Phonon Frequency and its Modification by Magnon-Phonon Coupling from All-Temperature Theory of Magnon

TL;DR

This paper extends the all-temperature magnon theory to analyze how magnon-phonon coupling affects phonon frequencies in ferromagnets across temperatures, revealing potential negative frequency modifications near transition points.

Contribution

It derives new expressions for phonon frequency modifications due to magnon-phonon coupling within the all-temperature magnon theory framework.

Findings

Frequency modification can be negative near the Curie temperature.

The ratio of frequency change to phonon frequency relates to energy surface curvatures.

The theory aligns better with experimental critical exponents than mean-field approaches.

Abstract

The all-temperature magnon (ATM) theory [J. Phys. Condens. Matter 21, 336003/1-14, 2009] has been used to analyze the temperature dependence of magnetization as well as the internal energy components of a mono-domain ferromagnetic solid. The critical exponents have been in better agreement with experiment than their mean-field theory and critical phenomenon theory counterparts, and unlike in the latter theories, vary from one ferromagnet to another. Expressions have been derived for the thermally averaged spin-center force constants and their break-up in terms of the base-line related (solid) and exchange-cum-field mediated (magnetic) components. These components give rise to expressions for phonon frequencies and their modifications by magnon-phonon coupling. The derived expressions are suitable for a correct quantum chemical evaluation of the involved properties. A detailed numerical calculation using spin configurations at varying crystal geometries is hardly possible even today and beyond the scope of the present work. The focus here is on the correctness and explaining the trends of properties. It has been shown that the frequency modification by magnon-phonon interaction can be negative for certain phonon branches near the ferromagnetic transition temperature. Also, the ratio of frequency modification and phonon frequency is approximately proportional to the ratio of curvatures of the involved energy surfaces.