Junjiro Kanamori

Superexchange interaction and symmetry properties of electron orbitals

Abstract The relation between the symmetry of electron orbitals and superexchange interaction is discussed. It is shown that the sign of the superexchange interaction is closely connected with the cation orbital state, when the cation is subject to the crystalline field arising from octahedrally or tetrahedrally surrounding anions. In some cases, the sign of the superexchange interaction is definitely determined from the symmetry relations. The cases in which each cation is subject to an octahedral cubic field and the lines connecting the interacting cations to the intervening anion make an angle of either 180° or 90° are discussed in particular. Our discussion of the 180° case is applicable to crystals of the perovskite type and NaCl type and that of the 90° case to anhydrous chlorides. In the case where each cation is subject to a tetrahedral cubic field, there is a definite relation between the symmetry of the cation electron orbitals and superexchange interaction, if only the s -orbital of anion participates in the superexchange interaction. TiH 2 is an example of this case. The interaction between nearest-neighbor cations in the crystal of the NaCl type is also discussed.

Electron Correlation and Ferromagnetism of Transition Metals

The electron correlation in a narrow energy band is discussed taking into account the multiple scattering between two electrons. The discussion is an adaptation of Brueckners theory of nuclear matter. It is assumed that electrons interact with each other only when they are at the same atom. The effect of the electron correlation depends in an intricate way on the energy spectrum of a given band. An approximate expression of the effective magnitude of the interaction is derived. The condition for the occurrence of ferromagnetism is investigated for various types of bands. The ferromagnetism of Ni and the paramagnetism of Pd can be understood reasonably through the present approach. The degeneracy of the d bands is taken into account in the discussion of these metals. (auth)

Crystal Distortion in Magnetic Compounds

A theoretical overview of the cooperative Jahn-Teller effect in the insulating phase is given. We obtain an effective Hamiltonian of an interaction between the orbital states of the Jahn-Teller ions through a canonical transformation, which associates each electronic state with a local lattice distortion, and by use of the mean field approximation. The effective Hamiltonian yields a simple unified picture of cooperative distortions of various types. The competing effect of the spin-orbit coupling is discussed also. Electron itinerancy is briefly discussed at the end.

An Application of the Coherent Potential Approximation to Ferromagnetic Alloys

An application of the coherent potential approximation (CPA) to ferromagnetic alloys is discussed on the basis of a nondegenerate tight-binding model. The intraatomic coulomb interaction is taken into account self-consistently within the framework of the Hartree-Fock approximation. Numerical calculations are carried out for the case of Ni 1- x Fe x . The results are in good agreement with the experimental data of the magnetic moments of Ni and Fe in concentrated alloys and also with the data of the electronic specific heat. The instability of the nonmagnetic state at a finite temperature is discussed by calculating the susceptibility in CPA. Possible experiments to check the calculated electronic structure are discussed. An extention to a degenerate (multiband) tight-binding model is also presented.

Calculation of Electronic Structure of Fe Base bcc Ferromagnetic Alloys in the Coherent Potential Approximation

The electronic structure of Ni base ferromagnetic alloys with Co, Fe, Mn and Cr is discussed on the basis of the coherent potential approximation combined with the Hartree-Fock approximation for the electron-electron interaction. Calculations are carried out by use of a tight-binding single band model. With consistent choices of parameters the concentration dependences of the average magnetic moment of each constituent atom, the saturation magnetization, and the density of states at the Fermi level are calculated. The significance and limit of the coherent potential approximation for calculating the electronic structure of the ferromagnetic alloys are discussed in some detail.

A Theory of the Cooperative Jahn-Teller Effect –Crystal Distortions in Cu1-xNixCr2O4 and Fe1-xNixCr2O4–

A theory of the cooperative Jahn-Teller effect is developed for the purpose of discussing the spontaneous crystal distortions in mixed chromites, Cu 1- x Ni x Cr 2 O 4 and Fe 1- x Ni x Cr 2 O 4 . The static Jahn-Teller effect only is taken into account. The theory separates the self-energy of each ion having a degenerate orbital level from those energies which contribute to the cooperative phenomenon; the former energy represents the effect of a local distortion which persists in the cubic phase. The theory also distinguishes between the contributions of the Jahn-Teller coupling with the bulk strains and with relative displacements of ions. The theory explains successfully various features of the phase diagrams of the mixed chromites such as the concentration dependences of the transition temperatures and the magnitudes of the distortions, the appearance of an orthorhombic phase, etc. The temperature dependence of elastic moduli is also discussed to point out that it will yield additional informations, pa...

A general mechanism underlying ferromagnetism in transition metal compounds

Among various magnetic orderings exhibited by transition-metal compounds, ferromagnetism is the most important particularly in technological aspects. Recent intensive studies on the colossal magnetoresistance (CMR) have given even another reason for the importance of ferromagnetism. It would be very useful if one could elucidate mechanisms for the stability of ferromagnetism. Zener’s mechanism based on the s-d model and the double exchange mechanism originally proposed also by Zener are such examples. In the present article, we point out that a new general mechanism underlies robust ferromagnetism in a group of transition metal compounds which include MnAs, ordered double perovskites such as A2FeMO6 with A=Ca,Sr and Ba and M=Mo and Re, and organic compound V(TCNE)2· 12CH2Cl2. For some of these materials, the Curie temperature Tc is quite high despite the fact that the magnetic ions are far separated by nonmagnetic ions. The stability of ferromagnetism for these materials can be predicted by the band-structure calculations 9) based on the density functional theory, which automatically take into account several mechanisms for exchange interactions, such as RKKY (including Zener’s mechanism), double exchange and even superexchange. This fact, however, does not mean that the mechanism for the stability of ferromagnetism is clarified. Below we present a physical picture for a new mechanism which may lead to the stability of ferromagnetism. We start with a simple case where the mechanism may be most clearly understood. We assume that the typical elements with the p states as the valence states intervene between the transition-metal elements and that the d states coming from the latter elements split energetically into fully occupied majority-spin state and empty minority-spin one due to the strong intraatomic exchange interaction irrespective of the magnetic ordering. This assumption is reasonable even in the metallic substances of present interest where the d states make bands, because the large inter-atomic distance between transition-metal atoms in these systems may make the d band width smaller than the exchange Fig. 1. A schematic illustration of the situation where ferromagnetism is strongly stabilized.The solid lines denote the density of states when the p-d mixing is switched off. The broken lines denote the valence p density of states with the p-d mixing. The electrons in the shaded area in the majority spin state are transferred to the shaded area in the minority spin state, leading to negative spin polarization in the valence p state and stabilization of ferromagnetic state with respect to antiferromagnetic state.

Electronic Structure of Impurities in Ferromagnetic Iron. I. s, p Valence Impurities

The electronic structure of impurity atoms (atomic number Z =1–56) in ferromagnetic iron is calculated self-consistently by the Green function method in the local density approximation of the spin-density functional approach. The calculation * corresponds to an improvement and the extension of the previous work by Yoshida, Terakura and Kanamori. The mechanism proposed by them for the observed systematic variation in the hyperfine field and the nuclear spin-lattice relaxation time of impurity nuclei for substitutional impurities with Z ≥10 is confirmed on the basis of the present self-consistent calculation which is carried out systematically for the first time.

Strain wave in pure chromium

Abstract New satellites were observed in a single crystal of pure chromium below the Neel point by X-ray diffraction at the positions apart by 2 δ from the Bragg points. The observed anisotropy of the satellite intensity around the Bragg point and the temperature dependence of 2 δ yield the conclusion that the lattice spacing in the direction of the wave vector of the spin density wave are modulated periodically and that the period is a half of that of the SDW. The experimental results coincide with the prediction made by a theory of the exchange striction in transition metal alloys.

Hyperfine field of positive muon in ferromagnetic nickel

Abstract The electronic structure around a positive muon μ + at an octahedral interstitial site of a ferromagnetic nickel is calculated from first principles. The hyperfine field experienced by μ + at absolute zero is calculated to be −0.72 kG in satisfactory agreement with the experimental value of −0.64 kG. The calculation also explains the temperature dependence of the hyperfine field. It is shown that the temperature dependence of the hyperfine field coupling constant A(T) is predominantly determined by single particle excitations.