Spin-Peierls transition in the frustrated spinels ZnCr2O4 and MgCr2O4

TL;DR

This study combines computational methods and simulations to analyze the spin-Peierls transition in frustrated spinels MgCr2O4 and ZnCr2O4, revealing a three-dimensional mechanism that reduces magnetic frustration.

Contribution

It provides a detailed computational analysis of the spin-Peierls transition in these spinels, establishing precise Hamiltonian parameters and linking structural distortion to magnetic energy gain.

Findings

Simulations align with experimental transition temperatures.

Structural distortions reduce magnetic frustration.

The transition mechanism is a three-dimensional spin-Peierls process.

Abstract

The chromium spinels MgCr2O4 and ZnCr2O4 are prime examples of the highly frustrated pyrochlore lattice antiferromagnet. Experiment has carefully established that both materials, upon cooling, distort to lower symmetry and order magnetically. We study the nature of this process by a combination of density-functional-theory based energy mapping and classical Monte Carlo simulations. We first computationally establish precise Heisenberg Hamiltonian parameters for the high temperature cubic and the low temperature tetragonal and orthorhombic structures of both spinels. We then investigate the respective ordering temperatures of high symmetry and low symmetry structures. We carefully compare our results with experimental facts and find that our simulations are remarkably consistent with a type of spin-Peierls mechanism, adapted to three dimensions, where the structural distortion is mediated by a magnetic energy gain due to a lower degree of frustration.