Solid State Phenomena Vols. 168-169

Abstract: . Magnetic properties of carbon–metal nanocomposites were investigated depending on the preparation conditions, metal content and phase composition. Their correlation with the structure is discussed.

Abstract: Nanocrystalline lithium-substituted manganese-zinc ferrites Li0.5xMn0.4Zn0.6-xFe2+0.5xO4 were prepared by the sol-gel autocombustion method. X-ray diffraction analysis (XRD) confirmed that samples are single-phase and that only a spinel phase is present. The saturation magnetization increases while the cell parameter of the cubic phase decreases with Li concentration. Magnetic permeability and dielectric permittivity of all samples were measured at room temperature as a function of frequency. Reflection loss calculations show that the prepared samples are good electromagnetic wave absorbers in microwave range. Li substitution plays an important role in changing the structural and magnetic properties of these MnZn ferrites.

Abstract: We study the phase transitions and critical phenomena in 3D site-diluted (with nonmagnetic impurities) Potts model with spin states q=4 by Monte-Carlo method. The systems with linear sizes L=20-32 and spin concentrations p=1.00, 0.90, 0.65 are examined. Using the Binder cumulants method the forth order it is shown that the second-order phase transition is observed in strongly diluted model at spin concentration p=0.65; the pure model (p=1.00) and weakly diluted one (p=0.90) reveals the first-order phase transition. On the basis of finite-size scaling theory the static critical parameters of heat capacity, susceptibility, magnetization, and correlation length exponent are calculated.

Abstract: The possibility of obtaining a nanostructured composite in the Ni-Mg-O system by ion-beam sputtering has been investigated. The structural, magnetic and magnetoresistive properties of obtained samples have been investigated in a wide concentration range. The presence of the nanostructure in the obtained samples with Ni nanogranules (2-3 nm) has been confirmed by transmission electron microscopy. There is no observation of any magnetic or magnetoresistive properties at room temperature in the Nix(MgO)100-x composites. These properties were observed at 77 K. The obtained data mean that Curie temperature of the Ni nanogranules is lower then 298 K. This is due to small size of nickel granules and low value of exchange interaction energy in nickel.

Abstract: In the paper, we will present experimental results on the Fe73.5CuNb3Si13.5B9 ferromagnetic resonance (FMR) line width as a function of annealing conditions, as well as the annealing-temperature dependence of the crystallite size, coercive force, and the magnetic anisotropy field. We shall show the interrelationship of these quantities, demonstrate that these parameters are all traceable to the nanoparticle crystallization, and indicate how they can be examined by extending the random anisotropy theory. To obtain quantitative parameters of random magnetic anisotropy we investigate the magnetization curves approaching saturation. This technique allows one to quantitatively determine the local magnetic anisotropy field (Ha) and magnetic anisotropy correlation radius (Rc), as well as to estimate the averaged magnetic anisotropy field (L) and ferromagnetic correlation radius (RL).

Abstract: The magnetization correlation function for two-phase one-dimentional chain of the ferromagnetic exchange-coupled nanoparticles with a random anisotropy was calculated. The dependence of the magnetic correlation radius and the average magnetic anisotropy constant on the intrinsic size of the phase inhomogeneity was founded. The data allows estimating the applicability limits of the analytical equations for the magnetization curves approaching saturation of two-phase ferromagnetic nanomaterials.

Abstract: Magnetocaloric effect (T-effect) was studied by direct method on three samples of Sm0.55Sr0.45MnO3 manganite: ceramic (C) sample and two single crystals, annealed in oxygen (O) or in air atmosphere (A). The temperature dependence of T-effect T(T) of all the samples has maximum at Tmax equal to 143.3 K for A-sample, 244 K for O-sample, and 143 K for C-sample. In these maxima T values are 0.8 K, 0.41 K, and 0.4 K for A-, O- and C-samples respectively. In addition, the T(T) curve of A-sample has minimum at Tmin = 120 K and T-value in minimum is equal to - 0.1 K. The maximum value of T-effect increases with H up to the maximum field of measurement 14.2 kOe. When this takes place the rate of this increasing is higher at H > 8 kOe than at H < 8 kOe. The above listed peculiarities of T-effect are explained by the presence in the samples of ferromagnetic, antiferromagnetic A-type and antiferromagnetic CE-type clusters.

Abstract: We have studied the magnetic susceptibility, magnetization and electron paramagnetic resonance (EPR) in the diluted magnetic semiconductor Pb1-x-yMgxYbyTe. The magnetic susceptibility of this system is a superposition of the matrix diamagnetism, the paramagnetic contribution of electrons localized in the ytterbium band, and the Curie-Weiss paramagnetism provided by Yb3+ ions. The concentration of Yb3+ ions monotonically increases with the ytterbium content, while a significant fraction of ytterbium ions remains in a nonmagnetic Yb2+ state. A pronounced hyperfine structure was revealed in the EPR spectra. It was ascribed to the interaction between the electron magnetic moment of an unfilled electron shell of Yb3+ ions and nuclear magnetic moment of the 173Yb isotope. The estimated values of the effective g-factor (g1.997 0.002) and the hyperfine-interaction constant (173A=0,56 cm-1) were found to be almost independent of both alloy composition and temperature.

Abstract: Solid solutions 1T-CrxTi1-xSe2 (x = 0-0,83) were synthesized. Single crystals in the concentration range x = 0-0.69 were grown. Structure of these materials was studied on single- and polycrystalline samples. It was determined that chromium atoms substitute for titanium atoms in the TiSe2 host lattice. XPS measurements of the core levels together with the resonance valence band spectra of the 1T-CrxTi1-xSe2, as well as absorption spectra of Ti and Cr were performed. According to the experimental XAS data and multiplet atomic calculation, titanium and chromium atoms have oxidation numbers of +4 and +3, respectively. The local magnetic moment of about 3 eV is detected at Cr atoms. A transition into the spin glass state is observed. The modeling calculations of chromium local density of states were carried out. These results are in a good agreement with the experimental ones and suggest that 3d density of electronic states of chromium that substitutes for titanium in the host lattice is spin polarized and has a half-metallic character.

Abstract: The effect of the nanocrystallization conditions on the structure, magnetic characteristics and temperature stability of the Fe73.5Cu1Nb3Si13.5B9, Fe73.5Cu1Nb1.5Мo1.5Si13.5B9, Fe69Cu1Nb1.5Мo1.5Si13.5B9Co4.5, and (Fe0.6Co0.4)86Hf7B6Cu1 nanocrystalline soft magnetic alloys has been analyzed. The samples were nanocrystallized in the absence of a field, in a dc or ac magnetic field. It has been shown that the Fe73.5Cu1Nb1.5Мo1.5Si13.5B9 alloy has unique magnetic properties and a high temperature-time stability. It has been found that nanocrystallization of the (Fe0.6Co0.4)86Hf7B6Cu1 amorphous alloy in an ac magnetic field considerably improves its static and dynamic magnetic properties.