Magnetic properties of Hydrogenated Li and Co doped ZnO nanoparticles

The effect of hydrogenation on magnetic properties of Zn0.85Co0.05Li0.10O nanoparticles is presented. It was found that the sample hydrided at room temperature (RT) showed weak ferromagnetism (FM) while that hydrided at 400oC showed robust ferromagnetism at room temperature. In both cases reheating the sample at 400oC in air converts it back into paramagnetic state (P) completely. The characterization of samples by X-ray and electron diffraction (ED) showed that room temperature ferromagnetism observed in the samples hydrogenated at RT is intrinsic in nature whereas that observed in the samples hydrogenated at 400oC is partly due to the cobalt metal clusters.Comment: 10 pages, 3 figure

Ab initio study of magnetism at the TiO2/LaAlO3 interface

In this paper we study the possible relation between the electronic and magnetic structure of the TiO2/LaAlO3 interface and the unexpected magnetism found in undoped TiO2 films grown on LaAlO$_3$. We concentrate on the role played by structural relaxation and interfacial oxygen vacancies. LaAlO3 has a layered structure along the (001) direction with alternating LaO and AlO2 planes, with nominal charges of +1 and -1, respectively. As a consequence of that, an oxygen deficient TiO2 film with anatase structure will grow preferently on the AlO2 surface layer. We have therefore performed ab-initio calculations for superlattices with TiO2/AlO2 interfaces with interfacial oxygen vacancies. Our main results are that vacancies lead to a change in the valence state of neighbour Ti atoms but not necessarily to a magnetic solution and that the appearance of magnetism depends also on structural details, such as second neighbor positions. These results are obtained using both the LSDA and LSDA+U approximations.Comment: Accepted for publication in Journal of Materials Scienc

Electrical and Magnetoresistance Properties of Composites Consisting of Iron Nanoparticles within the Hexaferrites

Nanocomposites containing Fe or FeCo (Fe-rich) dispersed in hexaferrites (M, W, or Y phase) are realized by the heterogeneous solid-gas reduction under H^sub 2^ + N^sub 2^. Transmission electron microscopy (TEM) studies show that metal nanoparticles precipitate coherently as thin flakes along the a-b planes of the hexaferrite lattice above the characteristic reduction temperature, T^sub R^ > 375°C. The electrical resistivity measurements reveal that the charge transport mechanism in the composites is by tunneling, whereas samples having higher fractions of the alloy particles show metallic behavior. Controlled reduction at T^sub R^ leads to apparent insulator-metal changeover in the p versus T plot. This changeover persists even in the presence of a high magnetic field (7 T) and is ascribed to the percolation of metal particles caused by the difference in the coefficient of thermal expansion between the constituents. In the insulator regime, negative magnetoresistance (MR) of ~5-9% is observed at 25°C. Further, p-T curves by the two-probe method exhibit hysteretic behavior caused by large inhomogeneity in the distribution of metal content and the time-dependent charge accumulation (Coulomb blockade) at the metal granules for these composites. They also exhibit nonlinearity in the current-voltage (I-V) characteristics with the nonlinearity coefficient ranging from 1.2 to 1.4 at different temperatures

Structural and magnetic characteristics of cobalt ferrite-coated nano-fibrous γ−Fe2O3\gamma-Fe_2O_3

The presence of mesopores in $\gamma-Fe_2O_3$ is a limiting factor for its optimal magnetic properties. Therefore, high coercive $\gamma-Fe_2O_3$ nano-fibrous particles are obtained by coating cobalt ferrite on the surface. An aqueous solution cobalt nitrate+ammonium ferrous sulphate (in the ratio 1:2) precipitated at pH>12.5 in the presence of uniformly dispersed $\gamma-Fe_2O_3$ yields cobalt ferrite coating on the surface of the particles at $\sim90^oC.$ The nature of the coating, studied by X-ray diffraction and high-resolution electron microscopy (HREM), reveals the epitaxial growth of the $CoFe_2O_4$ layer on $\gamma-Fe_2O_3$ provided the extent of coating $(X_{Co})$ is maintained $\leq30 wt\%.$ The pores within the $\gamma-Fe_2O_3$ particles are coherently filled up as deciphered by the continuity in the HREM lattice fringes. The surface characteristics using X-ray photoelectron spectroscopy analysis further confirm the coherent coating of cobalt ferrite onto the $\gamma-Fe_2O_3$ surface. Studies of cobalt ferrite-coated samples by $M\H{o}$ossbauer spectra clearly reveal the hyperfine field characteristics of $\gamma-Fe_2O_3$ notwithstanding the $CoFe_2O_4$ coating at lower contents. As the thickness of the coating increases $(\sim30 wt\%),$ the spectrum consists of two overlapping sextets (one from the average hyperfine pattern of $\gamma-Fe_2O_3$ and A-site of $CoFe_2O_4$ and the second from the B-site in the bulk $CoFe_2O_4).$ The deposition of $CoFe_2O_4$-rich surface layer results in an increase in the coercive field (Hc = 585 Oe for XCo = 10 wt%; Hc = 1100 Oe for XCo = 30 wt%), which attributed to the formation of uniaxial anisotropy under the influence of the field created by the magnetization of the particles. Saturation magnetization $(\sigma_{s})$ also increases from 74.4 emu/g (uncoated) to $\sim78.6 emu/g$ (XCo = 30 wt%) with the extent of coating. In comparison, for Co-substituted nano-fibrous $\gamma-Fe_2O_3$ the presence of homogeneously distributed $Co^{2+}$ ions (10 at%) enhances the Hc (609 Oe) as a result of high magnetocrystalline anisotropy accompanied by shape anisotropy, whereas, $\sigma_{s}$ decreases $(\sim60 emu/g)$ due to the relaxation of spins on the surface atoms of slender particles with mesopores as revealed by $M\H{o}$ossbauer spectroscopy for $Co^{2+}$-substituted $\gamma-Fe_2O_3.

Precipitation of acicular hydrogoethite (α−FeOOH⋅xH2O;0.1<x<0.22)(\alpha-FeOOH·xH_2O; 0.1 < x < 0.22) using morphology controlling cationic additives

Acicular hydrogoethite $(\alpha-FeOOH·xH_2O; 0.1<x<0.22)$ particles of high aspect ratio of $\geq10$ and narrow size distribution with length ~0.5 μm are prepared by air oxidation of $Fe(OH)_2{\cdot}xH_2O$ (80<x<120) in presence of morphology controlling cationic additives (0.01 at.% of $Pd^{2+}$ or $Rh^{3+}$). Growth of hydrogoethite particle proceeds by the oxidation of $Fe(OH)_2{\cdot}xH_2O$ to an intermediate phase through the nucleation within the amorphous ferric oxy-hydroxide [FeO_x(OH)_3_-_2x{\cdot}yH_2O] and growth of $\alpha-FeOOH·(H_2O)_x$ acicular particles with the aid of morphology controlling agents, as evidenced from X-ray diffraction (XRD) and transmission electron microscope (TEM) studies. Preferential adsorption of additives on certain crystallographic planes and thereby retarding the growth in the perpendicular direction, allows the particles to acquire acicular shape with high aspect ratio ~10. Thermal analyses of hydrogoethite samples show excess weight loss (3–4%) compared to goethite samples. Presence of $H\cdot\cdot\cdot O\cdot\cdot\cdot H$ vibrational absorptions arising from strongly coordinated water molecules in hydrogoethite and its absence in goethite is evidenced from IR spectra. $H_2O$ molecules in hydrogoethite are located in the strands of channel formed in between the double ribbons of $4FeO_6$ octahedra running parallel to c-axis of goethite, wherein the water molecules are bridging between the nearest neighbour cations