On the theory of spin fluctuations around the magnetic instabilities -: Effects of zero-point fluctuations

Effects of temperature variation of the zero-point spin fluctuations on the self-consistent renormalization (SCR) theory of spin fluctuations around the magnetic instabilities are discussed. In two and three dimensional ferromagnets and three dimensional antiferromagnets the effect is mostly to renormalize the mode-mode coupling constant. The renormalized value, corresponding to the experimental value, has a finite upper bound corresponding to a divergent mean field value. In two-dimensional antiferromagnets the critical behaviors of the staggered susceptibility and the nuclear spin-lattice relaxation rate are modified by logarithmic corrections while this correction hardly appears in those of the electrical resistivity and the specific heat. The temperature variation of the mean square local amplitude of the spin density around the magnetic instability is studied by using a simple model for the dynamical susceptibility and is applied to discuss a general trend in the magneto-volume effects in weak itinerant magnets. Possible origins of the large thermal expansion and the large low temperature coefficient of the specific heat in magnetically frustrated systems are also discussed.