Temperature dependence of magnetically induced deformation of Ni-Mn-Ga martensite

In the present work the contributions of the temperature-dependent (i) crystal lattice parameters (related to the magnetic anisotropy energy), (ii) Young's modulus, (iii) saturation magnetization and (iv) thermal fluctuations of the microstress to the temperature dependence of the magnetic field induced strain (MFIS) in Ni-Mn-Ga martensite are considered in the framework of a statistical model. Both individual and cooperative effects of these factors on the achievable MFIS value and on the characteristic values of the magnetic fields, which trigger and saturate MFIS, are estimated. It is shown that all the factors affect both the achievable MFIS value and characteristic fields under the real experimental conditions, and none of them can be neglected in the quantitative theoretical analysis of the experimental strain-field dependencies obtained for different temperature values. In addition, the influence of specimen shape on the characteristic fields is illustrated for different temperature values. For the available experimental dependencies (i)–(iii) and the reasonable set of model parameters the switching magnetic field proved to be equal to 160 kA/m when the temperature was by 15 K below the martensite start temperature and raised to 320 kA/m when the temperature was by 45 K below the martensite start temperature. Obtained results agree with the experimental data reported by O. Heczko and L. Straka, in J. Appl. Phys. 94, 7139 (2003).