Chiral Phonons Enhance Ferromagnetism

TL;DR

This paper proposes a mechanism where chiral phonons coupled with magnons enhance ferromagnetic stability and coercivity, explaining experimental observations of increased magnetic anisotropy in magnetite with chiral molecules.

Contribution

It introduces a novel phonon-magnon coupling mechanism enabled by chirality that explains ferromagnetic enhancement in magnetite.

Findings

Coercivity increased by nearly 100% with chiral molecules.

Magnetic anisotropy linearly increases with temperature.

Chiral phonon-magnon coupling diverts thermal energy, stabilizing ferromagnetism.

Abstract

Recent experiments suggest that the conditions for ferromagnetic order in, e.g., magnetite, can be modified by adsorption of chiral molecules. Especially, the coercivity of magnetite was increased by nearly 100 \%, or 20 times the earth magnetic flux density, at room temperature. The coercivity was, moreover, demonstrated to increase linearly with temperature in a finite range around room temperature. Based on these results, a mechanism is proposed for providing the necessary enhancement of the magnetic anisotropy. It is shown that nuclear vibrations (phonons) coupled to ferromagnetic spin excitations (magnons) absorb the thermal energy in the system, thereby diverting the excess energy that otherwise would excite magnons in the ferromagnet. This energy diversion, not only restores the ferromagnetic order but also enhances its stability by increasing the anisotropy energy for magnon excitations. The coupling between phonons with magnons is enabled by chirality due to the lack of inversion symmetry.