High-resolution diffraction reveals magnetoelastic coupling and coherent phase separation in tetragonal CuMnAs

Abstract: Tetragonal CuMnAs was the first antiferromagnet where reorientation of the N\'eel vector was reported to occur by an inverse spin galvanic effect. A complicating factor in the formation of phase-pure tetragonal CuMnAs is the formation of an orthorhombic phase with nearly the same stoichiometry. Pure-phase tetragonal CuMnAs has been reported to require an excess of Cu to maintain a single phase in traditional solid state synthesis reactions. Here we show that subtle differences in diffraction patterns signal pervasive inhomogeneity and phase separation, even in Cu-rich Cu${1.18}$Mn${0.82}$As. From calorimetry and magnetometry measurements, we identify two transitions corresponding to the N\'eel temperature (T$N$) and an antiferromagnet to weak ferromagnet transition in Cu${1.18}$Mn${0.82}$As and CuMn${0.964}$As$_{1.036}$. These transitions have clear crystallographic signatures, directly observable in the lattice parameters upon in-situ heating and cooling. The immiscibility and phase separation could arise from a spinoidal decomposition that occurs at high temperatures, and the presence of a ferromagnetic transition near room temperature warrants further investigation of its effect on the electrical switching behavior.