Study of the Laser-Assisted Thermal Hysteresis in FeRh Films

Near equiatomic FeRh alloys, with their close to room temperature antiferromagnetic (AF) to ferromagnetic (FM) magnetostructural transition, have been studied in bulk, and in thin film form. The complex interplay between optical, electronic, magnetic, and structural properties underlying the phase transition makes FeRh a promising multifunctional material with possible applications in fields as different as spintronics, nanomagnetism, and magnetic cooling [1]. Whatever the aimed application, understanding FeRh phase change mechanism, and tackling the associated thermal hysteresis, is one of the key challenge for harnessing its functional properties. Here we study the AF-FM transition using a modulated reflectance experiment [2] with a laser driven phase change taking place at a rather long timescale (μs range) where only thermal effects are at play. An optical microscopy setup where a pump and a probe beam (532 nm laser with square modulation at 100 kHz, and 488 nm laser respectively), are focused on a 2 μm radius spot, is used to measure the material's reflectance. The DC component (f=0), and the AC component (f=100Hz) of the reflectance are measured while the temperature of the sample is slowly varied across the transition. The average transition temperature shifts by 2.2°C per mW of the laser pump and, while the DC component under cooling is unaffected by the pump power, the one under heating merge with the cooling curve when pump power gets above 4 mW. On the other hand, the AC signal, rather weak at low pump power, increases (two orders of magnitude) when pump power gets above 2 mW.