Phonon mediated spin-spin interactions

TL;DR

This paper explores phonon-mediated interactions between magnetic moments in insulators, revealing conditions for symmetric and anti-symmetric interactions and their potential to explain weak ferromagnetism.

Contribution

It introduces a mechanism for phonon-mediated spin-spin interactions, including conditions for anti-symmetric interactions requiring broken inversion symmetry.

Findings

Symmetric anisotropic interactions exist for all phonons.

Anti-symmetric interactions require broken inversion symmetry.

Interactions decay oscillatory with distance and are temperature-independent at low temperatures.

Abstract

Indirect long range interactions between localized magnetic moments are in metals mediated by itinerant electrons. In insulators and semi-conductor, such interactions need to be small, if not negligible, due to the absence of mediating carriers. The existence of magnetically ordered insulators, for instance, metal-oxides, is therefore an everlasting source for proposals of various mechanisms that may support the order. Here, phonon mediated interactions between localized magnetic moments is considered as a mechanism that can provide quantifiable symmetric and anti-symmetric anisotropic spin-spin interactions. It is demonstrated that while a symmetric anisotropic interaction exists for all types of phonons, the existence of anti-symmetric anisotropic interactions requires broken inversion symmetry. The latter mechanism may explain the weak ferromagnetic order observed in chiral, e.g., CuO and CoO compounds. Furthermore, the interaction is nearly independent of the temperature at low temperature while approaches a linear growth at high. Spatially, the interactions have an oscillatory power law decay with the inter-nuclei distance.