Axion-matter coupling in multiferroics

Abstract

Multiferroics (MFs) are materials with two or more ferroic orders, like spontaneous ferroelectric and ferromagnetic polarizations. Such materials can exhibit a magnetoelectric effect whereby magnetic and ferroelectric polarizations couple linearly, reminiscent of, but not identical to the electromagnetic E • B axion coupling. Here we point out a possible mechanism in which an external dark matter axion field couples linearly to ferroic orders in these materials without external applied fields. We find the magnetic response to be linear in the axion-electron coupling. At temperatures close to the ferromagnetic transition fluctuations can lead to an enhancement of the axion-induced magnetic response. Relevant material candidates such as the Lu-Sc hexaferrite family are discussed.P M as an "impedance matched" medium to interact with axion DM. Here, P (M ) is the electric (magnetic) polarization vector. Multiferroics with P M act as the "condensate" axion field seen by an arriving external axion. The external axion field linearly couples to the polarization fields and both changes net magnetization and excites the collective modes of the MF. The mechanism