New Publications

Granular high-temperature superconductors (HTSs) exhibit magnetotransport properties, including the clockwise magnetoresistance hysteresis. The hysteresis is explained by the concept of an effective field in the subsystem of grain boundaries, where the observed dissipation occurs. The effective field in the intergrain medium is determined by a superposition of the external field H and the field induced by the magnetic response of HTS grains. The magnetic flux compression in the intergrain medium provides an increase of the effective field. The magnetoresistance hysteresis of polycrystalline YBa2Cu3O7-δ has a bright feature: In a fairly wide external field range, the hysteresis width ΔH(H) is found to be almost linear, ΔH ≈ H. This behavior is considered to be universal over the entire temperature range corresponding to the superconducting state (the investigations have been carried out at temperatures of 77 K and 4.2 K). The analysis of the magnetoresistive and magnetic properties has shown that the upper boundary of the field range of the ΔH ≈ H regime is consistent with the field of complete penetration into HTS grains. This is indicative of the strong interrelation between the magnetotransport and magnetic properties of granular HTSs.

We report on the investigations of a system of 8-nm NiO particles representing antiferromagnetic (AFM) materials, which are weak magnetic in the form of submicron particles, but can be considered to be magnetoactive in the form of nanoparticles due to the formation of the uncompensated magnetic moment in them. The regularities of the behavior of magnetization switching in AFM nanoparticles are established by studying the magnetic hysteresis loops under standard quasi-static conditions and in a quasi-sinusoidal pulsed field of up to 130 kOe with pulse lengths of 4–16 ms. The magnetic hysteresis loops are characterized by the strong fields of the irreversible magnetization behavior, which is especially pronounced upon pulsed field-induced magnetization switching. Under the pulsed field-induced magnetization switching conditions, which are analogous to the dynamic magnetic hysteresis, the coercivity increases with an increase in the maximum applied field H0 and a decrease in the pulse length. This behavior is explained by considering the flipping of magnetic moments of particles in an external ac magnetic field; however, in contrast to the case of single-domain ferro- and ferrimagnetic particles, the external field variation rate dH/dt is not a universal parameter uniquely determining the coercivity. At the dynamic magnetization switching in AFM nanoparticles, the H0 value plays a much more important role. The results obtained are indicative of the complex dynamics of the interaction between magnetic subsystems formed in AFM nanoparticles.

In antiferromagnetic (AFM) nanoparticles, an additional ferromagnetic phase forms and leads to the appearance in AFM nanoparticles of a noncompensated magnetic moment and the magnetic properties typical of common FM nanoparticles. In this work, to reveal the regularities and differences of the dynamic magnetization switching in FM and AFM nanoparticles, the typical representatives of such materials are studied: CoFe2O4 and NiO nanoparticles with average sizes 6 and 8 nm, respectively. The high fields of the irreversible behavior of the magnetizations of these samples determine the necessity of using strong pulsed fields (amplitude to 130 kOe) to eliminate the effect of the partial hysteresis loop when studying the dynamic magnetic hysteresis. For both types of the samples, coercive force HC at the dynamic magnetization switching is markedly higher than HC at quasi-static conditions. HC increases as the pulse duration τP decreases and the maximum applied field H0 increases. The dependence of HC on field variation rate dH/dt = H0/2τP is a unambiguous function for CoFe2O4 nanoparticles, and it is precisely such a behavior is expected from a system of single-domain FM nanoparticles. At the same time, for AFM NiO nanoparticles, the coercive force is no longer an unambiguous function of dH/dt, and the value of applied field H0 influences more substantially. Such a difference in the behaviors of FM and AFM nanoparticles is caused by the interaction of the FM subsystem and the AFM “core” inside AFM nanoparticles. This circumstance should be taken into account when developing the theory of dynamic hysteresis of the AFM nanoparticles and also to take into account their practical application.

Ferromagnetic resonance was used to study three types of ferrihydrite nanoparticles: nanoparticles formed as a result of the cultivation of microorganisms Klebsiella oxytoca; chemically prepared ferrihydrite nanoparticles; chemically prepared ferrihydrite nanoparticles doped with Cu. It is established from the ferromagnetic resonance data that the frequency-field dependence (in the temperature range Т_P < T < T*) is described by the expression: 2πν/γ = Н_R + H^A_{(T = 0)}(1 – T/Т*), where γ is the gyromagnetic ratio, H_R is the resonance field. The induced anisotropy H^A is due to the spin-glass state of the near-surface regions. T_P temperature characterizes the energy of the interparticle interaction of nanoparticles.

Ferrihydrite is a low-crystalline nanoscale matter. The uncompensated magnetic moment of the ferrihydrite caused by the antiferromagnetic ordering of the magnetic moments of iron atoms and leads to the magnetic properties very similar to those of ferro- and ferrimagnetic nanoparticles. In this study, we investigated the biogenic ferrihydrite nanoparticles with the narrow size distribution and an average diameter of ≈2 nm obtained by the bacteria life cycle. The features caused by the surface effects and the inhomogeneous structure of ferrihydrite have been examined in the temperature range of 4–300 K using Mössbauer spectroscopy and magnetometry. Based on the Mössbauer data, we identified the superparamagnetic blocking temperature at the temperature of 30 K for the largest ferryhidrite particles. We established that the exceptional magnetic anisotropy of ferrihydrite (KV=1.2⋅105 erg/cm³ and KS=0.1 erg/cm²) is reached because of the highly developed ferrihydrite nanoparticles’ surface. According to the Mössbauer data, we propose a core-shell structural model of the biogenic ferrihydrite particles. We found that the size of the dense core depends on the particle size. The well-crystallized core is formed only for nanoparticles larger than ≈2 nm, whereas smaller particles consist entirely of a matter with a lower density of iron atoms.

The magnetic structure of the epsilon-Fe2O3 iron oxide polymorphic modification is collinear ferrimagnetic in the range from room temperature to similar to 150 K. As the temperature decreases, epsilon-Fe2O3 undergoes a magnetic transition accompanied by a significant decrease in the coercivity H-c and, in the low-temperature range, the compound has a complex incommensurate magnetic structure. We experimentally investigated the dynamic magnetization switching of the epsilon-Fe2O3 nanoparticles with an average size of 8 nm in the temperature range of 80-300 K, which covers different types of the magnetic structure of this iron oxide. A bulk material consisting of xerogel SiO2 with the epsilon-Fe2O3 nanoparticles embedded in its pores was examined. The magnetic hysteresis loops under dynamic magnetization switching were measured using pulsed magnetic fields H-max of up to 130 kOe by discharging a capacitor bank through a solenoid. The coercivity H-c upon the dynamic magnetization switching noticeably exceeds the H-c value under the quasi-static conditions. This is caused by the superparamagnetic relaxation of magnetic moments of particles upon the pulsed magnetization switching. In the range from room temperature to similar to 150 K, the external field variation rate dH/dt is the main parameter that determines the behavior of the coercivity under the dynamic magnetization switching. It is the behavior that is expected for a system of single-domain ferro- and ferrimagnetic particles. Under external conditions (at a temperature of 80 K) when the epsilon-Fe2O3 magnetic structure is incommensurate, the coercivity during the pulsed magnetization switching depends already on the parameter dH/dt and is determined, to a great extent, by the maximum applied field H-max. Such a behavior atypical of systems of ferrimagnetic particles is caused already by the dynamic spin processes inside the epsilon-Fe2O3 particles during fast magnetization switching.

The origin of the low magnetization anisotropy of the textured bulk samples consisting of highly anisotropic (Bi,Pb)₂Sr₂Ca₂Cu₃O_x (Bi-2223) high-temperature superconductor crystallites has been investigated. It has been established that the observed anisotropy is determined by the disordering of Bi-2223 crystallites in the sample. The measured anisotropy of the textured sample makes it possible to determine the magnetic angle characterizing the ordering of crystallites.

A model for describing the magnetoresistance behavior in a granular high-temperature superconductor (HTS) that has been developed in the last decade explains a fairly extraordinary form of the hysteretic R(H) dependences at T = const and their hysteretic features, including the local maximum, the negative magnetoresistance region, and the local minimum. In the framework of this model, the effective field B_eff in the intergrain medium has been considered, which represents a superposition of the external field and the field induced by the magnetic moments of HTS grains. This field can be written in the form B_eff(H) = H + 4παM(H), where M(H) is the experimental field dependence of the magnetization and α is the parameter of crowding of the magnetic induction lines in the intergrain medium. Therefore, the magnetoresistance is a function of not simply an external field, but also the “internal” effective field R(H) = f(B_eff(H)). The magnetoresistance of the granular YBa₂Cu₃O_7 – δ HTS has been investigated in a wide temperature range. The experimental hysteretic R(H) dependences obtained in the high -temperature range (77–90 K) are well explained using the developed model and the parameter α is 20–25. However, at a temperature of 4.2 K, no local extrema are observed, although the expression for B_eff(H) predicts them and the parameter α somewhat increases (~30–35) at this temperature. An additional factor that must be taken into account in this model can be the redistribution of the microscopic current trajectories, which also affects the dissipation in the intergrain medium. At low temperatures under the strong magnetic flux compression (α ~ 30–35), the microscopic trajectories of the current I_m can change and tunneling through the neighboring grain is preferred, but the angle between I_m and B_eff will be noticeably smaller than 90°, although the external (and effective) field direction is perpendicular to the macroscopic current direction.

The relaxation of the remanent magnetization of antiferromagnetically ordered ferrihydrite nanoparticles at the exchange bias effect implemented in these systems has been investigated. The magnetization relaxation depends logarithmically on time, which is typical of the thermally activated hoppings of particle magnetic moments through the potential barriers caused by the magnetic anisotropy. The barrier energy obtained by processing of the remanent magnetization relaxation data under the field cooling conditions significantly exceeds the barrier energy under standard (zero field cooling) conditions. The observed difference points out the possibility of using the remanent magnetization relaxation to analyze the mechanisms responsible for the exchange bias effect in antiferromagnetic nanoparticles and measure the parameters of the exchange coupling of magnetic subsystems in such objects.

Ferrihydrite nanoparticles (2–3 nm in size), which are products of the vital activity of microorganisms, are studied by the ferromagnetic resonance method. The “core” of ferrihydrite particles is ordered antiferromagnetically, and the presence of defects leads to the appearance of an uncompensated magnetic moment in nanoparticles and the characteristic superparamagnetic behavior. It is established from the ferromagnetic resonance data that the field dependence of the frequency is described by the expression = , where γ is the gyromagnetic ratio, is the resonance field, kOe, and K. The induced anisotropy is due to the spin-glass state of the near-surface regions.

The -Fe2O3 magnetic structure has been analyzed using the synchrotron radiation source. Time spectra of nuclear forward scattering for isolated nanoparticles with an average size of 8 nm immobilized in a xerogel matrix have been recorded in the temperature range of 4–300 K in applied magnetic fields of 0–4 T in the longitudinal direction at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). It has been found that the external magnetic field does not qualitatively change the Hhf (T) behavior, but makes a strong opposite impact on the hyperfine fields in the nonequivalent iron sites, leading to the divergence of Hhf polar angle dependences below 80 K. A complete diagram of the -Fe2O3 magnetic structure in the temperature range of 4–300 K is proposed. At 300 K, the -Fe2O3 compound is confirmed to be a collinear ferrimagnet. The experimental results show that the magnetic transition at 150–80 K leads to the formation of a noncollinear magnetic structure. Furthermore, in the range of the 80–4 K, the ground state of a magnetic spiral is established. The experimental results are supplemented by the analysis of the exchange interactions and temperature dependence of the magnetization in a magnetic field of 7 T.

The Fe3O4/CoFe2O4 nanoparticles with a core–shell structure with an average size of 5 nm have been obtained by codeposition from the iron and cobalt chloride solutions. An analysis of the magnetic properties of the obtained system and their comparison with the data for single-phase Fe3O4 (4 nm) and CoFe2O4 (6 nm) nanoparticles has led to the conclusion about a noticeable interaction between the soft magnetic (Fe3O4) and hard magnetic (CoFe2O4) phases forming the core and shell of hybrid particles.

 Nuclear gamma-resonance experiments with energy and time resolved detection are carried out with epsilon-F2O3 nanoparticles and a Co-57(Rh) laboratory Mossbauer source of gamma radiation and a 14.4125 keV synchrotron radiation source on the ID18 beamline (ESRF) in the temperature range of 4-300 K. Both methods show a tremendous increase in the hyperfine field in tetrahedrally coordinated iron positions during the magnetic transition in the range of 80-150 K. As a result, the splitting of the quantum beat peaks in the nuclear scattering spectra is observed in the time interval of 20-170 ns with a periodicity of similar to 30 ns. In addition, the first quantum beat is slightly shifted to shorter times. A correlation between the quadrupole shift and the orbital angular momentum of iron in epsilon-F2O3 nanoparticles is found. The magnetic transition leads to the rotation of the magnetic moment in the tetrahedral positions of iron around the axis of the electric field gradient by an angle of 44 degrees.

The -Fe2O3 iron oxide polymorph is a well-known magnetic material with a complex magnetic structure, which undergoes a series of magnetic transitions in different temperature ranges. However, the -Fe2O3 phase diagram is still unclear. We report on the magnetic properties of a sample consisting of -Fe2O3 nanoparticles with an average size of 8 nm embedded in a SiO2 xerogel matrix without an admixture of foreign phases. Along with the features typical of the well-known -Fe2O3 magnetic transition in the temperature range 80150 K, the temperature dependence of magnetization M(T) of -Fe2O3 includes other low-temperature anomalies. In an external field of H 70kOe, there is a noticeable temperature hysteresis of magnetization at 5090 K, and near T & approx; 50 K, the M(T) curves have a characteristic bending, which may be indicative of an additional magnetic transition. The ferromagnetic resonance spectra shows that, near 500 K, a magnetic phase transition occurs, which was previously thought to be a transition to the paramagnetic state. An analysis of the temperature dependence of the ferromagnetic resonance spectra shows that the magnetically ordered phase in -Fe2O3 exists up to about 800 K.

The magnetic properties of samples of NiO nanoparticles with average sizes of 23, 8.5, and 4.5 nm were investigated. Using the magnetization curves measured in strong (up to 250 kOe) pulsed magnetic fields, the contributions of the free spin and ferromagnetic subsystems were extracted. It has been found that the ferromagnetic contribution increases with a decrease in the nanoparticle size and is proportional to the fraction of uncompensated exchange-coupled spins. It is demonstrated that the uncompensated spins form in the antiferromagnetic NiO oxide due to an increase in the fraction of surface atoms in the nanoparticles with decreasing particle size and defects in the bulk of particles

The temperature evolution of the magnetoresistance hysteresis in the granular YBa₂Cu₃O_7-δ high-temperature (T_C ≈ 92 K) superconductor has been investigated. The measurements have been performed in the high-temperature region (78–90 K) and at the liquid helium temperature (4.2 K). The results obtained have been analyzed using the developed model of the behavior of transport properties of a granular high-temperature superconductor in an external magnetic field. Within the discussed model, the dissipation of the grain boundary subsystem is determined by the intergrain spacing-averaged effective field B_eff, which is a superposition of external field H and the field induced by the magnetic moments of superconducting grains. Such a consideration yields the expression B_eff(H) = H − 4πM(H) α for the effective field in the intergrain medium, where M(H) is the experimental hysteretic dependence of magnetization and α is the parameter of magnetic flux crowding in the intergrain medium. Here, the magnetoresistance is assumed to be proportional to the absolute value of the effective field: R(H) ~ |B_eff(H)|. Analysis of the experimental R(H) and M(H) dependences obtained under the same conditions for the investigated high-temperature superconductor sample showed that in the high-temperature region this parameter is α ≈ 25. At the low temperature (4.2 K), we may state that the degree of flux crowding increases and the estimated α value is ~ 50. The estimates made are indicative of the strong effect of flux compression in the intergrain medium on the magnetotransport properties of the investigated granular high-temperature superconductor system. Possible reasons for a discrepancy between the developed model concepts and experimentally observed low-temperature R(H) hysteresis are analyzed.

It is well-known that the fraction of surface atoms and the number of defects in an antiferromagnetic particle increase with a decrease in the particle size to tens of nanometers, which qualitatively changes the properties of the particle. Specifically, in antiferromagnetic nanoparticles, spins in the ferromagnetically ordered planes can partially decompensate; as a result, an antiferromagnetic particle acquires a magnetic moment. As a rule, uncompensated chemical bonds of the surface atoms significantly weaken the exchange coupling with the antiferromagnetic particle core, which can lead to the formation of an additional magnetic subsystem paramagnetic at high temperatures and spin-glass-like in the low-temperature region. The existence of several magnetic subsystems makes it difficult to interpret the magnetic properties of antiferromagnetic nanoparticles. It is shown by the example of NiO nanoparticles with an average size of 8 nm that the correct determination of the contributions of the magnetic subsystems forming in antiferromagnetic nanoparticles requires magnetic measurements in much stronger external magnetic fields than those commonly used in standard experiments (up to 60–90 kOe). An analysis of the magnetization curves obtained in pulsed magnetic fields up to 250 kOe allows one to establish the contributions of the uncompensated particle magnetic moment μ_un, paramagnetic subsystem, and antiferromagnetic particle core. The μ_un value obtained for the investigated NiO particles is consistent with the Néel model, in which μ_un ∼ N^1/2 (N is the number of magnetically active atoms in a particle), and thereby points out the existence of defects on the surface and in the bulk of a particle. It is demonstrated that the anomalous behavior of the high-field susceptibility dM/dH of antiferromagnetic NiO nanoparticles, which was observed by many authors, is caused by the existence of a paramagnetic subsystem, rather than by the superantiferromagnetism effect.

The trivalent iron oxide ε-Fe₂O₃ is a fairly rare polymorphic iron oxide modification, which only exists in the form of nanoparticles. This magnetically ordered material exhibits an intriguing magnetic behavior, specifically, a significant room-temperature coercivity H_C (up to ~20 kOe) and a magnetic transition in the temperature range of 80–150 K accompanied by a sharp decrease in the H_C value. Previously, the temperature of the transition to the paramagnetic state for ε-Fe₂O₃ was believed to be about 500 K. However, recent investigations have shown that the magnetically ordered phase exists in ε-Fe₂O₃ also at higher temperatures and, around 500 K, another magnetic transition occurs. Using the data on the magnetization and temperature evolution of the ferromagnetic resonance spectra, it is shown that the temperature of the transition of ε-Fe₂O₃ particles 3–10 nm in size to the paramagnetic state is ~850 K.

The ultrafine (d = 4 nm) magnetite ferrofluid with a narrow nanoparticle size distribution has been synthesized in one stage at room temperature from a solution of iron(II) and (III) chlorides in dimethylsulfoxide (DMSO) with the propylene epoxide admixture. This is the first example of obtaining a stable concentrated ultrafine magnetite/DMSO ferrofluid at room temperature. X-ray diffraction, transmission electron microscopy, ferromagnetic resonance, Mössbauer spectroscopy, and magnetostatic study have been used to elucidate the role of DMSO and the H₂O/DMSO ratio in the formation of a stable colloid with a desired nanoparticle size. The initial stages of the magnetite nanoparticles formation have been investigated by the ferromagnetic resonance technique.

The stability of a catalyst for partial H₂S oxidation has been studied by the ferromagnetic resonance (FMR) technique combined with transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy, and magnetostatic investigations. The ε-Fe₂O₃ iron oxide nanoparticles supported on silica have been examined for their stability under the selective H₂S oxidation conditions. The combination of the physicochemical methods has been used to study the state of reacted catalysts. The ε-Fe₂O₃ phase has been found to remain stable under the selective H₂S oxidation conditions at temperatures up to 300 °C. The active phase state during the catalytic reaction has been explored using in situ FMR experiments. It has been established that the ε-Fe₂O₃ nanoparticles retain their structure and magnetic properties in the presence of H₂S at high temperatures. During the in situ FMR experiments, the ε-Fe₂O₃ sulfidation process has been studied.

The dynamic magnetization switching of antiferromagnetic nickel oxide nanoparticles with a characteristic size of 8 nm has been experimentally investigated by pulsed field magnetometry. It is shown that, due to the presence of defects in NiO nanoparticles, as in other antiferromagnetic particles, the uncompensated magnetic moment is induced by the incomplete compensation of spins at the antiferromagnetic ordering. The dynamic magnetic hysteresis loops have been studied in pulsed fields with the maximum field (H_max) of up to 130 kOe and pulse lengths (τP) of 4, 8, and 16 ms. According to the results obtained, the coercivity (H_C) depends on both the τP and Hmax values. The observed increase in the H_C value with decreasing pulse length (i.e., with increasing switching field frequency) is unambiguously related with the relaxation processes typical of single-domain ferromagnetic nanoparticles. However, the observed effect of the maximum applied field (H_max) on the H_C value is assumed to be a feature of antiferromagnetic nanoparticles.

Orthorhombic Pb₂Fe₂Ge₂O₉ antiferromagnetic single crystals have been synthesized by a modified pseudo-flux technique and their magnetic, thermodynamic, and magnetodielectric properties have been investigated. It has been found that, below the Neel temperature (45.2 K), iron moments are arranged in a canted antiferromagnetic structure with a weak ferromagnetic moment parallel to the a axis. According to the specific heat measurement data, the T_N value remains invariable in applied magnetic fields of up to 50 kOe within the experimental accuracy. The magnetic entropy in the investigated crystals attains 2Rln(2S + 1) right above T_N, which is indicative of a purely magnetic nature of the transition. It has been shown that the weak ferromagnetic moment is induced by the interplay between the single-ion anisotropy and antisymmetric Dzyaloshinskii–Moriya exchange interaction, with the latter contribution being dominant. It has been established from the angular dependences of the magnetization in three orthorhombic planes that the symmetries of the magnetic and crystal structure are identical. The magnetodielectric properties of the Pb₂Fe₂Ge₂O₉ single crystals have been studied at different mutual orientations of the electric and magnetic fields. The most prominent anomalies have been observed in the vicinity of the spin-flop transition in a magnetic field applied along the c axis.

The ε-Fe₂O₃ iron oxide is a fairly rare polymorphic modification, which only exists in the form of nanoparticles embedded, as a rule, into a silica gel matrix. This magnetically ordered iron oxide, which exhibits a significant room-temperature coercivity, is a ferroelectric; therefore, the magnetoelectric and magnetodielectric properties of this material evoke keen interest. In this work, we investigate the magnetodielectric (MD) effect in a metamaterial consisting of xerogel SiO₂ with embedded ε-Fe₂O₃ nanoparticles 9 nm in size on average in a concentration of 20 mass.%. This bulk material exhibits the MD effect in a wide temperature range. The temperature behavior of the permittivity is related to the magnetic state of the ε-Fe₂O₃ oxide, which undergoes the magnetic transition from the magnetically hard to magnetically soft phase in the temperature range of 80–150 K, indicating the interplay of the ε-Fe₂O₃ magnetic and charge subsystems.

The dynamic magnetization switching of ferrihydrite nanoparticles has been investigated by a pulsed magnetometer technique in maximum fields H_max of up to 130 kOe with pulse lengths of 4, 8, and 16 ms. Ferrihydrite exhibits antiferromagnetic ordering and defects cause the uncompensated magnetic moment in nanoparticles; therefore, the behavior typical of magnetic nanoparticles is observed. The dynamic hysteresis loops measured under the above-mentioned conditions show that the use of pulsed fields significantly broadens the temperature region of existence of the magnetic hysteresis and the coercivity can be governed by varying the maximum field and pulse length. This behavior is resulted from the relaxation effects typical of conventional ferro- and ferrimagnetic nanoparticles and the features typical of antiferromagnetic nanoparticles.

A novel method for synthesizing a new metamaterial based on ε-Fe₂O₃ nanoparticles immobilized in the xerogel matrix was proposed. Samples with different contents of ε-Fe₂O₃ nanoparticles dispersed in silica xerogel were synthesized by impregnation of as prepared hydrogel with iron (II) salts with the subsequent calcination. The structure and magnetic properties of the prepared composites were studied by transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy, and static magnetic measurements. The absence of other iron oxide polymorphs, controllable particle size distribution, and high ε-Fe₂O₃ nanoparticle concentration in combination with the weak interparticle magnetic interactions ensured the preservation of the unique magnetic properties of individual ε-Fe₂O₃ nanoparticles and allowed us to obtain a novel metamaterial. The high optical transparency and homogeneity of the prepared composites made it possible to detect the magnetic circular dichroism (MCD) of the magnetic silica xerogel, which is typical of the ε-Fe₂O₃-based systems.

Nanoparticles of antiferromagnetically ordered materials acquire the uncompensated magnetic moment caused by defects and surface effects. A bright example of such a nano-antiferromagnet is nanoferrihydrite consisting of particles 2–5 nm in size, the magnetic moment of which amounts to hundreds of Bohr magnetons per particle. We present a brief review of the studies on magnetic properties of ferrihydrite produced by bacteria. Special attention is focused on the aspects of possible biomedical applications of this material, i.e., the particle elimination, toxicity, and possible use for targeted drug delivery.

The changes in separation performance and magnetic characteristics of separation products is traced along the processing circuit of Abagur concentrator at a laboratory scale in order to determine the limit content of magnetite iron in impurity aggregates in the concentrate. The wet magnetic analysis is carried out in the field of H = 175 kA/m, and the magnetic characteristics are determined in the vibration magnetic detector in the field up to 800 kA/m. The concentrate impurity content is governed by the relative content of barren rock and ore aggregates removable in concentration at the given level of the technology.

Granular high-temperature superconductors (HTSs) are characterized by the hysteretic behavior of magnetoresistance. This phenomenon is attributed to the effective field in the intergrain medium of a granular HTS. At the grain boundaries, which are, in fact, weak Josephson couplings, the dissipation is observed. The effective field in the intergrain medium is a superposition of the external field and the field induced by magnetic moments of HTS grains. Meanwhile, analysis of the field width of the R(H) magnetoresistance hysteresis ΔH = H_dec  −  H_inc at H_dec = const, where H_inc and H_dec are increasing and decreasing branches of the R(H) hysteretic dependence, shows that the effective field in the intergrain medium exceeds by far both the external field and the field induced by magnetic moments of HTS grains. This situation suggests the magnetic flux compression in the intergrain medium because of the small length of grain boundaries, which amounts to ∼1 nm, i.e., is comparable with the coherence length and corresponds to Josephson tunneling in HTS materials. In this work, using the previously developed approach, we examine experimental data on the magnetoresistance and magnetization hysteresis in the granular YBa₂Cu₃O₇ HTS compound in the range from 77 K to the critical temperature. According to the results obtained, the degree of magnetic flux compression determined by the parameter α in the expression for the effective field B_eff(H) = H − 4π M(H) α in the intergrain medium remains constant over the investigated temperature range. All the features of the observed evolution of the R(H) hysteretic dependences are explained well within the proposed approach when the expression for B_eff(H) contains the experimental M(H) magnetization data and the parameter α of about 20–25. The latter is indicative of the dominant effect of magnetic flux compression in the intergrain medium on the transport properties of granular HTS materials.