H. Akai

Fast Korringa-Kohn-Rostoker coherent potential approximation and its application to FCC Ni-Fe systems

A fast KKR CPA method is explained and its convergence properties are examined numerically. It is shown that a step number of N 300, which determines the number of k-points used for the numerical integration in the k-space as well as the number of iteration steps in determining the coherent t-matrix, is sufficient for most purposes, including total-energy calculations. As a typical application the electronic structure of ferromagnetic Ni-Fe systems is calculated in the framework of the KKR CPA combined with the local spin density approximation, which demonstrates the feasibility of the present method.

Ab-initio calculations of the electronic structure of impurities and alloys of ferromagnetic transition metals

Abstract We review recent theoretical work on the electronic structure and the magnetic properties of ferromagnetic transition-metal alloys. All calculations are based on density-functional theory in the local-spin-density approximation. We report about calculations for dilute alloys using the KKR-Greens function method and for concentrated disordered alloys using the charge-self-consistent KKR-CPA method.

Hyperfine fields of impurities in ferromagnets

Abstract Due to the development of Greens function methods the calculation of hyperfine fields of impurities in ferromagnets has become feasible in the last years. We present the result of recent calculations for sp- and d-impurities and their nearest neighbours in Fe. The calculations are based on density functional theory; the potentials of both the impurity and the neighboring host atoms are determined self-consistently. For not too heavy impurities ( Z ≤ 50) the calculations well produce the observed trend of hyperfine fields. We discuss the strong correlation between the local magnetic moment of magnetic impurities and the hyperfine field, observed experimentally, in the light of the calculations. For sp-impurities the so-called systematic behavior of the hyperfine field with increasing Z is explained in terms of the bonding properties between impurity s and host d orbitals.

Nuclear spin-lattice relaxation of impurities in ferromagnetic iron

Due to the development of Greens function method the calculation of the nuclear spin-lattice relaxation time of impurities in ferromagnets has become feasible in the last years. We present the result of calculations for allsp andd impurities in ferromagnetic iron. The calculations are based on the density functional formalism. They well, reproduce the experimental trend of the relaxation timeT1 for bothsp andd impurities. By decomposing the relaxation rate into various contributions, we explain the observed systematic behavior ofT1T in terms of the local electronic structure.

SELF-CONSISTENT CALCULATION OF HYPERFINE FIELDS AND ADIABATIC POTENTIAL OF IMPURITIES IN IRON

Hyperfine fields of impurities of the atomic number Z=1–56 at the substitutional site and those of light impurities of Z=1–9 at the interstitial sites in ferromagnetic iron are calculated by the KKR method adapted to the system containing a single impurity atom. The potential of the impurity atom is determined self-consistently by use of the local spin density functional formalism. The results for nonmagnetic sp valence impurities agree with those of the previous nonself-consistent calculation by Katayama-Yoshida, Terakura and Kanamori except for a few cases, confirming their theory of the systematic variation of hyperfine fields. The calculation for magnetic impurities of transition elements is presented for the first time in this paper. The calculations mentioned so far assume that impurities are situated at the center of each site. For the purpose of discussing the stability of the impurity positions, the change of the adiabatic potential due to displacements from the center is calculated by carrying out similar self-consistent calculations for off-center impurity positions. It is concluded that positive muon and some light impurities including boron will be displaced from the center when trapped in a vacancy.

First-principles calculation of the Curie temperature Slater–Pauling curve

It is well known that the magnetizations as a function of the valence electron number per atom of 3d transition metal substitutional alloys form the so-called Slater-Pauling curve. Similarly, the Curie temperatures of these alloys also show systematic behaviour against the valence electron number. Though this fact has long been known, no attempt has been made so far to explain this behaviour from first principles. In this paper we calculate T(C) of 3d transition metal alloys in the framework of first-principles electronic structure calculation based on the local density approximation.

Ab initio calculations for impurities in Cu and Ni

Abstract By using the KKR-Green function method, the electronic structure of impurities in transition metals has been calculated within the local spin-density approximation. The electronic structure of 3d impurities in Cu are discussed, with particular reference to the formation of local moments and their interactions. An evaluation of the electronic structure of d and sp impurities in ferromagnetic Ni is given and results are presented for the hyperfine fields of the impurities. The calculations reproduce well the experimentally observed trends for the local moments and the hyperfine fields.

Monte Carlo simulations of diluted magnetic semiconductors using ab initio exchange parameters.

Co doped ZnO (Zn(1-x)Co(x)O) is studied as a prototype material for transition metal doped II-VI diluted magnetic semiconductors (DMSs) from first-principles and Monte Carlo simulations. The exchange interactions are calculated using the Korringa-Kohn-Rostoker (KKR) Greens function method. The exchange coupling constants thus obtained are treated in the classical Heisenberg model and the magnetic phase transitions are studied by the Monte Carlo technique. Our results show that the defect free substitutional DMSs of Zn(1-x)Co(x)O do not sustain magnetization at low concentration. At high concentration, we find layered magnetic structures. Ferromagnetism, with Curie temperature below room temperature, is stable at intermediate Co concentrations. First-principles studies with the generalized gradient approximation (GGA) and the GGA together with the Hubbard U are discussed with respect to structural and electronic properties of ZnO.

Calculated electronic structures and Néel temperatures of half-metallic diluted antiferromagnetic semiconductors

The possibility of half-metallic diluted antiferromagnetic semiconductors of II-VI compounds is investigated on the basis of first-principles electronic structure calculation. The electronic structures of ZnS, ZnSe, ZnO, CdS and CdSe doped with two kinds of 3d transition metal ions are calculated using the Korringa-Kohn-Rostoker (KKR) method and their magnetic transition temperatures are determined using a cluster-type approximation. It is predicted that II-VI compound semiconductors doped with two kinds of magnetic ions might be good candidates for half-metallic antiferromagnets.

Hyperfine field calculation for various alloy systems

The hyperfine field and the isomer shift of Fe−X (X=SC, Ti, V, Cr, Mn, Co, Ni, Cu) disordered alloys are calculated by use of the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) combined with the density functional theory in the local density approximation. The agrements with available experimental data are fairly well if the well-known systematic discrepancy arising in the Fe hyperfine field is properly biased. The overall trends of the concentration dependence of the Fe hyperfine field is qualitatively explained in terms of the local and the transferred contributions to the hyperfine field.