Yu. V. Knyazev

Electronic structure, magnetic, and optical properties of the intermetallic compounds R2Fe17 (R=Pr,Gd)

In this paper we report comprehensive experimental and theoretical investigation of magnetic and electronic properties of the intermetallic compounds Pr2Fe17 and Gd2Fe17. For the first time electronic structure of these two systems was probed by optical measurements in the spectral range of 0.22-15 micrometers. On top of that charge carriers parameters (plasma frequency and relaxation frequency) and optical conductivity s(w) were determined. Self-consistent spin-resolved bandstructure calculations within the conventional LSDA+U method were performed. Theoretical interpetation of the experimental s(w) dispersions indicates transitions between 3d and 4p states of Fe ions to be the biggest ones. Qualitatively the line shape of the theoretical optical conductivity coincides well with our experimental data. Calculated by LSDA+U method magnetic moments per formula unit are found to be in good agreement with observed experimental values of saturation magnetization.

Crystal structure and magnetic anisotropy of ludwigite Co2FeO2BO3

Co3O2BO3 and Co2FeO2BO3 single crystals with a ludwigite structure are fabricated, and their crystal structure and magnetic properties are studied in detail. Substituted ludwigite Co2FeO2BO3 undergoes two-stage magnetic ordering at the temperatures characteristic of Fe3O2BO3 (TN1 ≈ 110 K, TN2 ≈ 70 K) rather than Co3O2BO3 (TN = 42 K). This effect is explained in terms of preferred occupation of nonequivalent crystallographic positions by iron, which was detected by X-ray diffraction. Both materials exhibit a pronounced uniaxial magnetic anisotropy. Crystallographic direction b is an easy magnetization axis. Upon iron substitution, the cobalt ludwigite acquires a very high magnetic hardness.

Optical conductivity and magnetic parameters of the intermetallic compounds R2Fe17−xMx (R=Y, Ce, Lu; M=Al, Si)

Abstract Optical properties of the intermetallic compounds R 2 Fe 17− x M x (R=Y, Ce, Lu; M=Al, Si; x =0, 1.7) were investigated in the energy range 0.083–5.64 eV. It was shown that the Curie temperature enhancement for substitutional alloys is accompanied by a decrease in the plasma frequency of the conduction electrons, which indicates a decrease in the density of states at the Fermi level N ( E F ). This correlation was explained using the spin-fluctuation model of Mohn and Wohlfarth. From the analysis of recent results on the electronic, structural and magnetic properties of the R 2 Fe 17− x M x compounds, a supposition is made about the connection of N ( E F ) with the distance between Fe ions in the ‘dumbbell’ crystal site.

Electronic structure and optical properties of TbNi5−xCux

In this paper we present theoretical investigation of optical conductivity for intermetallic TbNi_{5-x}Cu_x series. In the frame of LSDA+U calculations electronic structure for x=0,1,2 and on top of that optical conductivities were calculated. Disorder effects of Ni for Cu substitution on a level of LSDA+U densities of states (DOS) were taken into account via averaging over all possible Cu ion positions for given doping level x. Gradual suppression and loosing of structure of optical conductivity at 2 eV together with simultaneous intensity growth at 4 eV correspond to increase of Cu and decrease of Ni content. As reported before [Knyazev et al., Optics and Spectroscopy 104, 360 (2008)] plasma frequency has non monotonic doping behaviour with maximum at x=1. This behaviour is explained as competition between lowering of total density of states on the Fermi level N(E_F) and growing of number of carriers. Our theoretical results agree well with variety of recent experiments.

Uniaxial anisotropy and low-temperature antiferromagnetism of Mn2BO4 single crystal

Abstract The Mn 2 BO 4 single crystals have been grown by the flux technique. A careful study of the crystal structure and magnetic properties have been carried out. The antiferromagnetic transition at T N =26xa0K has been traced through the dc magnetization and specific heat temperature dependences. Magnetic uniaxial anisotropy has been found with easy axis magnetization lying in the ab-plane. The obtained value of effective magnetic moment is assigned to the non-quenched orbital moment of Jahn–Teller Mn 3+ ions. The discussion of magnetic properties is based on the superexchange interaction calculations.

Crystal and local atomic structure of MgFeBO4, Mg0.5Co0.5FeBO4 and CoFeBO4: Effects of Co substitution

Single crystalline MgFeBO_4, Mg_0.5Co_0.5FeBO_4 and CoFeBO_4 have been grown by the flux method. The samples have been characterized by X-ray spectral analysis, X-ray diffraction and X-ray absorption spectroscopy. The X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra have been measured at the Fe andCoK-edges over a wide temperature range (6.5 - 300 K). The composition, the charge state and local environment of both Fe and Co atoms have been determined. The effects of Mg substitution by Co on the local structural distortions have been revealed experimentally and the M-O bond anisotropy has been found.

Effect of the diamagnetic dilution on the magnetic ordering and electrical conductivity in the Co 3 O 2 BO 3 : Ga ludwigite

Single crystals of cobalt ludwigite Co3O2BO3 with diamagnetic substitution of Ga3+ ions for a part of the cobalt ions have been grown by the flux method. A detailed investigation of the crystal structure and magnetic properties of the compound has been carried out. A preferred character of the occupation of nonequivalent crystallographic positions by gallium has been revealed. It has been found that the effective magnetic moment and the magnetic ordering temperature are decreased compared to those in the original crystal of the Co3O2BO3 ludwigite. It has been noted that the pronounced magnetic anisotropy observed in the crystallographic ab plane of the original material of the Co3O2BO3 composition disappears in the presence of gallium.

Copper-doping effects in electronic structure and spectral properties of SmNi5

The electronic structure and optical properties of the SmNi5–xCux (x = 0, 1, 2) compounds are studied. The band spectra of the studied intermetallics were calculated with LDA + U + SO method supplementing the local density approximation with a correction for strong electron interaction on the shell of the rare-earth element. Optical properties were studied by ellipsometry method in the wide wavelength range. It was found that the substitution of copper for nickel leads to local changes in the optical conductivity spectra. Both the spectroscopic measurements and theoretical calculations demonstrate the presence of a broad absorption band around 4 eV associated with the Cu 3d → Ni 3d electron transitions and increasing with the grown of copper content. The experimental dispersion curves of optical conductivity in the interband absorption region were interpreted using the results of the calculations.

Evolution of the mössbauer spectra of ludwigite Co3 − xFexO2BO3 with substitution of iron for cobalt

A concentration series of single crystals of iron-cobalt ludwigites Co3 − xFexO2BO3 (x = 0.0125, 0.025, 0.050, 0.10, 1.0) has been synthesized. The structure has been studied using X-ray diffraction and Mössbauer effect. A preferred occupation of nonequivalent crystallographic positions by iron in the ludwigite structure has been revealed. It has been found that the valence of substituting iron ions is three. It has been revealed that the structure of the γ-resonance spectrum of Co2FeO2BO3 is complicated due to a composition disorder in the system.