Negative Thermal Expansion in a Magnetically Frustrated Spinel

The vast majority of materials expand when heated. However, some like water ice, elastic bands, and the alloy invar show the unusual phenomenon of negative thermal expansion (NTE). Recent experiments find that the magnetic spinel oxide CdCr₂O₄ also exhibits NTE within its ordered half-magnetisation plateau phase, a phenomenon which has been explained by a strong coupling between a flat band of classical spin-wave excitations and the lattice [1]. Here we use classical Monte Carlo simulations to directly calculate the NTE. Also, we extend calculations to the quantum case by performing spin wave calculations. We find that the theory developed in [1] is robust against the inclusion of thermal and quantum fluctuations, and argue that it provides a scenario for NTE in a wide range of frustrated magnets.

[1] L. Rossi et al., Phys. Rev. Lett. 123, 027205 (2019).