S. V. Stolyar

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.

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.

Mechanism of the formation of an uncompensated magnetic moment in bacterial ferrihydrite nanoparticles

The magnetic properties of antiferromagnetic nanoparticles of FeOOH · nH2O with sizes of 3–7 nm, which are products of vital functions of Klebsiella oxytoca bacteria, have been studied. Particles exhibit a superparamagnetic behavior. The characteristic blocking temperature is 23 K. Analysis of the magnetization curves shows that the mechanism of the formation of the uncompensated magnetic moment of particles is the random decompensation of magnetic moments of Fe3+ ions both on the surface and in the bulk of the antiferromagnetic particle. In this mechanism, the exchange coupling between the uncompensated magnetic moment of the particle and its antiferromagnetic “core” is implemented. It has been found that the temperature dependence of the uncompensated magnetic moment has the form 1 — constT2.

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.

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).

Change in the magnetic properties of nanoferrihydrite with an increase in the volume of nanoparticles during low-temperature annealing

The results of the investigation into the effect of low-temperature annealing of a powder of nanoparticles of bacterial ferrihydrite on its magnetic properties have been presented. It has been found that an increase in the time (up to 240 h) and temperature (in the range from 150 to 200°C) of annealing leads to a monotonic increase in the superparamagnetic blocking temperature, the coercive force, and the threshold field of the opening of the magnetic hysteresis loop (at liquid-helium temperatures), as well as to an increase in the magnetic resonance line width at low temperatures and in the magnetic susceptibility at room temperature. At the same time, according to the results of the analysis of the Mössbauer spectra, the annealing of ferrihydrite does not lead to the formation of new iron oxide phases. Most of these features are well consistent with the fact that the low-temperature annealing of ferrihydrite causes an increase in the size of nanoparticles, which is confirmed by the results of transmission electron microscopy studies.

Specific features of magnetic properties of ferrihydrite nanoparticles of bacterial origin: A shift of the hysteresis loop

The results of the experimental investigation into the magnetic hysteresis of systems of superparamagnetic ferrihydrite nanoparticles of bacterial origin have been presented. The hysteresis properties of these objects are determined by the presence of an uncompensated magnetic moment in antiferromagnetic nanoparticles. It has been revealed that, under the conditions of cooling in an external magnetic field, there is a shift of the hysteresis loop with respect to the origin of the coordinates. These features are associated with the exchange coupling of the uncompensated magnetic moment and the antiferromagnetic “core” of the particles, as well as with processes similar to those responsible for the behavior of minor hysteresis loops due to strong local anisotropy fields of the ferrihydrite nanoparticles.

Magnetic and resonance properties of ferrihydrite nanoparticles doped with cobalt

Powders of undoped ferrihydrite nanoparticles and ferrihydrite nanoparticles doped with cobalt in the ratio of 5: 1 have been prepared by hydrolysis of 3d-metal salts. It has been shown using Mössbauer spectroscopy that cobalt is uniformly distributed over characteristic crystal-chemical positions of iron ions. The blocking temperatures of ferrihydrite nanoparticles have been determined. The nanoparticle sizes, magnetizations, surface anisotropy constants, and bulk anisotropy constants have been estimated. The doping of ferrihydrite nanoparticles with cobalt leads to a significant increase in the anisotropy constant of a nanoparticle and to the formation of surface rotational anisotropy with the surface anisotropy constant Ku = 1.6 × 10–3 erg/cm2.

The effect of low-temperature heat treatment on the magnetic properties of biogenic ferrihydrite nanoparticles

We have studied the influence of low-temperature heat treatment (annealing) on the magnetic properties of superparamagnetic nanoparticles of biogenic ferrihydrite. It is established that the proposed treatment leads to an increase in the blocking temperature and magnetic susceptibility of samples. After subsequent exposure in aqueous medium, the magnetic properties of annealed sol remain constant. The character of changes in the magnetic properties of samples studied shows that low-temperature heat treatment allows nanoparticle dimensions to be increased in a controlled way.

Unidirectional anisotropy in ferromagnetic-ferrimagnetic film structures

A mechanism of unidirectional anisotropy formation in an exchange-coupled ferromagnetic-ferrimagnetic film structure with orthogonal effective magnetizations in the layers is investigated. The reason for unidirectional anisotropy is the magnetic heterogeneity of the ferrimagnetic layer in the compensation range. Magnetization reversal in the magnetically soft layer of an (REE–transition metal)/NiFe film structure is discussed based on a model of uniform rotation of magnetization. It is found that unidirectional anisotropy sharply decreases the magnetic noise level in the magnetically soft layer. The field of application of these materials is outlined.