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

Uncompensated moments (UMs) 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 magnetothermal microscopy to image micron-scale uncompensated moments in thin films of FeRh, a room-temperature antiferromagnet that exhibits a first-order phase transition to a ferromagnetic (FM) state near 400 K. FeRh provides the unique opportunity to study both uncompensated moments in the antiferromagnetic (AF) phase and the interaction of uncompensated moments with emergent ferromagnetism within a relatively broad (390-420 K) 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 Neel order. This exchange coupling leads to the nucleation of unusual configurations in which FM domains are pinned in different in-plane orientations, even in the presence of a nominally saturating magnetic field, before suddenly collapsing into a state uniformly parallel to the field.