Tunable all-angle negative refraction using antiferromagnets

We show how all-angle negative refraction can occur in a uniaxial antiferromagnet close to the magnon resonance frequency. This behavior is based on the fact that, in such cases, the antiferromagnet acts as an indefinite permeability medium, i.e., not all its permeability tensor components are of the same sign. If an external magnetic field is applied, the angle of refraction becomes tunable, and can be made to change sign. We illustrate these effects using the example of MnF2 at low temperature, and show that in this system negative refraction should occur with a large figure of merit.