Time evolution of magnetization in the FeRh system near antiferromagnetic-ferromagnetic transition temperature (abstract)

Abstract

The body-centered cubic FeRh is known to exhibit a first-order phase transition from antiferromagnetic to ferromagnetic at about 400 K, accompanied by a volume expansion of about 1%. The time evolution of the magnetization M of ordered Fe0.5Rh0.5 and FeRh0.958Pt0.042 just below and above the transition temperature (T0) was measured as functions of temperature and magnetic field. Around T0, the magnetization increased logarithmically with time up to M*(=41 emu/g), the value of which indicates that the ferromagnetic grains occupy one third of the antiferromagnetic matrix. When M=M*, there appeared a jump in M. After that, M varies as M(t) =M∞[1−exp(−t/τ)], where τ is a relaxation time. It should be noted that the value of M* is independent of both temperatures and external magnetic fields. Microscopic observation and x-ray diffraction measurement showed that the ferromagnetic grains nucleated in the antiferromagnetic matrix began to grow with time just below T0. In this work, the time evolution of the phase transition was analyzed by considering the magnetic free energy, the elastic energy, and the strain energy at the surface of the grains. The above mentioned magnetic relaxation process is deeply related to the time evolution of the grain size.