Deterministic Switching in Altermagnets via Asymmetric Sublattice Spin Current

TL;DR

The paper addresses deterministic, field-free switching of altermagnetic order in even-parity altermagnets by exploiting asymmetric sublattice spin currents (ASSC) that arise from direction-dependent, nonrelativistic spin splitting. It develops a generic mechanism applicable to centrosymmetric films and weak spin–orbit coupling, demonstrated through a realistic FeSb$_2$ model and Landau–Lifshitz–Gilbert dynamics, predicting 180$^ ext{o}$ Néel-vector reversal on ~40 ps timescales. The approach relies on unequal sublattice torques generated by spin currents injected from a heavy metal, with switching controllable by crystal orientation and current polarity, and detectable via a sign change in the anomalous Hall effect. These results broaden the material platforms and device concepts for ultrafast, low-power spintronic memories based on altermagnetism, beyond relativistic spin–orbit torque paradigms.

Abstract

We demonstrate a deterministic switching mechanism in collinear altermagnets driven by asymmetric sublattice spin currents. Unlike conventional antiferromagnets, where combined parity-time-reversal symmetry enforces purely staggered sublattice spin torques, altermagnets host symmetry-protected nonrelativistic spin splitting that produces unequal torques on the two sublattices. Using doped FeSb$_2$ as a representative $d$-wave altermagnet, our Landau--Lifshitz--Gilbert simulations show that these torques enable magnetic-field-free and deterministic 180$^\circ$ Néel vector reversal over picosecond timescale. The mechanism is generic to even-parity altermagnets and remains effective even in centrosymmetric, weak spin-orbit coupled systems, where the Néel spin-orbit torque mechanism fails. Our results establish an experimentally accessible mechanism for switching of altermagnetic order, opening pathways for realizing ultrafast, low-power altermagnet spintronic devices.

Figures (5)

• Figure 1: Asymmetric sublattice spin current driven Néel switching in altermagnets. (a) Direction-dependent spin conduction in an altermagnet. Conduction occurs when the injected spin polarization matches the spin character of the anisotropic Fermi surface. (b) Proposed altermagnet/heavy-metal (AM/HM) junction device, where the asymmetric sublattice spin current enables deterministic switching of the Néel vector.
• Figure 2: Origin of asymmetric sublattice spin current (ASSC) in doped $d$-wave altermagnet FeSb2. (a) Crystal structure and Brillouin zone. (b) Band structure of doped FeSb2 along $Z$-$\Gamma$-$\rm S$-$\rm R$-$\rm T$-$\rm Y$-$\rm S$-$\rm X$-$\rm U$-$\rm R$-$\rm Z$, showing nonrelativistic spin splitting along the $\Gamma$-$\rm S$ and $\rm Z$-$\rm R$ directions. (c) Spin-projected Fermi surfaces at $\mu=0.16~\rm eV$ in the $k_z=0$ plane, revealing the $d$-wave symmetry of the spin splitting. (d) Sublattice-resolved longitudinal spin conductivities ($\hat{z}$-polarized) along $[110]$ and $[1\Bar{1}0]$ versus chemical potential. The unequal spin response on the $A/B$ sublattice along specific directions is a direct signature of ASSC.
• Figure 3: Switching mechanism in an altermagnet/heavy-metal heterostructure device with a $d$-wave altermagnet. (a)--(d) Torque profiles and effective field components governing the switching of the Néel vector by asymmetric sublattice spin current. Red and blue arrows indicate both the magnitude and polarization of spin currents acting on sublattices $A$ and $B$, respectively. (e) In the initial configuration (C0), the spin current polarization in the HM matches the anisotropic Fermi surface spin polarization of the AM along the current direction. This matching enables spin current transmission into the AM, generating ASSC-driven torques that switch the Néel vector ${\cal N} = ({\bm m}_A - {\bm m}_B)/2$ from C0 to C1. After switching, the Fermi surface polarization reverses, blocking further spin current transmission and stabilizing the high-resistance state. We verify this explicitly in Fig. \ref{['fig_4']}. (f) The reverse transition (C1 $\rightarrow$ C0) is triggered by an oppositely polarized spin current, demonstrating deterministic control of the Néel vector via bias polarity reversal across the HM.
• Figure 4: Ultrafast Néel vector switching in the doped FeSb$_2$ device. (a) Time evolution of the Néel vector components, showing full reversal of the $\hat{z}$ component within $40$ ps. (b) 3D trajectory of the sublattice magnetizations during the switching process. Together, (a) and (b) confirm robust and deterministic Néel vector reversal driven by ASSC-induced nonrelativistic torques. (c) Switching time of the Néel vector ($T^{\rm switch}$) as a function of torque efficiency ratio ($\xi^{\rm DL}/\xi^{\rm FL}$) for different asymmetric sublattice spin current ratios ($J^s_B/J^s_A$). (d) Persistent oscillation of the in-plane components of the Néel vector for a torque efficiency ratio of $\xi^{\rm DL}/\xi^{\rm FL}=0.5$ and $J^s_B/J^s_A=0.25$.
• Figure 5: Néel vector polarity dependent anomalous Hall effect. Anomalous Hall conductivity of doped FeSb$_2$ as a function of chemical potential $\mu$ for the Néel vector oriented parallel (red curve) and antiparallel (green curve) to the [001] axis. The anomalous Hall response reverses sign across a broad range of $\mu$, providing a robust electrical signature of Néel vector reversal in the altermagnet. The vertical line marks the $\mu$ value, used for calculations in Fig. \ref{['fig_3']}.