Starchikov, Sergey Sergeevich

Lyubutin I.S., Starchikov S.S., Lyubutina M.V., Markina M.M., Vasiliev A.N.

Starchikov S.S., Zayakhanov V.A., Troyan I.A., Bykov A.A., Bulatov K.M., Vasiliev A.L., Perekalin D.S., Snegirev N.I., Kulikova E.S., Davydov V.A., Lyubutin I.S.

Specific features of thermal transformations of ferrocene Fe(C5H5)2 at a pressure of 10 GPa upon laser heating to 2200 K have been investigated in diamond anvil cells. Maps of the temperature distribution on the sample during the heating have been obtained. The structure and properties of the transformation products have been studied by X-ray diffraction analysis, transmission electron microscopy, and Mössbauer spectroscopy. It is established that a characteristic feature of ferrocene transformations upon laser heating is the simultaneous formation of nanoparticles of iron (α-Fe) and iron carbide (Fe7C3) crystalline phases. The presence of α-Fe in the products of thermal transformations of ferrocene at high pressures has been observed for the first time. Possible mechanisms of the simultaneous formation of these nanoparticles during ferrocene transformations are discussed.

Stepanova A.V., Mironov A., Bogach A., Azarevich A., Presniakov I.A., Sobolev A.V., Pankratov D.A., Zayakhanov V., Starchikov S., Verchenko V., Shevelkov A.V.

A van der Waals telluride, NbFeTe2, has been synthesized using chemical vapor transport reactions. The optimized synthetic conditions yield high-quality single crystals with a novel monoclinic crystal structure. Monoclinic NbFeTe2...

Zayakhanov V.A., Starchikov S.S., Lyubutina M.V., Lin C., Chen Y., Chen B., Vasiliev A.L., Lyubutin I.S.

A new one-stage anhydrous sol-gel method for the synthesis of core-shell nanoparticles based on Fe3C and carbon is proposed. Powders of iron (III) nitrate Fe(NO3)3·9 H2O and polyacrylonitrile (PAN) were used as initial reagents, dimethylacetamide (DMAc) was used as a solvent. The synthesis was carried out in an inert argon atmosphere at various temperatures (Tsyn) up to 1000 °C. The morphology, structure, elemental and phase compositions, magnetic and electronic properties of the obtained core-shell nanocomposites were studied by various methods, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDXS), Raman and Mössbauer spectroscopy, and magnetometry. It was found that at temperatures Tsyn ≈ 400–500 °C, iron oxide γ-Fe2O3 predominates in the synthesis product. At Tsyn ≈ 600–700 °C, iron carbide Fe3C is formed, whose content increases and reaches a maximum value (about 75%) with increasing Tsyn to 800–1000 °C. At Tsyn ≈ 1000 °C, nanocomposites of the core@shell type – Fe3C@C and Fe2O3 @C, as well as those with a double shell Fe3C@Fe2O3 @C – are formed. In the Fe3C@Fe2O3 @C composites, Fe3C is a core, Fe2O3 and carbon C are shells. As Tsyn increases from 500 °C to 1000 °C, the amount of iron oxide γ-Fe2O3 decreases, and at Tsyn ≈ 1000 °C, γ-Fe2O3 oxide completely transforms into α-Fe2O3. In the sample synthesized at Tsyn ≈ 1000 °C, particles with a crystallite size of 29.6 nm exhibit magnetic properties with the highest saturation magnetization value of 68 emu/g. The temperature dependence of the magnetization M(T) of this sample has a specific character – there are three sharp drops at temperatures close to the N é el/Curie point for bulk Fe3C, Fe3O4, and α-Fe. Such peculiar behavior of magnetization seems to be important, since it can find application in technology.

Starchikov S.S., Funtov K.O., Zayakhanov V.A., Frolov K.V., Klenov M.G., Bondarenko I.Y., Lyubutin I.S.

One of the problems in the use of closed-cycle cryostats for applied and basic scientific research is the transmission of mechanical vibrations to the sample. This is particularly relevant for Mössbauer spectroscopy and optical research methods since vibrations lead to broadening of spectral lines. This paper presents various engineering approaches to reducing mechanical vibrations on a sample in closed-cycle cryostats, in particular for Mössbauer spectroscopy. The broadening of the spectral lines of the reference absorber, α-Fe foil, was analyzed and a comparison of the spectra of a FeBO3 single crystal of high structural quality before and after updating the cryostat was made. The obtained results can be used to develop new cryostats or improve existing ones.

Snegirev N.I., Chuev M.A., Lyubutin I.S., Starchikov S.S., Yagupov S.V., Strugatsky M.B.

The Mössbauer spectra of FeBO3 single crystals are studied at temperatures above and below the magnetic transition point at different orientations of the crystals with respect to the propagation direction of γ rays. To describe the Mössbauer spectra, a theoretical model is developed with allowance for different orientations of magnetic moments in the crystal plane. It is found that the magnetic domain structure in iron borate significantly affects the shape of the Mössbauer spectra and the intensity of resonant transitions. The proposed model may be useful for determining the configuration of the magnetic domain structure of materials from Mössbauer spectroscopy data.

Snegirev N.I., Starchikov S.S., Lyubutin I.S., Chuev M.A., Yagupov S.V., Strugatskii M.B.

The parameters of the hyperfine interaction of 57Fe nuclei in single crystals of iron borate FeBO3 and its isostructural solid solution Fe0.91Ga0.09BO3 have been determined by Mössbauer spectroscopy. A theoretical model has been developed to describe resonant transitions of iron nuclei in the approximation of a combined magnetic dipole and electric quadrupole hyperfine interaction, taking into account the statistical distribution of Ga and Fe over octahedral positions in the Fe0.91Ga0.09BO3 crystal. It has been established that even a low Ga concentration leads to a significant change in the hyperfine structure of the 57Fe nuclei in FeBO3, which manifests itself in the appearance of additional components in the Mössbauer spectra of the Fe0.91Ga0.09BO3 single crystal.

Starchikov S.S., Zayakhanov V.A., Lyubutin I.S., Vasiliev A.L., Lyubutina M.V., Chumakov N.K., Funtov K.O., Kulikova L.F., Agafonov V.N., Davydov V.A.

This work describes structural and magnetic properties of nanoparticles obtained during conversion of ferrocene Fe(C5H5)2 under a high pressure 8 GPa and a high temperature 900 °C (HP-HT treatment) for 10–10000 s, and subsequent self-oxidation in air. The magnetic, structural, and electronic properties of the nanocomposites were studied by XRD, TEM, HAADF-STEM, ED, EDXS, Mössbauer spectroscopy and magnetization measurements. Conversion of ferrocene leads to the formation of “pure” and carbon-encapsulated iron carbide (Fe7C3@C) nanoparticles embedded in carbon matrices with varying degrees of structural ordering. Depending on the size and structure of these nanoparticles different products can be obtained as a result of the self-oxidation of iron carbides. Along with solid iron oxide nanoparticles, hollow iron oxide particles were found in the oxidation products. The formation of hollow nanoparticles can be explained by the Kirkendall effect. It is known that magnetite Fe3O4 and maghemite γ-Fe2O3 are ferrimagnets with a high Neel point TN, while wüstite FeO is an antiferromagnet with TN about 198 K. By varying the content of these components in nanoparticles, it is possible to obtain materials with desired magnetic properties, which is of great importance for technological and biomedical applications of such nanostructures.

Lyubutin I.S., Snegirev N.I., Chuev M.A., Starchikov S.S., Smirnova E.S., Lyubutina M.V., Yagupov S.V., Strugatsky M.B., Alekseeva O.A.

• Precision studies of the temperature dependence of the Mössbauer spectra of FeBO 3 single crystals have been carried out. • Parameters of hyperfine interaction in FeBO 3 a wide temperature range were accurately determined. • Theoretical analysis of the peculiarities of the hyperfine structure formation of Mössbauer spectra in iron borate was performed. • A technique for correcting Mössbauer spectra taking into account the effective thickness of the absorber was developed. • Debye temperature for cations in the structure of FeBO 3 was determined. In this work, Mössbauer spectroscopy and X-ray diffraction were used to determine the precision values of the hyperfine interaction parameters and the crystal structure of FeBO 3 single crystals in a wide temperature range, including the region of magnetic phase transitions ( T N ). A theoretical model has been developed to describe nuclear resonance transitions in iron atoms in the approximation of a combined magnetic dipole and electric quadrupole hyperfine interaction. A technique for correcting the Mössbauer spectra, considering the effective thickness of the absorber, has also been elaborated. It is shown that the appearance of two additional resonance lines in the hyperfine structure significantly affects the shape of the FeBO 3 spectra near the N é el temperature ( Т N ). The characteristic Debye temperatures for Fe and B cations in the iron borate structure have been determined. The developed technique and the results obtained are extremely important for the use of FeBO 3 crystals in new high-tech branches of science and technology, including optoelectronics and synchrotron technologies.

Mikheev A.V., Burmistrov I.A., Zaitsev V.B., Artemov V.V., Khmelenin D.N., Starchikov S.S., Veselov M.M., Klyachko N.L., Bukreeva T.V., Trushina D.B.

Composite microcapsules based on polyelectrolytes and nanoparticles of iron oxides are synthesized, and the release of encapsulated high-molecular dextran under the influence of a low-frequency alternating magnetic field due to the magnetomechanical actuation of nanoparticles in a polymer shell is investigated. As a result of the chemical condensation of ferrous and ferric iron, single-domain magnetic spherical Fe3O4 nanoparticles are synthesized and characterized by transmission electron microscopy, dynamic light scattering, powder X-ray diffraction, and Mössbauer spectroscopy. Polyelectrolyte microcapsules from polyallylamine hydrochloride and sodium polystyrene sulfonate are modified with magnetic nanoparticles due to electrostatic adsorption on an oppositely charged polyelectrolyte layer. Dextran, labeled with tetramethyl rhodamine-5-isothiocyanate (TRITC-dextran), is used as the model substance for encapsulation; it is incorporated into CaCO3 particles (soluble cores for the formation of capsules) by coprecipitation. The capsule samples are examined by scanning electron microscopy, dynamic light scattering, and fluorescence confocal microscopy. The capsules are exposed to an alternating magnetic field with an amplitude of 100 mT and frequencies of 30–110 Hz. The content of labeled dextran in the shell of the microcapsules and the supernatant is determined using fluorimetry and fluorescence confocal microscopy. The duration of exposure and the frequency of the magnetic field, at which the maximum release of dextran from composite capsules is achieved, are established. Exposure to a low-frequency alternating magnetic field can lead to significant deformation of the shell of polyelectrolyte microcapsules modified with magnetic nanoparticles and successful release of the encapsulated substance.

Snegirev N., Smirnova E., Lyubutin I., Kiiamov A., Starchikov S., Yagupov S., Strugatsky M., Alekseeva O.

X-ray structural analysis, Mössbauer spectroscopy, and ab initio calculations are used to study the structural properties and refine the orientation of intracrystalline fields in FeBO 3 crystals in the region of the magnetic phase transition. It is found that in the temperature range of 293–403 K, the trigonal lattice parameters increase monotonically. Analysis of the electron density distribution maps does not show visible local disordering over the entire investigated temperature range. It is found that iron borate has an axially symmetric electric field gradient (EFG) whose main axis is directed along [001]. This orientation is maintained above and below the Néel point. In the magnetically ordered state of the crystal, the main axis of the EFG is orthogonal to the direction of the hyperfine magnetic field at iron nuclei. The results obtained will be used to develop a theoretical model of the formation of hyperfine structure in iron borate, which is important for applications of such crystals in next-generation synchrotron technologies.

Burmistrov I.A., Veselov M.M., Mikheev A.V., Borodina T.N., Bukreeva T.V., Chuev M.A., Starchikov S.S., Lyubutin I.S., Artemov V.V., Khmelenin D.N., Klyachko N.L., Trushina D.B.

Nanosystems for targeted delivery and remote-controlled release of therapeutic agents has become a top priority in pharmaceutical science and drug development in recent decades. Application of a low frequency magnetic field (LFMF) as an external stimulus opens up opportunities to trigger release of the encapsulated bioactive substances with high locality and penetration ability without heating of biological tissue in vivo. Therefore, the development of novel microencapsulated drug formulations sensitive to LFMF is of paramount importance. Here, we report the result of LFMF-triggered release of the fluorescently labeled dextran from polyelectrolyte microcapsules modified with magnetic iron oxide nanoparticles. Polyelectrolyte microcapsules were obtained by a method of sequential deposition of oppositely charged poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) on the surface of colloidal vaterite particles. The synthesized single domain maghemite nanoparticles integrated into the polymer multilayers serve as magneto-mechanical actuators. We report the first systematic study of the effect of magnetic field with different frequencies on the permeability of the microcapsules. The in situ measurements of the optical density curves upon the 100 mT LFMF treatment were carried out for a range of frequencies from 30 to 150 Hz. Such fields do not cause any considerable heating of the magnetic nanoparticles but promote their rotating-oscillating mechanical motion that produces mechanical forces and deformations of the adjacent materials. We observed the changes in release of the encapsulated TRITC-dextran molecules from the PAH/PSS microcapsules upon application of the 50 Hz alternating magnetic field. The obtained results open new horizons for the design of polymer systems for triggered drug release without dangerous heating and overheating of tissues.

Snegirev N.I., Starchikov S.S., Lyubutin I.S., Ogarkova Y.L., Lyubutina M.V., Lin C.-.

Gallium ferrite nanoparticles have been synthesized by chemical combustion. The morphology, phase composition, and magnetic properties of particles have been studied using electron microscopy, X-ray diffraction analysis, and Mössbauer spectroscopy. The presence of iron-containing Fe3O4, FeGa2O4, FeGaO3, and α-Fe phases in these particles is established. The cation distributions in the main phase FeGa2O4 over tetrahedral and octahedral crystallographic sites were found to be (Fe $$_{{0.78}}^{{2 + }}$$ Ga $$_{{0.24}}^{{3 + }}$$ ) and [Fe $$_{{0.28}}^{{2 + }}$$ Ga $$_{{1.78}}^{{3 + }}$$ ], respectively. Materials based on (Fe,Ga)3O4 gallium ferrites can be used in modern biomedical technologies.

Starchikov S.S., Zayakhanov V.A., Vasiliev A.L., Lyubutin I.S., Baskakov A.O., Nikiforova Y.A., Funtov K.O., Lyubutina M.V., Kulikova L.F., Agafonov V.N., Davydov V.A.

The core@shell nanostructures were obtained in the process of transformation of ferrocene Fe(C5H5)2 at high pressure (HP) of 8 GPa and high temperature (HT) of 900 °C with an isothermal exposure time t varying from 10 to 10000 s. At t > 300 s, the iron carbide o-Fe7C3 nanoparticles with an orthorhombic crystal structure (sp.gr. Pnma) can be created, which are dispersed in highly defective carbon matrix. After opening the high-pressure cell, a series of redox reactions occurs, leading to a formation of iron oxides on the surface of the iron carbide core. When the size of Fe7C3 nanoparticle is less than critical one the nanoparticle is fully oxidized, while in the larger particle an amorphous iron oxide shell is formed. A sequential increase in t initiates crystallization processes both in the iron carbide subsystem and in the carbon subsystem, resulting in the formation of core@shell Fe7C3/FexOy/C structures. Iron oxides with a cubic spinel-type structure (Fe3O4/γ-Fe2O3) appear in the shell. However, under oxygen reduction, part of magnetite can be transformed into wustite FeO. The magnetic properties of magnetite and wustite are radically different, and by varying the thickness of these layers, structures with the desired functional properties can be obtained.

Nikiforova Y.A., Ivanova A.G., Frolov K.V., Lyubutin I.S., Chareev D.A., Baskakov A.O., Starchikov S.S., Troyan I.A., Lyubutina M.V., Naumov P.G., Abdel-Hafiez M.

• Structural phase transitions in the FeSe0.89S0.11 superconductor at high pressures up to 18.5 GPa. • Most sample part’s recrystallization into the hexagonal phase (sp. gr. P63/mmc) at decompression. • Strong hysteresis of the structural properties during a phase transition under pressure. We report on the structural phase transitions in the FeSe 0.89 S 0.11 superconductor with T C = 11 K observed by powder synchrotron X-ray diffraction at high pressures up to 18.5 GPa under compression and decompression modes. It was found that at ambient pressure and room temperature, FeSe 0.89 S 0.11 has a tetragonal structure (space group P4/n). Under compression, in the region of 10 GPa, a phase transition from the tetragonal into the orthorhombic structure (sp. gr. Pnma) is observed, which persists up to 18.5 GPa. Our results strongly suggest that, at decompression, as the applied pressure decreases to 6 GPa and then is completely removed, most of the sample recrystallizes into the hexagonal phase of the structural type NiAs (sp. gr. P6 3 /mmc). However, the other part of the sample remains in the high pressure orthorhombic phase (sp. gr. Pnma), while the tetragonal phase (sp. gr. P4/n) is not restored. These observations illustrate a strong hysteresis of the structural properties of FeSe 0.89 S 0.11 during a phase transition under pressure.

Pham D.V., Seo P.W., Phan D., Bhatti A.H., Yun D., Kang K.H., Park S.

Shah B.S., Chaki S.H., Deshpande M.P.

Tadic M., Lazovic J., Panjan M., Tadic B.V., Lalatonne Y.

Bhaskaraiah M., Kaleemulla S.

Xu W., Wang H., Li H., Xiao J., Bi S., Xiao W.

Yang Y., Wang C., Zheng S., Yao J., Dai J., Chen H., Han C., Al-Anazi A., Ji H., Huang Y.

Nizam T., Joseph A., Krishnan R.R.

Aboushoushah S.F.

Kim S.A., Park K.M., Jung K.

Transition metal/porous carbon composite is good electrode candidate since porous carbon provides high surface porosity which promotes the access of electrolyte ions, and transition metal enables redox reactions to improve specific capacitance and energy density. In this study, iron/carbon nanofiber (CNF) composite electrodes were prepared by grafting ferrocenecarboxaldehyde to the CNFs which were fabricated by electrospinning and thermal treatment of polyacrylonitrile (PAN). The presence of iron on the CNF surface was confirmed by SEM/EDS, ICP-MS and XPS. Electrochemical performance was evaluated using a three-electrode cell with 1 M Na2SO4 as an electrolyte. Iron-grafted CNFs exhibited a high specific capacitance of 358 F g−1 and an energy density of 49.7 Wh kg−1 at 0.5 A g−1, which is significantly higher than those for untreated CNFs (68 F g−1 and 9.4 Wh kg−1). This demonstrates that this iron/CNF composite is promising candidate for supercapacitor electrode with outstanding energy storage performance.

Sánchez J., Cortés-Hernández D.A., Reyes-Rodríguez P.Y., Hurtado-López G.F.

Purwar S., Paul S., Vijay K., Venkatesh R., Banik S., Mahadevan P., Thirupathaiah S.

Mahna A.

This review explores the emerging applications of low-frequency magnetic fields (LFMFs) and magnetic nanoparticles (MNPs) in the field of biomedicine, including in cancer treatment, controlled drug delivery, proliferation of cells, and wound healing. LFMFs, which may be found in both natural and manmade sources, have the ability to penetrate objects and may have physiological consequences. In addition, magnetic nanoparticles with low frequencies display a sensitive reaction to magnetic fields from outside sources. This presents opportunities for precise drug administration, imaging, and hyperthermia treatment in a range of biological applications. The use of precise drug delivery and controlled release mechanisms in cancer therapy, as well as the application of magnetic fields to accelerate tissue regeneration in wound healing, are advantageous for these medical treatments. In addition, this study examines the importance of liposome release and permeability in improving the availability and specificity of drugs. The authors expect that this study will provide valuable guidance to scholars in strategizing their next investigations in the same realm.

Zhang H., Jiang Y., Yi C., Li B., Huang Z., Wang D., He K., Tang J., Zhang B., Zhang Z., Li S., Zhang L., Shen D., Liu M., Saeed M.Z., et. al.

González Ochea R.A., Zandalazini C.I., Oliva M.I., Benzaquén T.B., Encina E.R.

Kuang Q., Zhang B., Dong B., Men X., Yang B., Zhou Y., Li Z., Shang X., Yang T., Huang J., Li D., Zhang Z.

In the past few decades, a development of organic magnets with room-temperature strong ferromagnetism is challenged by the difficulty of creating three-dimensional (3D) long-range magnetic orderings in organic materials at a temperature higher than room temperature. Here, we report room-temperature ferrimagnetism of a tetragonal organic–inorganic hybrid Fe14Se16(tepa)III (tepa = tetraethylenepentamine), where III represents a coordination of a tepa molecule with a Fe3+ ion for an organic complex. The layered hybrid in a nanoplate-like shape is formed by periodic incorporation of tetragonal β-Fe3Se4 inorganic layers and organic spacing layers consisting of tepa and Fe3+. Fe14Se16(tepa)III shows a saturation magnetization MS of 7.2 emu g−1 at 300 K and a record-high Néel temperature TN (>560 K) in the organic magnets reported experimentally. A Mössbauer spectrum confirms a 3D long-range magnetic ordering of Fe2+ [S = 2 (71.4%)] and Fe3+ ions [S = 5/2 (21.7%) and 1/2 (4.0%)] in β-Fe3Se4 layers and organic spacing layers of Fe14Se16(tepa)III,9. First-principles calculations explain that the 3D long-range antiferromagnetic interactions between interlayer and intralayer irons result in the strong ferrimagnetism of Fe14Se16(tepa)III. This study unveils the possibility of tuning magnetic couplings of interlayer and intralayer high-spin Fe3+ and Fe2+ for enhancing the ferrimagnetism of layered hybrids and, thus, for future room-temperature magnetic/spintronic applications.

Bagramov R.H., Filonenko V.P., Zibrov I.P., Skryleva E.A., Kulnitskiy B.A., Blank V.D., Khabashesku V.N.

Nanoparticles of iron carbides and nitrides enclosed in graphite shells were obtained at 2 ÷ 8 GPa pressures and temperatures of around 800 °C from ferrocene and ferrocene–melamine mixture. The average core–shell particle size was below 60 nm. The graphite-like shells over the iron nitride cores were built of concentric graphene layers packed in a rhombohedral shape. It was found that at a pressure of 4 GPa and temperature of 800 °C, the stability of the nanoscale phases increases in a Fe7C3 > Fe3C > Fe3N1+x sequence and at 8 GPa in a Fe3C > Fe7C3 > Fe3N1+x sequence. At pressures of 2 ÷ 8 GPa and temperatures up to 1600 °C, iron nitride Fe3N1+x is more stable than iron carbides. At 8 GPa and 1600 °C, the average particle size of iron nitride increased to 0.5 ÷ 1 μm, while simultaneously formed free carbon particles had the shape of graphite discs with a size of 1 ÷ 2 μm. Structural refinement of the iron nitride using the Rietveld method gave the best result for the space group P6322. The refined composition of the samples obtained from a mixture of ferrocene and melamine at 8 GPa/800 °C corresponded to Fe3N1.208, and at 8 GPa/1650 °C to Fe3N1.259. The iron nitride core–shell nanoparticles exhibited magnetic behavior. Specific magnetization at 7.5 kOe of pure Fe3N1.208 was estimated to be 70 emu/g. Compared to other methods, the high-pressure method allows easy synthesis of the iron nitride cores inside pure carbon shells and control of the particle size. And in general, pressure is a good tool for modifying the phase and chemical composition of the iron-containing cores.

Starchikov S.S., Funtov K.O., Zayakhanov V.A., Frolov K.V., Klenov M.G., Bondarenko I.Y., Lyubutin I.S.

One of the problems in the use of closed-cycle cryostats for applied and basic scientific research is the transmission of mechanical vibrations to the sample. This is particularly relevant for Mössbauer spectroscopy and optical research methods since vibrations lead to broadening of spectral lines. This paper presents various engineering approaches to reducing mechanical vibrations on a sample in closed-cycle cryostats, in particular for Mössbauer spectroscopy. The broadening of the spectral lines of the reference absorber, α-Fe foil, was analyzed and a comparison of the spectra of a FeBO3 single crystal of high structural quality before and after updating the cryostat was made. The obtained results can be used to develop new cryostats or improve existing ones.

Davydov V.A., Agafonov V.N., Plakhotnik T., Khabashesku V.N.

Starchikov S.S., Zayakhanov V.A., Lyubutin I.S., Vasiliev A.L., Lyubutina M.V., Chumakov N.K., Funtov K.O., Kulikova L.F., Agafonov V.N., Davydov V.A.

This work describes structural and magnetic properties of nanoparticles obtained during conversion of ferrocene Fe(C5H5)2 under a high pressure 8 GPa and a high temperature 900 °C (HP-HT treatment) for 10–10000 s, and subsequent self-oxidation in air. The magnetic, structural, and electronic properties of the nanocomposites were studied by XRD, TEM, HAADF-STEM, ED, EDXS, Mössbauer spectroscopy and magnetization measurements. Conversion of ferrocene leads to the formation of “pure” and carbon-encapsulated iron carbide (Fe7C3@C) nanoparticles embedded in carbon matrices with varying degrees of structural ordering. Depending on the size and structure of these nanoparticles different products can be obtained as a result of the self-oxidation of iron carbides. Along with solid iron oxide nanoparticles, hollow iron oxide particles were found in the oxidation products. The formation of hollow nanoparticles can be explained by the Kirkendall effect. It is known that magnetite Fe3O4 and maghemite γ-Fe2O3 are ferrimagnets with a high Neel point TN, while wüstite FeO is an antiferromagnet with TN about 198 K. By varying the content of these components in nanoparticles, it is possible to obtain materials with desired magnetic properties, which is of great importance for technological and biomedical applications of such nanostructures.

Verchenko V., Stepanova A.V., Bogach A., Kirsanova M., Shevelkov A.V.

Transition metal-based two-dimensional nanomaterials with competing magnetic states are at the cutting edge of spintronic and low power memory devices. In this paper, we present Fe-rich NbFe1+xTe3 layered telluride (x...

Zhao B., Ngaloy R., Ghosh S., Ershadrad S., Gupta R., Ali K., Hoque A.M., Karpiak B., Khokhriakov D., Polley C., Thiagarajan B., Kalaboukhov A., Svedlindh P., Sanyal B., Dash S.P.

Tang X., Shen H., Zhao S., Li N., Liu J.

Brain–computer interfaces—which allow direct communication between the brain and external computers—have potential applications in neuroscience, medicine and virtual reality. Current approaches are, however, based on conventional rigid electronics and are limited by their intrinsic mechanical and geometrical mismatch with brain tissue. Flexible electronics, which can have mechanical properties compatible with the brain, could address these limitations and be used to create the next generation of brain–computer interfaces. Here we explore the use of flexible electronics in the development of brain–computer interfaces. We examine the unique advantages of flexible, stretchable and soft electronics in such interfaces and consider the potential impact of the technology on neuroscience, neuroprosthetic control, bioelectronic medicine, and brain and machine intelligence integration. We also explore the challenges in materials, device fabrication and system integration that need to be addressed to develop flexible brain–computer interfaces of general applicability. This Perspective explores the use of flexible electronics in the development of brain–computer interfaces, considering their potential impact on neuroscience, neuroprosthetic control, bioelectronic medicine, and brain and machine intelligence integration.

Snegirev N.I., Bogach A.V., Lyubutin I.S., Chuev M.A., Yagupov S.V., Mogilenec Y.A., Selezneva K.A., Strugatsky M.B.

Bulatov K.M., Zinin P.V., Machikhin A.S., Kutuza I.B.

The new method of quick temperature surface distribution measurement based on eight-colour video camera is presented. Measurement frequency is 80 Hz. The mathematical apparatus is developed for determination of temperature distribution in the camera field of view without using radiation capacity data. Statistical temperature measurement error is 10 % at 1170 K and 3 % at 1500 K. It is demonstrated that the multi-spectral measurement method is viable and allows us to obtain temperature distributions of objects with unknown composition.

Tadic M., Panjan M., Lalatone Y., Milosevic I., Tadic B.V., Lazovic J.

• Hollow iron oxide particles are prepared by annealing of akaganeite nanorods. • The β-FeOOH are synthesized by forced hydrolysis of FeCl3/HCl solution. • The hollow particles have superparamagnetic and weak-ferromagnetic properties. • A low-cytotoxicity and MRI relaxivity reveal potential for biomedical applications. • The results highlight the nanoparticle structure-dependent magnetic properties. We investigate synthesis, phase evolution, hollow and porous structure and magnetic properties of quasi-amorphous intermediate phase (QUAIPH) and hematite (α-Fe 2 O 3 ) nanostructure synthesized by annealing of akaganeite (β-FeOOH) nanorods. It is found that the annealing temperature determines the phase composition of the products, the crystal structure/size dictates the magnetic properties whereas the final nanorod morphology is determined by the starting material. Annealing of β-FeOOH at ∼300 °C resulted in the formation of hollow QUAIPH nanorods. The synthesized material shows low-cytotoxicity, superparamagnetism and good transverse relaxivity, which is rarely reported for QUAIPH. The QUAIPH nanorods started to transform to porous hematite nanostructures at ∼350 °C and phase transformation was completed at 600 °C. During the annealing, the crystal structure changed from monoclinic (akaganeite) to quasi-amorphous and rhombohedral (hematite). Unusually, the crystallite size first decreased (akaganeite → QUAIPH) and then increased (QUAIPH → hematite) during annealing whereas the nanorods retained particle shape. The magnetic properties of the samples changed from antiferromagnetic (akaganeite) to superparamagnetic with blocking temperature T B = 84 K (QUAIPH) and finally to weak-ferromagnetic with the Morin transition at T M = 244 K and high coercivity H C = 1652 Oe (hematite). The low-cytotoxicity and MRI relaxivity (r 2 = 5.80 mM −1 s −1 (akaganeite), r 2 = 4.31 mM −1 s −1 (QUAIPH) and r 2 = 5.17 mM −1 s −1 (hematite)) reveal potential for biomedical applications.

Seleznyova K., Smirnova E., Strugatsky M., Snegirev N., Yagupov S., Mogilenec Y., Maksimova E., Alekseeva O., Lyubutin I.

• Consistent approach to the interpretation of thermal expansion in magnetic crystals is developed. • FeBO 3 – a trigonal antiferromagnet with weak ferromagnetism – is used as a model object. • Different character of the thermal expansion above and below the magnetic ordering temperature is described taking into account magnetostrictive deformations. • A model based on the symmetry of the crystal and its domain structure is put forward. • The temperature dependences of magnetoelastic constants over a wide temperature range and the hitherto unknown constants are determined. Iron borate FeBO 3 – a trigonal antiferromagnet with weak ferromagnetism – was used as a model object for experimental and theoretical studies of thermal expansion. X-ray diffraction analysis indicated that the character of the thermal expansion changes significantly when passing through the Néel point. In the magnetically ordered state below the phase transition temperature, the thermal changes in the crystal lattice parameters are additionally affected by magnetostrictive deformations. A theoretical model was developed that allows interpreting experimental data and determining the magnetoelastic constants using X-ray diffraction analysis. Temperature dependences of some of the magnetoelastic constants of iron borate were determined, and the hitherto unknown constants were found. These data are essential to describe various phenomena associated with magnetoelastic interactions in magnetically ordered materials.

Zubov V.E., Kudakov A.D., Bulatov D.A., Strugatskii M.B., Yagupov S.V.

The Faraday effect in the rhombohedral weak ferromagnet FeBO3, which is due to the magnetization component parallel to the С3 axis of the crystal, is predicted and experimentally observed for the first time. This magnetization component is almost three and a half orders of magnitude smaller than the magnetization in the basal plane. The measured effect is six orders of magnitude smaller than the Faraday effect caused by the magnetization in the basal plane. The origin of the significant difference in the magnitudes of the Faraday effect due to the magnetization in the basal plane and to the magnetization parallel to the С3 axis is discussed.

Fujita R., Bassirian P., Li Z., Guo Y., Mawass M.A., Kronast F., van der Laan G., Hesjedal T.

Magnetic domain formation in two-dimensional (2D) materials gives perspectives into the fundamental origins of 2D magnetism and also motivates the development of advanced spintronics devices. However, the characterization of magnetic domains in atomically thin van der Waals (vdW) flakes remains challenging. Here, we employ X-ray photoemission electron microscopy (XPEEM) to perform layer-resolved imaging of the domain structures in the itinerant vdW ferromagnet Fe5GeTe2 which shows near room temperature bulk ferromagnetism and a weak perpendicular magnetic anisotropy (PMA). In the bulk limit, we observe the well-known labyrinth-type domains. Thinner flakes, on the other hand, are characterized by increasingly fragmented domains. While PMA is a characteristic property of Fe5GeTe2, we observe a spin-reorientation transition with the spins canting in-plane for flakes thinner than six layers. Notably, a bubble phase emerges in four-layer flakes. This thickness dependence, which clearly deviates from the single-domain behavior observed in other 2D magnetic materials, demonstrates the exciting prospect of stabilizing complex spin textures in 2D vdW magnets at relatively high temperatures.

Lyubutin I.S., Snegirev N.I., Chuev M.A., Starchikov S.S., Smirnova E.S., Lyubutina M.V., Yagupov S.V., Strugatsky M.B., Alekseeva O.A.

• Precision studies of the temperature dependence of the Mössbauer spectra of FeBO 3 single crystals have been carried out. • Parameters of hyperfine interaction in FeBO 3 a wide temperature range were accurately determined. • Theoretical analysis of the peculiarities of the hyperfine structure formation of Mössbauer spectra in iron borate was performed. • A technique for correcting Mössbauer spectra taking into account the effective thickness of the absorber was developed. • Debye temperature for cations in the structure of FeBO 3 was determined. In this work, Mössbauer spectroscopy and X-ray diffraction were used to determine the precision values of the hyperfine interaction parameters and the crystal structure of FeBO 3 single crystals in a wide temperature range, including the region of magnetic phase transitions ( T N ). A theoretical model has been developed to describe nuclear resonance transitions in iron atoms in the approximation of a combined magnetic dipole and electric quadrupole hyperfine interaction. A technique for correcting the Mössbauer spectra, considering the effective thickness of the absorber, has also been elaborated. It is shown that the appearance of two additional resonance lines in the hyperfine structure significantly affects the shape of the FeBO 3 spectra near the N é el temperature ( Т N ). The characteristic Debye temperatures for Fe and B cations in the iron borate structure have been determined. The developed technique and the results obtained are extremely important for the use of FeBO 3 crystals in new high-tech branches of science and technology, including optoelectronics and synchrotron technologies.

Tadic M., Lazovic J., Panjan M., Kralj S.

Controlled spatial arrangements of superparamagnetic iron oxide nanoparticles (SPIONs) in complex nanostructures determine fine tuning of physico-chemical properties which, in turn, may lead to new practical applications. We report here on newly observed properties of hierarchical SPIONs nanostructure with bundle-like morphology, also known as nanobundles. Colloidal chemical processes and sol-gel synthesis were used for the synthesis of nanobundles, i.e. i) self-assembly of SPIONs into magnetic nanoparticle clusters, ii) their magnetic assembly to the nanochains, and finally iii) formation of bundle-like hierarchical nanostructure. An XRPD measurements show spinel crystal structure of maghemite/magnetite nanoparticles, EDS analysis reveals Fe, Si and O as main elements whereas SEM/TEM analysis show silica-coated magnetic nanoclusters (∼100 nm) and their hierarchical assemblies with bundle-like morphology of ∼8 μm length and ∼1 μm width. TEM analysis revealed core-shell nature of iron oxide nanoparticle clusters with their size of around 80 nm that were coated by an amorphous silica shell with thickness of ∼15 nm. The nanoclusters in the core are constructed of maghemite/magnetite nanoparticle assembly with primary iron oxide nanoparticle size about 10 nm. The magnetization M data as a function of an applied external magnetic field H were successfully fitted by the Langevin function, whence the magnetic moment μ p = 19256 μ B , and the diameter d = 9.6 nm of nanoparticles were determined. Microsized bundle-like particles are superparamagnetic, magnetically guidable and possess high transverse relaxivity of r 2 = 397.8 mM −1 s −1 . Magnetic properties and such high value of transverse relaxivity holds promise for nanobundles application in MRI imaging (MRI contrast agent), as nanobundles may enhance the magnetic field in their surroundings and enhance proton relaxation processes. Our nanobundles can open new opportunities in the biomedical applications, magnetic separation, photonic crystals and magnetic liquid manipulation and can be inspiration for synthesizing novel self-assembled nanoparticle structures.