J. Przewoznik

Magnetoresistance in FeCoZr–Al2O3 nanocomposite films containing ‘metal core–oxide shell’ nanogranules

Temperature and magnetic field dependences of electrical conductivity are systematically studied in granular films (Fe45Co45Zr10)x(Al2O3)100?x (28???x???64) containing crystalline metallic ?-FeCo-based nanoalloy cores encapsulated in an amorphous oxide shell embedded in an amorphous Al2O3 matrix. Formation of ?metallic core?oxide shell? nanogranules is confirmed by transmission electron microscopy (TEM) and HRTEM. The structure of core and shell is governed with the difference in the oxidation states of Fe and Co ions investigated with EXAFS, XANES and M?ssbauer spectroscopy. A considerable negative magnetoresistance (MR) effect of spin-dependent nature is observed in the whole range of x values. Its increase with decreasing temperature is correlated with the magnetic saturation of superparamagnetic metallic nanogranules. The enhanced MR effect in ?core?shell? granular films is related to the percolation of oxide shells and their influence through spin filtering processes. A considerable high field MR at low temperatures and the resulting deviation of MR and squared magnetization are attributed to a magnetic randomness and/or strong magnetic anisotropy of the magnetic oxide shell.

Growth-induced non-planar magnetic anisotropy in FeCoZr-CaF2 nanogranular films: Structural and magnetic characterization

The relation between nanoscale structure, local atomic order and magnetic properties of (FeCoZr)x(CaF2)100−x (29u2009≤u2009xu2009≤u200973 at.u2009%) granular films is studied as a function of metal/insulator fraction ratio. The films of a thickness of 1–6u2009μm were deposited on Al-foils and glass-ceramic substrates, by ion sputtering of targets of different metal/insulator contents. Structural characterization with X-ray and electron diffraction as well as transmission electron microscopy revealed that the films are composed of isolated nanocrystalline bcc α-FeCo(Zr) alloy and insulating fcc CaF2 matrix. They grow in a columnar structure, where elongated metallic nanograins are arranged on top of each other within the columns almost normal to the substrate surface. Mossbauer spectroscopy and magnetometry results indicate that their easy magnetization axes are oriented at an angle of 65°–74° to the surface in films with x between 46 and 74, above the electrical percolation threshold, which is attributed to the growth-induced sh...

Structural and hyperfine characterization of σ-phase Fe–Mo alloys

A series of nine samples of sigma-Fe_{100-x}Mo_x with 44<x<57 were synthesized by a sintering method. The samples were investigated experimentally and theoretically. Using X-ray diffraction techniques structural parameters such as lattice constants, atomic positions within the unit cell and populations of atoms over five different sublattices were determined. An information on charge-densities and electric field gradients at particular lattice sites was obtained by application of the Korringa-Kohn-Rostoker (KKR) method for electronic structure calculations. Hyperfine quantities calculated with KKR were successfully applied to analyze Mossbauer spectra measured at room temperature.

Structural and electronic properties of the μ-phase Fe–Mo compounds

Abstract Structural (lattice parameters and sub lattice occupancies) and electronic (charge-density and electric field gradient) properties in a series of μ-Fe 100− x Mo x (37.5xa0⩽xa0 x xa0⩽xa044.5) compounds were studied experimentally (X-ray diffraction and Mossbauer spectroscopy) and theoretically (charge and spin self-consistent Korringa–Kohn–Rostoker Green’s function method). The lattice parameters a and c showed a linear increase with x while all five lattice sites were found to be populated by both alloying elements: A(1a) and B(6h) predominantly by Fe atoms whereas C(2c) and D(2c′) by Mo atoms hardly depending on the composition. The population of Fe atoms on the site E(2c″) was ranging between ∼50% at x xa0=xa037.5 and ∼20% at x xa0=xa044.5. Fe-site charge-density (isomer shift) and the electric field gradient (quadrupole splitting, QS ) were revealed to be characteristic of the lattice site and both of them were almost x -independent. The difference in the charge-density at Fe-atoms at the sites B (the highest value) and those at the sites D (the lowest value) was estimated as high as 0.18xa0e. The average charge-density increases linearly with x . The largest QS -values were those at the sites A and C, while the smallest ones at the site D. The average QS -value was 0.25xa0mm/s.

The influence of hydrogen on55Mn hyperfine fields in YMn2 hydrides

The NMR spin ccho measurements at 4.2 K for YMn2Hx (x=1, 2, 3) compounds are reported. The obtained values of55Mn hyperfine fields are analysed in dependence on the by hydrogen atoms and flips of neighbouring manganese moments. The strong increase of hyperfine field with increasing number of the nearest hydrogen atoms is explained by the change of orbital contribution to hyperfine field.