On the theory of magnetic anisotropic exchange interactions

Abstract

A generalization of expressions for the anisotropic exchange interaction is developed forPaCCMi3TpkiBaeTCII 0606we~kie BbIBOAa AJIH aHki30TpOn~Or0 06MeHHOr0 B~~H M O -AetCTBHH AJXH WY ¶aH HepaBHbIX CnHHOB B OCHOBHOM ki BOa6yM~eHHbIX COCTOHHUIIX. the case of nonequal spins of separate ions for the ground and excited states.

Pressure Dependence of Exchange Parameters and Neel Temperature in La2CuO4

“…The theory predicts strong pressure dependence of exchange parameter J at n = 7 ( dJ/dP ≅ 36 K/GPa), which contradicts the data in ref 2. One of the apparent reasons of a discrepancy between the theory (eq 12) and experiment 2 for the baric coefficient dJ/dP is that the coefficient ρ in ref 6 was determined at an essentially lower pressure than in ref 19. The reduced sensitivity of the superexchange interaction J to P in ref 2 at the highest pressures may arise from transformations of the crystal structure at room temperature as the pressure‐dependent orthorhombic‐tetragonal phase transition occurs at P crit = 4.3 GPa.…”

“…If the two copper ions have the ground o i , o k and excited e i , e k states, the interaction J pd comes from the third‐order perturbation process with terms that are linear in the exchange interaction of ions i and k and quadratic in the spin‐orbit coupling of ions i and k 6,12 plus terms that are Hermitian conjugate to the above terms and in which ion i is interchanged with ion k . Generally the numerical values of anisotropy parameters J a depend on four distinctly different perturbation theory contributions I‐IV 6 . Historically a situation emerged 12 in which type‐I contribution, corresponding to the first term in eq 3, was used without rigorous substantiation for any ions and spin values.…”

“…The antisymmetric exchange interaction D causes not only an output of Cu moments from a basic layer on an angle ∼ D /2 J ≅ 0.17°, but also the presence of a weak orthorhombic anisotropy ∼ D 2 / J < J a . The dominant contribution to D arises from a second‐order perturbation process 6b,17 with terms that are linear in the exchange interaction ( H ex ) and the spin‐orbit coupling ( V so ) for ions i and k . The corresponding microscopic expression for parameter D can be written as …”

“…In this study, the microscopic mechanisms responsible for the observable pressure dependencies of T N and the intralayer exchange parameters in La 2 CuO 4 are investigated. Within the framework of Anderson theory of the superexchange 5 and the advanced theory of anisotropic exchange interactions, 6 the expressions describing obvious dependence of the intralayer exchange parameters and T N on the structural parameters are obtained. The proposed theoretical dependencies and existing experimental data on pressure dependencies of structural parameters in La 2 CuO 4 4 explain, qualitatively and quantitatively, pressure dependencies of magnetic parameters and the T N observed experimentally.…”

Exchange interactions in polynuclear complexes

“…Exchange-coupled chromium pairs in corundum [9,17], yttrium and lanthanum aluminates [18,19] or the gallium spinel [20,21] have long been the object of particular attention for both experimental and theoretical [9,[22][23][24][25] reasons. On the one hand, chromium (III) is well known to show sharp R lines in the near infra-red region and is therefore a good candidate for observing small perturbations due to exchange interaction.…”