MAGNETIC-FIELD-INDUCED SIGN REVERSAL OF TRANSIENT PHOTOREFLECTANCE INCD1-XMNXTE - PARAMAGNETIC SHIFT AT LOW MANGANESE CONCENTRATION

Two thermomagnetic modulation mechanisms were revealed in photoreflect ance modulation spectroscopy of Cd1-xMnxTe at low temperature. One is the thermal modulation of the magnetic redshift (paramagnetic shift) o f the gap, the other is the magnetic-field-induced thermal modulation of the so-called giant spin splitting of the free exciton. In Faraday configuration and sigma(+) polarization, these two magnetic contributi ons cancel each other for a particular magnetic field B-c. This causes a sign reversal of the photoreflectance signal around B-c. Analysis o f the sign reversal is proposed as a tool for studying the paramagneti c shift at manganese concentrations x much lower than those reported s o far. By using a model of the paramagnetic Shift that takes into acco unt interferences between Bloch waves scattered by spin-correlated ion s, we obtain a good agreement with experimental results for x between 0.015 and 0.05. These interferences are shown to reduce the paramagnet ic shift. For the lowest concentration studied this reduction amounts to a factor of approximately 1.7.