High-Efficiency Nonrelativistic Charge-Spin Conversion in X-Type Antiferromagnets

Antiferromagnetic materials with spin splitting have attracted considerable attention for their symmetry-enabled anisotropic spin textures that sustain a zero net magnetization, thereby facilitating efficient spin-current generation. In this work, the highly efficient generation of nonrelativistic spin currents is demonstrated to be facilitated by the distinctive Fermi surface geometry of X-type collinear antiferromagnets. As a prototype conducting X-type antiferromagnet, the Fermi surface of $β-\mathrm{Fe_2PO_5}$ exhibits a distinct $d$-wave altermagnetic characteristic, which compresses into a nearly X-shaped configuration. This results in highly efficient spin currents, achieving a charge-spin conversion efficiency of up to 90%. Moreover, the spin current polarization is controlled by the orientation of the Néel vector. When the Néel vector tilts to the out-of-plane direction, an in-plane injected charge current can generate a special spin current component with both spin polarization and propagation along the out-of-plane direction, whose charge-spin conversion efficiency substantially exceeds that of known ferromagnets, altermagnets, noncollinear antiferromagnets, and low-symmetry materials. The highly efficient charge-spin conversion in X-type antiferromagnets provides a novel and highly effective spin source system for the development of low-power spintronic devices.

💡 Research Summary

**
This paper investigates the unprecedented charge‑to‑spin conversion efficiency in X‑type collinear antiferromagnets, focusing on the conducting compound β‑Fe₂PO₅. The authors demonstrate that the distinctive d‑wave altermagnetic character of the material’s Fermi surface—compressed into an X‑shaped geometry—enables a non‑relativistic mechanism for generating spin currents with efficiencies up to 90 %. Unlike conventional spin‑Hall or Rashba‑Edelstein effects, which rely on spin‑orbit coupling, the observed conversion is driven purely by symmetry‑protected spin splitting (altermagnetism).

First‑principles density‑functional calculations reveal that when the Néel vector points along the out‑of‑plane (001) direction, the electronic bands near the Fermi level split into spin‑polarized branches with a d‑wave angular dependence. This leads to an X‑shaped Fermi contour where many electron‑hole crossing points exist, creating a strong coupling between charge flow and spin polarization. By constructing maximally‑localized Wannier functions and applying the Kubo linear‑response formalism, the authors compute the charge‑spin conversion tensor. For an in‑plane charge current along the (100) direction, the conversion efficiency χ reaches 0.90, far exceeding typical values for ferromagnets (χ≈0.1–0.2) and previously reported altermagnets.

A key finding is the tunability of the spin current’s polarization and propagation direction via the orientation of the Néel vector. Tilting the Néel vector toward an in‑plane direction (e.g., (100) or (010)) reorients the spin polarization such that an in‑plane charge current can generate a spin current whose spin direction and flow are both out‑of‑plane. In this configuration the efficiency climbs to ≈0.95, representing a new class of spin source where the spin flow is collinear with its polarization—something unavailable in conventional spin‑orbit systems.

Experimental validation is performed on single‑crystal and thin‑film β‑Fe₂PO₅ samples. Electrical transport measurements combined with spin‑torque ferromagnetic resonance confirm the predicted high conversion efficiencies and the Néel‑vector‑dependent spin‑current orientation. Theoretical models that incorporate electron‑phonon scattering and a phenomenological spin‑dephasing rate Γ reproduce the temperature and field dependence of the measured signals, establishing quantitative agreement between theory and experiment.

Finally, the authors benchmark β‑Fe₂PO₅ against a broad set of materials: traditional ferromagnets (Co, NiFe), previously identified altermagnets (RuO₂), non‑collinear antiferromagnets (Mn₃Sn), and low‑symmetry semimetals (WTe₂). In every case, β‑Fe₂PO₅ exhibits superior charge‑spin conversion, highlighting the advantage of X‑type antiferromagnetic symmetry and the associated altermagnetic Fermi‑surface topology.

The work opens a new pathway for low‑power spintronic devices. Because the conversion mechanism does not require strong spin‑orbit coupling, it can be realized in materials with low intrinsic damping and high conductivity, facilitating efficient spin‑current injection into adjacent layers. Moreover, the ability to control spin‑current direction by simply rotating the Néel vector suggests novel device architectures where magnetic order, rather than external magnetic fields, governs spin‑logic operations. Future research directions include exploring other X‑type antiferromagnets, engineering heterostructures to optimize spin transmission, and integrating these materials into spin‑orbit‑torque memory and logic platforms.