Observation of the evolution of magnetic domain structures related to magnetic polaron cluster formation in a single crystal of EuB₆

EuB₆ is a ferromagnetic semi-metal which shows colossal magnetoresistance at a temperature of T_c1~15.5K and also exhibits nanoscale phase separation between conducting ferromagnetic and insulating paramagnetic domains. Here, we have performed high sensitivity magneto-optical imaging of EuB₆ which images the local magnetic field distribution on the surface of a sample. From our measurements, we identify three characteristic boundaries, T^*(H), T^*_c1(H) and T_c2(H), in a field - temperature magnetic phase diagram and identify their behavior with field. Using scaling and modified Arrott’s plot analysis of isothermal bulk magnetization data, we identify critical transition into a ferromagnetic state below T_c2 = 12 ± 0.2 K. High-resolution imaging of magnetic domains reveals large magnetized domains below T_c2. With increasing T (> T_c2) the magnetic domains disintegrate into finger-like patterns before fragmenting into disjoint magnetized puddles at T^*_c1 and ultimately disappearing at T*. At T^*_c1 we observe a significant increase in the spatial inhomogeneity of the local magnetic field distribution associated with the magnetic domains disintegrating into smaller magnetized structures. We explain our results via the formation and the subsequent coalescing of magnetic polaronic clusters.