Size dependence of martensite transformation temperature in ferromagnetic shape memory alloy FePd

Martensite transformation temperature of ferromagnetic shape memory alloy FePd was studied in the shape of nanoparticle and the polycrystalline samples with grain size in micrometers based on the x-ray diffraction and magnetic measurement as a function of sample size. Both the forward transformation start temperature Ms and reverse transformation finish temperature Af of polycrystalline sample monotonically decreased with decreasing grain size and were not observed in the nanoparticles. The size dependence of transformation temperature is explained based on the change in transformation mode, i.e., the decrease in sample size induces the change from the multivariant mode to single variant mode in which the strain energy is large. In the small sample, the strain energy becomes large, and thus the large driving force is required for the transformation. As a result, the large amount of undercooling occurs, which leads to the lowering of Ms. The measurement of heat capacity indicates that the strain energy is elastically stored without dispersion in the transformation process even in the small size because the volume change of FePd at the transformation is very small. The large elastic strain energy works as the driving force to reverse transformation, and then the lowering of Af is induced. Therefore, the lowering of transformation temperature in FePd is characterized by the lowering of thermodynamic equilibrium temperature.