Studying the Temperature Dependence of the Transient Reflectance of FeRh Thin Films on the Subpicosecond Timescale*

FeRh films exhibit a magnetic phase transition with respect to temperature. The magnetic order of FeRh is antiferromagnetic (AF) for temperatures below the transition and ferromagnetic (FM) for temperatures above the transition. Using time-domain thermoreflectance (TDTR), we have studied the temperature dependence of the transient reflectance (△R(t)/R) of FeRh on the subpicosecond timescale using small temperature excursions. Modeling ΔR(t)/R using both a fast decaying signal and a slowly decaying signal, we have found that both the magnitude and thermalization time of △R(t)/R strongly depend on the magnetic order of the material. The magnitude of △R(t)/R is negative in the AF phase and positive in the FM phase. In the AF phase, both the fast and slowly decaying signals are detected. However, in the FM phase, only the slowly decaying signal is detected since the decay rate of the fast-decaying signal is quicker than the detection limit of our TDTR system. The phase dependence of the magnitude of △R(t)/R is explained by differences in the electronic density of states of the AF and FM phases above the Fermi energy. In addition, using the two-temperature model, the phase-dependent characteristics of the fast-decaying transient signal are attributed to changes in the electron-phonon coupling factor and the electronic and lattice heat capacities. The authors would like to acknowledge the support of David G. Cahill, Nadya Mason, and Thomas A. Searles.