Temperature Dependence of Anisotropy in Ti and Gd Doped NiMnGa-Based Multifunctional Ferromagnetic Shape Memory Alloys

The temperature dependence of magnetocrystalline anisotropy was investigated in detail for the polycrystalline Ni50Mn25Ga25, Ni50Mn25Ga20Ti5 and Ni50Mn25Ga20Gd5 ferromagnetic shape memory alloys in the temperature range of 50–400 K. The effective anisotropy constant was estimated from a series of high field magnetization curves based on the fitting procedure according to the law of approach to magnetic saturation. The low temperature martensitic phase was found to have a significantly higher anisotropy energy in comparison to a high temperature austenitic phase, which was observed through a sudden, distinct drop of anisotropy energy. The calculated values of the effective anisotropy constant were comparable to the results published by other authors. Moreover, the strong influence of chemical composition on the first-order phase transition and the second-order ferromagnetic to the paramagnetic transition was revealed. Finally, the strong coupling between the temperature dependence of the coercive field and the temperature dependence of magnetocrystalline anisotropy was also shown and discussed in the present study.

EFFECT OF THICKNESS ON THE STRUCTURAL, MICROSTRUCTURAL, ELECTRICAL AND MAGNETIC PROPERTIES OF Ni FILMS ELABORATED BY PULSED ELECTRODEPOSITION ON Si SUBSTRATE

We have studied the effect of thickness on the structural, microstructural, electrical and magnetic properties of Ni films electrodeposited onto [Formula: see text]-Si (100) substrates. A series of Ni films have been prepared for different potentials ranging from [Formula: see text]1.6[Formula: see text]V to [Formula: see text]2.6[Formula: see text]V. Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), four point probe technique, atomic force microscopy (AFM) and vibrating sample magnetometry (VSM) have been used to investigate the physical properties of elaborated Ni thin films. From the analysis of RBS spectra, we have extracted the films thickness [Formula: see text] ([Formula: see text] ranges from 83[Formula: see text]nm to 422[Formula: see text]nm). We found that the Ni thickness, [Formula: see text] (nm), linearly increases with the applied potential. The Ni thin films are polycrystalline and grow with the [Formula: see text] texture. The lattice parameter [Formula: see text] (Å) monotonously decreases with increasing thickness. However, a positive strain was noted indicating that all the samples are subjected to a tensile stress. The mean grain sizes [Formula: see text] (nm) and the strain [Formula: see text] decrease with increasing thickness. The electrical resistivity [Formula: see text] ([Formula: see text]cm) increases with [Formula: see text] for [Formula: see text] less than 328[Formula: see text]nm. The diffusion at the grain boundaries may be the important factor in the electrical resistivity. From AFM images, we have shown that the Ni surface roughness decreases with increasing thickness. The coercive field [Formula: see text], the squareness factor [Formula: see text], the saturation field [Formula: see text] and the effective anisotropy constant [Formula: see text] are investigated as a function of Ni thickness and grain sizes. The correlation between the magnetic and the structural properties is discussed.

Magnetic and Microstructural Properties of Cobalt Substituted NiZn Ferrite Powders

This paper presents the results of magnetic and microstructural study of cobalt-substituted NiZn ferrite powders, Ni0.5Zn0.3Co0.2Fe2O4, synthesized with the solid-state reaction method. The lattice parameter decreased with Co substitution, as the ionic radius of cobalt is less than that of zinc. Raman spectroscopy showed clear peaks of the A1g, Eg, and F2g modes, revealing cubic spinel structure and good crystallinity of the samples. The saturation magnetization reduced in the substituted sample to 50.28 emu/g at room temperature. In comparison to the bulk ferrite, the powder sample showed pronounced coercivity even at room temperature. The temperature dependence of the magnetization had superparamagnetic curvature, which yielded the effective anisotropy constant equal to 8.832 kJ/m3.

The dependence of critical current density of GdFeCo layer on composition of thermally assisted STT-RAM

Amorphous rare earth–transitional metal (RETM) GdFeCo memory layer with RE- and TM-rich compositions was fabricated in stacks of GdFeCo (10 nm)/Cu (3 nm)/[Co(0.2 nm)/Pd(0.4 nm)]6. Their magnetic properties and spin transfer torque (STT) switching of magnetization were investigated. The maximum magneto-resistance (MR) was around 0.24% for the TM-rich Gd[Formula: see text] (Fe[Formula: see text]Co[Formula: see text])[Formula: see text] memory layer and was −0.03% for the RE-rich Gd[Formula: see text] (Fe[Formula: see text]Co[Formula: see text])[Formula: see text] memory layer. The critical current densities [Formula: see text] to switch the GdFeCo memory layers are in the range of [Formula: see text] A/cm2–[Formula: see text] A/cm2. The dependence of critical current density [Formula: see text] and effective anisotropy constant [Formula: see text] on Gd composition were also investigated. Both [Formula: see text] and [Formula: see text] have maximum values in the Gd composition range from 21–29 at.%, suitable for thermally assisted STT-RAM for storage density exceeding Gb/inch2.

Magnetic and Microstructure Study of Thin Films of FeCuNbMoSiB FINEMET Alloy

Thin films of FeCuNbMoSiB have been sputtered on Corning glass substrates with thicknesses varying from 10 to 200 nm with post annealing at 450 °C and 550 °C. Annealing in the presence of the magnetic field applied along the plane of a substrate develops an uniaxial magnetic anisotropy with the in-plane easy axis. Estimation of the effective anisotropy constant from the magnetization measurements gave Keff = 3.23 kJ/m3. Structure and surface of the films were investigated with the X-ray powder diffraction (XRD), resistivity measurements, and Raman spectroscopy. XRD and resistivity analyses show that thermal annealing at 550 °C improves the crystalline fraction and Fe-Si grain size. Raman spectra identified hematite, goethite, magnetite, as well as graphite contamination of film surfaces.

Effective Anisotropy Constant of Bilayer Film

A reasonable composition of different kinds of magnetic anisotropy of bilayer film has been carried out. The criterium of magnetic uniaxiality of bilayer film contained the first-order and the second-order crystallographic anisotropy has been defined. It has been demonstrated that the effective constants of the first-order and the second-order magnetic anisotropy of bilayer film has a non-linear dependence on the corresponding constants of crystallographic anisotropy of the layers, angles between them and constants of interlayer exchange interaction.

Microstructure and Superparamagnetic Properties of Mg-Ni-Cd Ferrites Nanoparticles

Magnesium substituted nickel cadmium ferrite nanoparticles MgxNi0.6−xCd0.4Fe2O4(fromx= 0 to 0.6 with step 0.1) have been synthesized by the chemical coprecipitation route. X-ray diffraction (XRD) and infrared spectroscopy (FTIR) revealed that the obtained powders have a single phase of cubic spinel structure. The crystallite sizes calculated from XRD data have been confirmed using transmission electron microscopy (TEM) showing that the powders are consisting of nanosized grains with an average size range 5–1.5 nm. Magnetic hysteresis loops were traced at 6.5 K as well as at room temperature using VSM. It was found that, due to the Mg2+-ions substitution, the values of saturation magnetizationMsfor the investigated samples were decreased, whereas the coercive fieldHcincreased. Both zero field cooling (ZFC) and field cooling (FC) curves are measured in the temperature range (6.5–350 K) and the values of blocking temperatureTBwere determined. No considerable variation in the values ofTBwas observed with increasing Mg-content, whereas the values of the effective anisotropy constantKeffwere increased.

On the Calculation of Effective Anisotropy Constant of Nanoparticle

A correct composition of two kind of magnetic anisotropy of ellipsoidal nanoparticles: crystalline anisotropy and shape anisotropy - has been carried out. It was shown that according to the shape anisotropy the effective anisotropy constant can change non-monotonically. The temperature dependence of effective anisotropy constant on the angle defining the position of effective axis was calculated. It was found out if the angle between the crystalline anisotropy axis and the long axis of nanoparticle exceedsπ/4, then the effective constant as well as the position of effective axis has to change non-monotonically according to changing of temperature.