Modification of magnetic anisotropy in metallic glasses using high-energy ion beam irradiation

Heavy ion irradiation in the electronic stopping power region induces macroscopic dimensional change in metallic glasses and introduces magnetic anisotropy in some magnetic materials. The present work is on the irradiation study of ferromagnetic metallic glasses, where both dimensional change and modification of magnetic anisotropy are expected. Magnetic anisotropy was measured using Mössbauer spectroscopy of virgin and irradiated Fe₄₀Ni₄₀B₂₀ and Fe₄₀Ni₃₈Mo₄B₁₈ metallic glass ribbons. 90 MeV ¹²⁷I beam was used for the irradiations. Irradiation doses were 5×10¹³ and 7.5×10¹³ ions/cm². The relative intensity ratios D ₂₃ of the second and third lines of the Mössbauer spectra were measured to determine the magnetic anisotropy. The virgin samples of both the materials display in-plane magnetic anisotropy, i.e., the spins are oriented parallel to the ribbon plane. Irradiation is found to cause reduction in magnetic anisotropy. Near-complete randomization of magnetic moments is observed at high irradiation doses. Correlation is found between the residual stresses introduced by ion irradiation and the change in magnetic anisotropy.     

Synthetic magnetic opals

We present studies of novel nanocomposites of BiNi impregnated into the structure of opals as well as inverse opals. Atomic force microscopy and high resolution elemental analyses show a highly ordered structure and uniform distribution of the BiNi filler in the matrix. These BiNi-based nanocomposites are found to exhibit distinct ferromagnetic-like ordering with transition temperature of about 675 K. As far as we know there exists no report in literature on any BiNi compound which is magnetic.     

Magnetization curves for general cylindrical samples in a transverse field

Using the recent results for the surface current density on cylindrical surfaces of arbitrary cross-section producing uniform interior magnetic field we propose a method for obtaining solutions of Bean’s critical state model for general cylindrical samples. The method uses the technique of conformal mapping to express the sample surface and the flux-fronts in terms of a set of coefficients that depend on a parameter. The flux-fronts are to be determined by solving a system of nonlinear ordinary differential equations for the coefficients. Retaining only a certain finite number of leading coefficients we get an approximate solution. The procedure is illustrated by considering two cyclindrical samples — one with an elliptical cross-section and the other with a non-elliptical cross-section. The virgin curve and small and large magnetization hysteresis loops for the two samples are obtained.     

Low-field vortex dynamics in various high-T c thin films

We present a novel ac susceptibility technique for the study of vortex creep in superconducting thin films. With this technique we study the dynamics of dilute vortices in c-axis oriented Y-123, Hg-1212, and Tl-1212 thin films, as well as a axis oriented Hg-1212 thin films. Results on the Hg-1212 and Tl-1212 thin films indicate that dislocation-mediated plastic flux creep of single vortices dominates at low temperatures and fields. As the temperature (or the field) is increased, the increasing vortex-vortex interactions promote a collective behavior, which can be characterized by elastic creep with a non-zero μ exponent. Also, in some of these samples effects of thermally assisted quantum creep are visible up to 45 K in some of these samples. In Y-123 thin films, creep is found to be collective down to the lowest temperatures and fields investigated, while the quantum creep persists only up to 10–11 K.     

Identification of local magnetic contributions in a Co2FeBO5 single crystal by XMCD spectroscopy

The temperature dependences of the X-ray absorption spectra (XAS) and of the spectra of X-ray magnetic circular dichroism (XMCD) are measured near the L _3,2 absorption edges of Co and Fe in ludwigite Co₂FeBO₅ single crystals. The antiparallel orientation of the magnetic moments of cobalt and iron is demonstrated. The coercive fields related to cobalt and iron ions are determined. The orbital (m _l ) and (m _s ) spin contributions to the total magnetic moments of cobalt and iron ions are identified. The ratios and relative directions of m _l and m _s are found.     

Determination of the effective nuclear charge from EPR data using a modified crystal-field theory

We propose a scheme for determining the effective nuclear charge Z _ef^CF of a 3d ion placed in a crystal field of arbitrary symmetry. As an example, we consider the Co²⁺ ion in a matrix MCO₃ (M = Ca, Cd). We show that the effective nuclear charge Z _ef^CF correlates with a change in the degree of the bond covalence.     

Intermediate-spin state of a 3d ion in the octahedral environment and generalization of the Tanabe-Sugano diagrams

Electronic spectra of 3d(n) transition ions in an octahedral ligand surrounding have been studied using the modified crystal field approach (MCFA), which includes a relativistic spin-orbital interaction. A new variable parameter, the effective nuclear charge Z(eff) of a metal ion that allows accounting implicitly the covalence degree of a metal-ligand bond, has been introduced. Energy diagrams similar to the Tanabe-Sugano ones have been calculated. To study the spin state evolution of the metal ion in an arbitrary distorted octahedral complex, a spin state diagram approach has been proposed. The intermediate-spin (IS) state problem for 3d(4), 3d(5), and 3d(6) metal ions has been considered and conditions for the IS state realization have been formulated. The regions of the mixed high-, intermediate-, and low-spin states have been found. The possibility of coexistence of the different spin states of 3d ions in the octahedral complexes has been considered using crystallography data for the YBaCo(2)O(5.5) layered cobaltite.     

Temperature dependence of the spin state of a Co3+ Ion in RCoO3 (R = La,Gd) cobaltites

Changes in the spin state of Co³⁺ ions in LaCoO₃ and GdCoO₃ compounds are studied through the use of the temperature dependence of the magnetic susceptibility and the modified crystal field theory. It is shown that the spin subsystem of Co³⁺ ions in LaCoO₃ and GdCoO₃ undergoes the spin-crossover type transition between the high-spin (S = 2) and low-spin (S = 0) states without any contribution of the intermediate-spin state (S = 1).     

Microscopic evidence of spin state order and spin state phase separation in layered cobaltites RBaCo2O5.5 with R=Y, Tb, Dy, and Ho

We report muon-spin relaxation measurements on the magnetic structures of RBaCo2O(5.5) with R=Y, Tb, Dy, and Ho. Three different phases, one ferrimagnetic and two antiferromagnetic, are identified below 300 K. They consist of different ordered spin state arrangements of high-, intermediate-, and low-spin Co3+ of CoO6 octahedra. Phase separation into well separated regions with different spin state order is observed in the antiferromagnetic phases. The unusual strongly anisotropic magnetoresistance and its onset at the FM-AFM phase boundary is explained.     

Determination of the effective nuclear charge for free ions of transition metals from experimental spectra

The refined set of values of the effective nuclear charge (\(\bar Z_{ef} \)) and the set of one-electron spin-orbital constants (ξ _3d ^teor ) are obtained for ions of transition metals with the help of a semiempirical method of calculation of the spectrum of free ions with allowance for the spin-orbital interaction. The effective nuclear charge, which is a variable theoretical parameter, was determined from a comparison of theoretically calculated ion absorption spectra with experimental ones. The form of the potential for the calculation of the spin-orbital constant was chosen such that the calculated value ξ _3d ^teor would coincide with the experimental one ξ _3d ^exp . The calculated set of values \(\bar Z_{ef} \), ξ _3d ^teor } can be used in semiempirical calculations of energy levels of 3d ions (ions with an incomplete 3d shell).