# Imaging uncompensated moments and exchange-biased emergent ferromagnetism in FeRh thin films

## TL;DR

This study uses magneto-thermal microscopy to directly image uncompensated magnetic moments in FeRh thin films, revealing their interaction with emergent ferromagnetism near the phase transition temperature.

## Contribution

It provides the first direct imaging of uncompensated moments in FeRh and elucidates their role in exchange bias and emergent ferromagnetism during the phase transition.

## Key findings

- Pinned uncompensated moments cause local exchange bias.
- Unpinned moments exhibit enhanced coercivity due to exchange coupling.
- Emergent ferromagnetism is exchange-coupled to antiferromagnetic order near T_C.

## Abstract

Uncompensated moments in antiferromagnets are responsible for exchange bias in antiferromagnet/ferromagnet heterostructures; however, they are difficult to directly detect because any signal they contribute is typically overwhelmed by the ferromagnetic layer. We use magneto-thermal microscopy to image uncompensated moments in thin films of FeRh, a room-temperature antiferromagnet that exhibits a 1st-order phase transition to a ferromagnetic state near 100~$^\circ$C. FeRh provides the unique opportunity to study both uncompensated moments in the antiferromagnetic phase and the interaction of uncompensated moments with emergent ferromagnetism within a relatively broad (10-15~$^\circ$C) temperature range near $T_C$. In the AF phase below $T_C$, we image both pinned UMs, which cause local vertical exchange bias, and unpinned UMs, which exhibit an enhanced coercive field that reflects exchange-coupling to the AF bulk. Near $T_C$, where AF and FM order coexist, we find that the emergent FM order is exchange-coupled to the bulk N\'eel order. This exchange coupling leads to the nucleation of unusual configurations in which different FM domains are pinned parallel, antiparallel, and perpendicular to the applied magnetic field before suddenly collapsing into a state uniformly parallel to the field.