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Publications (25) View all
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Article: Structural, static and dynamic magnetic properties of dextran coated γ-Fe(2)O(3) nanoparticles studied by (57)Fe NMR, Mössbauer, TEM and magnetization measurements.
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ABSTRACT: The structural and magnetic properties and spin dynamics of dextran coated and uncoated γ-Fe(2)O(3) (maghemite) nanoparticles have been investigated using high resolution transmission electron microscopy (HRTEM), (57)Fe nuclear magnetic resonance (NMR), Mössbauer spectroscopy and dc magnetization measurements. The HRTEM observations indicated a well-crystallized system of ellipsoid-shaped nanoparticles, with an average size of 10 nm. The combined Mössbauer and magnetic study suggested the existence of significant interparticle interactions not only in the uncoated but also in the dextran coated nanoparticle assemblies. The zero-field NMR spectra of the nanoparticles at low temperatures are very similar to those of the bulk material, indicating the same hyperfine field values at saturation in accord with the performed Mössbauer measurements. The T(2) NMR spin-spin relaxation time of the nanoparticles has also been measured as a function of temperature and found to be two orders of magnitude shorter than that of the bulk material. It is shown that the thermal fluctuations in the longitudinal magnetization of the nanoparticles in the low temperature limit may account for the shortening and the temperature dependence of the T(2) relaxation time. Thus, the low temperature NMR results are in accord with the mechanism of collective magnetic excitations, due to the precession of the magnetization around the easy direction of the magnetization at an energy minimum, a mechanism originally proposed to interpret Mössbauer experiments in magnetic nanoparticles. The effect of the surface spins on the NMR relaxation mechanisms is also discussed.Journal of Physics Condensed Matter 03/2012; 24(15):156001. · 2.55 Impact Factor -
Article: Structural, static and dynamic magnetic properties of dextran coated γ-Fe2O3 nanoparticles studied by 57Fe NMR, M¨ossbauer, TEM and magnetization measurements
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ABSTRACT: The structural and magnetic properties and spin dynamics of dextran coated and uncoated -Fe2O3 (maghemite) nanoparticles have been investigated using high resolution transmission electron microscopy (HRTEM), 57Fe nuclear magnetic resonance (NMR), M¨ossbauer spectroscopy and dc magnetization measurements. The HRTEM observations indicated a well-crystallized system of ellipsoid-shaped nanoparticles, with an average size of 10 nm. The combined M¨ossbauer and magnetic study suggested the existence of significant interparticle interactions not only in the uncoated but also in the dextran coated nanoparticle assemblies. The zero-field NMR spectra of the nanoparticles at low temperatures are very similar to those of the bulk material, indicating the same hyperfine field values at saturation in accord with the performed M¨ossbauer measurements. The T2 NMR spin–spin relaxation time of the nanoparticles has also been measured as a function of temperature and found to be two orders of magnitude shorter than that of the bulk material. It is shown that the thermal fluctuations in the longitudinal magnetization of the nanoparticles in the low temperature limit may account for the shortening and the temperature dependence of the T2 relaxation time. Thus, the low temperature NMR results are in accord with the mechanism of collective magnetic excitations, due to the precession of the magnetization around the easy direction of the magnetization at an energy minimum, a mechanism originally proposed to interpret M¨ossbauer experiments in magnetic nanoparticles. The effect of the surface spins on the NMR relaxation mechanisms is also discussed.Journal of Physics Condensed Matter 03/2012; 24:156001. · 2.55 Impact Factor -
Article: Spin order and lattice frustration in optimally doped manganites. A high temperature NMR study.
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ABSTRACT: The physics underlying the complex glassy phenomena, which accompany the formation of polarons in optimally doped manganites (ODM) is a cumbersome issue with many unexplained aspects. In this article we present $^{139}$La and $^{55}$Mn NMR in the temperature range 80K - 900K of ODM La$_{0.67}$Ca$_{0.33}$MnO$_3$. We show that local lattice distortions, established in the Paramagnetic (PM) phase for $T<700$K, induce a genuine spin-glass state, which for $T<T_c$ consolidates with the Ferromagnetic (FM) state into a single thermodynamic phase. Comparative NMR experiments on La$_{0.77}$Ca$_{0.23}$MnO$_3$, La$_{0.59}$Ca$_{0.41}$MnO$_3$, and La$_{0.70}$Sr$_{0.30}$MnO$_3$ demonstrate the dominant role of lattice distortions, which appear to control (i) the stability of the spin glass phase component and (ii) the kind (1st or 2nd order) of the PM-FM phase transition. The experimental results are in agreement with the predictions of the compressible random bond - random field Ising model, where consideration of a strain field induced by lattice distortions, is shown to invoke at $T_c$ a discontinuous (1st order like) change of both the FM and the "glassy" Edwards-Anderson (EA) order parameters. Comment: 8 pages, 8 figures09/2009; -
Conference Proceeding: Study of Physico-chemical properties of Self-cleaning Cement using NMR and Ultrasonic Technique
1st Panhellenic Conference on Construction Materials and Elements, Athens, Greece; 05/2008 -
Article: Magnetic nanoparticles for biomedical applications
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ABSTRACT: Magnetite nanoparticles covalently coated with polysaccharide arabic acid are investigated as possible constituents of aqueous ferrofluids for biomedical applications. The nanoparticles have been characterized by transmission electron microscopy and with magnetic measurements. 1 H nuclear magnetic resonance T 1 relaxation experiments of the coating which cover the nanoparticles have been also carried out in an attempt to probe the dynamic (superparamagnetic) behaviour of the magnetic nanoparticles.Journal of Optoelectronics and Advanced Materials 01/2007; 9(3):527-531. · 0.46 Impact Factor
