Spin-fluctuation theory for weak itinerant-electron ferromagnets: revisited

An explicit self-consistent calculation of the zero-point (ZP) and thermally excited (TE) contributions to spin fluctuations in weak itinerant-electron (WI) ferromagnets in the presence and absence of an external magnetic field, based on the version of spin-fluctuation theory that makes use of the Ginzburg-Landau formalism, is presented. These calculations get rid of certain major deficiencies of the conventional spin-fluctuation theories by bringing out clearly the roles of ZP and TE excitations. The results so obtained demonstrate that zero-point spin fluctuations have a major share in renormalizing the Landau coefficients of the Stoner theory, are relatively insensitive to magnetic field, and make an appreciable contribution to the temperature dependence of magnetization in WI ferromagnets. By contrast, thermally excited collective electron-hole pair excitations almost entirely account for the dependences of magnetization on temperature and field, and get strongly suppressed by magnetic field. The present theoretical approach, in addition, for the first time, yields an analytical expression for the suppression of thermally excited spin fluctuations by field for temperatures just outside the critical region but below the Curie point