R. S. Iskhakov

Magnetic microstructure of amorphous, nanocrystalline, and nanophase ferromagnets

The magnetic microstructure of nanostructured ferromagnets is represented by an ensemble of stochastic magnetic domains—regions with dimensions of the length of magnetic orientation coherency. It is shown that the curves displaying the approach of magnetization to saturation make it possible to determine the dimension of the element of the micromagnetic structure, i.e., the size of the stochastic domain and the constant of the effective anisotropy in this element, the size of the element of the nanostructure and the constant its local anisotropy, as well as the dimensionality of the exchange-coupled ferromagnetic nanoparticles.

Fe Nanowires in Carbon Nanotubes as an Example of a One-dimensional System of Exchange-Coupled Ferromagnetic Nanoparticles

The cooperative phenomena revealed in the field and temperature dependences of the magnetization in a system of iron nanoparticles in carbon nanotubes were studied experimentally. The character of the temperature dependences of the magnetization indicates that the ferromagnetic Fe particles in carbon nanotubes are exchange-coupled. In the region where the magnetization approaches saturation, the magnetization curves reveal the power dependence ΔM∼ H−3/2 typical for a one-dimensional system of exchange-coupled ferromagnetic nanoparticles.

Magnetic properties and the mechanism of formation of the uncompensated magnetic moment of antiferromagnetic ferrihydrite nanoparticles of a bacterial origin

The magnetic properties of the superparamagnetic ferrihydrite nanoparticles that form as a result of the vital activity of Klebsiella oxytoca bacteria are studied. Both an initial powder with an average number of iron atoms NFe ∼ 2000–2500 in a particle and this powder after annealing at 140°C for 3 h in air are investigated. The following substantial modifications of the magnetic properties of the ferrihydrite nanoparticles are detected after annealing: the superparamagnetic blocking temperature increases from 23 to 49.5 K, and the average magnetic moment of a particle increases (as follows from the results of processing of magnetization curves). The particles have antiferromagnetic ordering, and the magnetic moment resulting in the superparamagnetism of the system appears due to random spin decompensation inside the particle. For this mechanism, the number of uncompensated spins is proportional to the number of magnetically active atoms raised to the one-half power, and this relation holds true for the samples under study at a good accuracy. The possible causes of the detected shift of magnetic hysteresis loops at low temperatures upon field cooling are discussed.

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.

Magnetic properties of Fe3C ferromagnetic nanoparticles encapsulated in carbon nanotubes

The low-temperature dependences of magnetic characteristics (namely, the coercive force Hc, the remanent magnetization Mr, local magnetic anisotropy fields Ha, and the saturation magnetization Ms) determined from the irreversible and reversible parts of the magnetization curves for Fe3C ferromagnetic nanoparticles encapsulated in carbon nanotubes are investigated experimentally. The behavior of the temperature dependences of the coercive force Hc(T) and the remanent magnetization Mr(T) indicates a single-domain structure of the particles under study and makes it possible to estimate their blocking temperature TB = 420–450 K. It is found that the saturation magnetization Ms and the local magnetic anisotropy field Ha vary with temperature as ∼T5/2.

Fractal magnetic microstructure in the (Co41Fe39B20)x(SiO2)1−x nanocomposite films

Magnetostructural methods are applied to determine the exchange bond percolation limit in (Co41Fe39B20)x(SiO2)1−x nanocomposites (xc = 0.30 ± 0.02), which separates the phase plane along the metal concentration axis into a superparamagnetic region and a ferromagnetic region. It is shown that, with respect to the singularities of the magnetization up to the magnetization saturation curves, the ferromagnetic region is further subdivided into three regions differing in the character of the spatial propagation of the magnetization ripples or in the magnetic correlation function characteristics. The fractal dimension of the nanocomposite magnetic microstructure near the percolation threshold is determined.

Magnetization curve and magnetic correlations in a nanochain of ferromagnetic grains with random anisotropy

The magnetization curve and magnetization correlation function are calculated for a ferromagnetic chain of single-domain nanoparticles with a randomly oriented anisotropy axis for different ratios between the exchange correlation and anisotropy energies. It is shown that the coercive force decreases as the exchange correlations increase. For strong exchange correlations, the magnetization curve is described by the following three successive magnetization processes as the applied field is increased: (i) nonuniform rotation of the magnetization of stochastic domains, (ii) collapse of the magnetic solitons, and (iii) nonuniform rotation of exchange-correlated magnetization vectors of the nanoparticles. For high fields, the calculated correlation function of the transverse magnetization components coincides with that predicted from linear theory. At low and zero fields, the main parameters of the correlation function (the variance and correlation radius) tend to certain finite values rather than diverge (as is the case in linear theory). The irreversible variation in the magnetization at low fields (the hysteresis loop) and the hysteresis of the main parameters of the correlation function are calculated.

Study of Magnetic Correlations in Nanostructured Ferromagnets by Correlation Magnetometry

We propose a theoretically justified experimental magnetometric technique for determining the size of stochastic domains spontaneously formed in the spin system of nanostructured ferromagnets and for evaluating the effective anisotropy in these magnetically correlated regions. The method is based on monitoring the ΔM∼ H−2 relationship in the low-field part of the integral magnetization curve.

Mössbauer investigation of temperature transformations in bacterial ferrihydrite

Ferrihydrite nanoparticles formed as a result of the microorganism activity have been studied using Mössbauer spectroscopy, X-ray powder diffraction analysis, and X-ray fluorescence analysis. Three positions of trivalent iron with nonoverlapping ranges of quadrupole splittings have been revealed in bacterial ferrihydrite: QS{Fe3+(1)} = 0.49–0.83 mm/s, QS{Fe3+(2)} = 0.84–1.10 mm/s, and QS{Fe3+(3)} = 1.25–1.73 mm/s. It has been experimentally demonstrated that the Fe3+(3) positions are the centers of nucleation of the hematite phase in the course of heat treatment.