2014 L'activité électrique des joints de grains dans le germanium polycristallin a été étudiée pa... more 2014 L'activité électrique des joints de grains dans le germanium polycristallin a été étudiée par la technique SEM/EBIC. L'effet de traitements thermiques successifs (T = 810 °C, t = 2 h et à T = 400 °C, t = 50 h) a été analysé. Des profils de courant EBIC à travers des joints de grains inactifs ont été enregistrés à différentes températures entre 140 et 300 K et pour plusieurs valeurs de courant primaire. Une inversion de contraste a été observée pour les niveaux d'injection élevés et à basses températures. Cette observation a été associée à une transition de processus piégeage/recombinaison sur un niveau Et proche de la bande de valence (Et = Ev + 0,03 eV). Abstract. 2014 The electrical activity of grain boundaries in polycrystalline germanium has been studied from SEM/EBIC observations. The effect of two successive thermal treatments (T = 810 °C, t = 2 h and T = 400 °C, t = 50 h) has been examined. Measurements of EBIC profiles across unactive grain boundaries were carried out at different temperatures between 140 K and 300 K and for several injection currents. The inversion of contrast observed at high injection level and low temperature has been analysed in terms of a trapping/recombination transition on a near valence band level (Et = Ev + 0.03 eV).
We report here the first results on magnetic nanoparticles fabricated from basalt fragments of th... more We report here the first results on magnetic nanoparticles fabricated from basalt fragments of the eruption that took place in Lanzarote (Canary Islands) between 1730-1736, to be used in technologies of biomedicine. This approach was inspired by the strong and extremely stable remanent magnetization of slowly cooled rocks from basaltic lavas containing finely members of the hematite-ilmenite (Fe 2 O 3 -FeTiO 3 ) series. Besides, the presence of titanium may promote good biocompatibility and an adequate corrosion resistance, as it does in prosthesis. The green, simple, fast, and cost-effective synthesis of magnetic nanoparticles was attempted using solar vapor-phase condensation. This technique allows the preparation of large volumes of nanoparticles presenting a narrow particle size distribution without purification steps. Alternatively, high-energy ball-milling of rock powder in the presence of organic surfactants was also studied as a potential method to produce, in a one-step approach, large quantities of nanostructured particles for biomedical applications. As a proof of concept, we explore these materials as candidates to be used in hyperthermia therapy, which promotes selective necrosis of cancer cells by rais-ing the temperature of the tissue slightly above 43ºC. Results are correlated to their chemical and structural properties and compared to initial igneous rock characteristics.
Magnetic Hysteresis in Novel Magnetic Materials, 1997
ABSTRACT The fabrication and characterisation of nano-sized magnetic particles are the subject of... more ABSTRACT The fabrication and characterisation of nano-sized magnetic particles are the subject of intense research motivated not only by its interesting properties, quite different from those of the corresponding bulk materials, but also from the point of view of its promising technological applications, such as magnetic recording media, ferrofluids, soft magnetic materials, etc.[1].
We study the impact of vacancies on the magnetism of MgO and TiO 2 nanoparticles, and develop fis... more We study the impact of vacancies on the magnetism of MgO and TiO 2 nanoparticles, and develop fist-principle calculations to ascertain the origin of the induced polarization. Theoretically, we expect ferromagnetism only for the case of MgO with cation defects. Experimentally, we observed a small magnetic signal probably due to imperfections in the lattice order, perhaps at the nanoparticles surface.
... At the same time high field irreversibility in MH curves and zero field cooled-field cooled (... more ... At the same time high field irreversibility in MH curves and zero field cooled-field cooled (ZFC-FC) processes has also been detected. A low temperature spin-glass-like transition is evidenced at T F ~42 K with strong irreversibility even at H = 55 kOe. ...
In this work we carefully analyze the role of the microstructure on the magnetic properties of Co... more In this work we carefully analyze the role of the microstructure on the magnetic properties of Co-doped ZnO nanoparticles prepared by the vaporization-condensation method in a solar reactor. We show that a close correlation exists between microstructural features and the appearance of ferromagnetism. Both shape and size of the particles, as well as the microstructure, can be controlled by changing the pressure inside the evaporation chamber, as evidenced by transmission electron microscopy micrographs and high resolution electron microscopy ͑HREM͒. X-ray diffraction patterns and HREM make evident the absence of any significant Co segregation or any other phase different from würtzite type ZnO. On the other hand, electron energy loss spectroscopy analyses performed on several particles of würtzite type ZnO yielded an average Co concentration in good agreement with the nominal composition. Samples prepared in low pressure ͑Ϸ10 Torr͒ exhibit a very homogeneous microstructure and are ferromagnetic at low temperature but they have a very small saturation moment, well below that expected for a Co 2+ ion. Conversely, samples prepared at higher pressure conditions ͑Ϸ70-100 Torr͒ show a defective microstructure and are paramagnetic and increasing the Co content does not induce ferromagnetism.
ABSTRACT We have studied the hysteresis loops of three YBa2Cu3O7-x ceramics characterized by diff... more ABSTRACT We have studied the hysteresis loops of three YBa2Cu3O7-x ceramics characterized by different grain sizes (2, 7 and 10 mum). The measurements are based on 16 bits analog to digital conversion, the applied magnetic field being alternating at a frequency of 20 Hz and with a maximum amplitude of 300 Oe. We observed that the irreversible phenomena (area of the loops) largely increase when reducing the grain size. Considering that the measured magnetization can be separated into two terms characteristics of the inter- and intragranular regions, we suggest that at 4.2 K and for H
Nanomedicine: Nanotechnology, Biology and Medicine, 2010
A one-step process for the production of nanoparticles presenting advanced magnetic properties ca... more A one-step process for the production of nanoparticles presenting advanced magnetic properties can be achieved using vapor condensation. In this article, we report on the fabrication of Fe particles covered by a uniform MgO epitaxial shell. MgO has a lower surface energy than Fe, which results in a core-shell crystal formation. The particles satisfy a few of technical requirements for the practical use in real clinics, such as a high biocompatibility in living cells in-vitro, an injection through blood vessels without any clothing problems in murine model, a high absorption rate for magnetic hyperthermia at small particle concentration, and the potential to be used as contrast agent in the field of diagnostic magnetic imaging. They are also able to be used in drug delivery and magnetic-activated cell sorting.
The effects of exchange bias on core/shell structured nanoparticles are analyzed. Nanoparticles a... more The effects of exchange bias on core/shell structured nanoparticles are analyzed. Nanoparticles are integrated with high moment Fe-Co crystallites covered epitaxially with MgO shells. It is observed that the coercive field H C (FeCo) > H C (Co) > H C (Fe); however, the exchange bias field H E of the Co sample is higher than that of the FeCo one, while H E = 0 for the Fe sample. It is suggested that the exchange bias is induced by the formation of a (Co, Mg)O solid solution. In fact, we show that it is possible to modify the exchange bias properties by manipulating the level of Mg dusting at the interface, as recently reported for thin films.
We report on the structural and magnetic properties of Co-doped ZnO particles prepared by vaporiz... more We report on the structural and magnetic properties of Co-doped ZnO particles prepared by vaporization-condensation in the solar furnace in Odeillo. X-ray diffraction data show no traces of Co segregation or any other phase different from ZnO. High-resolution electron microscopy (HREM) and transmision electron microscopy (TEM) techniques have also been used to characterize particles. Irrespective of their composition, the shape and size of the obtained particles, as well as their magnetic properties, clearly depend on the preparation conditions. The samples prepared in vacuum exhibit hysteretic behavior with low coercivity (about 100 Oe) at T ¼ 5 K and saturation magnetization well below that expecte for Co 2+ in a tetrahedral crystal field. On the other hand, samples prepared at high pressure (70-100 Torr inside the balloon) are paramagnetic. r
2014 L'activité électrique des joints de grains dans le germanium polycristallin a été étudiée pa... more 2014 L'activité électrique des joints de grains dans le germanium polycristallin a été étudiée par la technique SEM/EBIC. L'effet de traitements thermiques successifs (T = 810 °C, t = 2 h et à T = 400 °C, t = 50 h) a été analysé. Des profils de courant EBIC à travers des joints de grains inactifs ont été enregistrés à différentes températures entre 140 et 300 K et pour plusieurs valeurs de courant primaire. Une inversion de contraste a été observée pour les niveaux d'injection élevés et à basses températures. Cette observation a été associée à une transition de processus piégeage/recombinaison sur un niveau Et proche de la bande de valence (Et = Ev + 0,03 eV). Abstract. 2014 The electrical activity of grain boundaries in polycrystalline germanium has been studied from SEM/EBIC observations. The effect of two successive thermal treatments (T = 810 °C, t = 2 h and T = 400 °C, t = 50 h) has been examined. Measurements of EBIC profiles across unactive grain boundaries were carried out at different temperatures between 140 K and 300 K and for several injection currents. The inversion of contrast observed at high injection level and low temperature has been analysed in terms of a trapping/recombination transition on a near valence band level (Et = Ev + 0.03 eV).
We report here the first results on magnetic nanoparticles fabricated from basalt fragments of th... more We report here the first results on magnetic nanoparticles fabricated from basalt fragments of the eruption that took place in Lanzarote (Canary Islands) between 1730-1736, to be used in technologies of biomedicine. This approach was inspired by the strong and extremely stable remanent magnetization of slowly cooled rocks from basaltic lavas containing finely members of the hematite-ilmenite (Fe 2 O 3 -FeTiO 3 ) series. Besides, the presence of titanium may promote good biocompatibility and an adequate corrosion resistance, as it does in prosthesis. The green, simple, fast, and cost-effective synthesis of magnetic nanoparticles was attempted using solar vapor-phase condensation. This technique allows the preparation of large volumes of nanoparticles presenting a narrow particle size distribution without purification steps. Alternatively, high-energy ball-milling of rock powder in the presence of organic surfactants was also studied as a potential method to produce, in a one-step approach, large quantities of nanostructured particles for biomedical applications. As a proof of concept, we explore these materials as candidates to be used in hyperthermia therapy, which promotes selective necrosis of cancer cells by rais-ing the temperature of the tissue slightly above 43ºC. Results are correlated to their chemical and structural properties and compared to initial igneous rock characteristics.
Magnetic Hysteresis in Novel Magnetic Materials, 1997
ABSTRACT The fabrication and characterisation of nano-sized magnetic particles are the subject of... more ABSTRACT The fabrication and characterisation of nano-sized magnetic particles are the subject of intense research motivated not only by its interesting properties, quite different from those of the corresponding bulk materials, but also from the point of view of its promising technological applications, such as magnetic recording media, ferrofluids, soft magnetic materials, etc.[1].
We study the impact of vacancies on the magnetism of MgO and TiO 2 nanoparticles, and develop fis... more We study the impact of vacancies on the magnetism of MgO and TiO 2 nanoparticles, and develop fist-principle calculations to ascertain the origin of the induced polarization. Theoretically, we expect ferromagnetism only for the case of MgO with cation defects. Experimentally, we observed a small magnetic signal probably due to imperfections in the lattice order, perhaps at the nanoparticles surface.
... At the same time high field irreversibility in MH curves and zero field cooled-field cooled (... more ... At the same time high field irreversibility in MH curves and zero field cooled-field cooled (ZFC-FC) processes has also been detected. A low temperature spin-glass-like transition is evidenced at T F ~42 K with strong irreversibility even at H = 55 kOe. ...
In this work we carefully analyze the role of the microstructure on the magnetic properties of Co... more In this work we carefully analyze the role of the microstructure on the magnetic properties of Co-doped ZnO nanoparticles prepared by the vaporization-condensation method in a solar reactor. We show that a close correlation exists between microstructural features and the appearance of ferromagnetism. Both shape and size of the particles, as well as the microstructure, can be controlled by changing the pressure inside the evaporation chamber, as evidenced by transmission electron microscopy micrographs and high resolution electron microscopy ͑HREM͒. X-ray diffraction patterns and HREM make evident the absence of any significant Co segregation or any other phase different from würtzite type ZnO. On the other hand, electron energy loss spectroscopy analyses performed on several particles of würtzite type ZnO yielded an average Co concentration in good agreement with the nominal composition. Samples prepared in low pressure ͑Ϸ10 Torr͒ exhibit a very homogeneous microstructure and are ferromagnetic at low temperature but they have a very small saturation moment, well below that expected for a Co 2+ ion. Conversely, samples prepared at higher pressure conditions ͑Ϸ70-100 Torr͒ show a defective microstructure and are paramagnetic and increasing the Co content does not induce ferromagnetism.
ABSTRACT We have studied the hysteresis loops of three YBa2Cu3O7-x ceramics characterized by diff... more ABSTRACT We have studied the hysteresis loops of three YBa2Cu3O7-x ceramics characterized by different grain sizes (2, 7 and 10 mum). The measurements are based on 16 bits analog to digital conversion, the applied magnetic field being alternating at a frequency of 20 Hz and with a maximum amplitude of 300 Oe. We observed that the irreversible phenomena (area of the loops) largely increase when reducing the grain size. Considering that the measured magnetization can be separated into two terms characteristics of the inter- and intragranular regions, we suggest that at 4.2 K and for H
Nanomedicine: Nanotechnology, Biology and Medicine, 2010
A one-step process for the production of nanoparticles presenting advanced magnetic properties ca... more A one-step process for the production of nanoparticles presenting advanced magnetic properties can be achieved using vapor condensation. In this article, we report on the fabrication of Fe particles covered by a uniform MgO epitaxial shell. MgO has a lower surface energy than Fe, which results in a core-shell crystal formation. The particles satisfy a few of technical requirements for the practical use in real clinics, such as a high biocompatibility in living cells in-vitro, an injection through blood vessels without any clothing problems in murine model, a high absorption rate for magnetic hyperthermia at small particle concentration, and the potential to be used as contrast agent in the field of diagnostic magnetic imaging. They are also able to be used in drug delivery and magnetic-activated cell sorting.
The effects of exchange bias on core/shell structured nanoparticles are analyzed. Nanoparticles a... more The effects of exchange bias on core/shell structured nanoparticles are analyzed. Nanoparticles are integrated with high moment Fe-Co crystallites covered epitaxially with MgO shells. It is observed that the coercive field H C (FeCo) > H C (Co) > H C (Fe); however, the exchange bias field H E of the Co sample is higher than that of the FeCo one, while H E = 0 for the Fe sample. It is suggested that the exchange bias is induced by the formation of a (Co, Mg)O solid solution. In fact, we show that it is possible to modify the exchange bias properties by manipulating the level of Mg dusting at the interface, as recently reported for thin films.
We report on the structural and magnetic properties of Co-doped ZnO particles prepared by vaporiz... more We report on the structural and magnetic properties of Co-doped ZnO particles prepared by vaporization-condensation in the solar furnace in Odeillo. X-ray diffraction data show no traces of Co segregation or any other phase different from ZnO. High-resolution electron microscopy (HREM) and transmision electron microscopy (TEM) techniques have also been used to characterize particles. Irrespective of their composition, the shape and size of the obtained particles, as well as their magnetic properties, clearly depend on the preparation conditions. The samples prepared in vacuum exhibit hysteretic behavior with low coercivity (about 100 Oe) at T ¼ 5 K and saturation magnetization well below that expecte for Co 2+ in a tetrahedral crystal field. On the other hand, samples prepared at high pressure (70-100 Torr inside the balloon) are paramagnetic. r
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