Dynamic axion field in a trivial magnetoelectric insulator

It has been shown that the magnetoelectric susceptibility in the topologically trivial insulator Cr₂O₃ can be decomposed into a pseudoscalar component and a tensor component. The former component has been identified as the axion piece. We use a nonlinear magneto-optical effect to experimentally investigate the axion and tensor components of the magnetoelectric susceptibility. Fluctuations of the axion piece originate from fluctuations of the antiferromagnetic order parameter. The fluctuations of the axion piece contribute to fluctuations in the electric field induced magnetization. Measuring the latter as a function of temperature allows separating pseudoscalar and tensor contributions.