Ni Mn Ga Particles Research Articles

Magnetocaloric effect in NiMnGa particles produced by spark erosion

The magnetic entropy change of tetragonal and orthorhombic NiMnGa fine particles made by spark erosion was investigated in this paper. It was found that the structure and crystalline phase transformation temperatures can be strongly affected by the compositions of the particles, while Curie temperature is less sensitive to the compositions. Due to the possible distribution of the particle size and compositions in these particles, the magnetic entropy changes observed are much broader and smaller than those of bulk NiMnGa alloys. The maximum absolute value of entropy change ΔS=2JKg−1K−1 was observed for tetragonal structure NiMnGa particles at 95°C in a field of 2T.

Large energy absorption in Ni–Mn–Ga/polymer composites

Ferromagnetic shape memory alloys can respond to a magnetic field or applied stress by the motion of twin boundaries and hence they show large hysteresis or energy loss. Ni–Mn–Ga particles made by spark erosion have been dispersed and oriented in a polymer matrix to form pseudo 3:1 composites which are studied under applied stress. Loss ratios have been determined from the stress-strain data. The loss ratios of the composites range from 63% to 67% compared to only about 17% for the pure, unfilled polymer samples.

Analytical model for field-induced strain in ferromagnetic shape-memory alloy polycrystals

Single crystals of Ni–Mn–Ga have shown promising properties for actuators with field-induced strains up to 6% [Murray et al., Appl. Phys. Lett. 77, 886 (2000)]. There is interest in achieving large field-induced actuation in textured polycrystals as well as in composites of oriented Ni–Mn–Ga particles in a polymer matrix. A simple analytical model has been developed to predict an upper limit to the field-induced strain achievable in ensembles of particles having specified orientations distributions. It is found that in a uniform distribution of orientations (no texture) the strain can reach at most 21% of the single-crystal strain for a given field. Distributions in which the twin plane normal subtends a fixed angle of 55° with the field direction have upper bounds for the strain of 38%. Similarly, orientation distributions in which the normal to the twin plane is the same for all crystals have an upper bound of 50%.