Atomic and magnetic configurational energetics by the generalized perturbation method

Abstract

It is shown that, using the generalized perturbation method (GPM) with screened Coulomb interactions that ensures its consistency with the force theorem, one is able to obtain effective interactions that yield an accurate and physically transparent description of configurational energetics in the framework of the Korringa-KohnRostoker method within the atomic sphere and coherent potential approximations. This is demonstrated with calculations of ordering energies, short-range order parameters, and transition temperatures in the CuZn, CuAu, CuPd, and PtCo systems. Furthermore, we show that the GPM can be used to obtain Heisenberg exchange interaction parameters, which, for instance, capture very well the magnetic configurational energy in bcc Fe.

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

“…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]. 3 .…”

“…As a consequence, the Curie temperature, estimated within the spherical model (equivalent to the RPA), amounts to T C = 2700 K [155], substantially higher than the experimental and theoretical values in table 1. A very recent analysis [50] of this approach clarified the main reason for this overestimation: if the fully renormalized form of the exchange parameters [155,156] is replaced by an unrenormalized one (corresponding to an infinitesimal rotation of magnetic moments described by the second-order term in an expansion of band-energy changes with respect to the CPAmedium), the resulting Curie temperatures come out in the range T C = 1090 − 1330 K, i.e., only slightly higher than experiment. This comparison indicates that the two particular configurations of a pair of atomic moments (parallel and antiparallel) used for a quantitative analysis in Ref.…”

“…The underlying physical picture assumes that the direction of the local magnetic moment and of the effective magnetic field acting on it coincide; this is fully justified, e.g., for two magnetic impurities in a paramagnetic host metal in the limit of a large separation of the impurity atoms. It has been proved that ferromagnets satisfy this assumption only if the exchange splitting is not too small as compared to the band-width [30] (see also section 5); a simple check can be based on evaluating the pair interactions (5) from other reference states (antiferromagnetic, DLM) besides the FM ground state [50]. Similar checks can be performed concerning the role of the neglected Stoner excitations.…”

Anab initioeffective Hamiltonian for magnetism including longitudinal spin fluctuations

“…4, however they also allow for the following criteria for the FM-MFT to provide accurate Curie temperatures to be formulated: The FM-MFT should work for strong local ferromagnets, where it will agree with DLM-MFT, and not otherwise. The results of Table I show that this is indeed the case, with the Curie temperatures for Co being close in both approaches but there existing a large difference for Ni and Fe.…”

Exchange Coupling and Exchange Bias in FM/AFM Bilayers for a Fully Compensated AFM Interface

“…The Liechtenstein-Katsnelson-Gubanov method (LKGM) [4], which is the most widely used technique for obtaining exchange interactions for a Heisenberg Hamiltonian, results in exchange parameters valid in the small angle limit. The generalized-perturbation-method (GPM) [5] provides an alternative method of producing parameters strictly valid only for the paramagnetic state. Our discussion in this article is based on the LKGM mapping.…”