Microwave Faraday Rotation in Antiferromagnetic MnF2

Abstract

Microwave Faraday rotation has been observed in Mn${\mathrm{F}}_2$ from 30°K up to 300°K. The antiferromagnetic resonance frequency is computed from the rotation measurements below the Néel point. The results are in good agreement with direct observation of the antiferromagnetic resonance and molecular field theory extra-polations except close to the transition where evidence for magnetic clustering is found. The Kramers-Kronig relations are applied to antiferromagnetic media and it is shown that the Faraday rotation depends both on the shift of the resonance with field and on very small field-induced intensity changes which are present. A study of Faraday ellipticity establishes the need for using terms in the equation of motion which relax the magnetization toward the instantaneous field direction.