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 FeBO 3 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.

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 degrees 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, Mo center dot ssbauer 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 gamma-Fe2O3 are ferrimagnets with a high Neel point TN, while wustite 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.

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 degrees 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/gamma-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. (C) 2021 Elsevier Ltd. All rights reserved.

A physico-mathematical model for processing the Mossbauer spectra of the langasite family compounds with a helical magnetic structure was developed. It was shown that the Mossbauer spectra demonstrate high information content and sensitivity to details of the magnetic structure even in the case of polycrystalline samples and in the absence of an external magnetic field. In addition, they make it possible to monitor the dynamics of the magnetic structure in the entire temperature range below the Neel point. As an example, the helical magnetic structure in the langasite Ba3SbFe3Si2O14 was experimentally investigated. It was established that during translation along the c axis, the magnetic moments of iron in the (ab) planes rotate at an angle of 72 degrees, forming a spiral with a period of 5c. The directions of the main axis of the electric field gradient (EFG) and the local axis of magnetic anisotropy were determined, which are very important characteristics of the frustrated magnetic structure of langasites.

Carbon-coated iron oxides core@shell nanoparticles were synthesized by a one-step process of thermal pyrolysis. The influence of the synthesis temperature on the structural and magnetic properties of FexOy@carbon core-shell nanoparticles was investigated by X-ray diffraction, transmission electron microscopy (TEM), Raman and Mossbauer spectroscopy, as well as by magnetic measurements. At reaction temperatures (T-R) from 360 to 385 degrees C, spherical and nearly monodispersed nanoparticles of magnetite with an average diameter of about 40 nm covered with an amorphous carbon shell about 5 nm thick were obtained. When the reaction temperature rises above 385 degrees C, the nanoparticles of wiistite phase begin to form, and its concentration increases at the expense of the magnetite phase with further T-R increasing. In a narrow temperature range from 390 to 400 degrees C, the wiistite concentration increases from 10 to 40%. At that time magnetite nanoparticles become smaller and superparamagnetic. The Mossbauer spectra indicate that the excess [Fe3+](B), ions in octahedral B-sites of magnetite which associated with the particle surface do not participate in the electron hoping [Fe3+ reversible arrow Fe2+](B) above the Verwey temperature. The wiistite phase appears and grows with a corresponding decrease in the fraction of [Fe3+](B) that does not undergo the electronic exchange. This proves that the wilstite phase is generated and formed on the surface of magnetite particles under the influence of carbon. The Mossbauer data allow one to monitor the dynamics of reduction of iron oxide from Fe3O4 to FeO under the influence of carbon.

The magnetic structure of the Ba3TaFe3Si2O14 multiferroic of the langasite family has been studied using the Mossbauer spectroscopy and theoretical analysis within the full Hamiltonian of the combined hyperfine magnetic dipole and electric quadrupole interaction in the ground and excited states of Fe-57 nuclei. The model is based on an antiferromagnetic triangular magnetic lattice with a 120. orientation of the iron spins in the (a, b)-plane, which occurs below the Neel temperature T-N = 27.2K. It was found that during translation along the c-axis, the magnetic moments of iron ions in triangular clusters rotate at an angle of 51.4 degrees. This angle remains constant with further translation, and a helical magnetic structure with a magnetic cell period of about 7c is formed. Excited states of magnetic moments arising due to a finite temperature are also considered. It has been established that the helical structure is well described by the proposed model in the entire temperature range T < T-N. Copyright (C) EPLA, 2020.

The iron-containing crystals of the langasite family A(3)MFe(3)X(2)O(14) (A =Ba, Sr, M = Sb, Nb, Ta, X= Si, Ge) are a new class of multiferroics in which ferroelectricity can be induced by magnetic ordering. We have measured Mossbauer spectra of a powder sample Ba3NbFe3Si2O14 of this family in a wide temperature range and developed a physico-mathematical model for analyzing the complex Mossbauer spectra shape. This model allows establishing a helical magnetic structure at all temperatures in the magnetic ordering region (T < T-N). It is shown that the experimental Mossbauer spectra are well described by the model of a helical magnetic structure with a constant angle of rotation of the moment (rotation step) during its transfer between adjacent planes. The angle value is about 51 degrees, which agrees with the previous results of neutron experiments. The excited states of the magnetic moments that appear at finite (nonzero) temperatures are also taken into account, which makes it possible to study the magnetic dynamics at different temperatures. The value of the angle between the main axis of the electric field gradient (EFG) at the iron nuclei and the crystal axis c (theta = 36 degrees), which does not change with the temperature in the region T < T-N, is established. This indicates that in the region of magnetic ordering of this langasite no structural transitions occur. The developed model promotes a deeper understanding of the magnetic properties of the multiferroic Ba3NbFe3Si2O14 and can be used to study the magnetic structure of other compounds where iron ions form a similar helical structure.

A series of nickel-chromium-ferrite NiFe2-xCrxO4 (with x = 1.25) nanoparticles (NPs) with a cubic spinel structure and with size d ranging from 1.6 to 47.7 nm was synthesized by the solution combustion method. A dual structure of all phonon modes revealed in Raman spectra is associated with metal cations of different types present in the spinel lattice sites. Mössbauer spectra of small NPs exhibit superparamagnetic behavior. However, the transition into the paramagnetic state occurs at a temperature that is unusually high for small particles (TN is about 240 K in the d = 4.5 nm NPs). The larger NPs with d > 20 nm do not exhibit superparamagnetic properties up to the Neel temperature. From the magnetic and Mössbauer data, the cation occupation of the tetrahedral (A) and octahedral [B] sites was determined (Fe0.75Ni0.25)[Ni0.75Cr1.25]O4. The saturation magnetization MS in the largest NPs is about (0.98-0.95) μB, which is more than twice higher the value in bulk ferrite (Fe)[CrNi]O4. At low temperatures the total magnetic moment of the ferrite coincides with the direction of the B-sublattice moment. In the NPs with d > 20 nm, the compensation of the magnetic moments of A- and B-sublattices was revealed at about Tcom = 360-365 K. This value significantly exceeds the point Tcom in bulk ferrites NiFexCr2-xO4 (about 315 K) with the similar Cr concentration. However, in the smaller NPs NiFe0.75Cr1.25O4 with d ≤ 11.7 nm, the compensation effect does not occur. The magnetic anomalies are explained in terms of highly frustrated magnetic ordering in the B sublattice, which appears due to the competition of AFM and FM exchange interactions and results in a canted magnetic structure.

Single crystalline iron sulfide nanoparticles doped with chromium Fe1-xCrxS (0 ≤x≤ 0.15) have been successfully prepared by a thermal decomposition method. The particles are self-organized into the single crystalline plates with the accurate hexagonal shape and dimensions up to 1 μ in plane and about 30-40 nm in thickness. The samples have the NiAs-type crystal structure (P63/mmc) at all Cr concentrations up to x = 0.15. Fe(57)-Mössbauer spectroscopy data reveal four nonequivalent iron sites in these nanocrystals related to the different number of cation vacancies in neighboring of the iron atoms. A 2C-type superstructure or a mixture of 2C and 3C superstructures of vacancy ordering can appear in these samples. It was established that in the Fe1-xCrxS series chromium prefers to replace iron in the cation layers containing vacancies at 0.00 < x < 0.10 and Cr atoms occupy both iron and vacant sites at x > 0.10. The specific magnetic properties, which can be tuned by chromium doping, enable potential applications of these nanoparticles in technical devices using the material with thermally activated magnetic memory, for example, switches or storages.

Arrays of vertically aligned carbon nanotubes (CNTs) grown on n-doped silicon substrates using an aerosol-assisted catalytic chemical vapor deposition (CCVD) technique have been tested as supercapacitor electrodes. Electrochemical properties of the electrodes were shown to be significantly dependent on the array thickness. At scan rate of 20 mV/s the largest specific capacitance of 124 F/g was achieved for the similar to 280-mu m array, while increase in the thickness to similar to 1100 mu m caused a drop in electrode capacitance by four times. It was shown that in a sulfuric acid electrolyte, the redox processes with iron nanoparticles encapsulated in CNTs contribute significantly to the capacitance of array. From the Mossbauer spectroscopy, these nanoparticles are present as alpha-Fe, gamma-Fe, and Fe3C phases. X-ray photoelectron spectroscopy revealed that during the electrode charging and discharging sulfates of Fe(II) and Fe(III) are formed in surficial layers of the nanoparticles. (C) 2014 Elsevier Ltd. All rights reserved.

Hollow microcapsules with the shell composed of biodegradable polyelectrolytes modified with the maghemite nanoparticles were fabricated by in situ synthesis. The nanoparticles were synthesized from the iron salt and the base directly on the capsule shells prepared by "layer by layer" technique. An average diameter of the capsule was about 6.7 μm while the average thickness of the capsule shell was 0.9 μm. XRD, HRTEM, Raman and Mössbauer spectroscopy data revealed that the iron oxide nanoparticles have the crystal structure of maghemite γ-Fe2O3. The nanoparticles were highly monodisperse with medium size of 7.5 nm. The Mössbauer spectroscopy data revealed that the nanoparticles have marked superparamagnetic behavior which was retained up to room temperature due to slow spin relaxation. Because of that, the microcapsules can be handled by an external magnetic field. Both these properties are important for target drug delivery. Based on the Mössbauer spectroscopy data, the spin blocking temperatures TB of about 90K was found for the particles with size D≤5 nm and TB≈250 K for particles with D≥6 nm. The anisotropy constants K were determined using the superparamagnetic approximation and in the low temperature approximation of collective magnetic excitation.

Iron selenide nanoparticles with the NiAs-like crystal structure were synthesized by thermal decomposition of iron chloride and selenium powder in a high-temperature organic solvent. Depending on the time of the compound processing at 340 °C, the nanocrystals with monoclinic (M)-Fe3Se4 or hexagonal (H)-Fe7Se8 structures as well as a mixture of these two phases can be obtained. The magnetic behavior of the monoclinic and hexagonal phases is very different. The applied-field and temperature dependences of magnetization reveal a complicated transformation between ferrimagnetic (FRM) and antiferromagnetic (AFM) structures, which can be related to the spin rotation process connected with the redistribution of cation vacancies. From XRD and Mössbauer data, the 3c type superstructure of vacancy ordering was found in the hexagonal Fe7Se8. Redistribution of vacancies in Fe7Se8 from random to ordered leads to the transformation of the magnetic structure from FRM to AFM. The Mössbauer data indicate that vacancies in the monoclinic Fe3Se4 prefer to appear near the Fe(3+) ions and stimulate the magnetic transition with the rotation of the Fe(3+) magnetic moments. Unusually high coercive force Hc was found in both (H) and (M) nanocrystals with the highest ("giant") value of about 25 kOe in monoclinic Fe3Se4. This is explained by the strong surface magnetic anisotropy which is essentially larger than the core anisotropy. Such a large coercivity is rare for materials without rare earth or noble metal elements, and the Fe3Se4-based compounds can be the low-cost, nontoxic alternative materials for advanced magnets. In addition, an unusual effect of "switching" of magnetization in a field of 10 kOe was found in the Fe3Se4 nanoparticles below 280 K, which can be important for applications.

A thermal pyrolysis method has been developed to synthesize the tetragonal phase of chalcopyrite magnetic semiconductor CuFeS2 nanoparticles. All nanoparticles have the same anisotropic brick-like shape, and the “bricks” are self-organized in a certain orientation, creating well-ordered nanocomposites. High-resolution transmission electron microscopy and electron diffraction data show that every nanobrick is a single crystal with a layered atomic structure and characteristic dimensions of about 5nm×20nm in plane. Magnetic measurements support the antiferromagnetic spin structure and reveal the appearance of a small ferromagnetic component below 60K. Magnetic anomalies observed in the zero-field cooled magnetization curves at low temperatures may be related to an appearance of magnetic moment at the Cu ion site. The Mössbauer spectra show that only about 50% of Fe atoms are in the magnetically ordered α-phase of chalcopyrite. The remaining Fe is non-magnetic and may be located either in the γ-phase of chalcopyrite or in isocubanite.

Iron sulfide nanoparticles Fe3S4 with the spinel-type crystal structure were synthesized by the polyol mediated process. The particle size depends on preparation conditions and varies from 9 to 20 nm. Mössbauer data have revealed that the dominating fraction of iron ions in the 9-nm sample is in the high-spin ferric state. This implies an occurrence of the cation vacancies in nonstoichiometric greigite. The stoichiometric phase of greigite Fe3S4 dominates in the 18-nm-size nanoparticles. Magnetic measurements have shown a ferrimagnetic behavior of all samples at temperatures between 78 and 300 K. The estimated value of magnetic moment of the stoichiometric greigite nanoparticles is about 3.5 μB per Fe3S4 unit. The Mössbauer spectra indicate a superparamagnetic behavior of small particles, and some fraction of superparamagnetic phase is observed in all samples synthesized which may be caused by the particle size distribution. The blocking temperatures of T B ≈ 230 and 250 K are estimated for the 9 and 14 nm particles, respectively. The Mössbauer parameters indicate a great degree of covalency in the Fe–S bonds and support the fast electron Fe3+ ⇆ Fe2+ exchange in the B-sites of greigite. An absence of the Verwey transition at temperatures between 90 and 295 K is established supporting a semimetal type of conductivity. The temperature and magnetic field dependences of the magnetic circular dichroism (MCD) of optical spectra were measured in Fe3S4 for the first time. The spectra differ substantially from that of the isostructural oxide Fe3O4. It is supposed that the MCD spectra of greigite nanoparticles result from the collective electron excitations in a wide band with superimposed peaks of the d–d transitions in Fe ions.