Natalia E . Gervits - Academia.edu (original) (raw)

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Papers by Natalia E . Gervits

We studied dependences of T 2 relaxation time on magnetic field and concentration of nanoparticle... more We studied dependences of T 2 relaxation time on magnetic field and concentration of nanoparticles. It was found that nanocontrast media are effective under the influence of the magnetic fields in the range 0.3-7 T. Data of electron paramagnetic resonance confirm the assumption on aggregation of nanoparticles not coated with proteins in high magnetic fields.

The hollow microcapsules made of silica-coated CoFe 2 O 4 nanoparticles were synthesized using ch... more The hollow microcapsules made of silica-coated CoFe 2 O 4 nanoparticles were synthesized using chemical co-precipitation, followed by the sol-gel method. Poly(MMA-co-MAA) microspheres were used as a core template which can be completely removed after annealing at 450 °C. The microcapsules are monodisperse with the outer diameter of about 450 nm and the thickness of the shell is about 50 nm. The nanoparticles of Co-ferrite are single crystalline. The size of the nanoparticles and magnetic properties of CoFe 2 O 4 /SiO 2 hollow spheres can be tuned with high accuracy at the annealing stage. The Mössbauer data indicate that CoFe 2 O 4 ferrite is an inverse spinel, in which Fe 3+ and Co 2+ ions are distributed in both octahedral and tetrahedral sites with the inversion degree close to the bulk ferrite value. At low temperature the CoFe 2 O 4 /SiO 2 nanoparticles are in antiferromagnetic (AFM) state due to the canted or triangular magnetic structure. Under heating in the applied field, AFM structure transforms to the ferrimagnetic(FM) structure, that increases the magnetization. The Mössbauer data revealed that the small size CoFe 2 O 4 /SiO 2 particles do not show superparamagnetic behavior, but they transit to the paramagnetic state by the jump-like first order magnetic transition (JMT). This effect is a specific property of the magnetic nanoparticles isolated by inert material. The suggested method of synthesis can be modified with various bio-ligands on the silane surface, and such materials can find many applications in diagnostics and bio-separation.

The evolution of the electronic structure and magnetic properties with Co substitution for Fe in ... more The evolution of the electronic structure and magnetic properties with Co substitution for Fe in the solid solution Fe 1−x Co x Ga 3 was studied by means of ab initio band-structure calculations and nuclear spin-lattice relaxation 1/T 1 of the 69,71 Ga nuclei. The 69,71 (1/T 1) was studied as a function of temperature in a wide temperature range of 2-300 K for the concentrations x = 0.0, 0.5, and 1.0. In the parent semiconducting compound FeGa 3 , the temperature dependence of the 69 (1/T 1) exhibits a huge maximum at about T ∼ 6 K indicating the existence of in-gap states. The opposite binary compound, CoGa 3 , demonstrates a metallic Korringa behavior with 1/T 1 ∝ T. In Fe 0.5 Co 0.5 Ga 3 , the relaxation is strongly enhanced due to spin fluctuations and follows 1/T 1 ∝ T 1/2 , which is a unique feature of weakly and nearly antiferromagnetic metals. This itinerant antiferromagnetic behavior contrasts with ab initio band-structure calculations, where a ferromagnetic state with an ordered moment of 0.5 μ B /f.u. is predicted. The results are discussed in terms of the interplay between the localized and itinerant magnetism including in-gap states and spin fluctuations.

BiFeO 3 is a perovskite-like compound with magnetic and electrical long-range order with antiferr... more BiFeO 3 is a perovskite-like compound with magnetic and electrical long-range order with antiferromag-netic transition temperature about 670 K. It has space modulated magnetic structure of cycloidal type with the period λ = 620 ± 20 Å incommensurate with the lattice parameter leading to specific NMR line shape. Substitution of strontium for bismuth atoms destroys the space modulated structure. Several samples with substitution of strontium for bismuth Bi 1−x Sr x FeO 3 were studied by means of zero field NMR and Mössbauer spectroscopy. A significant broadening of 57 Fe NMR spectrum was observed at x = 0.10 while Mössbauer spectroscopy demonstrates this effect at x = 0.07. This phenomena evidence for the destruction of the spatial modulated magnetic structure at x ∼ 0.07-0.10 by the heterovalent substitution of strontium for bismuth without the destruction of crystal rhombohedral structure.

BiFeO 3 is a perovskite-like compound with magnetic and electrical long-range order with antiferr... more BiFeO 3 is a perovskite-like compound with magnetic and electrical long-range order with antiferromagnetic transition temperature about 670 K. It has space modulated magnetic structure of cycloidal type with the period λ = 620 ± 20 Å incommensurate with the lattice parameter leading to specific NMR line shape. Substitution of strontium for bismuth atoms destroys the space modulated structure. Several samples with substitution of strontium for bismuth Bi 1−x Sr x FeO 3 were studied by means of zero field NMR and Mössbauer spectroscopy. A significant broadening of 57 Fe NMR spectrum was observed at x = 0.10 while Mössbauer spectroscopy demonstrates this effect at x = 0.07. This phenomena evidence for the destruction of the spatial modulated magnetic structure at x ∼ 0.07-0.10 by the heterovalent substitution of strontium for bismuth without the destruction of crystal rhombohedral structure.

We studied dependences of T 2 relaxation time on magnetic field and concentration of nanopar-ticl... more We studied dependences of T 2 relaxation time on magnetic field and concentration of nanopar-ticles. It was found that nanocontrast media are effective under the influence of the magnetic fields in the range 0.3-7 T. Data of electron paramagnetic resonance confirm the assumption on aggregation of nanoparticles not coated with proteins in high magnetic fields.try of Health of the Russian Federation; 2 D. I. Mendeleev University of Chemical Technology of Russia; 3 P. N. Lebedev Physical Institute of the Russian Academy of Sciences; 4 S. P. HELPIC, Moscow, Russia. Address for correspondence: anna.bagdinova@gmail.com. A. N. Bagdinova MRI diagnostics and precise detection of tumor boundaries in different organs at the early stages require the use of contrasting media that accumulate in tumors and modifying their NMR characteristics, in particular , transverse relaxation time T 2. High accuracy of diagnostics can be achieved with innovative iron based nanocontrasts with targeted delivery [2,4,5]. The use of contrasts for MRI in the clinical practice has become popular worldwide, and to date, almost all contrast compounds are based on paramagnetic ions containing gadolinium and bound with low-molecular-weight ligands reducing their toxicity and increasing the time of their circulation in the blood. Such contrast agents increase the signal intensity in the region of their accumulation by shortening the longitudinal spin-lattice relaxation time T 1. A fundamentally different type of contrast agents is based on iron oxide nanoparticles. These preparations reduce the transverse relaxation time T 2 and the intensity of the signal, thus being a "negative" contrast agent. T 2-contrast agents have a number of advantages: 1) exogenous and toxic free forms of gadolinium contrasts are replaced with bio-compatible and biodegradable iron oxide compounds, 2) relaxation for T 2-contrast agents is tenfold higher than for gadolinium compounds which allows significantly reducing the dose, 3) iron based contrasts are characterized by longer blood circulation time; 4) passive accumulation in tumors due to the effect of increased permeability and retention unique to nanoparticles. It should be noted that an attempt was previously undertaken to bring a contrast agent based on nanopar-ticles of iron oxide to the market. However, the drug was commercially successful due to imperfection of chemical design and restricted application (liver pathologies). Nevertheless, many complexes based on nanoparticles are now developed, which confirms the relevance and prospects of their usage in clinical practice. The use of nanocontrasts and appropriate software improves spatial resolution to fractions of

The magnetic ground states of the isostructural paper-chain compounds Ba 3 Cu 3 In 4 O 12 and Ba ... more The magnetic ground states of the isostructural paper-chain compounds Ba 3 Cu 3 In 4 O 12 and Ba 3 Cu 3 Sc 4 O 12 were recently found to be ordered antiferromagnetically (AFM), while both quantum magnets reveal positive asymptotic Curie-Weiss temperatures suggestive of ferromagnetic (FM) interactions at high temperatures. Due to the AFM-FM competition the saturation magnetization in both compounds is reached already in modest magnetic fields with a nontrivial succession of two spin-flop and two spin-flip-like transitions in Ba 3 Cu 3 In 4 O 12. We argue that the paper-chains in both compounds can be described as a system of Cu I-2Cu I I trimers with a low-spin S = 1/2 ground state which implies a nonmagnetic spin-singlet Cu I I dimer state and a spin-polarized Cu I ion. To validate the model we performed extensive nuclear resonance-nuclear quadrupole resonance-nuclear magnetic resonance (NQR-NMR)-measurements in Ba 3 Cu 3 In 4 O 12. Two different types of 63,65 Cu nuclear resonance spectra were observed in the magnetically ordered state at zero external magnetic field: (i) a pure NQR spectrum in the frequency range of 24-30 MHz and (ii) a zero-field NMR spectrum in the frequency range of 50-65 MHz. This result unambiguously indicates that one of the two types of copper ions in Ba 3 Cu 3 In 4 O 12 is in a nonmagnetic spin state below T N. It provides a unique criterion to test any theoretical model describing the ground-state spin structure in Ba 3 Cu 3 In 4 O 12. We attribute the pure NQR spectrum to Cu I I ions forming nonmagnetic spin-singlet 2Cu I I dimers and the zero-field NMR spectrum to spin-polarized Cu I ions. Both NQR and zero-field NMR spectra point to an existence of at least two nonequivalent sets of the copper trimers, probably due to displacements of the Cu I ions along the cavity in the c direction.

The evolution of the electronic structure and magnetic properties with Co substitution for Fe in ... more The evolution of the electronic structure and magnetic properties with Co substitution for Fe in the solid solution Fe 1−x Co x Ga 3 was studied by means of ab initio band-structure calculations and nuclear spin-lattice relaxation 1/T 1 of the 69,71 Ga nuclei. The 69,71 (1/T 1 ) was studied as a function of temperature in a wide temperature range of 2-300 K for the concentrations x = 0.0, 0.5, and 1.0. In the parent semiconducting compound FeGa 3 , the temperature dependence of the 69 (1/T 1 ) exhibits a huge maximum at about T ∼ 6 K indicating the existence of in-gap states. The opposite binary compound, CoGa 3 , demonstrates a metallic Korringa behavior with 1/T 1 ∝ T . In Fe 0.5 Co 0.5 Ga 3 , the relaxation is strongly enhanced due to spin fluctuations and follows 1/T 1 ∝ T 1/2 , which is a unique feature of weakly and nearly antiferromagnetic metals. This itinerant antiferromagnetic behavior contrasts with ab initio band-structure calculations, where a ferromagnetic state with an ordered moment of 0.5 μ B /f.u. is predicted. The results are discussed in terms of the interplay between the localized and itinerant magnetism including in-gap states and spin fluctuations.

BiFeO 3 is a perovskite-like compound with magnetic and electrical long-range order with antiferr... more BiFeO 3 is a perovskite-like compound with magnetic and electrical long-range order with antiferromagnetic transition temperature about 670 K. It has space modulated magnetic structure of cycloidal type with the period λ = 620 ± 20 Å incommensurate with the lattice parameter leading to specific NMR line shape. Substitution of strontium for bismuth atoms destroys the space modulated structure. Several samples with substitution of strontium for bismuth Bi 1−x Sr x FeO 3 were studied by means of zero field NMR and Mössbauer spectroscopy. A significant broadening of 57 Fe NMR spectrum was observed at x = 0.10 while Mössbauer spectroscopy demonstrates this effect at x = 0.07. This phenomena evidence for the destruction of the spatial modulated magnetic structure at x ∼ 0.07-0.10 by the heterovalent substitution of strontium for bismuth without the destruction of crystal rhombohedral structure.

The magnetic ground states of the isostructural paper-chain compounds Ba 3 Cu 3 In 4 O 12 and Ba ... more The magnetic ground states of the isostructural paper-chain compounds Ba 3 Cu 3 In 4 O 12 and Ba 3 Cu 3 Sc 4 O 12 were recently found to be ordered antiferromagnetically (AFM), while both quantum magnets reveal positive asymptotic Curie-Weiss temperatures suggestive of ferromagnetic (FM) interactions at high temperatures. Due to the AFM-FM competition the saturation magnetization in both compounds is reached already in modest magnetic fields with a nontrivial succession of two spin-flop and two spin-flip-like transitions in Ba 3 Cu 3 In 4 O 12 . We argue that the paper-chains in both compounds can be described as a system of Cu I -2Cu I I trimers with a low-spin S = 1/2 ground state which implies a nonmagnetic spin-singlet Cu I I dimer state and a spin-polarized Cu I ion. To validate the model we performed extensive nuclear resonance-nuclear quadrupole resonance-nuclear magnetic resonance (NQR-NMR)-measurements in Ba 3 Cu 3 In 4 O 12 . Two different types of 63,65 Cu nuclear resonance spectra were observed in the magnetically ordered state at zero external magnetic field: (i) a pure NQR spectrum in the frequency range of 24-30 MHz and (ii) a zero-field NMR spectrum in the frequency range of 50-65 MHz. This result unambiguously indicates that one of the two types of copper ions in Ba 3 Cu 3 In 4 O 12 is in a nonmagnetic spin state below T N . It provides a unique criterion to test any theoretical model describing the ground-state spin structure in Ba 3 Cu 3 In 4 O 12 . We attribute the pure NQR spectrum to Cu I I ions forming nonmagnetic spin-singlet 2Cu I I dimers and the zero-field NMR spectrum to spin-polarized Cu I ions. Both NQR and zero-field NMR spectra point to an existence of at least two nonequivalent sets of the copper trimers, probably due to displacements of the Cu I ions along the cavity in the c direction.

BiFeO 3 is a perovskite-like compound with magnetic and electrical long-range order with antiferr... more BiFeO 3 is a perovskite-like compound with magnetic and electrical long-range order with antiferromagnetic transition temperature about 670 K. It has space modulated magnetic structure of cycloidal type with the period λ = 620 ± 20 Å incommensurate with the lattice parameter leading to specific NMR line shape. Substitution of strontium for bismuth atoms destroys the space modulated structure. Several samples with substitution of strontium for bismuth Bi 1−x Sr x FeO 3 were studied by means of zero field NMR and Mössbauer spectroscopy. A significant broadening of 57 Fe NMR spectrum was observed at x = 0.10 while Mössbauer spectroscopy demonstrates this effect at x = 0.07. This phenomena evidence for the destruction of the spatial modulated magnetic structure at x ∼ 0.07-0.10 by the heterovalent substitution of strontium for bismuth without the destruction of crystal rhombohedral structure.

The evolution of the electronic structure and magnetic properties with Co substitution for Fe in ... more The evolution of the electronic structure and magnetic properties with Co substitution for Fe in the solid solution Fe 1−x Co x Ga 3 was studied by means of ab initio band-structure calculations and nuclear spin-lattice relaxation 1/T 1 of the 69,71 Ga nuclei. The 69,71 (1/T 1 ) was studied as a function of temperature in a wide temperature range of 2-300 K for the concentrations x = 0.0, 0.5, and 1.0. In the parent semiconducting compound FeGa 3 , the temperature dependence of the 69 (1/T 1 ) exhibits a huge maximum at about T ∼ 6 K indicating the existence of in-gap states. The opposite binary compound, CoGa 3 , demonstrates a metallic Korringa behavior with 1/T 1 ∝ T . In Fe 0.5 Co 0.5 Ga 3 , the relaxation is strongly enhanced due to spin fluctuations and follows 1/T 1 ∝ T 1/2 , which is a unique feature of weakly and nearly antiferromagnetic metals. This itinerant antiferromagnetic behavior contrasts with ab initio band-structure calculations, where a ferromagnetic state with an ordered moment of 0.5 μ B /f.u. is predicted. The results are discussed in terms of the interplay between the localized and itinerant magnetism including in-gap states and spin fluctuations.

BiFeO 3 is a perovskite-like compound with magnetic and electrical long-range order with antiferr... more BiFeO 3 is a perovskite-like compound with magnetic and electrical long-range order with antiferromagnetic transition temperature about 670 K. It has space modulated magnetic structure of cycloidal type with the period λ = 620 ± 20 Å incommensurate with the lattice parameter leading to specific NMR line shape. Substitution of strontium for bismuth atoms destroys the space modulated structure. Several samples with substitution of strontium for bismuth Bi 1−x Sr x FeO 3 were studied by means of zero field NMR and Mössbauer spectroscopy. A significant broadening of 57 Fe NMR spectrum was observed at x = 0.10 while Mössbauer spectroscopy demonstrates this effect at x = 0.07. This phenomena evidence for the destruction of the spatial modulated magnetic structure at x ∼ 0.07-0.10 by the heterovalent substitution of strontium for bismuth without the destruction of crystal rhombohedral structure.

The magnetic ground states of the isostructural paper-chain compounds Ba 3 Cu 3 In 4 O 12 and Ba ... more The magnetic ground states of the isostructural paper-chain compounds Ba 3 Cu 3 In 4 O 12 and Ba 3 Cu 3 Sc 4 O 12 were recently found to be ordered antiferromagnetically (AFM), while both quantum magnets reveal positive asymptotic Curie-Weiss temperatures suggestive of ferromagnetic (FM) interactions at high temperatures. Due to the AFM-FM competition the saturation magnetization in both compounds is reached already in modest magnetic fields with a nontrivial succession of two spin-flop and two spin-flip-like transitions in Ba 3 Cu 3 In 4 O 12 . We argue that the paper-chains in both compounds can be described as a system of Cu I -2Cu I I trimers with a low-spin S = 1/2 ground state which implies a nonmagnetic spin-singlet Cu I I dimer state and a spin-polarized Cu I ion. To validate the model we performed extensive nuclear resonance-nuclear quadrupole resonance-nuclear magnetic resonance (NQR-NMR)-measurements in Ba 3 Cu 3 In 4 O 12 . Two different types of 63,65 Cu nuclear resonance spectra were observed in the magnetically ordered state at zero external magnetic field: (i) a pure NQR spectrum in the frequency range of 24-30 MHz and (ii) a zero-field NMR spectrum in the frequency range of 50-65 MHz. This result unambiguously indicates that one of the two types of copper ions in Ba 3 Cu 3 In 4 O 12 is in a nonmagnetic spin state below T N . It provides a unique criterion to test any theoretical model describing the ground-state spin structure in Ba 3 Cu 3 In 4 O 12 . We attribute the pure NQR spectrum to Cu I I ions forming nonmagnetic spin-singlet 2Cu I I dimers and the zero-field NMR spectrum to spin-polarized Cu I ions. Both NQR and zero-field NMR spectra point to an existence of at least two nonequivalent sets of the copper trimers, probably due to displacements of the Cu I ions along the cavity in the c direction.