New Publications

https://doi.org/10.1016/j.ceramint.2024.11.048

A representative series of samples consisting of fine ε-Fe₂O₃ nanoparticles uniformly distributed over the SiO₂ xerogel matrix with an iron oxide content of 5–33 wt % has been synthesized and studied by mutually complementary physical methods. It has been shown by the X-ray diffractometry and high-resolution electron microscopy examination that, with increasing iron oxide concentration, the average particle size <d> increases from 4 to 11 nm. According to the X-ray diffractometry and Mӧssbauer spectroscopy data, the samples with an iron oxide content of 5–20 wt % are single-phase, while at the highest Fe₂O₃ concentration (33 wt %), the β-Fe₂O₃ and α-Fe₂O₃ phases arise. As the average particle size <d> increases, a monotonic increase in coercivity H_C and remanent magnetization M_R of the synthesized systems at room temperature is observed, which is indicative of their magnetic hysteresis. The magnetic transition known to occur in the ε–Fe₂O₃ oxide manifests itself in all the investigated samples as a drastic change in the H_C and M_R values at 150–75 K. At the same time, a thorough analysis of the temperature dependence of the real part of the ac magnetic susceptibility χ′ has shown that the particles with a size smaller than a critical value of d_C ∼6.5 nm do not undergo the magnetic transition and have a much lower magnetic anisotropy constant as compared with coarser particles (d > d_C). It has been found that, in the low-temperature region, the magnetic moments of these fine particles experience superparamagnetic blocking. It has been established that the size effects that arise in ultra-fine ε–Fe₂O₃ particles has a serious impact on the macroscopic magnetic properties of the highly dispersed systems based on them.

https://doi.org/10.48550/arXiv.2408.16203

The relation of the magnetically dead layer and structural defects in ultrafine interacting NiFe2O4 nanoparticles (<d> = 4 nm) have been investigated using transmission electron microscopy, X-ray diffraction, ^57Fe Mössbauer spectroscopy, and dc magnetization and ac susceptibility measurements. According to the magnetic data, we found out three magnetic subsystems in NiFe2O4 nanoparticles. The first one with the lowest blocking (spin freezing) temperature (TS = 8 K) established by atomic magnetic moments of magnetically disordered particles with the d < 4 nm. The other two subsystems are formed by the magnetic moments of the "core" of nanoparticles having size more than 4 nm and correlated surface spins in nanoparticle clusters, correspondingly. Magnetic moments of ferrimagnetically ordered "core" are blocking at a higher temperature ( \approx 40 K). It has been shown that the most significant contribution to the energy dissipation is made upon blocking of the correlated nanoparticle surface spins from the magnetically dead layer on the nanoparticles' surface. By the magnetic data, the thickness of this layer is dmd \approx 1 nm for a particle with the <d> \approx 4 nm. At the same time or meanwhile, the ^57Fe Mössbauer spectroscopy has revealed a structural disorder penetrating to a depth of up to dcd \approx 0.6 nm in a particle with <d> = 4 nm. This evidences for a faster destruction of the magnetic order as compared with the crystal order upon moving away from the center of a particle to its periphery.

https://doi.org/10.1016/j.ceramint.2024.09.268

Solid-state synthesis is widely used in exploratory research to study various structural modifications that affect the properties (critical temperature, critical current density, irreversibility field, etc.) of superconductors. The popularity of this method is due to its relative simplicity and availability of the necessary equipment. Combining solid-state synthesis and top-seeded melt growth allows us to increase the grain size in a Tm- and Nd-based 1-2-3 superconductor. Samples with a grain size up to 0.1 mm have been obtained. X-ray diffraction, scanning electron microscopy and magnetization measurements have been used for investigating this superconducting material. The magnetization width ΔM has increased significantly in the synthesized samples. However the temperature dependence of the intragrain critical current density and the pinning force scaling give evidences that the pinning mechanism in the obtained superconductor is essentially the same as in polycrystalline superconductors synthesized by standard solid-state technology. The increase in grain size in the synthesized samples is the main reason for the high values of ΔM

https://doi.org/10.1134/S0031918X23603025

The paper examines the impact of interparticle interactions on the superparamagnetic relaxation of ultrasmall nanoparticle ensembles, using Fe₂O₃∙nH₂O iron oxyhydroxide (ferrihydrite) nanoparticles as an example. Two samples were analyzed: ferrihydrite of biogenic origin (with an average particle size of ⟨�⟩ ≈ 2.7 nm) with a natural organic shell, and a sample (with ⟨�⟩ ≈ 3.5 nm) that underwent low-temperature annealing, during which the organic shell was partially removed. The DC and AC magnetic susceptibilities (χ′(T), χ′′(T)) in a small magnetic field in the superparamagnetic (SPM) blocking region of the nanoparticles were measured. The results show that an increase in interparticle interactions leads to an increase in the SPM blocking temperature from 28 to 52 K according to DC magnetization data. It is shown that below the SPM blocking temperature, magnetic interactions of nanoparticles lead to the formation of a collective state similar to spin glass in bulk materials. The scaling approach reveals that the dynamics of correlated magnetic moments on the particle surface slow down with increasing interparticle interactions. Simulation of χ′′(T) dependence has shown that the dissipation of magnetic energy occurs in two stages. The first stage is directly related to the blocking of the magnetic moment of nanoparticles, while the second stage reflects the spin-glass behavior of surface spins and strongly depends on the strength of interparticle interactions.

https://doi.org/10.1016/j.jmmm.2024.171781

Contributions of different magnetic subsystems formed in the systems of synthetic ferrihydrite nanoparticles (characterized previously) with an average size of < d> ≈ 2.7 nm coated with polysaccharide arabinogalactan in different degrees have been separated by measuring the dependences of their magnetization M on magnetic field H of up to 250 kOe on vibrating sample and pulsed magnetometers. The use of a wide measuring magnetic field range has been dictated by the ambiguity in identifying a linear M(H) portion for such antiferromagnetic nanoparticle systems within the conventional field range of 60–90 kOe. The thorough analysis of the magnetization curves in the temperature range of 100–250 K has allowed the verification of the contributions of (i) uncompensated magnetic moments µ_un in the superparamagnetic subsystem, (ii) the subsystem of surface spins with the paramagnetic behavior, and (iii) the antiferromagnetic susceptibility of the antiferromagnetically ordered ferrihydrite particle core. As a result, a model of the magnetic state of ferrihydrite nanoparticles has been proposed and the numbers of spins corresponding to magnetic subsystems (i)–(iii) have been estimated. An average magnetic moment μ_un of ∼ 145 μ_B (μ_B is the Bohr magneton) per particle corresponds approximately to 30 decompensated spins of iron atoms in a particle (about 3 % of all iron atoms), which, according to the Néel’s hypothesis μ_un ∼ <d>^3/2, are localized both on the surface and in the bulk of an antiferromagnetically ordered particle. The fraction of free (paramagnetic) spins is minimal in the sample without arabinogalactan coating of the nanoparticle surface (7 %) and is attained 20 % of all iron atoms in the sample with the highest degree of spatial separation of particles. According to this estimation, paramagnetic spins are located mainly on the edges and protruding areas of particles. Most magnetic moments of iron atoms are ordered antiferromagnetically and the corresponding magnetic susceptibility of this subsystem behaves as in an antiferromagnet with the randomly distributed crystallographic axes, i.e., increases with temperature.

https://doi.org/10.1134/S1063776123120075

The magnetic-field dependence of the superparamagnetic-blocking temperature T_B of systems of antiferromagnetically ordered ferrihydrite nanoparticles has been investigated and analyzed. We studied two powder systems of nanoparticles: particles of “biogenic” ferrihydrite (with an average size of 2.7 nm), released as a result of vital functions of bacteria and coated with a thin organic shell, and particles of biogenic ferrihydrite subjected to low-temperature annealing, which cause an increase in the average particle size (to 3.8 nm) and burning out of the organic shell. The character of the temperature dependences of magnetization, measured after cooling in a weak field, as well as the shape of the obtained dependences T_B(H), demonstrate peculiar features, indicating the influence of magnetic interparticle interactions. A detailed analysis of the dependences T_B(H) within the random magnetic anisotropy model made it possible to estimate quantitatively the intensity of magnetic particle–particle interactions and determine the magnetic anisotropy constants of individual ferrihydrite particles.

https://doi.org/10.1016/j.nanoso.2023.101089

The results of a study of the dynamic (alternating current magnetic susceptibility) and static magnetic properties, as well as ⁵⁷Fe Mössbauer spectrometry and ferromagnetic resonance of two-line ferrihydrite nanoparticle systems with varying intensities of magnetic interparticle interactions are reported. The strength of the magnetic interparticle interactions has been tuned by coating (with various degrees of coating) the ferrihydrite particles (2–4 nm in size and an average size ∼2.7 nm) of the initial synthetic sample by arabinogalactan. Also, a biogenic ferrihydrite sample (an average particle size of 2-nm) with a natural organic coating was studied and it has the weakest magnetic interparticle interactions among of all the samples. Relaxation times of the particle’s magnetic moment were determined by the data of static and dynamic magnetic susceptibilities and from analysis of ⁵⁷Fe Mössbauer spectrometry. Based on the temperature dependences of the relaxation times, it has been concluded that the predominantly collective processes of freezing of the particle magnetic moments occur under the action of the magnetic interparticle interactions. It is shown that an important role in these processes is played by a magnetic subsystem of the surface spins of the particles. The effect of the interplay between the surface spin and magnetic moment subsystems on the static magnetic properties (low-temperature magnetic hysteresis loops) and the parameters of the microwave absorption line under the magnetic resonance conditions is discussed.

https://doi.org/10.1016/j.jmmm.2023.171573

Arrays of iron nanowires (NWs) obtained by template-assisted electrodeposition constitute a promising composite material characterized by a combination of high magnetization in the filler and perpendicular magnetic anisotropy. The properties of these composites arise from the interplay between the behavior of individual NWs and their magnetostatic interactions. In this study, we investigated NW arrays with identical wire diameters but varying spatial arrangements. Major hysteresis loops were studied under various field directions relative to the NW axis. Key parameters such as the slope of the magnetization curve, saturation magnetization, and coercive force were quantified. Additionally, FORC (First Order Reversal Curve) measurements were conducted with the field oriented longitudinally with respect to the NW, offering insights into the inhomogeneity of the demagnetizing field influenced by the NW array's configuration. In the sample with the highest NW density, we observed isotropic behavior of the effective demagnetizing field, and we proposed an explanation for this phenomenon using the effective media approach. Micromagnetic simulations revealed that the magnetic behavior of individual NWs with a 100 nm diameter can be described as an interchange between volumes characterized by vortex and uniform magnetization patterns. Calculations of the demagnetizing field using the effective medium model demonstrated excellent agreement with experimental data across arrays featuring different NW densities. Remarkably, the quantitative consistency of coercive field values obtained from micromagnetic simulations and experimental measurements in the range of angles from 0° to 45° for the studied samples underscores the structural homogeneity of the obtained NWs.

https://doi.org/10.1007/s10948-023-06608-2

Granular high-temperature superconductors (HTSs) are characterized by the hysteretic field dependences of magnetoresistance R(H) and critical current I_C(H). These hysteretic effects are described within the concept of an effective field in the intergrain medium. The effective field is a superposition of external magnetic field H and the field induced by the magnetic moments of superconducting grains into intergrain spacings (grain boundaries). The magnetization of superconducting grains is determined by two contributions: Meissner (shielding) currents (MC) and trapped magnetic fluxes (Abrikosov vortices (AV)). To develop the concept of an effective field in the intergrain medium, the magnetotransport properties (R and I_C) have been compared for two cases: (AV) the magnetization of superconducting grains is only determined by the trapped magnetic flux (zero external field) and (MC) HTS grains are in the Meissner state (the external field is weaker than the first critical field of grains). In a set of experiments, the main features of the hysteretic R(H) and M(H) dependences have been illustrated and the external conditions for implementing the AV and MC states have been established. It has been found that the effects of the Abrikosov vortices and intragrain Meissner currents on an effective field in the intergrain medium at the same magnetization values are noticeably different. This is a nontrivial fact that requires a thorough study of the impact of the anisotropy of the superconducting properties of grains on the configuration of the Meissner currents in them, as well as on the orientation of vortices both inside grains and near their surface. We suggest the explanation of observed stronger effect of the Meissner currents on the intergrain medium as compared with the effect of the Abrikosov vortices.

https://doi.org/10.1134/S1063776123010132

We present a brief review of investigations and analysis of magnetization curves M(H) for NiO and ferrihydrite antiferromagnetic nanoparticles in external fields up to 250 kOe. For correct interpretation of magnetic properties of systems of antiferromagnetic nanoparticles, it is important to take into account the segment of M(H) dependences, which corresponds to high fields (exceeding 100 kOe). We analyze the regularities in the formation of additional magnetic subsystems in antiferromagnetically ordered nanoparticles due to the influence of size effects. These additional subsystems (the ferromagnetic subsystem associated with uncompensated magnetic moment and the subsystem of surface free spins) are estimated quantitatively. It is shown that antiferromagnetic nanoparticles with a size of 5 nm acquire the properties of “nanomagnets,” which are not inferior to those for iron-oxide ferromagnetic nanoparticles of the same size.

https://doi.org/10.1016/j.physb.2023.414901

In this study, nanoparticles of initial synthetic ferrihydrite have been coated with arabinogalactan. The synthesized series of samples with different degrees of coverage of particles has been characterized by X-ray photoelectron spectroscopy, Mössbauer spectroscopy, transmission electron microscopy and magnetometry. The superparamagnetic blocking temperature decreases monotonically with an increase in the degree of coverage of ferrihydrite particles, which is unambiguously related to the different role of the interparticle magnetic interactions in the investigated powder systems. Analysis of the field dependence of the blocking temperature within the random anisotropy model has shown that an increase in the degree of coverage of ferrihydrite particles leads to a decrease in the size of a cluster in which the behaviors of the nanoparticle magnetic moments are correlated. The results obtained have shown the possibility of effective control of the strength of magnetic interparticle interactions in powder ferrihydrite systems by coating nanoparticles with arabinogalactan.

https://doi.org/10.1103/PhysRevB.107.115413

Features of the spin dynamics in ensembles of interacting (FH-chem) and weakly interacting (FH-coated) magnetic ultrasmall ($⟨d⟩\sim 2$ nm) ferrihydrite nanoparticles have been explored. The dc and ac magnetic susceptibilities $[{\chi }^{\prime }\left(T\right)$ and ${\chi }^{\text{'}\text{'}}\left(T\right)]$ of the investigated samples have been thoroughly measured in a weak magnetic field (2 Oe) around the temperatures of superparamagnetic blocking of the nanoparticle magnetic moments (19 and 50.4 K for FH-coated and FH-chem, respectively, according to the dc magnetization data). It has been shown that the magnetic interactions between nanoparticles induce the formation of the cluster spin-glass state below the superparamagnetic blocking temperature ($T_g=18$ and 49.5 K for FH-coated and FH-chem, respectively). It has been found that coating of nanoparticles increases the critical scaling index from $z\nu =5.9$ (FH-chem) to $z\nu =8.0$ (FH-coated). This indicates a general slowdown of the dynamics of correlated spins, which is also expressed as an increase in relaxation time ${\tau }_0$ after switching on the interparticle interactions. We attribute this phenomenon to a consequence of a change in the volume of correlated spins with the increasing size of a cluster of interacting nanoparticles. It has been demonstrated using the simulated ${\chi }^{\text{'}\text{'}}\left(T\right)$ dependence that the dissipation of the magnetic energy occurs in two independent stages. The first stage is directly related to the blocking of the nanoparticle magnetic moments, while the second stage reflects the spin-glass behavior of surface spins and depends strongly on the intensity of the interparticle interactions.

https://doi.org/10.3390/nano12244366

Mixed Co-Ni bimetallic systems with the structure of a solid substitution solution have been synthesized using the supercritical antisolvent precipitation (SAS) method, which uses supercritical CO₂ as an antisolvent. The systems obtained have been characterized in detail using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Fourier-transform infrared (FTIR) spectroscopy, and magnetostatic measurements. It has been found that Co-enriched systems have a defective hexagonal close-packed (hcp) structure, which was described by a model which embedded cubic fragments of packaging into a hexagonal close-packed (hcp) structure. It has been shown that an increase in water content at the precipitation stage leads to a decrease in the size of cubic fragments and a more uniform distribution of them in Co-enriched systems. It has also been shown that mixed systems have the greatest coercivity in the line of samples. Ni-enriched bimetallic systems have a cubic close-packed (ccp) structure with modified crystal lattice parameters.

https://doi.org/10.1007/s10948-022-06454-8

A misalignment of anisotropic crystallites causes small values of anisotropy and decreases the critical current density of textured polycrystalline superconductors. To relate the crystallite misalignment and out-plane anisotropy, the magnetic properties of the textured Bi2223 polycrystalline superconductor were investigated. A distribution of orientation angles of crystallites was determined using different data: scanning electron microscopy images and hysteresis magnetization loops when an external magnetic field was applied at different angles with respect to the texturing plane of the sample. It was demonstrated that the standard deviation of the distribution and the magnetic disorder angle of crystallites in textured samples can be determined from the magnetization data in perpendicular directions. These data may be either the irreversible magnetization measured for two different orientations of the sample or the simultaneously measured magnetization projections parallel and perpendicular to the magnetic field.

The low-temperature spin dynamics of the orthorhombic TbFeO3 perovskite has been studied. It has been found that the inelastic neutron scattering (INS) spectrum contains two modes corresponding to different sublattices in the compound. The iron subsystem orders antiferromagnetically at TN = 632 K and exhibits the high-energy magnon dispersion. Magnetic dynamics of this subsystem has been described using the linear spin wave theory and our solution yields sizable anisotropy between in-plane and out-of-plane exchange interactions. This approach was previously used to describe the magnon dispersion in the TmFeO3 compound. Three non-dispersive crystal electric field levels corresponding to Tb3+ ions have been established in the region below 40 meV at about 17, 26, and 35 meV. Study of diffuse scattering at different temperatures has elucidated the behavior of the magnetic correlation length. The behavior of the Tb3+ ion subsystem has been numerically described in the framework of the point charge model. The numerical data agree satisfactorily with the experiment and with the general concept of the single-ion approximation applied to the rare-earth subsystem of orthorhombic perovskites.

https://doi.org/10.1134/S1063776122060097

The structural and magnetic properties of a niobium nitride (NbN) film prepared by reactive sputtering onto a quartz substrate are investigated. It is shown using scanning electron microscopy that the film has a columnar structure with a diameter of crystallite columns of about 50 nm. The film magnetization loops are measured for the field orientation parallel and perpendicular to its surface. Based on the experimental data, the critical current densities of the film are estimated in both cases. For the field parallel to the film surface, the estimate is 6.5 × 104 A/cm2 at the liquid helium temperature. For the field perpendicular to the surface, the critical current density is close to the depairing current density (107 A/cm2). Analysis of the results based on different models of magnetic vortex pinning in superconductors shows that in the former case, pinning occurs at the boundaries of columns in the bulk of the sample, while in the latter case, it is determined by the influence of the surface barrier.

https://doi.org/10.1016/j.jallcom.2022.166346

https://doi.org/10.12989/anr.2022.12.6.605

We prepared two samples of ultrafine ferrihydrite (FH) nanoparticle ensembles of quite a different origin. First is the biosynthesized sample (as a product of the vital activity of bacteria Klebsiella oxytoca (hereinafter marked as FH-bact) with a natural organic coating and negligible magnetic interparticle interactions. And the second one is the chemically synthesized ferrihydrite (hereinafter FH-chem) without any coating and high level of the interparticle interactions. The interparticle magnetic interactions have been tuned by modifying the nanoparticle surface in both samples. The coating of the FH-bact sample has been partially removed by annealing at 150℃ for 24 h (hereinafter FH-annealed). The FH-chem sample, vice versa, has been coated (1.0 g) with biocompatible polysaccharide (arabinogalactan) in an ultrasonic bath for 10 min (hereinafter FH-coated). The changes in the surface properties of nanoparticles have been controlled by XPS. According to the electron microscopy data, the modification of the nanoparticle surface does not drastically change the particle shape and size. A change in the average nanoparticle size in sample FH-annealed to 3.3 nm relative to the value in the other samples (2.6 nm) has only been observed. The estimated particle coating thickness is about 0.2-0.3 nm for samples FH-bact and FH-coated and 0.1 nm for sample FH-annealed. Mössbauer and magnetization measurements are definitely shown that the drastic change in the blocking temperature is caused by the interparticle interactions. The experimental temperature dependences of the hyperfine field <hhf>(T) for samples FH-bact and FH-coated have not revealed the effect of interparticle interactions. Otherwise, the interparticle interaction energy Eint estimated from the <hhf>(T) for samples FH-chem and FH-annealed has been found to be 121kB and 259kB, respectively.

https://doi.org/10.1016/j.apt.2022.103692

Ferrihydrite nanoparticles were synthesized using Klebsiella oxytoca microorganisms under various cultivation conditions. The cultivation of bacteria was carried out under various lighting conditions, and the duration of cultivation varied from 3 to 56 days. Biogenic ferrihydrite nanoparticles were studied by Mössbauer spectroscopy, magnetometry, and small-angle X-ray scattering. The process of formation of ferrihydrite nanoparticles and the states arising during the cultivation of microorganisms have been investigated. The results of Mössbauer spectroscopy showed that, depending on the time of cultivation, three different states of ferrihydrite can be realized. States differ both in the ratio of defective and non-defective positions, and the size of the particle. Experimental results indicate that ferrihydrite nanoparticles are a system of variable composition and pass through several structural (or morphological) states during the cultivation of microorganisms. A model of the structure of ferrihydrite nanoparticles is proposed, which consists in the presence of an antiferromagnetic dense core with a high Néel temperature and a friable shell with a significantly lower temperature of magnetic ordering.

https://doi.org/10.1016/j.rinp.2022.105340

Ferrihydrite is characterized by the antiferromagnetic ordering and, in ferrihydrite nanoparticles, as in nanoparticles of any antiferromagnetic material, an uncompensated magnetic moment is formed. We report on the investigations of ferrihydrite powder systems with an average particle size of ∼ 2.5 nm obtained (i) as a product of the vital activity of bacteria (sample FH-bact) and (ii) by a chemical method (sample FH-chem). In the first approximation, these samples can be considered to be identical. However, in sample FH-chem, particles contact directly, while in sample FH-bact, they have organic shells; therefore, the interparticle magnetic interactions in these samples have different degrees. The main goal of this work has been to establish the effects of the interparticle magnetic interactions and individual characteristics of ferrihydrite nanoparticles on ferromagnetic resonance (FMR) spectra. The FMR spectra have been measured at different (9.4–75 GHz) frequencies in a wide temperature range. It has been found that, at low temperatures, the field-frequency dependence ν(H_R) of the investigated systems has a gap ν/γ = H_R + H^A, where H_R is the resonance field and H^A is the induced anisotropy, which decreases with increasing temperature. To estimate a degree of the effect of interparticle interactions on the results obtained and to correctly determine the temperature range of the superparamagnetic (or blocked) state, the static magnetic measurement and Mössbauer spectroscopy data have been obtained and analyzed. It has been shown that the most striking feature of the FMR spectra - a gap in the field-frequency dependences - is a manifestation of individual characteristics of ferrihydrite nanoparticles. The induced anisotropy is caused by freezing of a subsystem of surface spins and its coupling with the particle core, which is observed in both samples at a temperature of ∼80 K. The temperature range (below 80 K) in which the gap exists corresponds to the blocked state in the FMR technique. In sample FH-bact, the ratio between the FMR parameters H^A and linewidth ΔH obeys the standard expression H^A ∼ (ΔH)³. In sample FH-chem, however, the interparticle magnetic interactions dramatically affect the behavior of parameters of the FMR spectra, which change nonmonotonically upon temperature variation. This fact is attributed to the collective freezing of the magnetic moments of particles under the conditions of sufficiently strong interactions, which follows from the temperature dependence of the particle magnetic moment relaxation time determined from the Mössbauer spectroscopy and static magnetometry data obtained in weak magnetic fields.

https://doi.org/10.1134/S1063783421070192

https://doi.org/10.1016/j.jmmm.2021.168343

-The analysis of the M(H) magnetization curves of antiferromagnetic nanoparticles yields information about magnetic subsystems formed in these objects, which are characterized by a large fraction of surface atoms. However, in the conventionally investigated experimental magnetic field range of up to 60–90 kOe, this analysis often faces the ambiguity of distinguishing the Langevin function-simulated contribution of uncompensated magnetic moments μ_un of particles against the background of a linear-in-field dependence (the antiferromagnetic susceptibility and other contributions). Here, this problem has been solved using a pulsed technique, which makes it possible to significantly broaden the range of external fields in which the μ_un contribution approaches the saturation. Nanoparticles of a typical NiO antiferromagnet with an average size of <d> ~ 4.5 nm have been investigated. Based on the thorough examination of the M(H) magnetization curves measured in pulsed fields of up to 250 kOe, a model of the magnetic state of NiO nanoparticles of such a small size has been proposed. The average moment is ~130 μ_B (μ_B is the Bohr magneton) per particle, which corresponds to 60–70 decompensated spins of nickel atoms localized, according to the Néel hypothesis (μ_un ~ <d>^3/2), both on the surface and in the bulk of a particle. A part of the surface spins unrelated to the antiferromagnetic core form another subsystem, which behaves as free paramagnetic atoms. Along with the antiferromagnetic core, an additional linear-in-field contribution has been detected, which is apparently related to superantiferromagnetism, i.e., the size effect inherent to small antiferromagnetic particles.

https://doi.org/10.1016/j.jallcom.2021.161623

Samples of synthetic ferrihydrite with an average nanoparticle size of 2.7 nm have been examined by magnetometry and Mössbauer spectroscopy. Ferrihydrite is characterized by the antiferromagnetic interactions between the magnetic moments of iron atoms. In ferrihydrite nanoparticles, as in any other antiferromagnetic ones, structural defects induce the formation of an uncompensated magnetic moment, which determines the magnetic properties typical of single-domain ferro- and ferrimagnetic particles. The manifestation of the magnetic interactions between ferrihydrite nanoparticles in the magnetic properties of the material and in the temperature evolution of Mössbauer spectra has been in focus. The results obtained on synthetic ferrihydrite have been compared with the data for the biogenic ferrihydrite sample with a similar average size of particles surrounded by a polysaccharide shell, which weakens and screens the interparticle magnetic interactions. A clear manifestation of the effect of the interparticle magnetic interactions on the transition to the blocked state is the presence of a significant contribution of the relaxation component in the Mössbauer spectra at temperatures of the transition from the superparamagnetic to blocked state. The temperature dependence of the particle relaxation time obtained from the Mössbauer spectra points out the collective effect of freezing of the magnetic moments of particles due to the magnetic interactions between them.

Superconducting and paramagnetic contributions to the magnetization of polycrystalline Y1−xHoxBa2Cu3O7−δ samples were investigated. The superconductivity is responsible for a partial screening of magnetic ions from an external magnetic field and for a possible sinking of antiferromagnetic correlations between these ions. Magnetic moments of Ho ions influence on a peak effect induced by the order–disorder transition of the Abrikosov vortex lattice. The critical current density and the critical temperature of YBCO are not changed by the Ho doping.

https://doi.org/10.1007/s10948-021-05954-3

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 ТP < T < T*) is described by the expression: 2πν/γ = НR + HA(T = 0)(1 – T/Т*), where γ is the gyromagnetic ratio, HR is the resonance field. The induced anisotropy HA is due to the spin-glass state of the near-surface regions. TP temperature characterizes the energy of the interparticle interaction of nanoparticles.