Effects of Spin Fluctuations on Magnetic Properties

Abstract

In this chapter, we show a number of interesting magnetic properties are derived based on the idea that the local spin fluctuation amplitude is conserved, independent of temperature and irrespective of the presence of externally applied magnetic field. As an interesting example, the magnetic isotherm in the ground state is determined under the influence of the zero-point spin fluctuation amplitude. The temperature dependence of the magnetic susceptibility and the fourth expansion coefficient of the free energy with respect to the magnetization is also derived. Meanwhile, as an effect of the anomalous critical thermal amplitude at the Curie temperature, the critical magnetic isotherm is shown to be derived. Its critical index is consistent with the scaling law relation of the critical phenomena. These properties are also quantitatively compared with experiments. At the end of this chapter, crossover behaviors of the magnetic susceptibility around the quantum critical point is discussed.