L. A. Chekanova

Dimensionality of a System of Exchange-Coupled Grains and Magnetic Properties of Nanocrystalline and Amorphous Ferromagnets

Characteristics of random magnetic anisotropy in ferromagnetic films of amorphous Co90P10 and nanocrystalline Ni75C25, Fe80B4C16, and Co80C20 alloys and also in multilayer films [Co93P7(x)/Pd(14 Å)]20 and [Co90P10(x)/Pd(14 Å)]20 obtained by various technological procedures were studied experimentally. It was found that the spatial dimensionality (d) of the system of ferromagnetically coupled grains (2Rc) in the materials under study determined the exponent in the power dependence of the approach of magnetization to saturation in the region of fields H<2A/MRc2. The dependence ΔM∼H−1/2 was observed for nanocrystalline and amorphous films with a three-dimensional grain arrangement. The approach to saturation in multilayer films with a two-dimensional grain arrangement in an individual magnetic layer follows the law ΔM∼H−1. The main micromagnetic characteristics of random anisotropy, such as the ferromagnetic correlation radius Rf and the average anisotropy 〈K〉 of a ferromagnetic domain with a size of 2Rf, were determined for multilayer Co/Pd films. Correlation was found between the coercive field and these characteristics of random anisotropy.

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.

The Magnetic Structure of Ferromagnetic Filaments of a CoNi(P) Alloy in a Porous Silicon Matrix

The magnetic and resonance properties of CoNi(P) alloys, synthesized by chemical deposition as films on single crystal silicon substrates and as filaments in linear pores of porous silicon substrates, were studied by magnetization and ferromagnetic resonance measurements. It is established that CoNi(P) alloys of the same composition but different morphologies occur in states characterized by different degrees of nonequilibrium, which is manifested by different modes of the magnetization approach to saturation.

Spin-wave resonance in multilayer films (one-dimensional magnon crystals). Identification rules

The characteristic modification of the spectrum of the exchange spin waves has been revealed in ferromagnet/ferromagnet multilayer films with a thickness of N(d1 + d2) by the spin-wave resonance method. This modification is due to the first bandgap at the wavenumber kb = π/(d1 + d2) of a magnon crystal, which is formed by one-dimensional modulation of the magnetization. It has been shown that the transformation of the multilayer film with thermal annealing to the film of a single-phase alloy is accompanied by the disappearance of this modification of the spectrum.

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.

Spin-wave resonance in Co/Pd magnetic multilayers and NiFe/Cu/NiFe three-layered films

The spectrum of standing spin waves has been detected by the ferromagnetic resonance method in NiFe(740 Å)/Cu/NiFe(740 Å) three-layered film structure in the perpendicular configuration for the copper thickness dCu ≤ 30 Å. At thicknesses dCu > 30 Å, the resonance absorption curve is a superposition of two spinwave resonance spectra from individual ferromagnetic NiFe layers. For Co/Pd multilayer films, united spinwave responance spectra have also been observed at thicknesses of the paramagnetic palladium layer up to dPd < 30 Å. The partial exchange stiffness has been calculated for a spin wave propagating across the Pd layer (APd = 0.1 × 10−6 erg/cm). This value is always positive (up to the critical thickness of the palladium interlayer dPd < dc) or equal to zero (dPd > dc).

Multilayer Co/Pd films with nanocrystalline and amorphous Co layers: Coercive force, random anisotropy, and exchange coupling of grains

The values of saturation magnetization Ms, exchange coupling constant A, local magnetic anisotropy field Ha, random anisotropy correlation radius Rc, and coercive force Hc were independently measured for multilayer Co/Pd films with nanocrystalline and amorphous Co layers. It is shown that variation of the coercive force Hc(tCo) as a function of the cobalt layer thickness tCo is related to changes in characteristics of the magnetic microstructure. The main factor determining changes in the ferromagnetic correlation radius Rf and the average anisotropy 〈K〉 of a magnetic block in multilayer Co/Pd films is variation of exchange coupling constant A(tCo).

High-Pressure Phases in Nanocrystalline Co(C) Films Obtained by Pulsed Plasma Vaporization

The phase composition of nanocrystalline Co(C) films obtained by a new pulsed plasma vaporization technique was found by studying their atomic structure and magnetic properties. The films deposited at the substrate temperature T=50°C were of heterophase structure and consisted of a supersaturated solid Co(C) solution and the metastable Co3C carbide. The films obtained at T=150°C represented a mechanical mixture of the metastable Co3C and Co2C carbides. The metastable Co3C and Co2C carbides obtained in a nanocrystalline state were high-pressure phases (∼100 kbar). The thermal stability ranges of these metastable phases were determined.

Spin-Wave Spectroscopy Study of Spatial Magnetization Fluctuations in Metastable Nanocrystalline Films of Fe-Based Alloys

The spectrum of standing spin waves is investigated in nanocrystalline Fe films prepared by the pulsed plasma-spraying method. The dispersion relation of these waves is determined in the wave-vector range (0.2–3.2)×106 cm−1 and is found to be affected by spatial magnetization fluctuations 100 Å in size. These fluctuations are supported as being due to the inhomogeneous distribution of C atoms in the atomic structure of nanocrystalline Fe films.

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.