Interference between distortion and magnetization for the reentrant martensitic transition in Heusler-type alloys Co2Cr(Ga,Si)

Heusler-type CoxCr78-xGa11Si11 alloys exhibit the reentrant martensitic transition, i.e., the forward martensitic transition from the paramagnetic austenite (Para-A) phase into the paramagnetic martensite (Para-M) phase and the successive reverse martensitic transition from the Para-M phase into the ferromagnetic austenite (Ferro-A) phase with decreasing temperature T. In the present paper, this complicated transition is analyzed by use of phenomenological free energy, which is expanded in powers of two order parameters, i.e., the tetragonal distortion e3 and the magnetization M. It is assumed that an interference between e3 and M through the repulsive magnetostructural interaction, Ge32M2(>0), in the free energy is strong enough to suppress out the ferromagnetic martensite (Ferro-M) phase with both nonvanishing e3 and M. With the presence of this repulsive interaction, the observed phase diagram in the T-x plane is found to be successfully explained. The T dependencies of e3 and the entropy are also calculated to see the equilibrium state. The inverse magnetic susceptibility in the martensite phase χ-1 is derived by taking into account that the term Ge32M2(>0) opposes the magnetization by an external magnetic field in the martensite phase. The calculated χ-1 exhibits a negative paramagnetic Curie temperature as was observed, which verifies directly the existence of Ge32M2(>0). An essential role of Ge32M2(>0) in the phase stabilities is also found in the calculated T dependence of the elastic constant (c11-c12). On the basis of the present analyses, the conditions for the appearance of the reentrant martensitic transition are described.