Magnetic and magnetoelastic properties of Ni-Mn-Ga - Do they need a revision?

Abstract

The magnetic anisotropy, magnetostriction and behaviour of elastic constants in magnetic field are important for fundamental understanding of martensitic transformation and magnetic shape memory effects. Here we present the measurement of the properties of cubic austenite and tetragonal martensite as a function of temperature and compare it with previously published results. In austenite, while the magnetostriction broadly agrees with published data, measured anisotropy was about ten times smaller. Using single variant tetragonal martensite the magnetic anisotropy constants were revisited. The magnetic anisotropy of modulated (10M and 14M) and tetragonal martensites can be related using tetragonal distortion within the concept of adaptive phase consisting of nanotwinned tetragonal building blocks. The elastic constants measured by pulse echo methods vary only slightly with magnetic field with apparent exception of shear constant c', which is field dependent. However, due to very low value of c', the pulse echo method is not suitable and other method as resonant ultrasound spectroscopy has to be used.