Phase transitions in Ni–Mn–Ga alloys with the account of crystal lattice modulation

Abstract

Using the Ginzburg–Landau theory, phenomenological phase diagrams of Heusler alloys Ni–Mn–Ga are theoretically investigated. We take into account the magnetic, elastic and micro-modulation order parameters in order to understand their coupling in the real system. We focus on the aspect of crystal modulation which appears at c/ a<1 only, and is never observed for tetragonal distortions characterized by c/ a>1. The influences of the parameters are discussed on the basis of phase diagrams. From different phase diagrams we choose those corresponding to the experimental situation and draw a conclusion that two opposite tetragonal distortions, c/ a<1 and c/ a>1, must be driven by different instabilities having different microscopic origins.