Temperature-dependent Eu spin reorientations in the tetragonal A-type antiferromagnet EuGa$_4$ induced by small ab-plane magnetic fields

The body-centered-tetragonal antiferromagnet EuGa$_4$ exhibits A-type antiferromagnetic order below its N\'eel temperature $T_{\rm N} = 16.4$ K in magnetic field $H = 0$ where the moments are ferromagnetically aligned in the $ab$-plane with the Eu moments in adjacent Eu planes aligned antiferromagnetically. Previous magnetization versus field $M_{ab}(H)$ measurements revealed that the moments exhibit a spin-reorientation transition at a critical field $H_{c1}$ where the Eu moments become perpendicular to an in-plane magnetic field while still remaining in the $ab$ plane. A theory for $T=0$ K was previously presented that successfully explained the observed low-field moment-reorientation behavior at $T = 2$ K. Here we present a theory explaining the observed $T$ dependence of $M_{ab}(H,T<T_{\rm N})$ in the [1,0,0] direction for $H\leq H_{\rm c1}(T)$ from 2 to 14 K arising from a $T$-dependent anisotropy energy.
Comment: 4 pages, 4 captioned figures, 16 references; Equation (12) corrected