Substrate-dependent Magneto-Thermoelectric Properties in FeRh Thin Films
during Antiferromagnetic-Ferromagnetic Phase Transition
Abstract
FeRh is an intermetallic magnetic compound featuring a B2-ordered
structure and exhibiting a first-order antiferromagnetic (AFM) to
ferromagnetic (FM) phase transition near room temperature. Here, we
explore the magneto-thermoelectric properties encompassing the
resistivity, Seebeck coefficient, and Thomson coefficient in
near-equimolar FeRh thin films grown on three different substrates,
namely Al2O3 (sapphire), SiO2/Si, and MgO. The substrate affects the
phase transition temperature by inducing in-plane strain (compressive
for MgO, tensile for sapphire and SiO2/Si), while film thickness shifts
the transition to higher temperatures. At the same thickness of 80 nm,
the MgO films have the highest phase transition temperature, the
hysteresis loop of sapphire films starts at around 300K, and that of
SiO2/Si shifts to 200K. While the Seebeck coefficient in the FM phase
remains negative consistently, the Seebeck coefficient of the AFM phase
varies in sign depending on the substrate, film quality, and thickness.
The larger negative Seebeck coefficient values in the FM phase are
attributed to the larger FM density of states around the Fermi level. We
discuss the role of crystal quality in designing a pathway for
heightened Thomson response in FeRh thin films. Within the monitored
temperature range, Thomson coefficient values as large as -250 µV.K−1
were observed with non-negligible values in a 50 K temperature range. We
also observe a larger Nernst thermopower in the FM phase, the
lower-mobility phase, and an unusual sign change in the AFM phase.