  Time-Domain Thermoreflectance (TDTR) characterization of FeRh throughout its
first-order antiferromagnetic (AF) to ferromagnetic (FM) transition shows that
the transient reflectance, $\Delta$R(t)/R, strongly depends on the magnetic
order of the sample. Using TDTR, which uses optical pulses to induce small
temperature excursions, we have found that the $\Delta$R(t)/R of the AF phase
exhibits a large negative response, while the response of the FM phase is
positive. This magnetic phase sensitivity has allowed us to study the transient
response of both the AF and FM phase to the pump pulse excitation and the mixed
phase of the material. These results are significant since the ultrafast
properties of antiferromagnetic materials and mixed antiferromagnetic and
ferromagnetic materials are difficult to detect using other conventional
techniques.We have found that the AF phase exhibits a strong subpicosecond
signal not observed in the FM phase. The magnetic phase dependence of the sign
of $\Delta$R(t)/R is qualitatively explained using the results of ab-initio
density functional theory (DFT) calculations. Using the two-temperature model,
we found that the change in the thermalization time across the transition is
caused by differences in both the electronic heat capacity and the
electron-phonon coupling factor of the AF and FM phases. The electron-phonon
coupling constant in the AF phase is also determined using the two-temperature
model conducted using the NTMpy code package. For the FM phase, we provide
boundaries for the magnitude of the electron-phonon coupling factor for the FM
phase. These results indicate that TDTR can be used to study the transient
properties of magnetic materials that are otherwise challenging to probe.
