新規磁性体・超伝導体の開拓とその物性評価

Abstract

The brief history of itinerant-electron magnetism has been reviewed in terms of spin fluctuations. In the research field of itinerant-electron magnetism, the effects of spin fluctuations are very important to understand the experimental results. Among itinerant-electron theories, the self-consistent renormalization theory of spin fluctuations (the SCR theory) has developed since 1973 by Moriya’s group. Now, we can use the SCR theory in order to analyze our experimental data quantitatively by means of 4 spin fluctuation parameters, ps, F₁, T₀, TA, by which we can calculate Curie temperature TC, inverse magnetic susceptibility 1/χ, nuclear spin-lattice relaxation rate 1/T1, specific heats C, etc. quantitatively. In fact, the quantitative discussion is very important to characterize itinerant-electron magnets which distribute from weak-itinerant to localized-moment regimes. Furthermore, Takahashi’s theory of spin fluctuations has been developed since 1986 by setting up two hypotheses in order to solve some serious problems existing in previous itinerant theories including the SCR theory. One is that total amplitudes of spin fluctuations should be conserved (TAC). The second is global consistency (GC), implying that the continuity of magnetization and inverse magnetic susceptibility should be preserved through TC. These two assumptions result in two equations among these 4 spin-fluctuation parameters, by which we can estimate 4 parameters quite easily only by using static magnetic measurements like magnetization and magnetic susceptibility. At the same time, the assumption TAC may result in the unified picture of itinerant ferromagnetism. The effects of spin fluctuations are also important to understand the exotic superconductors of which mechanism is not of BCS-type but of magnetic origin. The superconducting transition temperatures TC of those exotic superconductors can be explained universally by T₀ which is the energy width of the spin-fluctuation spectrum. Here, the effects of spin fluctuations are explained quantitatively in itinerant-electron magnetism as well as exotic superconductivity in the review.