Magnetic-field-controlled growth of magnetoelastic phase domains in FeRh

Magnetic phase transition materials are relevant building blocks for developing green technologies such as magnetocaloric devices for solid-state refrigeration. Their integration into applications requires a good understanding and controllability of their properties at the micro- and nanoscale. Here, we present an optical microscopy study of the phase domains in FeRh across its antiferromagnetic-ferromagnetic phase transition. By tracking the phase-dependent optical reflectivity, we establish that phase domains have typical sizes of a few microns for relatively thick epitaxial films (200 nm), thus enabling visualization of domain nucleation, growth, and percolation processes in great detail. Phase domain growth preferentially occurs along the principal crystallographic axes of FeRh, which is a consequence of the elastic adaptation to both the substrate-induced stress and laterally heterogeneous strain distributions arising from the different unit cell volumes of the two coexisting phases. Furthermore, we demonstrate a magnetic-field-controlled directional growth of phase domains during both heating and cooling, which is predominantly linked to the local effect of magnetic dipolar fields created by the alignment of magnetic moments in the emerging (disappearing) FM phase fraction during heating (cooling). These findings highlight the importance of the magnetoelastic character of phase domains for enabling the local control of micro- and nanoscale phase separation patterns using magnetic fields or elastic stresses.

Automated summary

In this study, we used optical microscopy to investigate the phase domains in FeRh during its antiferromagnetic-ferromagnetic phase transition. The size of the phase domains was found to be several microns, allowing detailed observation of the nucleation, growth, and percolation processes. The growth of phase domains preferentially occurred along the principal crystallographic axes of FeRh, which was a result of the elastic adaptation to substrate-induced stress and laterally heterogeneous strain distributions due to the different unit cell volumes of the two coexisting phases. Additionally, we demonstrated a magnetic-field-controlled directional growth of phase domains during both heating and cooling, which was primarily influenced by the local effect of magnetic dipolar fields created by the alignment of magnetic moments in the emerging (disappearing) ferromagnetic phase fraction during heating (cooling).