Semiconducting and optical properties of ferromagnetic CdCr 2S4 and CdCr2Se4

Abstract

Electrical conductivity, Hall effect, magnetoresistance and thermoelectric properties have been studied on a number of undoped and doped polycrystalline CdCr2S4 and CdCr2Se4 samples. Undoped CdCr2S4 and CdCr2Se4 are n-type and p-type, respectively. With suitable doping, the conductivity of CdCr2Se4 can be changed by about six orders of magnitude [10-5 to 10 (Ω·cm)-1] without changing the magnetic properties. In Ag-doped p-type CdCr2Se4 the mobility at room temperature is 32 cm2/V·sec and rises with decreasing temperature. By doping with In, CdCr2Se4 can be made n-type. The mobility of electrons is found to be two orders of magnitude lower than the mobility of holes. The conductivity of p-type CdCr 2Se4 decreases monotonically with decreasing temperature without any anomaly around the Curie temperature (Tc=130°K 1). However, the conductivity of n-type CdCr2Se 4 has a minimum at 146°K and rises sharply with further decreasing temperature. This n-type material also exhibits a large negative transverse magnetoresistance effect which has a maximum at the Curie temperature (Δρ/ρ0=-0.8 for H=6 kG). The magnetoresistance of the p-type material is positive in the paramagnetic range and negative in the ferromagnetic range. Optical transmission experiments on thin single crystals show that the absorption edges of CdCr2S4 and CdCr 2Se4 are at 1.6 and 1.3 eV, respectively, at room temperature.2 For the ferromagnetic range an anomalously strong shift of the absorption edge takes place to higher energies in CdCr2S 4 and to lower energies in CdCr2Se4. The edge is shifted further in both materials by an externally applied magnetic field. This additional field shift exhibits a linear magnetic dichroism and has its maximum at the Curie temperature. The observed anomalous optical properties and also the strong increase in conductivity of n-type CdCr2Se 4 in the ferromagnetic range are closely related to magnetic ordering. Apparently the strong shift of the absorption edge decreases the energy difference between the donor level and conduction band and results therefore in an increase of the carrier density. A complete account of this work will be published elsewhere. © 1967 The American Institute of Physics.