Giant magnetoresistance in electrodeposited multilayers

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Preparation, magnetic studies and band structure calculation of NiFe2O4 nanoparticles

We have undertaken a comprehensive theoretical study of the band structure, density of states, dependence of the Curie point and saturation magnetization on the size of NiFe2O4 nanoparticles prepared by the conventional ceramic method. Commercially available NiFe2O4 powder was first annealed in an oxygen environment in a furnace at 1100oC for 3h. The X-ray diffraction pattern indicated that the sample was single-phase at this stage. The average grain size estimated by scanning electron microscopy (SEM) was in the range of 300 to 350nm. The magnetic behavior of the sample at room temperature was studied by means of a superconducting quantum interference device (SQUID). The Curie temperature of the nickel ferrite powder was measured using an LCR meter. The measurement of the Curie temperature and saturation magnetization indicated that a decrease in the grain size leads to a decrease in the Curie temperature and in the saturation magnetization. The small value of saturation magnetization was attributed to a spin-glass-like surface layer on the nanocrystalline nickel ferrite with a ferrimagnetically aligned core (H.Nathani and S.Gubbala 2004 J.Mater. Sci. and Engin. B 111 95). Good agreement was obtained between theory and experimental results

Structural Design and Characterization of BaMgxCo2−xFe16O27BaMg_{x}Co_{2-x}Fe_{16}O_{27} Hexaferrites Based on ab initio Computations

Structural design of barium hexaferrites $BaMg_xCo_{2-x}Fe_{16}O_{27}$ (x=0.0, 1, 2) has been studied, and the magnetic and electronic structure of that has then been investigated using first principle total energy calculation. All calculations are based on the density functional theory. In order to improve the description of strongly correlated 3d electrons of iron, the general gradient approximation plus Hubbard U (GGA+U) method is used. We found that in the lowest energy configuration Mg and Co ions preferentially occupy the 6g sites. With the increase of Mg content x, the energy gap of $BaMg_xCo_{2-x}Fe_{16}O_{27}$ increases but the lattice constant of unit cell decreases. The magnetic moment of the unit cell for Mg content x=0, 1, and 2 are calculated to be 52, 49 and 46 $μ_{B}$/cell, respectively, in agreement with previous experimental results. The substitutions of Mg and Co at the $BaFe_2^{2+}Fe_{16}^{3+}O_{27}$ decrease electrical conductivity and transit it from a half-metal to semiconductor material. Based on our calculations on electronic band structure, the $BaFe_2Fe_{16}O_{27}$ (BFFO) is a weak half-metal, but $BaMg_2Fe_{16}O_{27}$ (BMFO), $BaMgCoFe_{16}O_{27}$ (BMCFO) and $BaCo_2Fe_{16}O_{27}$ (BCFO) are semiconductors. The electrical resistivity increases by increasing Mg and Co contents due to increase in porosity which prevents the hopping of charge carriers