Abstract: Magnetic and thermal anomalies of Invar alloys

Abstract

The anomalous thermal expansion coefficient of Fe-Ni Invar alloys has been well explained1) in terms of the two-state model of an Fe atom.2) The analysis of the thermal expansion coefficient (α) of disordered phase of Fe-Pt Invar alloys in terms of the Grüneisen equation shows that the anomalous part of α has negative values in the whole temperature region with monotonic temperature dependence and remains even at high tempertures above the ferromagnetic Curie point. These behaviors are similar to that of Fe-Ni Invar alloys. The ferromagnetic moment becomes unstable in the Invar region. The calculation of a molecular field approximation in terms of the two-state model, assuming the distribution of the ferromagnetic nature of the high spin state as well as the low spin state, shows that the magnetic moment at 0 °K and Curie point decrease and the temperature dependence of the magnetization deviates from the Brillouin function in the Invar region. The temperature dependence of the susceptibility of Fe-Ni alloys has been measured. The inverse susceptibility deviates from the Curie-Weiss law at high temperatures. The deviation is due to the temperature dependence of the number of the low spin states of Fe atoms. It has been clarified by means of the measurement of the integrated intensity of the X-ray diffraction experiment of Fe-Ni Invar alloys that the Debye temperature decreases at low temperatures, regarded as a lattice softening effect. The static deviation effect from the ordinary lattice point is also contained in the integrated intensity of the X-ray diffraction. The static deviation of atoms is large in Invar alloys and comes from the mixing of the two states of an Fe atom. 1 M. Matsui and S. Chikazumi J. Phys. Soc. Japan 45, 453 (1978). 2 R. J. Weiss, Proc. Phys. Soc. 32, 281 (1963).