E. A. Denisova

Multilayer nanogranular films (Co40Fe40B20)50(SiO2)50/α-Si:H and (Co40Fe40B20)50(SiO2)50/SiO2: Magnetic properties

Magnetic properties of multilayers, consisting of nanogranular (Co40Fe40B20)50(SiO2)50 layers as thin as magnetic granule diameter alternating the α-Si:H or SiO2 layers and the single layer film (Co40Fe40B20)50(SiO2)50 with the thickness much larger than the magnetic granule diameter are reported and compared. The thick single layer film is ferromagnetic but the multilayer film with the ultrathin granular layers and SiO2 spacer is superparamagnetic. This is interpreted as the result of increasing percolation threshold in the 2D granular media above 50% concentration of magnetic granules in the multilayer with the nonmagnetic and dielectric SiO2 spacer. The multilayer with the α-Si:H spacer is superparamagnetic at 300 K but it becomes ferromagnetic, when temperature is below 250 K. It is assumed to be resulted from the exchange interaction of magnetic granules through the semiconductor α-Si:H layers. The value of exchange interaction through the semiconductor spacer is estimated.

Magnetic Anisotropy in the Films of Oriented Carbon Nanotubes Filled with Iron Nanoparticles

Films of carbon nanotubes oriented perpendicularly to the substrate surface and filled with iron nanoparticles have been synthesized and studied. Morphological features of these nanocomposite films lead to the appearance of an easy magnetization axis, which is perpendicular to the film plane. A method for enhancement of this effect is suggested and successfully tested.

Ferromagnetic resonance and magnetic microstructure in nanocomposite films of Cox(SiO2)1 − x and (CoFeB)x(SiO2)1 − x

This paper reports on the results of the investigation of the relation between the magnetic microstructure and ferromagnetic resonance (FMR) in ferromagnetic metal-insulator composites by using granular alloys (Co41Fe39B20)x(SiO2)1 − x and Cox(SiO2)1 − x as an example. A comparative analysis of the properties of FMR spectra and parameters of random magnetic anisotropy leads to correlations between these quantities. It has been found that the main mechanism that determines the FMR line width in the films under investigation is the exchange narrowing mechanism.

Accelerated Mechanical Alloying of Mutually Insoluble Metals: Co-Cu System

We propose a new method for accelerating the process of mechanical alloying in the Co-Cu system possessing positive enthalpy of mixing and exhibiting no mutual solubility of components under equilibrium conditions. For this purpose, highly disperse powders of composite particles representing a Co-P amorphous alloy core covered with a nanocrystalline copper shell were prepared by chemical deposition. These composite powders were mechanically alloyed by processing in a ball mill. Investigation of the atomic structure and magnetic properties of the composite powders upon milling showed that the formation of supersaturated Co-Cu solid solutions under such conditions requires a much shorter milling time as compared to that for the conventional mechanical alloying processes.

Width of the ferromagnetic resonance line in highly dispersed powders of crystalline and amorphous Co-P alloys

The resonance characteristics (inhomogeneous FMR linewidth ΔH) in highly dispersed (d=0.1–3 µm) powders of crystalline and amorphous Co-P alloys are investigated as a function of the composition, particle size, and atomic structure. It is established that ΔH for powders of amorphous Co-P alloys is two to three times larger than ΔH for crystalline Co-P powders. According to the investigations performed, this is caused by thermodynamically stimulated segregation of nonmagnetic Co2P inclusions, apparently an effective relaxation channel, in the amorphous state of Co-P powders.

Magnetic Anisotropy of Co-Nanostructures Embedded in Matrices with Different Pores Size and Morphology

Composite materials with Co (P) particles embedded into pores of silica and track etched polycarbonate membranes were fabricated by an electroless reduction. The magnetic and structural properties of the composite materials are characterized by scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometer. The macroscopic and local magnetic anisotropy of the Co (P) particles electroless deposited in the pores of the polycarbonate membrane and silica is studied. The composite materials with linear pores exhibit uniaxial magnetic anisotropy. The easy axis lies along the Co (P) rods, the shape anisotropy dominates over the intrinsic crystalline anisotropy. Information on local anisotropy field and the grain size was obtained from investigation of approach to saturation magnetization law. The local anisotropy field for all the samples depends on P content. For Co (P) rods the local anisotropy value is also determined by nominal pore sizes. It was found that the investigated Co (P) rods is nanocrystalline. The effects of different pores morphology on the FMR-spectra characteristics are studied.

Cobalt ferrite nanoparticles in a mesoporous silicon dioxide matrix

We have studied magnetic nanoparticles of cobalt ferrite obtained by the extraction-pyrolysis method in a mesoporous silicon dioxide (MSM-41) molecular sieve matrix. The X-ray diffraction data show evidence for the formation of CoFe2O4 particles with a coherent scattering domain size of ∼40 nm. Measurements of the magnetization curves showed that powders consisting of these nanoparticles are magnetically hard materials with a coercive field of Hc(4.2 K) = 9.0 kOe and Hc(300 K) = 1.8 kOe and a reduced remanent magnetization of Mr/Ms(4.2 K) = 0.83 and Mr/Ms(300 K) = 0.49. The shape of the low-temperature (4.2 K) magnetization curves is adequately described in terms of the Stoner-Wohlfarth model for randomly oriented single-domain particles with a cubic magnetic anisotropy.

Ferromagnetic resonance linewidth in powders consisting of core–shell particles

The dependence of the ferromagnetic resonance linewidth on the thickness of nonmagnetic shells in powders consisting of ferromagnetic core–nonferromagnetic shell composite particles is investigated. It is found that an increase in shell thickness reduces the ferromagnetic resonance linewidth by several times, down to values comparable to those for coatings with compositions similar to that of the particle’s core. The observed effect is assumed to result from suppression of the inhomogeneity of demagnetizing fields in a powder consisting of magnetic particles.

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.

Micro Grid Frame of Electroless Deposited Co-P Magnetic Tubes

The morphology and magnetic characterization results of electroless deposited CoP coating on the copper microgrid with the mesh size of 50 microns are presented and discussed. The deposited coating is found to be well uniform by the thickness. Magnetic anisotropy in the plane of the grid has been studied by the hysteresis loops and the FMR spectra and discussed with the modeling of the grid as the frame grid system of magnetic tubes.