MAGNETOOPTICAL EFFECTS AT THE LOW-TEMPERATURE PHASE-TRANSITION OF CR3+-DOPED POTASSIUM ALUM

Faraday rotation and magnetic circular dichroism have been determined as a function of temperature in chromium-doped potassium alum. Measurements show strong anomalies at T0 = 59 K which are caused by birefringent domains. Below the phase transition birefringence decreases the Faraday rotation. In contrast to direct birefringence measurements this decrease does not depend on the sign of domains. Thus changes of the Faraday rotation are a more reliable measure of the temperature dependence of the order parameter. The occurrence of birefringence below T0 reveals apparative limits in the measurement of magnetic circular dichroism which is three orders of magnitude smaller than the Faraday rotation.