Negative mode-mode coupling among spin fluctuations and the magneto-volume 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.