MAGNETIC TRANSITION IN THE FERROMAGNETIC STATE OF Ni-Fe-Ga SINGLE CRYSTALS

摘要

Temperature dependences of alternating current impedance, stress-induced magnetic induction and internal friction have been studied in Ni-21.5Fe-27Ga (at. %) single crystals. The Curie temperature (T_C) and the temperature of the start of the martensitic transformation (M_s) are affected by the degree of L2_1 atomic order and range from 340 to 360 K for the former one and from 145 to 190 K for the latter one. We have observed strong and rapid variations with temperature of the electrical impedance of the samples, as well as stress induced magnetic induction and magnetomechanical damping in the austenitic ferromagnetic phase. These variations can be described as a magnetic transition: the low-temperature state is characterized by lower values of initial magnetic permeability and of magnetomechanical hysteretic damping than the high-temperature one. We have found that the heat treatments affect the temperature of this magnetic transition much stronger than the temperature of martensitic transformation: the transition is situated around 350 K for samples air cooled from 1070 K, and is at around 240 K for samples quenched from 970 K with the lowest degree of atomic order. The transition presents a small hysteresis between cooling and heating scans.