Cobalt(II) enters the trigonal channels of calcium-lanthanum and strontium-lanthanum germanate apatites, forming the nonlinear dioxocobaltate(II) ion [OCoO]2-. Samples reveal two magnetization relaxation processes: SR1 characterized by a Ueff of 47-48 cm-1 and SR2 with considerably extended relaxation times and a weak temperature dependence. The relaxation times of SR1 show an unusual steplike drop with increasing temperature, which correlates with the SR2 contribution to the magnetization. Temperature and field dependence of the magnetization are described by an axial zero-field splitting model, yielding negative D values corresponding well to the Ueff values obtained. Modeling the electronic structure of the dioxocobaltate(II) ion provides information about the Co-O distance ranges corresponding to two different ground states, a moderately anisotropic spin-only one with spin S = 3/2 and an extremely anisotropic non-Aufbau one with S = 3/2 and an orbital angular moment L = 3. Experimental D and g|| values correspond to certain Co-O distances in the spin-only state. The distances follow a qualitative relation with estimated Coulomb forces acting between ions in the host compound. These findings suggest that to convert the dioxocobaltate(II) anion into the desired non-Aufbau state, one must construct a host apatite crystal from ions with lower charges and larger sizes.

Composites of exfoliated graphite (EG) with metals are promising magnetic sorbents for oil sorption and recovery. This work presents a method for preparation of iron-containing EG from graphite intercalation compound (GIC) with iron chloride and ammonia/amines complexes. GICs with FeCl3 were treated by liquid ammonia, methylamine and ethylamine, followed by rapid heat treatment in a nitrogen atmosphere. Thermal decomposition of ammonia-treated adduct resulted in the formation of metallic alpha-Fe on the EG surface, while methylamine-treated adduct produced a mixture of alpha-Fe and iron carbide Fe3C. The structure of both GIC and EG were investigated by XRD analysis and Mossbauer spectroscopy. Ammonia and methylamine react with iron chloride and form the complex compounds within the graphite matrix, whereas ethylamine shows weak reaction with GIC. The presence of alkyl groups in the amines influences both the composition of the iron-containing phase and the EG surface morphology. Iron interacts with carbon to form a gamma-(Fe,C) alloy and iron carbide Fe3C and amorphous carbon coats the EG surface leading to increasing hydrophilicity. The resulting EG/Fe composites exhibited high saturation magnetization (up to 50 emu/g) and exceptional hydrocarbon sorption capacity (up to 30 g/g), demonstrating their potential as efficient magnetic sorbents for oil remediation.

The name of one of the authors of the article by Grigoryeva et al. [ J. Appl. Cryst. (2026), 59 , 1129–1138] is corrected.

Dysprosium-doped calcium-strontium vanadate(V) hydroxyapatites (Ca1-ySry)10(VO4)6(OH)2:Dy, y = 0-0.4, were synthesized by the solid-state reaction at temperatures between 900 and 1000 °C. Dy3+ substitutes for Ca2+ at the 6h Wyckoff site (Ca2) and displaces strongly toward the isolated oxygen anion imbedded in the trigonal channel. This results in the formation of dysprosyl ion DyO+ with a short bond length of 2.15 Å. In zero external magnetic field and below 65 K, the compounds exhibit slow relaxation of the magnetization. With increasing strontium content y, the energy barrier for remagnetization grows from 614 cm-1 to 699 cm-1, the magnetization blocking temperature changes from 3 to 5.5 K, and the magnetization hysteresis at T = 2 K extends from 12 to 16 kOe. The photoluminescence bands exhibit a large crystal field splitting that increases with y. The electronic energy level diagram of Dy3+ obtained from the luminescence data agrees well with the measured magnetic properties. That is the first example of the DyO+ single-ion magnet (SIM) in a nonphosphate compound. This provides an opportunity for comparative studies to reveal new relations between crystal structure details and SIM parameters.

Polycrystalline ceramic and small single-crystal samples of Co-containing calcium vanadate(V) with apatite structure were prepared for the first time. The Co2+ ions enter the apatite trigonal channels formally substituting protons of the OH- groups and form separate O-Co-O atomic groups elongated in the c direction, Co being additionally weakly coordinated to an oxygen atom of a VO4 group. At a high Co content, the hexagonal apatite structure undergoes a triclinic distortion followed by partial ordering of the Co2+ ions. The dc magnetic data fit well to a model of a zero-field split S = 3/2 state with a large negative D of -22 to -25 cm-1, suggesting a strong easy-axis magnetic anisotropy. The ac susceptibility measurements below T = 10 K reveal a multichannel slow relaxation of the magnetization in non-zero dc field. The temperature dependence of the relaxation time can be described by an Orbach process with the remagnetization energy barrier Ueff being equal to experimentally determined 2|D|. Modelling of the electronic structure shows that, with a small increase of the crystal field strength, the high-spin Co2+ ion changes its ground state from one with an unquenched orbital moment L = 3 to a fully orbitally quenched one. Both states are characterized by easy-axis magnetization vectors directed approximately perpendicular to each other with a smooth rotation of the vector at intermediate crystal fields. The model explains the weak magnetic anisotropy observed in the triclinic single crystal as well as the earlier reported ability of the dioxocobaltate(II) ion to behave like a single-ion magnet with either a high Ueff of hundreds of cm-1 or a moderate one of tens of cm-1. To the best of our knowledge that represents the first instance of the conversion of the d-element ground electronic state from orbitally degenerate to non-degenerate by a slight variation of the crystal field.

Single-phase barium hexaferrite powders with crystallite sizes in a single-domain region and with the general composition BaFe12-xMnxO19, where x = 0, 2, 4, 6, were synthesized applying a citric sol-gel auto-combustion technique with final annealing temperatures of 900 - 1200 degrees C. The crystal structures were refined, and the magnetic properties were studied. The observed variations in atomic positions with the Mn-for-Fe substitution revealed presence of Mn in three oxidation state +2, +3, and +4, with a preference of Mn2+ to the tetrahedral 4f(1) site and Mn4+ to the octahedral 2a and 12k sites. With the Mn-doping, the samples' magnetization decreased, while coercivity increased and reached 8.4 kOe for x = 6. The rise of the annealing temperature resulted in a slight growth of magnetization with a general tendency of the coercivity to decrease. A Curie temperature decreased with the Mn-doping remaining above room temperature for the maximal doping.

High-spin Fe 3+ ion imbedded in the extended solid displays field-induced slow relaxation of magnetization with enhanced relaxation time values.

Known preparation methods of the magnetic sorbents based on exfoliated graphite (EG) with metallic phase take several hours and requires the use of a large amount of reducing gas at a high temperature, which is not technologically advanced. The aim of the work is to obtain EG modified with iron, cobalt, and nickel using new simple method. This method includes the thermal treatment in nitrogen atmosphere of the mixture of expandable graphite, Mn+ nitrates (Mn+ = Fe3+, Co2+, Ni2+) and melamine. The thermal decomposition of melamine leads to formation of ammonia, which reduces the products of the Mn+ nitrates decomposition (α-Fe2O3, Co3O4/CoO, NiO) to metallic iron, cobalt and nickel. The use of different amount of melamine as reducing agent leads to the formation of mixtures with different compositions containing iron oxides Fe3O4, FeO, metallic iron α-Fe, Fe-C alloy γ-(Fe,C) and iron carbide Fe3C, which were confirmed by Mossbauer spectroscopy. Metallic Co and Ni on the EG surface is formed in nitrogen atmosphere by reduction with carbon even without melamine. Obtained EG/Fe has the highest saturation magnetization of 54.7emu/g. EG/Co and EG/Ni have lower saturation magnetization of 40.1 and 12.0emu/g, which is due to lower saturation magnetization of the individual metals.

Doping of the (Sr,Ba)10(PO4)6(OH)2 apatite ceramics with a small quantity of Dy2O3 was studied. Formation of the Dy3+ containing high-energy single-ion magnet (SIM) in the apatite structure was confirmed. Partial replacement of Ba for Sr in the structure resulted in a regular increase of the remagnetization energy barrier Ueff from 1043 to 1119 cm-1, while the solubility of Dy3+ in the compound dropped drastically. Ueff followed simple relations with the alkaline-earth metal cation size and the compound composition, highlighting predictability of SIM parameters.

Dy3+ doped SrLaGaO4 exhibits unusually slow relaxation of magnetization determined by two widely separated excited Kramers doublets with a second remagnetization energy barrier of 223 cm-1. This value considerably exceeds that for analogous Ca(Y,Dy)AlO4 in spite of the apparently enlarged Dy3+ coordination sphere.

A small amount of cobalt was incorporated into the lanthanum calcium silicate apatite structure by annealing at 1500 °C in argon. The compound exhibits easy-axis magnetic anisotropy with a zero-field splitting parameter 2D of −60 cm−1 and field- induced slow relaxation of magnetization with a remagnetization energy barrier of 58–63 cm−1. Thereby, for the first time, a cobalt-based single-ion magnet was created in silicate.

A novel ultra-high-entropy rare earth orthoferrite (UHE REO) of Sc1/16Y1/16La1/16Ce1/16Pr1/16Nd1/16Sm1/16Eu1/16Gd1/16Tb1/16Dy1/16Ho1/16Er1/16Tm1/16Yb1/16Lu1/16FeO3 nominal composition was successfully synthesized for the first time through a simple and efficient solution combustion approach. PXRD, Raman, and 57Fe Mössbauer spectroscopy confirmed the high chemical and phase purity of the synthesized UHE REO (hereafter denoted as ΣREFeO3), which belonged to the Pnma space group, typical of the perovskite-like rare earth orthoferrites. Despite the fact that the main X-ray reflections, vibration modes, and spectral Mössbauer components unambiguously indicate the single-phase nature of the sample, the results of SEM and TEM make it possible to establish the presence of a main (about 50 nm) and a minor ultrafine (about 10 nm) fraction of ΣREFeO3 nanoparticles. The bimodal size distribution of nanoparticles was also reflected in the magnetic behavior of this substance: the presence of several sextet components in the Mössbauer spectra, the hard single-domain magnetic nature of the main fraction of 50 nm UHE REO nanoparticles, and the superparamagnetic state of the minor fraction of 10 nm UHE REO nanoparticles. Thus, the unusual features of nanostructured ΣREFeO3 can potentially be used for the creation of new generations of transformers, magnetic memory systems, magnetic screens, radio devices, etc.

New magnetic nanocomposite sorbents were obtained by doping natural bentonite with nanosized CoFe2O4 spinel (10 and 20 wt.%). Nanocrystals of cobalt ferrite were synthesized by a citrate burning method. The structure and physical-chemical properties of the composites were characterized by XRD, XRF, TEM, BET, FTIR and Faraday balance magnetometry. During the formation of nanocomposites, 10–30 nm particles of cobalt ferrite occupied mainly the interparticle space of Fe-aluminosilicate that significantly changed the particle morphology and composite porosity, but at the same time retained the structure of the 2:1 smectite layer. A combination of two functional properties of composites, adsorption and magnetism has been found. The adsorption capacity of magnetic nanosorbents exceeded this parameter for bentonite and spinel. Despite the decrease in the adsorption volume, pore size and specific surface area of the composite material relative to bentonite, the sorption activity of the composite increases by 12%, which indicated the influence of the magnetic component on the sorption process. FTIR data confirmed the mechanism of formaldehyde sorption by the composite sorbent. The production of a magnetic nanosorbent opens up new possibilities for controlling the sorption processes and makes it possible to selectively separate the sorbent from the adsorption medium by the action of a magnetic field.

The first instance of a rare-earth single-ion magnet in a robust extended solid has been found, which possesses a crystal structure different from apatite. The compound exhibits slow relaxation of magnetization in a zero field revealing simultaneously two energy barriers for magnetization reversal.

Structural ordering in the concentrated magnetic colloids containing 50 × 5 nm hard magnetic disc-like SrFe12O19 nanoparticles was investigated by cryogenic scanning electron microscopy, optical microscopy, magnetic measurements, and small-angle X-ray scattering. It was revealed that macroscopically homogeneous magnetic liquid consists of dynamic threads of stacked nanoparticles. The threads align into quasiperiodic arrays with the distances between individual threads of a few micrometers. They also can form pseudodomain structures with ~ 90° domain boundaries realized through T-type thread interconnects. The effects of magnetic attraction and electrostatic repulsion on the equilibrium interplatelet distance in the threads were studied. It was demonstrated that this distance can be tuned by the control of the particles charge and electric double layer screening from Stern layer thickness (~ 1 nm) to tens of nanometers. It was shown that the permanent magnetic field is not able to cause any structural changes in the ordered magnetic liquid phase, while alternating field draws particles apart by their vibrations. External variation of interparticle distance up to 6% was achieved using an alternating magnetic field of low intensity. Experimental data were complemented by the theoretical models of screened electrostatic interactions between spherical and platelike magnetic particles. The last model provides good predictive power and correlates with the experimental data. The stabilization energy of the condensed phase in the order of 1–10 kBT was derived from the model. An approach allows controlling of an equilibrium interparticle distance and interparticle distance distribution by adjusting the magnetization and surface charge of the particles as well as the ionic strength of the solvent.

Herein, we demonstrate for the first time compact ferrite ceramics with giant coercivity. The materials are manufactured via sintering single-domain Sr0.67Ca0.33Fe8Al4O19 particles synthesized by a citrate-nitrate auto-combustion method. The obtained ceramics show coercivities up to 22.5 kOe and natural ferromagnetic resonance frequencies (NFMR) in a sub-THz range of 160-282 GHz. At a maximum density of 95%, the sample displays coercivity of 18.5 kOe, which is the highest value among dense ferrite materials reported so far. In addition, we report an unusual blueshift of the NFMR frequency from 160 to 200 GHz, which occurs during material sintering.

Fine particles of SrFe12−xGaxO19 (x = 0–6) were obtained via a citrate auto-combustion route. Their magnetic and microwave absorption properties, as well as the features of the crystal structure were studied in detail.

A fast method for the synthesis of ε-Fe2O3, yielding 100% pure material with a variable FMR frequency, is proposed.

Exchange-coupled hard/soft ferrite nanoparticles are prospective to squeeze out a part of expensive magnets based on rare-earth elements. However, the known exchange-coupled composite ferrite nanoparticles often suffer from the lack of a powerful enough hard magnetic core, high defectivity of magnetic phases, and a poor interface between them. Herein, we demonstrate the first efficient synthesis of sandwiched nanomagnets, which exhibit a pronounced exchange-coupling effect. This work is featured by the use of individual highly coercive strontium hexaferrite nanoplates prepared by a borate glass crystallization method as cores for the composite particles. The high crystal quality of the hexaferrite cores as the substrate promotes the epitaxial growth of CoFe2O4 layers on the 001 facets from an organic high-boiling solvent and results in the enhancement of the remanent magnetization and maximum energy product of the composite material. The results of this work open new prospects for the fabrication of multilayer oxide heterostructures with synergetic performance, which expands the applications of exchange-coupled composites.

Magnetically hard ferrites attract considerable interest due to their ability to maintain a high coercivity of nanosized particles and therefore show promising applications as nanomagnets ranging from magnetic recording to biomedicine. Herein, we report an approach to prepare nonsintered single-domain nanoparticles of chromium-substituted hexaferrite via crystallization of glass in the system SrO–Fe2O3–Cr2O3–B2O3. We have observed a formation of plate-like hexaferrite nanoparticles with diameters changing from 20 to 190 nm depending on the annealing temperature. We demonstrated that chromium substitution led to a significant improvement of the coercivity, which varied from 334 to 732 kA m−1 for the smallest and the largest particles, respectively. The results provide a new strategy for producing high-coercivity ferrite nanomagnets.