Surface Effects in Superparamagnetic Magnetite Particles *

Abstract. We have studied the properties of unshielded and shielded (hybrid) nanosized spherical magnetite particles with diameter 10 ± 2 nm, which are superparamagnetic at room temperature, through magnetometry, X-ray powder diffraction, magnetic force microscopy imaging and Mössbauer spectroscopy. The unshielded material was prepared by co-precipitation either in air or in inert atmosphere and part of it was shielded subsequently by β-cyclodextrin (β-C42H70O35). The studies indicated that in the unshielded particles there is a surface layer with a depth of 3 nm wherein an exponential rise in the number of vacancies is observed in the octahedral sublattice ([B]-sites), so that the particle surface is highly defective and could be represented by the general formula (Fe , where x = 0-0.3 and denotes vacancies. In contrast, the hybrid particles, being protected from oxidation, are structurally close to bulk magnetite so that the surface magnetic effects could be understood as due to the chaotic orientation of the magnetic moments at the magnetic-nonmagnetic material interface. An empirical model for a spherical single domain particle was developed in attempting to describe the evolution of structural defects in the surface layer

Effect of cation substitutions in Y-type Ba0.5Sr1.5Me2Fe12O22hexaferrites on the magnetic phase transitions

peer reviewedWe investigated the magnetic properties and magnetic phase transition in Y-type Ba0.5Sr1.5NiMgFe12O22 hexaferrite powder prepared by citrate sol-gel spontaneous combustion. The saturation magnetisation value of 32 emu/g at 4.2 K was lowered to 24 emu/g at 300 K. The magnetisations curves did not saturate even at a magnetic field of 50 kOe for both temperatures - 4.2 K and 300 K. A step-like behaviour appeared in the initial magnetisation curve at 4.2 K. A magnetic phase transformation from a spiral magnetic ordering to a conical spin one was observed at 40 K

Magnetic phase transitions in Ba0.5Sr1.5Zn2Fe11.92Al0.08O22hexaferrites

peer reviewedWe report studies on the effect of substituting the magnetic Fe3+ cations with nonmagnetic Al3+ cations in Y-type hexaferrite Ba0.5Sr1.5Zn2Fe11.92Al0.08O22 powders on their magnetic properties and especially on the magnetic phase transitions responsible for observing the magnetoelectric effect. In this research, the Y-type hexaferrite powders were synthesized by citric acid sol-gel auto-combustion. After the auto-combustion process, the precursor powders were annealed at 1170 °C in air to obtain the Y-type hexaferrite materials. The effects of Al substitution on the structural, microstructural properties and phase content were investigated in detail using X-ray powder diffraction and scanning electron microscopy. Hysteresis measurements were performed by a physical-property-measurement-system (PPMS) (Quantum Design) at 4.2 K and at room temperature. Dc-magnetic measurements of the temperature dependence of the magnetization at magnetic fields of 50 Oe, 100 Oe and 500 Oe were used to determine the effect of applying a magnetic field on the temperature of magnetic-phase transitions. We demonstrated that the helical spin state can be modified further by varying the magnetic field

Network structure of molybdate glasses by neutron and X-ray diffraction and reverse Monte Carlo modelling

Rare-earth molybdate glasses have been prepared by rapid quench technique, the network structure was investigated by neutron and high-energy X-ray diffraction. For data evaluation the reverse Monte Carlo simulation technique was applied to obtain a possible 3dimensional network configuration, which is consistent with the experimental data. From the modelling the partial atomic correlation functions g$_{iJ}$(r) and the coordination number distributions CN$_{iJ}$ have been revealed. Formation of MoO$_4$ (55%) and MoO$_6$ (25%) units was established for the binary 90MoO$_3$-10Nd$_2$O$_3$ glass. The B-O first neighbour distribution show a relatively broad first neighbour distance at 1.40A, the average coordination numbers show the presents of trigonal BO$_3$ and tetrahedral BO$_4$ groups. For 50MoO$_3$-25Nd$_2$O$_3$-25B$_2$O$_3$ sample mixed MoO$_4$-BO$_4$ and MoO$_4$-BO$_3$ linkages form pronounced intermediate-range order

Network structure of Mo-oxide glasses

The structure of molybdate glasses have been investigated by neutron and high-energy X-ray diffraction coupled with Reverse Monte Carlo (RMC) simulation technique. From the modelling the partial atomic correlation functions gij(r), the coordination number distributions CNij and bond angle distributions have been revealed. For binary 90MoO3-10Nd2O3 glass composition the fraction of MoO4/MoO6 was 0.55/0.25. Three type of ternary system have been studied, where the most important structural units was authenticated. For MoO3-Nd2O3-B2O3 sample mixed MoO4-BO4 and MoO4-BO3 linkages form pronounced intermediate-range order. In case of MoO3-ZnO-B2O3 series the BO3 and BO4 units are linked to MoO4 and/or ZnO4, forming mixed MoO4-BO4(BO3), MoO4-ZnO4 and ZnO4-BO4(BO3) bond-linkages

Magnetic cation distribution in a nanocrystalline Fe(3)O(4) spinel

Neutron diffraction study of nanostructured magnetite was performed at room temperature. The actual grain-size was determined from evaluation of X-ray diffraction profiles by the modified Williamson-Hall plot. The atomic structure parameters and the sublattice magnetic moments were determined using the Rietveld refinement of the neutron diffraction spectra. The sublattice magnetization at the octahedral sites decreases with decreasing grain-size, while it remains practically constant at the tetrahedral sites

Structural and electrochemical characterization of yttrium doped barium cerateBaCe0.85Y0.15O3-α for applications in solid oxide fuel cells

International audienceThe aim of this work is to achieve a deeper understanding of the conductivity mechanisms in BaCe0.85Y0.15O3-α (BCY15) material displaying good proton conductivity. The study is directly related to the application of BCY15 in an innovative and competitive concept for a high temperature fuel cell operated in reverse mode in the range of 600-700 °C. New approach for combining the information on atomic level by X-ray and neutron-diffraction (ND) and on macro level by impedance spectroscopy for deeper insight into the origin of its mixed (proton and oxide-ion) conductivity is applied. Single-phase BaCe0.85Y0.15O3-α samples with different porosity obtained by addition of graphite powders as pore former and changing sintering conditions were characterized in wet and dry atmosphere and their conductivity was measured in air and hydrogen in wide temperature range. The first neutron diffraction data collected at different temperatures show that the crystal structure of the samples adopts orthorhombic symmetry independently of their porosity and preparation technological conditions. Oxygen vacancies remain random at room temperature. Comparison of the proton and oxide-ion conductivity indicates that at operating temperatures (600-700 oC) they are equal. The results obtained from the electrochemical studies show that porosity of about 30-35% ensures an optimal microstructure in respect to conductivity and mechanical stability

Magnetic properties of nanosized MgFe2O4 powders prepared by auto-combustion

peer reviewedTargets were prepared to be used for magnetron sputtering and laser ablation and their microstructural and magnetic properties were investigated. The base material was nanosized MgFe2O4 powder produced by citrate auto-combustion synthesis. The auto-combusted powders were annealed at temperatures in the range 600 - 1000°C in air to study the effect of temperature on thofe formation MgFe2O4. The saturation magnetization Ms was 24.30 emu/g at room temperature. © Published under licence by IOP Publishing Ltd