N. A. Pichugin

Electronic structure of chromium-doped lead telluride-based diluted magnetic semiconductors

The crystal structure, composition, galvanomagnetic, and oscillatory properties of Pb1−x−ySnxCryTe (x=0,0.05–0.30; y⩽0.01) alloys are studied for different matrices and chromium impurity concentrations. It is shown that impurity chromium ions dissolve in the lattice in amounts below 1mol.%, while higher chromium concentrations lead to the appearance of microscopic regions with elevated chromium contents and inclusions of chromium-tellurium compounds. Reductions in the hole concentration, p−n conversion of the conductivity type, and stabilization of the Fermi level by the resonance level of chromium are observed with increasing chromium content. The initial rates of change of the charge carrier concentrations during doping are determined. A two-band Kane dispersion relation is used to calculate the electron concentration and Fermi level as functions of the tin content. A diagram showing the rearrangement of the electronic structure of the chromium doped alloys with varying matrix composition is constructed.

Metal-insulator transition in chromium-doped Pb1−xGexTe alloys

The electrical properties of chromium-doped n-Pb1−xGexTe alloys (x = 0.02–0.13) have been studied. A decrease in the free-electron concentration and a metal-insulator transition are observed as the germanium content of alloys increases. This is due to the Fermi level pinning by the chromium impurity level and to the flow of electrons from the conduction band to the impurity level. The experimental data obtained are used to calculate, in terms of the two-band Kane dispersion law, the dependences of the electron concentration and Fermi energy on the germanium content in the alloy. The motion rate of the chromium-related level with respect to the conduction band bottom is determined and a model of variation of the electronic structure with the matrix composition is suggested.

Ferromagnetism in diluted Pb1 − x − yGexCryTe magnetic semiconductors

Magnetic properties of Pb1 − x − yGexCry Te alloys (x = 0.02–0.20, y = 0.01–0.08) were studied. It was detected that the magnetic susceptibility of alloys consists of two contributions: the paramagnetic Curie-Weiss part (the temperature region is T < 50 K) caused, probably, by the Cr3+ ion paramagnetism, and the high-temperature ferromagnetic part (T < 300 K). The dependences of the concentration of magnetic centers on the composition of the matrix were obtained from the paramagnetic and ferromagnetic contributions. It was shown that a decrease in the concentration of paramagnetic centers can be, at least qualitatively, explained by the transformation of the electronic structure as the germanium concentration increases. A phenomenological model was suggested, which explains the behavior of magnetic properties as the chromium content increases, and possible mechanisms of ferromagnetic ordering in studied alloys are discussed.

On the resonant level of chromium in the rhombohedral and cubic phases of Pb1 − x − yGexCryTe alloys

The temperature dependences of the Hall coefficient (4.2 K ≤ T ≤ 300 K, B ≤ 0.07 T) in Pb1 − x − yGexCryTe alloys (x = 0.03–0.08, y ≤ 0.01) are studied. An increase in the absolute value of the Hall coefficient with an increase in temperature is found. This fact is indicative of a decrease in the concentration of free electrons as a result of the motion of the resonant level of chromium stabilizing the Fermi level relative to the conduction-band bottom. The temperature dependences of the Hall coefficient, in satisfactory agreement with the experimental ones, are calculated in the context of the two-band Kane dispersion law allowing for the structural phase transition upon increasing temperature. The energy position and temperature coefficients of the motion of the resonant level of chromium relative to the middle of the band gap in the rhombohedral and cubic phases are determined.

Magnetic Properties of Diluted Magnetic Semiconductors Pb1-xVxTe

The structural and magnetic properties as well as the electron paramagnetic resonance in Pb1-xVxTe (х0.7 at.%) solid solutions have been investigated. It was found that the magnetic field and the temperature dependences of magnetization have a paramagnetic character, connected obviously with the paramagnetic contribution of vanadium impurity isolated ions. Electron paramagnetic resonance spectra were measured and the temperature dependence of the g-factor in the temperature range 85-200 K was obtained.

Ferromagnetism in Diluted Magnetic Semiconductors Pb1-x-yGexCryTe

Magnetic and galvanomagnetic properties of Pb1-x-yGexCryTe (x=0.02-0.20, y=0.01-0.08) solid solutions have been investigated. It was found that the magnetic susceptibility of these alloys contains two contributions: a paramagnetic Curie-Weiss share (T<50 K) due to the paramagnetism of Cr3+ ions and a high temperature ferromagnetic share (T<300 K). Dependence of the concentration of paramagnetic centers on the composition of the matrix was obtained. The magnetic field dependences of the Hall coefficient in the vicinity of the metal-insulator transition were measured and the main parameters of charge carriers in terms of the two-band conduction model were estimated. The experimental results are discussed in the framework of the electronic structure model, assuming varying electrical and magnetic activities of Cr ions as function of germanium content in the alloys.

Room-temperature ferromagnetism in diluted magnetic semiconductor Pb1−xCrxTe

The temperature and magnetic field dependencies of magnetization (T=5-340 K, B≤0.5 T) as well as electron paramagnetic resonance (EPR) (T=80-400 K) in Pb1−xCrxTe (x≤0.045) have been studied. It was shown that all samples are ferromagnetic with a clear hysteresis loop up to the room temperature and magnetic saturation moment rises with increasing of the chromium content. Using temperature dependencies of magnetization M(T) ferromagnetic Curie temperatures up to 330 K were revealed. EPR spectra in the paramagnetic phase were satisfactory approximated by a single Dysonian line. Temperature dependence of magnetic susceptibility was estimated and paramagnetic Curie temperature was determined. In ferromagnetic phase pronounced distortion and splitting of the EPR spectra on two Dysonian lines were revealed. Using approximation by the Lorenz-type curves with Dysonian terms, temperature dependencies of the main parameters for each absorption line were obtained.

Magnetic properties of Ge1−xMnxTe ferromagnetic semiconductors doped with gadolinium

We report on magnetic measurements and X-band electron paramagnetic resonance for Ge1-x-yMnxGdyTe (x=0.15-0.40, y=0, 0.02) single crystals. Ferromagnetic behavior was observed with the Curie temperature TC=70-90 K depending on the alloy composition. Effective magnetic moment peff per Mn ion estimated from the Curie constant increases with increasing x from 2.8 up to 4.3 μB, while saturation magnetization per magnetic ion at 4.2 K gives 1.7-3.4 μB. EPR spectra at low temperatures are described by Dysonian type line, which splits into two lines at T ≈ 150 K. At temperature lower this value we observe distinct anomalies on temperature dependencies of the linewidths and g-factors. In paramagnetic region anomalous broadening of the linewidth with decreasing the temperature is revealed, while the g-factor is practically independent both on temperature and alloy composition.

Electronic structure and structural phase transition in Pb1−xGexTe:Cr under pressure

We investigate galvanomagnetic effects near the insulator-metal transition induced by pressure in Pb1−xGexTe (x=0.10, 0.13) alloys doped with chromium (4.2≤T≤300 K, B≤8 T). We observe nonlinearity in the magnetic field dependencies of Hall voltage and peaks of anomalous scattering in the temperature dependencies of resistivity. These are due to the conductivity via chromium impurity band and to the structural phase transition from the cubic NaCl phase to the rhombohedral GeTe phase. Magnetic field dependencies of the Hall coefficient RH(B) are calculated in the framework of two-band conduction model. The main parameters of charge carriers are determined. Pressure dependencies of the critical phase transition temperature are obtained.

Insulator-metal transition in Pb1-xGexTe doped with chromium under pressure

The galvanomagnetic effects in the n-Pb1-xGexTe (x=0.02-0.13) alloys, doped with chromium, at the temperatures in the range of 4.2≤T≤300 K and under hydrostatic compression of up to 17 kbar have been investigated. The insulator-metal transition, induced by pressure in Pb1-xGexTe: Cr (x=0.10) alloy, was revealed. Using the experimental data in the frame of two-band Kane dispersion relation the pressure dependences of electron concentration and Fermi energy were calculated. The pressure coefficient of chromium deep level energy was obtained and the diagram of the electronic structure reconstruction under pressure was proposed.