NEGATIVE MODE MODE-COUPLING AMONG SPIN FLUCTUATIONS AND THE MAGNETOVOLUME EFFECT IN AN ITINERANT-ELECTRON FERROMAGNET

By making use of the Landau-Ginzburg energy expanded up to the sixth power of magnetization density, the spontaneous volume magnetostriction and dependence on pressure P of the Curie temperature T(c) of an itinerant-electron ferromagnet with negative mode-mode coupling among spin fluctuations are discussed. The spontaneous moment is found to show first-order and second-order transitions at T(c), according to the values of the Landau coefficients. It is shown that the difference between the spontaneous volume magnetostrictions at T = 0 and T(c) is large and the P-dependence of T(c) becomes anomalously large when a certain condition is satisfied by the Landau coefficients. The results obtained in the present theory could explain the anomalous T-dependence of the thermal expansion and the strong P-dependence of T(c) observed in Invar alloys, some Laves phase Co compounds and others.