Solid State Phenomena Vols. 152-153

Abstract: A comparative study of electronic properties of the two isostructural narrow-band semiconductors FeSb2 and RuSb2 by means of experimental NQR spectroscopy as well as theoretical ab-initio band structure calculations is presented. The temperature dependence of 1/T1 consists of two distinct intervals: above 40 K (HT) with activated behavior for FeSb2 with D/kB @ 450 K and below 40 K (LT) with smooth maximum at 10 K. Here the relaxation is governed by in-gap states. We propose the model of inherent Sb-deficiency (as prepared non-stoichiometry) of both FeSb2 and RuSb2 as a possible reason of the in-gap states. This results in creation of a small portion of Fe (Ru) ions possessing formal oxidation number +3 with d5 configuration and forming narrow energy level of localized S = ½ spins near the bottom of the conduction band. Due to much higher gap value in RuSb2, the activation mechanism for RuSb2 in the HT range is inefficient and the 1/T1 dependence in the HT range is more close to T2 behavior characteristic for phonon relaxation mechanism by two-phonon (Raman) scattering.

Abstract: 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.

Abstract: 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.

Abstract: Compounds with the Ca3Ga2Ge4O14 structure containing Co2+, Mn2+ and Fe3+ ions in 3f position were synthesized and their magnetic properties were studied at low temperatures. Long range antiferromagnetic ordering in triangular net lattice has been observed with TN = 7–40 K.

Abstract: Structural, magnetic and transport properties of undoped and Mn-doped quasi one-dimensional ZnO nano-wires formed as dendrite crystals of different width have been studied. All Mn-doped nanowires (Mn content 16% and 27 %) exhibit a ferromagnetic behavior, which is significantly temperature dependent. A small magnetic signal detected up to the room temperature is supposed to be due to small amounts of magnetic impurities. Analysis of the temperature dependencies of magnetization shows that the observed magnetic properties can be caused by the host ferromagnetic matrix with Curie temperature TC  40 K.

Abstract: The transport and magnetic properties of the Eu1-xCaxB6 (0x0.4) single crystals were studied in the wide ranges of temperatures (1.8-300 K) and magnetic fields (up to 8 T). The experimental data allow to identify a metal-insulator transition (MIT) at the critical Ca concentration xMIT≈0.3, which agrees well with the predictions of double exchange model (V.M. Pereira et al. Phys. Rev. Lett., 93 (2004) 147202). A significant enhancement of magnetoresistive effect is observed below 100K for Eu1-xCaxB6 compounds corresponding to the metallic side of the MIT (xxMIT). The drastic decrease of paramagnetic Curie temperature evaluated from the magnetic and magnetotransport data is discussed in terms of quantum MIT scenario recently proposed for this low carrier density system.

Abstract: Based on Mn-doped chalcopyrite ZnSiAs2 the new dilute magnetic semiconductor with p-type conductivity was produced. The Curie temperature behavior of the produced semiconductor is distinctly dependent on the Mn concentration: 325 K for 1 wt.% and 337 K for 2 wt.% of Mn, consequently. Magnetization, electrical resistance, magnetic resistance and Hall effect of mentioned compositions were studied. Temperature dependence of magnetization M(T) have complicate behavior. For T  15 K the M(T) dependence is characteristic for superparamagnetic and at T > 15 K magnetization is sum of magnetizations of ensemble of superparamagnetic clusters and ferromagnetic phase contained frustration regions.

Abstract: The effect of glass coating thickness on the magnetoimpedance in Co-based amorphous microwires was studied. The microwires with different thickness of the glass layer were obtained by means of etching in hydrofluoric acid solution. It was observed that the value of the external mag¬netic field corresponding to the maximum of the impedance decreased with the thickness of the glass layer, whereas the magnetoimpedance ratio did not change significantly. The experimental results are interpreted in terms of changes in the radial distribution of the easy magnetization axes.

Abstract: The effect of noise on the reversal of a magnetic dipole is investigated on the basis of computer simulation of the Landau-Lifshits equation. It is demonstrated that at the reversal by the pulse with sinusoidal shape, there exists the optimal duration, which minimizes the mean reversal time (MRT) and the standard deviation (SD, jitter). Both the MRT and the jitter significantly depend on the angle  between the reversal magnetic field and the anisotropy axis. At the optimal angle the MRT can be decreased by a factor of 7 for damping =1 and up to 2 orders of magnitude for =0.01, and the jitter can be decreased from 1 to 3 orders of magnitude in comparison with the uniaxial symmetry case. It has been demonstrated that fluctuations can not only decrease the reversal time, as it has been known before for the magnetic systems and is correct for small angles only, but it can also significantly, up to the factor of two, increase the reversal time.

Abstract: The structure and microwave magnetic performance of Fe, Fe-Si-C, and Fe-Co-Si-C powders fabricated by mechanical milling has been studied. The study was aimed at revealing of the effect of shape, composition, and structure of the powder particles on the microwave frequency dispersion of permeability of the powder-filled composites in the frequency range of 0.1−3 GHz. At low frequencies, below 1 GHz, the main reason causing the differences in the microwave magnetic properties of the powders is the shape of powder particles. At higher frequencies, the magnetic performance is mainly due to the effect of eddy currents and is determined by the size of powder particles. The difference in the composition of the powders under study and, therefore, corresponding differences in intrinsic permeability of these have a minor effect on the microwave magnetic performance as compared to the effects of particle size and shape.