Semenova Y. Magnetic and Phase States in the Ferromagnetic Shape Memory Alloys Based on Nickel and Copper

The martensitic and intermartensitic transitions in ferromagnetic shape memory alloys are investigated in the present work. On the basis of the generalized results obtained using experimental methods, the sequence of magnetic and structural transitions is defined. This sequence, namely, TCA < MS < AS < TCM, provides a large difference of the magnetic moments of martensite and austenite phases and lets the alloys of Ni-Mn-Ga be used as magnetocaloric materials. It is also shown that the reduction in the width of the temperature hysteresis in Ni-Mn-Ga alloys is explained by a decrease in the shear modulus due to premartensitic lattice instability which takes place for both austenitic and martensitic phases, thereby reducing the elastic energy of martensite crystals. Thus, in the Ni54Mn24Ga22 alloy a record low temperature hysteresis width for the main martensitic transition, ?Т ? 2 К, was achieved. Taking into account the practical perspectives of FSMAs at temperature cycling, the changes of volume during the martensitic and intermartensitic transitions in the Ni-Mn-Ga alloy were determined. The changes of volume, in their turn, cause the functional degradation of the material. It is shown that the temperatures of intermartensitic transformations increase with pressure, thus intermartensitic transitions are accompanied by a decrease of the specific volume. It is also demonstrated that the nano-sized particles released during the high temperature aging and inherited by martensite phase, can be an effective mechanism for the hysteresis loop narrowing in the Cu-Mn-Al system, in which as a result of aging the system of dispersed ferromagnetic particles in the paramegnetic matrix is formed.