Combined effect of structural softening and magneto-elastic coupling on elastic coefficients of NiMnGa austenite

Using two complementary ultrasonic methods the complete set of elastic constants of the austenite phase of NiMnGa ferromagnetic shape memory alloy was measured in magnetic field up to saturation and in temperature range 295–400 K. While the coefficients c11 and c44 were nearly independent of the field and the temperature, the shear coefficient c′ increased strongly with increasing field and temperature. Experiment indicates that two independent mechanisms contribute to the anomalous shear softening of the c′ coefficient in NiMnGa austenite; the structural softening of the bcc lattice and the softening due to magneto-elastic coupling. Above the Curie point and in the saturation field only the structural softening took place with rate about 10 MPa/K. In the demagnetized state the combined softening was approximately 35 MPa/K. Similar dependencies on the temperature and the magnetic field were also observed for the ultrasound attenuation of the shear mode.

► Complete set of elastic coefficients of cubic Ni2MnGa austenite is experimentally determined as a function of magnetic field and temperature.
► Only shear coefficient c′ significantly increases with increasing temperature and magnetic field and it scales with magnetization square.
► Anomalous shear softening is due to combination of lattice softening and softening due to magnetoelastic coupling.
► Huge mechanical damping occurs in demagnetized state and it is suppressed in saturation.