Near-degenerate competing magnetic orders in EuAgAs: a tunable route to altermagnetism

TL;DR

EuAgAs exhibits near-degenerate magnetic states, including antiferromagnetic and altermagnetic phases, which can be tuned via pressure, making it a promising material for spintronics and topological applications.

Contribution

This study reveals the near-degeneracy of magnetic orders in EuAgAs and demonstrates its tunability to realize altermagnetism through pressure control.

Findings

Neutron diffraction shows AFM ground state with in-plane spin sequence.

DFT calculations find FM and AM states close in energy to AFM.

Pressure induces a transition to altermagnetic phase around 14 GPa.

Abstract

Altermagnets (AMs) have recently emerged as a distinct magnetic class bridging central features of ferromagnets (FMs) and antiferromagnets (AFMs), offering new opportunities for spin-based electronics. While they possess zero net magnetization like collinear AFMs, they simultaneously exhibit momentum-dependent spin splitting long thought exclusive to FMs. Despite intense theoretical interest, experimentally accessible materials hosting both altermagnetism and nontrivial band topology remain scarce. EuAgAs, crystallizing in space group $P6_3/mmc$, was previously identified via density functional theory (DFT) as a bulk altermagnetic Dirac semimetal. Contrary to these predictions, our neutron diffraction experiments reveal that the bulk ground state adopts a $\mathbf{q} = (0,0,\tfrac{1}{2})$ AFM structure with an in-plane $\uparrow\uparrow\downarrow\downarrow$ spin sequence. Systematic DFT calculations, however, uncover a remarkable near-degeneracy among competing magnetic orders: the FM and AM configurations lie only $0.11$ and $0.40~\text{meV/f.u.}$ above the AFM ground state, respectively. We further show that while a simple Heisenberg model favors a spin-spiral ground state, the inclusion of non-Heisenberg biquadratic coupling stabilizes the observed commensurate AFM phase. This near-degeneracy renders the magnetic state highly tunable, with DFT predicting a transition to the altermagnetic phase under hydrostatic pressure at approximately $14 \text{ GPa}$, establishing EuAgAs as a controllable platform for accessing topological altermagnetism.