New semiconductor spin glass Cu [formula omitted]Ge [formula omitted]Cr 2S 4: Comparison of magnetic and electric properties of the spin glasses Cu 0.5In 0.5Cr 2Se 4 and Cu [formula omitted]Ge [formula omitted]Cr 2S 4

Abstract

The magnetic and electric properties of a new semiconductor compound Cu 2 3 Ge 1 3 Cr 2S 4 have been studied. The data of the X-ray diffraction analysis and IR spectra show that this compound possesses a spinel-like structure with a crystallographic ordering of 1:1-type between the Cu 1+ and Ge 4+ ions in a tetrahedral sublattice. This compound has magnetic properties typical of spin glasses (SGs): a maximum initial susceptibility at a certain temperature, a dependence of low-temperature magnetic properties on the thermomagnetic history of the sample and the absence of spontaneous magnetization, the latter shown by the Belov-Arrot method. The critical behaviour has been studied by means of the nonlinear susceptibility X nl in the region of the freezing temperature T f, which is defined as the temperature of the maximum of the X nl( T) curve. The critical exponents have been determined: μ=3.9±0.1, δ=4.8±0.1, β=1±0.2. The experimental points satisfactorily fit the scaling function g( X) in the critical field range H⩽50 Oe and for normalized temperatures t= T − T f/ T f < 0.5. The freezing temperature T' f, determined from the maximum initial susceptibility in an ac field, depends on the measurement frequency according to the Vogel-Fulcher law, the constant T 0 in this law being close to T f. The dependence of T' f on the static magnetic field H for both the compound Cu 2 3 Ge 1 3 Cr 2S 4 and the previously studied Cu 0.5In 0.5Cr 2Se 4 obeys the relation of Almeida-Thouless. A giant negative magnetoresistance (MR), approaching 80 % in a field of 30 kOe, has been found in the semiconductor SGs Cu 2 3 Ge 1 3 Cr 2S 4 and Cu 0.5In 0.5Cr 2Se 4. A minimum is observed in the temperature dependence of the MR at T f. These experimental facts indicate that in the SGs inder investigation a phase transition SG-paramagnetism takes place. The nature of the SG state is discussed.