Temperature and field hysteresis of the antiferromagnetic-to-ferromagnetic phase transition in epitaxial FeRh films

TLDR

FeRh films exhibit a broad, defect‑induced antiferromagnetic‑to‑ferromagnetic transition that is further broadened by out‑of‑plane magnetic fields due to demagnetization effects. The study investigates the temperature and field hysteresis of the first‑order antiferromagnetic‑to‑ferromagnetic transition in FeRh films grown on c‑axis sapphire and MgO (001). A mean‑field Ising model is used to qualitatively describe the field‑dependent shift in transition temperature. FeRh on sapphire shows abrupt antiferromagnetic nucleation during cooling, remanent magnetization measurements reveal irreversible field‑induced changes upon heating but reversible changes upon cooling, and the model predicts a -10 K/T shift in transition temperature, matching the experimentally observed -8 to -9 K/T shifts for MgO and sapphire substrates.

Abstract

The temperature and field hysteresis of the magnetization in the first order antiferromagnetic to ferromagnetic phase transition in FeRh films grown onto $c$-axis sapphire and MgO (001) are investigated. The transition to the ferromagnetic state upon heating and antiferromagnetic state upon cooling is generally broad indicating a heterogeneous transition due to defects so that antiferromagnetic and ferromagnetic domains coexist during the transition. However, the nucleation of antiferromagnetic domains upon cooling is abrupt for FeRh on $c$-axis sapphire which is indicative of homogeneous nucleation and growth of antiferromagnetic domains. The transition is further broadened when measuring with fields applied out of plane of the sample due to internal demagnetization fields. Temperature dependent remanent magnetization measurements reveal that field induced magnetization changes are irreversible during heating, but reversible during cooling. The field dependence of the shift in transition temperature is qualitatively modeled with an Ising spin type model utilizing a mean field approach. From this calculation a shift of $\ensuremath{-}10\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{T}$ in transition temperature is determined in good agreement with the experimentally observed shift of $\ensuremath{-}8$ and $\ensuremath{-}9\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{T}$ for FeRh films grown onto MgO (001) and c-axis sapphire, respectively.