Electrical and magnetic properties of n-Cd(_1-x)Mn(_x) Te close to the metal-insulator transition

Abstract

Electrical transport and magnetic measurements have been made on n-Cd(_1-x)Mn(_x) Te (0.047 < X < 0.197) for samples doped with hi or In,Al. Results are presented for measurements made as a function of temperature (40 mK < T < 300 K), applied magnetic field (0 T < B < 6 T) and photogenerated carrier density. Low field magnetic susceptibility measurements have identified a transition from a paramagnetic phase to a spin glass phase at low temperatures. The measured temperature dependence of the magnetic susceptibility is consistent with a cluster glass model. Measurements of the spin glass freezing temperature have been carried out on four samples having different Mn fractions The results obtained are in agreement with the limited number of previous measurements on nominally undoped samples. At low temperatures photo-induced changes in magnetism have been measured in both the paramagnetic and the spin glass phase. The change in magnetisation on illumination is due to an increased number of bound magnetic polarons formed around quasi localised s-spins. High temperature susceptibility measurements have been used to examine the parameters characterising the paramagnetic phase, and their variation with Mn fraction. Electrical transport measurements at very low temperatures (T < 800 mK) have shown a strong temperature dependent electron localisation. This has resulted in the confirmation of insulating behaviour in a sample (x = 0.047) having n. At low temperatures and in zero field an activated form of the conductivity is observed. In applied magnetic fields (B > 50 mT) Efros-Shklovskii variable range hopping is observed in the insulating phase. These results are attributed to the formation of a hard gap in the density of states, having a magnetic origin. At higher fields an insulator-metal phase transition occurs. In the metallic phase the conductivity can be described by a quantum correction to the zero temperature conductivity due to the effect of electron-electron interactions. Results obtained before and after illumination are consistent with scaling theory of electron localisation, having a critical exponent close to unity, indicative of the importance of electron-electron interactions. A reduction in the value of the critical field is seen after increasing the carrier density (B(_c) = 2.0 and 1.3 T for n = 3.3 xl0(^17)cm(^-3) and 3.8 x10(^17) cm(^-3) respectively).At low temperatures an anisotropy in the resistivity has been measured for samples in the spin glass phase. Magnetoresistance measurements have shown results consistent with previous measurements, in addition to a large, low field component that is attributed to the effect of magnetic field on electrons in the variable range hopping regime.