Ab initio theory of exchange interactions in itinerant magnets

Abstract

The paper reviews an ab initio two-step procedure to determine thermodynamic properties of itinerant magnets. In the first step, the selfconsistent electronic structure of a system is calculated using the tight-binding linear muffin-tin orbital method combined with Green function techniques. In the second step, the parameters of the effective classical Heisenberg Hamiltonian are determined using the magnetic force theorem and they are employed in subsequent evaluation of magnon spectra, the spin-wave stiffness constants and the Curie/Nel temperatures. Applicability of the developed scheme is illustrated by investigations of selected properties of 3d metals Fe, Co, and Ni, diluted magnetic semiconductors (Ga,Mn)As, and 4f metals Gd and Eu.1. Introduction Practical implementation of density-functional formalism led to excellent parameterfree description of ground-state properties of metallic magnets, including traditional bulk metals and ordered alloys as well as systems without the perfect three-dimensional periodicity (disordered alloys, surfaces, thin films). On the other hand, an accurate quantitative treatment of excited states and finitetemperature properties of these systems remains a challenge for ab initio theory of solids. The complexity of the problem calls for additional assumptions and approximations the validity of which has to be checked in each particular case. The purpose of this article is to review a recently developed first-principles scheme of this kind [1][2][3]. A brief presentation of the formalism and of the underlying physical ideas is accompanied by examples of applications to various bulk systems including transition metals, diluted magnetic semiconductors, and rare-earth metals. Applications to thin magnetic films can be found elsewhere [2,4].

Electronic topological transition and noncollinear magnetism in compressed hcp Co

“…4 (upper panel). The zero-pressure values are in good agreement with prior DFT studies [39,40]. Between 0 and 80 GPa the nearest-neighbor couplings (J 1 ) remain almost unchanged.…”

Exchange interactions, spin waves, and transition temperatures in itinerant magnets

“…One can see dominating FM interactions for the first and second nearest-neighbour shells followed by weaker interactions of both signs and decreasing magnitudes for bigger distances d = |R − R | ( figure 1, left panel). The same qualitative features were found for other 3d ferromagnets: fcc Co, fcc Ni [15] and hcp Co [60]. Note that the calculated exchange interactions depend on the type of the exchange-correlation potential [50] and on the inclusion of non-spherical parts of oneelectron potentials and densities [61]; these effects can be especially pronounced in some systems, e.g., in fcc Fe with atomic volume close to a crossover from a FM high-spin state to a FM low-spin state [61].…”

“…Application of the TB-LMTO-CPA formalism to this system employs so-called empty spheres located at interstitial positions of GaAs semiconductor for matters of space filling, so that the zinc-blende structure is described in terms of four fcc sublattices with substitutional disorder only on the cation sublattice. The pair exchange interactions between Mn atomsJ MnMn RR in the (Ga 1−x Mn x )As alloy with x = 0.05 are shown in figure 8 (left panel) [17,22,60]; interactions between the other components are much smaller and negligible concerning their possible influence on magnetic properties. The first nearest-neighbour interaction is positive and bigger than the (mostly positive) interactions between more distant Mn atoms.…”

Temperature-dependent exchange stiffness and domain wall width in Co

“…The long-range pair-wise exchange parameters can be evaluated on the basis of ab-initio methods by mapping the electronic structure calculation onto the Heisenberg model. The first set of these parameters for HCP Co at T = 0 K was evaluated by Turek et al [24] and for FCC Co by Pajda et al [25] with experimental lattice parameters. Very recently a new set data has been published by Kvashnin et al [26].…”