Laser-induced random modulation of the antiferromagnetic/ferromagnetic phase transition in FeRh films

Recently, the magnetocaloric ferromagnetic-antiferromagnetic phase transition of FeRh, and its thermal hysteresis have been studied under fast laser-induced temperature pulses (i.e. with temperature sweep rates up to 107 Ks-1 ) [1] using a modulated reflectivity setup [2]. While the time-averaged response of the material is lead by the key quasi-static features of the hysteresis cycle with a remarkable compliance with return-point-memory [1], the time resolved behavior exhibits a complex random response. Using the ≈4% change in optical reflectivity between the antiferromagnetic and ferromagnetic phase, the relative phase-fraction is probed over a 4 µm radius spot of a 195 nm thick near equiatomic FeRh film grown on a MgO(001) substrate. The film temperature is controlled by a heating stage while the temperature over the spot is periodically changed using a square modulated pump-laser [2]. The phase modulation is recorded as a function of the laser-induced temperature increase ΔT, and at different locations across the phase coexistence interval. This is done by controlling the pump-laser power, and the heating stage temperature respectively. The entailed modulation of the FM phase fraction does not systematically follow the temperature but shows a random behavior. This has been systematically studied as a function of ΔT, and of the way it spans the phase-coexistence temperature interval. The statistics of the modulated FM phase (i.e. relative frequency, amplitude, and time delay of the FM phase nucleation bursts with respect to the temperature pulses) is studied, and its relationship with quasi-static hysteresis features is discussed.