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The relation of the magnetically dead layer and structural defects in ultrafine interacting NiFe2O4 nanoparticles ( = 4 nm) have been investigated using transmission electron microscopy, X-ray diffraction, ^57Fe M\"ossbauer spectroscopy, and dc magnetization and ac susceptibility measurements. According to the magnetic data, we found out three magnetic subsystems in NiFe2O4 nanoparticles. The first one with the lowest blocking (spin freezing) temperature (TS = 8 K) established by atomic magnetic moments of magnetically disordered particles with the d < 4 nm. The other two subsystems are formed by the magnetic moments of the "core" of nanoparticles having size more than 4 nm and correlated surface spins in nanoparticle clusters, correspondingly. Magnetic moments of ferrimagnetically ordered "core" are blocking at a higher temperature ( \approx 40 K). It has been shown that the most significant contribution to the energy dissipation is made upon blocking of the correlated nanoparticle surface spins from the magnetically dead layer on the nanoparticles' surface. By the magnetic data, the thickness of this layer is dmd \approx 1 nm for a particle with the \approx 4 nm. At the same time or meanwhile, the ^57Fe M\"ossbauer spectroscopy has revealed a structural disorder penetrating to a depth of up to dcd \approx 0.6 nm in a particle with = 4 nm. This evidences for a faster destruction of the magnetic order as compared with the crystal order upon moving away from the center of a particle to its periphery., Comment: 20 pages, 9 figures

HoFe1-xMnxO3 (0 < x < 1) single crystals have been grown by the optical floating zone technique. A structural transition from the orthorhombic to hexagonal modification has been established in the crystals in the concentration range of 0.7-0.8, which has been confirmed by the X-ray diffraction data. For a series of the rhombic crystals, the room-temperature Mossbauer study and magnetic measurements in the temperature range of 4.2-1000 K have been carried out. It has been observed that, with an increase in the manganese content in the samples, the temperature of the spin-reorientation transition increases significantly: from 60 K in the HoFeO3 compound to room temperature in HoFe0.6Mn0.4O3. The magnetic measurements have shown that, upon substitution of manganese for iron, the magnetic orientational type transition changes from a second-order transition (AxFyGz -> CxGyFz) to first-order one (AxFyGz -> GxCyAz) with a weak ferromagnetic moment only in the b direction (for Pnma notation). The growth of the spin-reorientation transition temperature has been attributed to the change in the value of the indirect exchange in the iron subsystem under the action of manganese, which has been found when studying the Mossbauer effect in the HoFe1-xMnxO3 (x < 0.4) compound.

The magnetic contribution of the Co3+ ions in Co3BO5 has been investigated using the Co (K-edge) XMCD, dc magnetic susceptibility, and heat capacity measurements. The crystal structure of Co3BO5 single crystal has been solved in detail at the T range 296-703 K. The results have been supplemented by the GGA+U calculations., Comment: 38 pages, 15 figures

Single crystalline Mn2-xFexBO4 with x = 0.3, 0.5, 0.7 grown by the flux method have been studied by means of X-ray diffraction and X-ray absorption spectroscopy at both Mn and Fe K edges. The compounds were found to crystallize in an orthorhombic warwickite structure (sp. gr. Pnam). The magnetization data have revealed a spin-glass transition at TSG= 11, 14 and 18 K for x= 0.3, 0.5 and 0.7, respectively., Comment: 9 pages, 2 tables, 9 figures

The Mn2BO4 single crystals have been grown using the flux technique. The careful study crystal structure and magnetic properties have been carried out. The antiferromagnet transition at TN = 26 K has been traced through the dc magnetization and specific heat temperature dependences. The magnetic uniaxial anisotropy has been detected with easy axis of magnetization lying in ab-plane. A reduction of the effective magnetic moment value is assigned to the non-quenched orbital moment of Jahn-Teller Mn3+ ions. Based on the superexchange interactions calculations the magnetic behavior is discussed., Comment: 20 pages, 5 figures, 3 tables

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., Comment: 21 pages, 11 figures, 4 tables

We report experimental and theoretical studies of magnetic and related properties of ErRuSi compound. Various experimental techniques such as neutron diffraction, magnetization, magneto-thermal, magneto-transport, optical have been used to study the compound. Neutron diffraction shows ferromagnetic ordering at low temperatures with moments aligned in ab plane. Neutron diffraction and magnetization data show reduction in magnetic moment, which may be due to crystalline electric field effects at low temperatures. The compound shows good magnetocaloric properties with a low field adiabatic temperature change of 4.7 K, which is larger than that of many proposed materials for magnetic refrigeration at low temperatures. Magnetoresistance shows large negative value at 8 K, which changes its sign and increases in magnitude, with decrease in temperature and/or increase in field. The positive MR at low temperatures attributed to the Lorentz force effect. The electronic structure calculations accounting for electronic correlations of the 4f electrons of Er reproduces the ferromagnetic ordering and effective magnetic moment. Interband transitions between the Ru and Er d states and Er f states in one spin projection are found to form the main features of the measured optical conductivity in this compound., Comment: 14 pages, 10 figures

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., Comment: 16 pages, 5 figures, 1 table