Anomalies in Thermal Expansion and Heat Capacity of Ferromagnets

Abstract

Polymorphic transformations (PT) f.c.c. + h.c.p. in Co are investigated on the basis of Born's thermodynamic theory. The spontaneous deformation u2p is the P T parameter; the volume exchange w is the additional variable. The P T problem is reduced to Landau's theory with the variable uZ3. The elastic moduli in this case are: (Cll -GIl2) + 0 and C,,(T) = -C,,(T) in the point T = Too (Too is the point of the instability). The mibxima of the thermal expansion coefficient (TEC) and the elastic moduli as functions of T are calculated in the case of the first-order PT.TEC anomalies are calculated by the method of near the orientational phase transition (OPT) point in Co and Ni. TEC is measured by the method of the continual T-exchange. Anomalies of the TEC are found in the region of the P T in Co and near OPT in Co and Ni (near 500 K). The TEC quantities are in accord with the theoretical estimations. The TEC peak in the P T region permits to find the parameters of anharmonicity.