The process of interaction of graphene with iron oxide nanoparticles was investigated. First, graphene oxide (GO) modified with magnetite Fe3O4 nanoparticles was successfully synthesized. Raman and ...Mössbauer spectroscopy revealed that the magnetite Fe3O4 in combination with GO became non-stoichiometric, and the maghemite phase γ-Fe2O3 appears. Subsequent reduction of graphene oxide by thermal treatment leads to an increase in the fraction of maghemite content and, in addition, the hematite phase α-Fe2O3 appears in the sample annealed at above 500 °C. Meanwhile, the core-shell nanocomposites of FexOy/G appear, were FexOy consists of a mixture of the Fe3O4, γ-Fe2O3 and α-Fe2O3 phases. The content of each phase can be varied by the annealing temperature. Magnetic, Mössbauer and Raman spectroscopy measurements indicate that graphene can interact with iron oxide. Charge-transfer from iron to graphene can occur due to delocalization of 3d electrons, which reduces the overall magnetic moment of the charge-transfer complexes. These properties can have potential applications in electronic such as supercapacitors, advanced anode materials for lithium-ion batteries, magnetically targeted drug delivery, photothermic therapy, and magnetic resonance imaging.
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•Graphene modified by iron oxide nanoparticles FexOy/G was successfully synthesized.•The nanocomposites FexOy/G have a core-shell type design.•High temperature treatment leads to reduction of the GO and γ-Fe2O3 formation.•Magnetite act as electron donors to graphene, and charge-transfer interaction between graphene and iron oxide takes place.•Charge-transfer occurs due to delocalization of 3d electrons in the interface layer of the core-shell nanocomposites.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
In this work, high-resolution triple-crystal X-ray diffraction and reciprocal space mapping were applied to study the effect of magnetoelastic interactions on the structural perfection and ...diffraction properties of magnetically ordered iron borate crystals. It was established that the presence of magnetic domains with different orientation of magnetization and near-surface magnetism effect in FeBO3 leads to the appearance of disordered areas in crystals and slight changes in the lattice parameters. This is affected in the broadening or splitting of the reciprocal lattice point and diffraction properties of the crystal. An applied weak external magnetic field alters the domain structure and significantly improves the diffraction characteristics of FeBO3 crystals. This effect is important for practical applications of iron borate crystals, including the use of these crystals as X-ray modulators.
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•Precision studies of the temperature dependence of the Mössbauer spectra of FeBO3 single crystals have been carried out.•Parameters of hyperfine interaction in FeBO3 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 FeBO3 was determined.
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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 FeBO3 single crystals in a wide temperature range, including the region of magnetic phase transitions (TN). 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 FeBO3 spectra near the N é el temperature (ТN). 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 FeBO3 crystals in new high-tech branches of science and technology, including optoelectronics and synchrotron technologies.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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 wüstite FeO. The magnetic properties of magnetite and wüstite are radically different, and by varying the thickness of these layers, structures with the desired functional properties can be obtained.
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Core-shell FexOy@C nanoparticles (NPs) modified with Ag were studied with x-ray diffraction, transmission electron microscopy, energy dispersive elemental mapping, Mössbauer spectroscopy, static ...magnetic measurements, and optical magnetic circular dichroism (MCD). FexOy@C NPs synthesized by the pyrolysis process of the mixture of Fe(NO3)3 · 9H2O with oleylamine and oleic acid were added to a heated mixture of oleylamine and AgNO3 in different concentrations. The final product was a mixture of iron oxide crystalline NPs in an amorphous carbon shell and Ag crystalline NPs. The iron oxide NPs were presented by two magnetic phases with extremely close crystal structures: Fe3O4 and γ-Fe2O3. Ag is shown to form crystalline NPs located very close to the iron oxide NPs. An assumption is made about the formation of hybrid FexOy@C-Ag NPs. Correlations were obtained between the Ag concentration in the fabricated samples, their magnetic properties and the MCD spectrum shape. Introducing Ag led to a approximately linear decrease of the NPs saturation magnetization depending upon the Ag concentration, it also resulted into the MCD spectrum shift to the lower light wave energies. MCD was also studied for the Fe3O4@C NPs synthesized earlier with the same one-step process using different heat treatment temperatures, and MCD spectra were compared for two series of NPs. A possible contribution of the surface plasmon excitation in Ag NPs to the MCD spectrum of the FexOy@C-Ag NPs is discussed.
The conditions for occurrence of the magnetoelectric, magnetoelastic, and piezomagnetic effects in crystals of the langasite family with a triangular magnetic lattice are considered on the basis of ...the phenomenological model. It is shown that the iron-containing langasites with a triangular magnetic lattice may have piezoelectric properties above the Neel temperature T sub(N), but they are not ferroelectrics. Occurrence of either ferroelectric or both ferroelectric and piezoelectric states is possible below T sub(N), and these crystals can be considered as multif erro ics with a helicoid of magnetic moments oriented along the c axis. The observation of electric polarization p sub(z) in these crystals at T < T sub(N) is possible at sufficiently large values of piezocoefficients e sub(zyz) (class 2) or e sub(zxx) (class 3) and as a result of mechanical deformations occurring due to magnetostriction. Moreover, because of magnetoelastic interactions (appearing due to lowering symmetry P321 arrow right P3 below T sub(N)), the electric polarization and the axis of the magnetic helicoid can both be parallel to the z axis even in the absence of the Dzyaloshinskii-Moriya (DM) effect. Possible ferroelastic and ferroelectric transitions are considered at T < T sub(N). The occurrence of the magnetic helicoid with its axis in the ab plane is also discussed. It may appear due to the spin-orbit interaction (DM effect). In this case, electric polarization must appear also in the ab plane.
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Iron sulfide nanoparticles Fe
3
S
4
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 Fe
3
S
4
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 μ
B
per Fe
3
S
4
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 Fe
3+
⇆ Fe
2+
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 Fe
3
S
4
for the first time. The spectra differ substantially from that of the isostructural oxide Fe
3
O
4
. 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.
•Parameters of the spin helicoid in multiferroic Ba3SbFe3Si2O14are obtained for all T < TN.•A physical model for the behavior of the Mössbauer spectra in spiral magnetic structures is developed.•This ...model gives fine information about complicated magnetic structures even in polycrystalline samples.•The magnetic anisotropy axis can be found in a triangle magnetic lattice.
A physico-mathematical model for processing the Mössbauer spectra of the langasite family compounds with a helical magnetic structure was developed. It was shown that the Mössbauer 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 Néel 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°, 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.
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The method and experimental cell for the X-ray diffraction diagnostic of crystalline materials exposed to magnetic fields have been described. It has been shown that the application of even weak ...magnetic fields (about several Oersteds) allows one to significantly reduce the diffraction contrast on X-ray topograms of FeBO
3
and Fe
0.94
Ga
0.06
BO
3
single crystals, which is due to a nonuniform distribution of magnetostriction strains, and thereby to improve the degree of their structural perfection. The magnetic domain structure of Fe
1 –
x
Ga
x
BO
3
solid solution single crystals has been visualized. It has been found that small concentrations of diamagnetic Ga ions do not significantly change the configuration of domain walls in FeBO
3
-based crystals.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Highly perfect FeBO
3
and Fe
0.91
Ga
0.09
BO
3
single crystals were studied in a wide temperature range using SQUID magnetometry. A theoretical model describing the temperature and field dependences ...of the magnetization of single crystals has been developed. It is found that even a small concentration of gallium, which is a diamagnetic impurity, substantially affects the magnetic properties of single FeBO
3
crystals. In particular, the Fe
0.91
Ga
0.09
BO
3
crystal differs from the pure FeBO
3
phase in by a lower magnetic phase transition temperature and a higher antiferromagnetic susceptibility at low temperatures.
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