Tuning spin currents in collinear antiferromagnets and altermagnets

TL;DR

This work addresses the challenge of generating nonrelativistic spin currents in collinear antiferromagnets and higher-order altermagnets, which are symmetry-forbidden in their pristine states. It introduces a spin-group symmetry framework showing that magnetoelectric, piezomagnetic, and piezomagnetoelectric-like couplings, realized via electric fields and strain, can lower symmetry and induce finite spin conductivity, $\sigma^{z}_{ij}$. Using density functional theory and Boltzmann transport, the authors demonstrate sizable spin-to-charge conversion in representative materials across AFM and altermagnetic classes, including $d$-wave AMs like KVSe$_2$O and RuF$_4$, AFMs like Cr$_2$O$_3$, and planar $g$-/i$-$wave AMs such as FeS$_2$ and MnPSe$_3$, with conversion efficiencies reaching up to nearly 100% in some cases. The results establish a practical, symmetry-guided route to engineer spin currents in a broad class of magnetic materials, enabling efficient, nonrelativistic spin-current generation for spintronic applications.

Abstract

Spin current generation through non-relativistic spin splittings, found in uncompensated magnets and d-wave altermagnets, is desirable for low-power spintronics. Such spin currents, however, are symmetry forbidden in conventional collinear antiferromagnets and higher-order altermagnets. Using spin point group analysis, we demonstrate that finite spin currents can be induced in these materials via magnetoelectric, piezomagnetic, and piezomagnetoelectric-like couplings. We utilize electric fields, strain, and their combinations to drive symmetry-lowering phase transitions into uncompensated magnetic or d-wave altermagnetic states, thereby enabling finite spin conductivity in a broader class of magnetic materials. We further substantiate this framework using density functional theory and Boltzmann transport calculations on representative magnetic materials - KV2Se2O, RuF4 , Cr2O3 , FeS2 , and MnPSe3 - spanning these different cases. The charge-to-spin conversion ratio reaches up to almost 100% via uncompensated magnetism and about 40% via d-wave altermagnetism under realistic conditions, highlighting the effectiveness of this approach for efficient spin current generation.

Figures (6)

• Figure 1: Schematic of electric field and strain-induced spin conductivity. (a) Spin-polarized Fermi surface of an antiferromagnet with $[C_2||P]$ symmetry. The red and blue isosurfaces correspond to the opposite spin directions. Fermi surfaces of an antiferromagnet under an applied electric field when: (b) the effective spin group of the material possesses a symmetry connecting opposite spin sublattices (e.g., $d$-wave altermagnet with $[C_2||M_y]$), and (c) no such symmetry exists (ferromagnetic or compensated ferrimagnetic). (d) Fermi surface of a higher-order planar $g$-wave state. Fermi surfaces of a higher-order $g$-wave altermagnet (d) without strain and under a strain field $\eta_{ij}$ when: (e) the effective spin group retains a symmetry connecting opposite spin sublattices (e.g., $d$-altermagnet with $[C_2||M_{xy}]$), and (f) no such symmetry remains. In the plots, Fermi surfaces are obtained from the minimal two-band models derived in Ref. roig2024minimal, with strain effects incorporated using $\textit{k}_i \rightarrow (\delta_{ij}+\eta_{ij})\textit{k}_j$.
• Figure 2: (a) The crystal and magnetic structures of KVSe$_2$O. (b) The Fermi surface of KVSe$_2$O projected on the $k_z=0$ plane, calculated without spin-orbit coupling. The red and blue curves denote spin-up and spin-down Fermi surfaces, respectively. (c) The charge and spin conductivity as a function of the scattering rate ($\Gamma$). (d) The spin conductivity and corresponding charge-to-spin conversion ratio as a function of the Fermi energy.
• Figure 3: (a) The crystal and magnetic structures of RuF$_4$. (b) The Fermi surface of RuF$_4$ projected on the $k_z=0$ plane, calculated without spin-orbit coupling. (c) The spin conductivity and (d) corresponding charge-to-spin conversion ratio as a function of the Fermi energy.
• Figure 4: (a) The crystal and magnetic structures of Cr$_2$O$_3$. The blue and red spheres are used for Cr and O atoms, respectively. (b) Spin-polarized band structure of [0001] film of Cr$_2$O$_3$ with and without the electric field, $\mathcal{E}_z$. (c) The spin conductivity and (d) corresponding charge-to-spin conversion ratio as a function of the Fermi energy with and without $\mathcal{E}_z$.
• Figure 5: (a) The crystal and magnetic structures of an FeS$_2$ monolayer (a) without and (b) with strain field, $\eta_{xy}$. The magnetic structures in (a) and (b) are shown by superimposing the $\odot$ and $\otimes$ symbols on the Fe atoms, representing spins that point in $+z$ (i.e out of the plane of paper) and $-z$ (into the plane of paper) directions, respectively. Also, in order to make the strain-induced distortions visible to the naked eye, we have exaggerated the strain to 20% in (b). The constant energy contours for (c) unstrained and (d) strained FeS$_2$ for E$_F-1.45$ eV with maximum charge-to-spin conversion. The (e) spin conductivity and (f) charge-to-spin conversion ratio as a function of energy.