Temperature and time dependent magnetic phenomena in a nearly stoichiometric Ni(2)MnGa alloy

Temperature Dependence of Anisotropy in Ti and Gd Doped NiMnGa-Based Multifunctional Ferromagnetic Shape Memory Alloys

The temperature dependence of magnetocrystalline anisotropy was investigated in detail for the polycrystalline Ni₅₀Mn₂₅Ga₂₅, Ni₅₀Mn₂₅Ga₂₀Ti₅ and Ni₅₀Mn₂₅Ga₂₀Gd₅ ferromagnetic shape memory alloys in the temperature range of 50–400 K. The effective anisotropy constant was estimated from a series of high field magnetization curves based on the fitting procedure according to the law of approach to magnetic saturation. The low temperature martensitic phase was found to have a significantly higher anisotropy energy in comparison to a high temperature austenitic phase, which was observed through a sudden, distinct drop of anisotropy energy. The calculated values of the effective anisotropy constant were comparable to the results published by other authors. Moreover, the strong influence of chemical composition on the first-order phase transition and the second-order ferromagnetic to the paramagnetic transition was revealed. Finally, the strong coupling between the temperature dependence of the coercive field and the temperature dependence of magnetocrystalline anisotropy was also shown and discussed in the present study.

Temperature dependence of magnetic susceptibility in the vicinity of martensitic transformation in ferromagnetic shape memory alloys

Temperature dependences of low-field quasistatic magnetic susceptibility in the vicinity of martensitic transitions in an NiFeGa alloy are studied both by experiment and analytically. Pronounced reversible jumps of the magnetic susceptibility were observed near the martensitic transition temperature. A general description of the temperature dependences of the susceptibility in ferromagnetic austenite and martensite phases and the susceptibility jump at the transition is suggested. As a result, the main factors governing the temperature dependences of the magnetic susceptibility in the magnetic shape memory alloys are revealed. The magnetic susceptibility jump value is found to be related to changes of: (i) magnetic anisotropy; (ii) magnetic domain wall geometrical constraints (those determined by the alignment and size of twin variants) and (iii) mean magnetic domain spacing.