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, p_s, F̅₁, T₀, T_A, by which we can calculate Curie temperature T_C, 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 T_C. 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 T_C 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.

We study several cases of the Heisenberg antiferromagnet by large-scale simulation of numerical- diagonalizations based on the Lanczos algorithm. This review paper presents recently obtained results for three cases: the S = 1/2 orthogonal-dimer system, the S = 1/2 kagome-lattice antiferromagnet, and the integer-spin one-dimensional antiferromagnet showing the Haldane gap. Concerning the S = 1/2 orthogonal-dimer system, our numerical-diagonalization results suggest the existence of an unknown boundary that is different from the edge of the exact-dimer phase and the edge of the Néel-ordered phase. The studies for the latter two cases treat extraordinarily large dimensions of the Hamiltonian matrices for the target systems. Calculations for the cases require use of almost all the resources in a modern powerful supercomputer.

The uniaxial magnetic anisotropy of the magnetoplumbyte-type (M-type) hard ferrite magnet is improved by the substitution of small amount of Co for Fe. The Co atoms occupy at least 3 sites among 5 crystallographically inequivalent Fe sites. Different magnetic functions of Co are expected at the different sites because of different oxygen coordinations around the sites. We have revealed from ⁵⁹Co nuclear magnetic resonance experiment that only Co located at the tetrahedrally coordinated 4f₁ site is responsible for the enhancement in the uniaxial anisotropy, demonstrating that the control of the Co occupation site is essentially important in the Co-doped M-type ferrite. If only the tetrahedrally coordinated 4f₁ site were occupied by Co atoms, the magnetic performance can be improved more efficiently.

We report synthesis and characterization of structural and physical properties in YbInCu₄ and Yb-rich Yb_1+xIn_1-xCu₄. We have measured the powder X-ray diffraction, the temperature dependence of the magnetic susceptibility between 2 and 300 K in the magnetic field of 1 T, the high-field magnetization up to 72 T and the temperature dependence of the electrical resistivity in the magnetic field of 0 and 14 T to study the change of physical properties by substitution and to understand the origin of the valence transition.

The powder X-ray diffraction patterns suggest that Yb_1+xIn_1-xCu₄ crystallized in the cubic C15b-type Laves structure. We newly discovered a two-step magnetic anomaly in the high-field magnetization process interpreted by crystal field splitting effects. We finally made a magnetic phase diagram in H-x, which is consisting of 4 phases. Three phase boundaries are defined by valence transition from Yb valence fluctuation to Yb trivalent localized magnetism, change of the ground state due to crystal field splitting effects and disturbance of Kondo lattice.

Mn perovskite oxides with half-integer valence (3.5+) exhibit charge ordering (CO) that is the origin of intriguing properties such as the colossal magnetoresistance effect. Although CO melts below the room temperature in most of the compounds, Bi_0.5Sr_0.5MnO₃ preserves the CO state up to 475 K. It is such explained that 6s² lone pair of Bi stabilizes the charge ordering. 6s² lone pair has a steric activity and disturbs the electron transfer through the Mn-O-Mn bond. In this study, we replaced Sr by Pb with 6s² lone pair, and investigated the properties of Bi_0.5Pb_0.5MnO₃ in order to clarify the contribution of 6s² lone pair to the CO. Synchrotron XRD measurement clarified the presence of superlattice structure originating from CO and orbital ordering (OO) up to 500 K. The CO state was preserved up to 550 K, much higher than Bi_0.5Sr_0.5MnO₃. It is confirmed that increase in the fraction of ions with lone pair at the A site of Mn perovskite makes the CO state stable.

Ln₂Co₁₂P₇ (Ln = Y, Nd, and Sm) shows a ferromagnetic transition of Co magnetic moments approximately at 150 K. To know how the Nd and Sm moments interact with the Co moments, the magnetocaloric effect of Ln₂Co₁₂P₇ was studied. For Ln = Nd and Sm, the value of the magnetic entropy change of Ln₂Co₁₂P₇ is positive below 50 K and increases with decreasing temperature. This behavior of the magnetic entropy change indicates that the Ln moments interact antiferromagnetically with the Co moments. For Ln = Sm, this is the first report to verify the antiferromagnetic interaction with the Co moments. The value of the magnetic entropy change steeply drops below 12 K for Ln = Nd and 20 K for Ln = Sm at 5 T, suggesting antiferromagnetic transitions.

Magnetization reversal by electric field has recently been confirmed in cobalt-substituted bismuth ferrite (BiFe_0.9Co_0.1O₃) thin films, which would be applicable to ultra-low energy consumption memory devices. In this study, we fabricated the nanodot array using pulsed laser deposition method with an anodized porous alumina mask to realize a unit of BiFe_0.9Co_0.1O₃ with single ferroelectric domain. BiFe_0.9Co_0.1O₃/SrRuO₃ nanodots with a diameter of ~60 nm which is as small as a typical scale of ferroelectric domain in BiFe_0.9Co_0.1O₃ films were successfully obtained.

PrMnGe was reported to exhibit three magnetic transitions at 415, 150, and about 80 K. The last transition was attributed to the ferro-antiferromagnetic (F-AF) transition. We have investigated the magnetization of PrMnGe as functions of temperature and magnetic field in detail using a pure polycrystalline sample. A first-order metamagnetic transition is observed below 80 K in magnetic fields lower than 5 T. Magnetocaloric effect for PrMnGe related to the F-AF transition is evaluated from the magnetization data. The maximum value of the entropy change from H = 5 to 0 T is calculated to be ΔS_M = 24 mJ/K cm³ at T_max = 69 K.

Stoichiometric Sr₃V₂O₇ was prepared by high-pressure synthesis method from carefully chosen raw materials. Unusual magnetic behavior reported previously, such as a kink of temperature dependence of magnetic susceptibility and thermal hyesteresis of the susceptibility, disappears in our sample, and instead a Pauli paramagnetic behavior appears. The intrinsic magnetic behavior was hidden in the previous study by large Curie-Weiss term at low temperatures likely due to oxygen nonstoichiometry and/or crystal defects.