Unified Description of Spin-Lattice Coupling and Thermodynamics in the Pyrochlore Heisenberg Antiferromagnet

TL;DR

This paper introduces an extended spin-lattice coupling model for pyrochlore Heisenberg antiferromagnets, successfully capturing phase transitions and thermodynamic behaviors, and offers a versatile framework for analyzing spin-phonon interactions.

Contribution

It develops a unified model interpolating between bond- and site-phonon models, accurately reproducing experimental phenomena and providing insights into dominant phonon modes in spin-lattice coupling.

Findings

Reproduces field-induced phase transitions

Explains negative thermal expansion and magnetocaloric effects

Identifies key phonon modes affecting spin-lattice interactions

Abstract

We study an extended model to describe the spin-lattice coupling, incorporating individual vibrations of bonds and atomic sites alongside distance-dependent exchange interactions. The proposed spin Hamiltonian can be effectively considered as an interpolation between two well-established minimum models, the bond-phonon model and the site-phonon model. The extended model, which treats bond phonons and site phonons on comparable footing, well reproduces successive field-induced phase transitions as well as the thermodynamic properties of a three-up-one-down state in the pyrochlore-lattice Heisenberg antiferromagnet, including negative thermal expansion, an enhanced magnetocaloric effect, and a sharp specific-heat peak. The present approach is broadly applicable to various spin models, providing a framework for identifying the primary phonon modes responsible for spin-lattice coupling and for understanding complex magnetic phase diagrams.