintronic devices with field-free perpendicular magnetization switching.

Figures (5)

• Figure 1: (a) Simplified Fermi surface of $[C_2||C_{4z}]$ altermagnet in the absence of electric field. The red and blue lines represent the Fermi surfaces of spin-up and spin-down electrons, respectively. The red and blue arrows illustrate the Fermi velocity of spin-up electrons and spin-down electrons, correspondingly. (b) Redistribution of electrons at the Fermi surface induced by the application of an electric field, indicated by variations in line thickness. The green arrows depict the associated spin currents. (c) Schematic diagram illustrating the generation of collinear spin current (CSC, $J_s$) under external electric field and CSC applied spin-splitting torque (SST) to the adjacent ferromagnetic layer.
• Figure 2: Atomic structures, spin density distributions, and permitted altermagnetic symmetric operations for (a) RuO$_2$, (b) KRu$_4$O$_8$, (c) Mn$_5$Si$_3$ and (d) CuF$_2$.
• Figure 3: Band structures along high-symmetry paths for (a) RuO$_2$, (b) KRu$_4$O$_8$, (c) Mn$_5$Si$_3$ and (d) CuF$_2$. The shape of the Brillouin zone and the coordinates of the high-symmetry points are provided in the section SIII of Supplemental Materials supplementary.
• Figure 4: Isoenergetic surface of RuO$_2$ ($E=E_F$) and CuF$_2$ ($E=-0.27$ eV). (a) Top view of spin-up electrons of RuO$_2$. (b) Top view of spin-down electrons of RuO$_2$. (c) Top view of spin-up electrons of CuF$_2$. (d) Top view of spin-down electrons of CuF$_2$. (e) Side view of spin-up electrons of CuF$_2$ and (e) side view of spin-down electrons of CuF$_2$. Isoenergetic surface of KRu$_4$O$_8$ and Mn$_5$Si$_3$ are presented in the Supplemental Materials supplementary.
• Figure 5: Calculated conductivity for RuO$_2$ (a-b), Mn$_5$Si$_3$ (c-d), KRu$_4$O$_8$ (e-f), CuF$_2$ (g-h). For RuO$_2$, Mn$_5$Si$_3$ and KRu$_4$O$_8$, the calculated $\sigma_{zx}^{\uparrow/\downarrow}$ and $\sigma_{zy}^{\uparrow/\downarrow}$ are both zero for spin-up electrons and spin-down electrons.