G. N. Kakazei

Rhombohedral-to-orthorhombic transition and multiferroic properties of Dy-substituted BiFeO3

Investigation of crystal structure, ferroelectric, and magnetic properties of polycrystalline Bi1−xDyxFeO3 (0.1≤x≤0.2) samples was carried out. X-ray diffraction study revealed composition-driven rhombohedral-to-orthorhombic R3c→Pnma phase transition at x∼0.15. Both structural phases were found to coexist in a broad concentration range. Piezoresponse force microscopy found suppression of the parent ferroelectric phase upon dysprosium substitution. Magnetometric study confirmed that the A-site doping induces appearance of a weak ferromagnetic behavior. Both the ferroelectric and magnetic properties were shown to correlate with a structural evolution.

Tunnel magnetoresistance and magnetic ordering in ion-beam sputtered Co80Fe20/Al2O3 discontinuous multilayers

Discontinuous multilayered Co80Fe20(t)/Al2O3(30u200aA) thin films have been prepared by ion-beam sputtering. We report on structural, magnetic, and transport (for current in plane geometry) results obtained in this system. With growing nominal thickness t of the metal layers, which effectively characterizes the granular structure, a transition from tunnel to metallic conductance is observed, indicating the onset of infinite conducting paths at t>18u200aA. At t 13u200aA was detected from the magnetization data which display here a transition from superparamagnetic to ferromagnetic behavior. The measurements of tunnel magnetoresistance (MR) show that a sharp maximum of MR sensitivity to field takes place at this thickness, reaching ∼24%/kOe at room temperature. At least, MR itself as a function of t has a break at the same value. All these features suggest that some specific kind of percolation with respect to magnetic order occurs in o...

Ferromagnetic resonance in granular thin films

A theoretical analysis has been done of the effects of granule size, shape, orientation, and concentration and the temperature effects on the ferromagnetic resonance (FMR) field Hr in granular thin films. The granular CoxAg1−x thin films with 0.2 fp. For the Co–Ag system, persistence of a considerable SPM fraction is revealed by the superconducting quantum interference device data up to the highest f, and the effect at f=fp consists in a discontinuous jump of the Hr(f) slope. Otherwise, the FMR data for granular Fe–SiO2 films reveal a discontinuous jump in H...

Transport properties of discontinuous cCo/sub 80/Fe/sub 20//Al/sub 2/O/sub 3/ multilayers prepared by ion beam sputtering

Ion beam sputtered Co/sub 80/Fe/sub 20/(t)/Al/sub 2/O/sub 3/ (30 /spl Aring/) multilayers were obtained. The Co/sub 80/Fe/sub 20/ layers become discontinuous for nominal thickness t/spl les/18 /spl Aring/. Tunnel magnetoresistance was measured in CIP and CPP geometries, reaching up to 6.5% at room temperature and 11% at 15 K, for as-deposited films in CIP geometry. The temperature dependence of MR was found quite different for the two geometries: fairly strong in the CIP case and almost absent in the CPP geometry. A model is proposed to explain these large differences in behavior.

Resistive switching in nanostructured thin films

Planar capacitor structures based on granular films composed of nanometric ferromagnetic grains embedded in an insulating Al2O3 matrix can switch between a high-conductance and a low-conductance state. The switching properties are induced by a forming process. The ON/OFF resistance ratio is as high as 104 under an electrical field of only 15 kV/m. This resistive switching is accompanied by a capacitive switching between two well-defined voltage-independent states, a behavior that is not readily explained by the filamentary type of conduction.

Measurements of spin reorientation in YbFeO3 and comparison with modified mean-field theory

Precise measurements of YbFeO_3 magnetization in the spin-reoirentation temperature interval are performed. It is shown that ytterbium orthoferrite is well described by a recently developed modified mean field theory developed for ErFeO_3. This validates the conjecture about the essential influence of the rare earth ions anisotropic paramagnetism on the magnetization behavior in the reorientation regions of all orthoferrites with Gamma{4} ->Gamma{24} ->Gamma{2} phase transitions.

Influence of co-evaporation technique on the structural and magnetic properties of CoCu granular films

Abstract Structural, magnetic and transport properties of granular Co x Cu 1 − x ( x = 0.11–0.45) films prepared by e-beam co-evaporation, were studied. Scanning tunneling microscopy showed the surface crystallites having an elongated shape. Magnetization and FMR data indicated a large in-plane uniaxial anisotropy in all the samples. A correlation between the orientation of the surface crystallites and the anisotropy easy axis has been found. Magnetoresistance in the fields up to 0.5 T is strongly different for transverse and longitudinal geometries.

Magnonic crystals composed of Ni80Fe20 film on top of Ni80Fe20 two-dimensional dot array

Modulated structures consisting of square arrays of 60u2009nm thick Ni80Fe20 circular dots underneath a continuous Ni80Fe20 film were fabricated using multi-level process based on deep ultraviolet lithography at 248u2009nm exposure wavelength. We observed a drastic change in both the static and dynamic properties of the modulated structures by varying the Ni80Fe20 film thickness t in the range from 5 to 60u2009nm. It was revealed via comparison between experimental results and micromagnetic simulations that the dots create perturbations of internal fields in the neighbor regions of the film which can be controlled by magnetic field and film thickness.

Evidences for direct magnetic patterning via diffusive transformations using femtosecond laser interferometry

The application of femtosecond laser interferometry to direct patterning of thin-film magnetic alloys is demonstrated. The formation of stripe gratings with submicron periodicities is achieved in Fe1−xVx (xu2009=u200918–34u2009wt. %) layers, with a difference in magnetic moments up to Δμ/μ ∼ 20 between adjacent stripes but without any significant development of the topographical relief (<1% of the film thickness). The produced gratings exhibit a robust effect of their anisotropy shape on magnetization curves in the film plane. The obtained data witness ultrafast diffusive transformations associated with the process of spinodal decomposition and demonstrate an opportunity for producing magnetic nanostructures with engineered properties upon this basis.

Ferromagnetic resonance experiments in an obliquely deposited FeCo–Al2O3 film system

Granular cermet films (Fe50Co50)x–(Al2O3)1−x fabricated using the electron-beam coevaporation technique at oblique incidence of FeCo and alumina atom fluxes have been found to exhibit both oblique and in-plane uniaxial magnetic anisotropy. This anisotropy first appears just below the percolation threshold due to a magnetic coupling of particles taking place at a certain stage of their growth and coalescence. The FeCo content x varied from 0.07 to 0.49. A simple model of the film microstructure is presented based on the results of magnetization measurements and ferromagnetic resonance at intermediate (9.4 GHz) and high (94 GHz) frequencies. At 94 GHz the concentration dependence of the effective anisotropy field follows the solid solution law, since then the magnetic field is sufficient to magnetize the films close to saturation. The 9.4 GHz data points deviate from the solid solution line below the percolation threshold due to both modification of the resonance fields by intergranular interactions in nons...