Chiral Phonons Enhance Ferromagnetism

Recent experiments suggest that the conditions for ferromagnetic order in magnetite can be modified by adsorption of chiral molecules. Especially, the coercivity of a ferromagnetic metal 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 the temperature in a finite range around room temperature. On the basis of these results, a mechanism is proposed for providing the necessary enhancement of 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.