Anomalous field dependence of the IR magneto-optical Kerr rotation in CeSb

The IR magneto-optical Kerr effect of CeSb is rich in anomalies. In particular, the Kerr rotation reaches the new record value of 90 at 0.46 eV at a temperature of 1.5 K with an external field of 5 T. At this maximum, the Kerr rotation jumps with no intermediate values from +90 to -90. We present measurements of the field dependencies of the Kerr rotation taken at 1.5 K for fields up to 10 T on both sides of the Kerr rotation jump, at 0.25 and 0.57 eV. Both field dependencies show three magnetic phase transitions. But the general shape of the field dependence of the Kerr rotation θK is quite different from the magnetization curve M, breaking the usually accepted proportionality between θK and M. The origins of this anomaly are discussed. (orig.)