The Suppression of Demagnetizing Field Heterogeneity in Ferromagnetic Powders

The two ways of reducing the dispersion of the demagnetizing fields in magnetic powders of cobalt alloys have been tested: the isolation of magnetic particles in nonmagnetic matrix and compaction of the powders. The ferromagnetic resonance (FMR) and the hysteresis loops of the pretreated magnetic powders were investigated. The FMR linewidth is significantly narrowed both with increasing of the nonmagnetic fraction in the samples prepared by the first way and with increasing of relative density of the compacts. The limit case of FMR linewidth in continuous magnetic film there is no dispersion of the demagnetizing fields is in good agreement with discussed data. The hysteresis loops of powder with the particles coated by the non-magnetic phase became narrow and easy saturated with increasing of nonmagnetic phase fraction.

Magnetic States of Nanoparticles with RKKY Interaction

Conditions of phase transitions in systems of identical ferromagnetic spherical nanoparticles randomly distributed in metal nonmagnetic matrix and superlattices of small number of nanoparticles with the Ruderman-Kittel-Kasuya-Yosida interaction are researched. In the framework of the Ising model the behavior of superspins is well described with the random interaction field method. The alteration of the effective magnetic moment due to the change in volume affects the choice of the magnetic state of the system: ferromagnetic spin glass or antiferromagnetic spin glass. The ground state of superlattice depends on the distance between particles.

Evidence for Magnetic Polarons in Hole-Doped Cobalt Perovskites

A substitution of La3+ by Sr2+ in LaCoO3 induces holes in the low-spin ground state of the Co ions, which behave like magnetic impurities with a very high spin value (13 μB per hole). In this work, using single-crystal neutron spectroscopy, we prove that the charges introduced by strontium doping do not remain localized at the cobalt sites. Instead, each hole not only creates Co4+ in low-spin state, but it also transforms the six nearest neighboring Co3+ ions to the intermediate-spin state thereby forming a magnetic seven-site (heptamer) polaron. Spin-state polarons behave like magnetic nanoparticles embedded in an insulating nonmagnetic matrix. Therefore, lightly doped La1-xSrxCoO3 is a natural analog to artificial structures composed of ferromagnetic particles in insulating matrices.

Microwave Properties of Sandwich Structure Based on Iron and Barium Nanoparticles

The high frequency intrinsic and effective permeability was calculated for spherical iron and barium particles in a nonmagnetic matrix (epoxy resin) by using the combination of Landau-Lifshitz-Gilbert equation and extended Bruggeman’s effective medium theory with the consideration of the skin effect. It was found that the microwave properties of the composite were influenced by many parameters, such as damping parameter, volume fractions and size of magnetic components. The sandwich microwave absorber with reflection loss < −10 dB over the range of 6-22 GHz was achieved. The thickness of absorber is only 4.25 mm.

Clausius-Mossotti approximation in the theory of sintered polar materials: A review

Clausius-Mossotti approximation is extended to describe the measured magnetic moment of an ellipsoidal sample containing magnetic or nonmagnetic ellipsoidal inclusions and magnetic or nonmagnetic matrix. The magnetic field in the matrix and inclusions is calculated. The magnetic energy of a system is calculated also. The equilibrium shape of a pore in a ferromagnetic sample is investigated. The phenomenon of a cavitation in porous ferromagnetic samples is described. The model is applied to calculate magnetic properties of granular superconductors. The effective electric conductivity of a sample, containing an arbitrary number of differently ordered distributions of ellipsoidal inclusions is calculated.

Microstructural evolution and magnetic properties of size-controlled nanocrystalline Ni in Ni(OH)2–ZrO2 composite

Size-controlled nanocrystallites of Ni varying in size from 2 to 26 nm distributed in a nonmagnetic matrix of Ni(OH)2–ZrO2have been prepared by changing the reduction reaction time and by adding different concentrations of ZrOCl2solution to the initial reaction mixture. X-ray photoelectron spectroscopy indicates the presence of B2O3, Ni(OH)2, and ZrO2with Ni nanocrystals in the as-prepared composites. The room-temperature saturation magnetization and coercivity of the composites increases from ∼1.5 to 10 emu g−1and 100 to 200 Oe, respectively, when the ZrOCl2concentration in the reaction mixture increases to 0.10 M. The nanocrystals prepared without the addition of ZrOCl2exhibit a typical ferromagnetic response at 300 K, while that prepared with the addition of 0.10 M ZrOCl2shows a ferromagnetic response up to 400 K with higher saturation magnetization and coercivity.