Anomalous magneto-optical Kerr hysteresis loops in Fe0.25TaS2

We have performed magneto-optical Kerr spectroscopy measurements on intercalated transition-metal dichalcogenide Fe0.25TaS2 in the polar Kerr geometry as a function of temperature, magnetic field, and wavelength. The Kerr angle exhibits pronounced peaks at ∼775 nm (1.6 eV) and ∼515 nm (2.4 eV), which we attribute to spin-dependent interband optical transitions arising from states in the vicinity of the Fermi energy. Below the ferromagnetic transition temperature (165 K) we observe a strongly wavelengthand magnetic-field-dependent Kerr signal. At a fixed wavelength, the magnetic-field dependence of the Kerr angle shows a clear hysteresis loop, but its shape sensitively changes with the wavelength. We propose a model that takes into account contributions from domain walls, which allowed us to derive a mathematical expression that successfully fits all the observed hysteresis loops.