Switchable Giant Spin Injection Current in Janus Altermagnet Fe$_2$SSeO

Generating and controlling spin current in miniaturized magnetic quantum devices remains a central objective of spintronics, due to its potential to enable future energy-efficient information technologies. Among the existing magnetic phases, altermagnetism have recently emerged as a highly promising platform for spin current generation and control, going beyond ferromagnetism and antiferromagnetism. Here, we propose a symmetry-allowed spin photovoltaic effect in two-dimensional (2D) altermagnetic semiconductors that enables predictable control of giant spin injection currents. Distinct from parity-time ($\mathcal{PT}$)-antiferromagnets, Janus altermagnetic semiconductors generate not only shift current but also a unique injection current with spin momentum locked in a specific direction under linearly polarized light – a mechanism absent in $\mathcal{PT}$-antiferromagnets. Through symmetry analysis and first-principles calculations, we identify Janus Fe$_2$SSeO as a promising candidate. Specifically, the monolayer Fe$_2$SSeO exhibits a polarization-dependent injection conductivity reaching $\sim$1,200~$μ$A/V$^{2}!\cdot!\hbar/2e$, and the giant spin injection current can be effectively switched by rotating the magnetization direction and engineering strains. These findings underscore the potential of 2D altermagnets in spin photovoltaics and open avenues for innovative quantum devices.

💡 Research Summary

The manuscript introduces a novel spin photovoltaic effect (SPVE) that enables the generation of large, controllable spin injection currents in two‑dimensional (2D) Janus altermagnets, with Fe₂SSeO monolayer identified as a prototypical material. Unlike parity‑time (𝒫𝒯) symmetric antiferromagnets, which can only produce a spin shift current under linearly polarized light, altermagnets break both spatial inversion (𝒫) and time‑reversal (𝒯) symmetries. Consequently, both spin shift and spin injection currents are symmetry‑allowed. The authors first perform a systematic symmetry analysis of all 125 magnetic layer point groups, isolating 30 groups that can support SPVE. They then focus on the Janus compound Fe₂SSeO, whose crystal structure (space group P4mm) lacks out‑of‑plane mirror symmetry while retaining in‑plane C₄ rotation, creating a natural platform for altermagnetism.

First‑principles density‑functional theory (DFT) calculations, including spin‑orbit coupling, reveal that Fe₂SSeO is an indirect‑gap semiconductor (≈0.53 eV) with pronounced spin splitting, a hallmark of altermagnetism that lifts Kramers degeneracy. Magnetic anisotropy calculations show an in‑plane easy axis along