Magnetomechanical damping in Ni–Fe–Ga poly and single crystals

Magnetomechanical damping has been studied in poly and single crystalline Ni–Fe–Ga ferromagnetic shape memory alloys in the austenitic state, with emphasis on the non-linear hysteretic component. The experiments were performed at an ultrasonic frequency, over a wide range of strain amplitudes, at room temperature with different transverse polarizing fields and from 255 to 365 K with different axial polarizing fields. We found that polycrystalline samples demonstrate higher damping level than single crystals. The contribution of the hysteretic magnetomechanical damping is low in polycrystals and damping properties in the austenite are predominantly controlled by existing defects, like dislocations, grain boundaries, and cracks. Single crystalline samples are characterized by substantially lower values of the total damping due to their higher degree of structural perfectness. In contrast to the polycrystals, the non-linear damping in the austenitic state is associated in single crystals with the hysteretic magnetomechanical damping.