Microscopic theory of far-infrared two-magnon absorption in antiferromagnets. I. Perturbation-theory search and application of fourth-order process to FeF2

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Abstract

A microscopic description of the mechanism responsible for an absorption line observed in antiferromagnetic FeF2 at 154 cm-1 is proposed. The mechanism appears to account for the properties of the observed line which are described in the preceding paper. In isolating the proposed mechanism, we set up a model which includes magnon, phonon, exciton, and photon fields and their interactions. The mechanism involves an indirect coupling between odd-parity excitons and magnons which we have previously called the odd-exciton-magnon interaction. This coupling is a consequence of the combined action of the spin-orbit interaction and the quadrupole-dipole part of the spin-orbit interaction. A search through fourth order in perturbation theory in this model yields several other possible processes. Our estimates indicate that the process we propose is the largest contributor in FeF2 and that a third process, involving a phonon, may be contributing to the absorption intensity. The proposed process is used to derive the spin Hamiltonian used in the preceding paper and to estimate the intensity and polarization ratio.

Original languageEnglish (US)
Pages (from-to)423-433
Number of pages11
JournalPhysical Review
Volume149
Issue number2
DOIs
StatePublished - 1966

Bibliographical note

Funding Information:
National Science Foundation Postdoctoral Fellow.

Copyright:
Copyright 2007 Elsevier B.V., All rights reserved.

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