The Crystallization Kinetics of Ni-Mn-Ga Magnetic Shape Memory Alloy Thin Films

Abstract

The crystallization mechanism of Ni-Mn-Ga thin films and the reason for the difference of crystallization temperatures of Ni-Mn-based magnetic shape memory alloys (MSMAs) are unknown. Here, the crystallization kinetics of Ni53Mn28Ga19 magnetic shape memory alloy thin films have been determined by non-isothermal and isothermal differential scanning calorimetry (DSC). The reason for the difference of crystallization temperatures of Ni-Mn-based MSMAs was studied by the first-principles methods. The crystal structure of annealed Ni53Mn28Ga19 thin films are 7 M martensite. In non-isothermal DSC, crystallization peak temperatures are 588.2 K, 593.7 K, 601.3 K, 604.6 K and 608.2 K at different heating rates. The apparent activation energy calculated by Kissinger’s method is 148.3 kJ/mol. For isothermal crystallization, the Avrami exponent of Ni53Mn28Ga19 thin films is approximately 1.6. The local Avrami exponents n(x) which range from 1.1 to 2.8 imply that the crystallization mechanism of Ni53Mn28Ga19 thin films changes from one-dimensional diffusion-controlled growth to two-dimensional and three-dimensional diffusion-controlled growth. Moreover, it is found that crystallization peak temperatures of Ni-Mn-based MSMAs increase with their increasing vacancy formation energy with Ni vacancy.