Positive Temperature Coefficient of Anisotropy due to $d^6L$ Groundstate in $ε$-Fe$_2$O$_3$

RESUMO

Positive anisotropy temperature coefficients ($dK/dT>0$) usually arise from hybridization between magnetic 3d states and strongly spin-orbit-coupled (SOC) subsystems. Yet $\epsilon$-Fe$_2$O$_3$ shows $dK/dT>0$ (125-200~K) without an obvious SOC partner. We study pure and Cr-doped (with weakened Fe-O hybridization) $\epsilon$-Fe$_2$O$_3$ across the transition from low temperature incommensurate to high-anisotropy magnetic phases. We identify a $d^6L$ groundstate in $\epsilon$-Fe$_2$O$_3$ while reduced hybridization yields $d^6 + d^6L^2$ in the Cr-doped system, highlighting metal-ligand hybridization as a route to tune magnetic, electronic and orbitronic properties.