Magneto-thermal imaging of exchange bias in Pt/FeRh bilayers

We use a recently-developed technique, magneto-thermal microscopy, to image exchange bias at the interface between coexisting antiferromagnetic (AF) and residual ferromagnetic (FM) order below the Neél temperature in 10 nm Pt/20 nm FeRh /MgO(001) bilayers. The 1^st-order AF-FM phase transition in FeRh near 100 °C may provide a simple way to deterministically switch the AF order by field-cooling from the FM phase. However, weak residual FM order below T_N complicates the process. We image this FM order with the anomalous Nernst effect, using 3 ps laser pulses focused to 700 nm diameter to generate local thermal gradients. Imaging as a function of field, we observe spatially-dependent magnetization-like (vertical) shifts in hysteresis loops at 25 °C, indicating exchange bias at a disordered AF/ FM interface. The exchange bias and the coercivity decrease at higher temperature until we observe an abrupt increase in signal at 100 °C from exchange-biased emergent FM order in the transition region. By indirectly imaging the AF order through exchange bias, our results provide new insight into the spatial structure of AF order in FeRh.