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Papers by Marian Grigoras
Journal of Optoelectronics and Advanced Materials, 2007
This paper reports some results concerning the influence of the Cu, Nb, and NbCu as additions M a... more This paper reports some results concerning the influence of the Cu, Nb, and NbCu as additions M and Nb, Ta and NbCu as buffer and capping layers N on the microstructure and magnetic properties of multilayer N/[NdFeB/(M)]-n/N thin films. The materials used as additions were 'mixed' with NdFeB alloy either by stratification as multilayer system or by disposing of the additions on the target's surface and simultaneously by stratification as multilayer system. When the additions were introduced in NdFeB alloy by stratification, the thickness of individual NdFeB layer was held fixed at 180 nm, while the M spacer layer thickness was varied from 3 to 10 nm. The Si/NbCu/[NdFeBNbCu(180nm)/NbCu (3nm)]-3/NbCu and Si/Ta/NdFeBNbCu(540nm)/Ta systems exhibit an enhanced coercivity of about 1510 kA/m and 1580 kA/m, respectively, in comparison with the NdFeB single layer having the same thickness.
2018 IEEE International Magnetic Conference (INTERMAG), 2018
Fe-oxide nanoparticles are of considerable interest nowadays because of their unique characterist... more Fe-oxide nanoparticles are of considerable interest nowadays because of their unique characteristics, such as superparamagnetism, high saturation fields, and extra anisotropy contributions, which arise from the effects of finite size and large surface area. Usually they are obtained by chemical methods, but more recently some groups reported on their successful preparation by wet high-energy ball-milling. It is also well known that as the size of the nanoparticles decreases, surface effects would become more significant due to the increasing surface relative to their volume. We report here our recent results on the effect of ligands on the induced surface anisotropies and magnetic properties of Fe/Fe2O3 and Fe/Fe3O4 core-shell nanoparticles functionalized with 3-aminopropyltriethoxysilane (APTS) for biomedical applications (image contrast agents in magnetic resonance imaging (MRI) and magnetic carriers for drug delivery). Core-shell nanoparticles have been prepared by high-energy ba...
Journal of Biomedical Materials Research Part B: Applied Biomaterials, 2020
Magnetic nanoparticles (MNP) are intensely scrutinized for biomedical applications due to their e... more Magnetic nanoparticles (MNP) are intensely scrutinized for biomedical applications due to their excellent biocompatibility and adjustable magnetic field (MF) responsiveness. Three-dimensional spheroid culture of ADSC improves stem cell proliferation and differentiation, increasing their potential for clinical applications. In this study we aimed to detect if MF levitated culture of ADSC loaded with proprietary MNP maintain the properties of ADSC and improve their performances. Levitated ADSC-MNP formed aggregates with increased volume and reduced number compared to nonlevitated ones. ADSC-MNP from levitated spheroid displayed higher viability, proliferation and mobility compared to nonlevitated and 2D culture. Levitated and nonlevitated ADSC-MNP spheroids underwent three lineage differentiation, demonstrating preserved ADSC stemness. Quantitative osteogenesis showed similar values in MNP-loaded levitated and nonlevitated spheroids. Significant increases in adipogenic conversion was observed for all 3D formulation. Chondrogenic conversion in levitated and nonlevitated spheroids produced comparable ratio glucosaminoglycan (GAG)/DNA. Increased chondrogenesis could be observed for ADSC-MNP in both levitated and nonlevitated condition. Taken together, ADSC-MNP levitated spheroids retain stemness and display superior cell viability and migratory capabilities. Furthermore, the method consistently increases spheroid maneuverability, potentially facilitating large scale manufacturing and automation. Levitated spheroid culture of ADSC-MNP can be further tested for various application in regenerative medicine and organ modeling. K E Y W O R D S differentiation, human adipose derived mesenchymal cells 1 , levitation 5 , magnetic field 4 , magnetic nanoparticle 3 , proliferation 6 , spheroid 2
Fe-Nb-Cr-B rapidly quenched melt-spun ribbons (MSRs) show glassy structures in the as-quenched st... more Fe-Nb-Cr-B rapidly quenched melt-spun ribbons (MSRs) show glassy structures in the as-quenched state and Curie temperatures (T<sub>C</sub>) of 290-330 K depending on the Cr content [1]. The medical applications are usually requiring submicron powders, which can be easily targeted to the sick body organs by means of the external magnetic field [2].<br/>The present work focusses on the preparation of Fe<sub>67.2</sub>Nb<sub>0.3</sub>Cr<sub>12.5</sub>B<sub>20</sub> submicron powders by high-energy ball milling from MSRs precursors, using different milling agents: oleic acid (OA), sodium oleate (NAOL) and stearic acid (SA). The main challenge is to preserve the glassy structure from MSRs through the milling process, because only the glassy structure guarantees the low T<sub>C</sub>. MSRs were annealed at 400<sup>0</sup>C for 1 h to facilitate the milling. The ratio stainless steel balls:powder was ...
Journal of Optoelectronics and Advanced Materials, 2004
The granular metal - insulator films are composite materials consisting of metallic nanograins em... more The granular metal - insulator films are composite materials consisting of metallic nanograins embedded in an insulating matrix. The ferromagnetic metal - insulator thin films with very high electrical resistivity are an excelent candidate for high permeability cores in thin film inductors which operate at high frequencies. A source of losses are eddy currents in the magnetic layers. A possible way to reduce these losses is to fabricate the magnetic/dielectric multilayers. This paper reports some results concerning the influence of the FeCoB and SiO 2 layers composition and annealing temperature on the resistive, magnetic and microstructural properties of [FeCoB/(SiO 2 )]×n multilayer thin films. As compared to the well known FeCoB alloys, widely used in the electromagnetic micro-devices which can operate at high frequencies, the [FeCoB/(SiO 2 )]×n (n ≥ 30) thin films exhibit high resistivity over 8 x 10 - 3 mΩ.m while superior soft magnetic properties specific for FeCoB thin films ...
Journal of Magnetism and Magnetic Materials
Abstract Results on the magnetic and structural properties of Ce10+xFe78−xB6 (x = 0, 2, 4, 6) rib... more Abstract Results on the magnetic and structural properties of Ce10+xFe78−xB6 (x = 0, 2, 4, 6) ribbons with good potential for the manufacturing of permanent magnets are presented. Ce–Fe–B based ribbons have been prepared by melt-spinning technique. The as-spun ribbons reveal either amorphous or nanocrystalline structure depending on the velocity of the wheel. In order to obtain ribbons with optimum nanocrystalline structure and magnetic properties, the as-spun ribbons with amorphous structure were annealed for different periods of time at temperatures ranging between 650 and 750 °C. It was found that the nanocrystalline ribbons obtained by optimal annealing of amorphous precursors presented better magnetic properties compared to the nanocrystalline ribbons obtained directly by rapid quenching from the melt. The best magnetic properties have been obtained for the amorphous Ce14Fe80B6 ribbons after annealing at 700 °C for 20 min. when the coercivity was about 6.08 kOe, remanence about 7.7 kG and energy product of 8.21 MGOe.
Journal of Magnetism and Magnetic Materials
Abstract Hybrid magnets of Nd 12 Fe 82 B 6 (2:14:1-phase)/Nd 9.4 Fe 59 Co 25.3 Ti 6.3 (3:29-phase... more Abstract Hybrid magnets of Nd 12 Fe 82 B 6 (2:14:1-phase)/Nd 9.4 Fe 59 Co 25.3 Ti 6.3 (3:29-phase) and Nd 12 Fe 82 B 6 /Sm 11.1 Co 65.8 Fe 8.9 Cu 10.7 Zr 3.5 (2:17-phase) with different weight ratio have been prepared by spark plasma sintering pressing technique from ball-milled powders obtained from melt-spun ribbons. Influence of the ratio between the two phases on the magnetic properties and thermal stability of the hybrid magnets was studied. It has been found that the ratio has a remarkable influence, especially on the thermal stability of the bulk magnets. However, the magnetic properties of such type of hybrid magnets result not only from the type and ratio of components but also from the interaction between them. It was found that in NdFeB/3:29 hybrid magnets with 15% content of 3:29-phase, the temperature coefficients of remanence (α) and of coercivity (β) are improved from −0.095 to −0.082 (%/°C) and from −0.57 to −0.47 (%/°C), respectively, as compared to the Nd 2 Fe 14 B single-phase counterpart. While for the NdFeB/2:17 hybrid magnets the content of 2:17-phase is not significantly influencing the temperature coefficient of induction (α), the temperature coefficient of °C (β) increases up to −0.41 (%/°C) for 10% content of 2:17-phase. The increase in the reversible temperature coefficients of hybrid magnets indicate a remarkable improvement of their thermal stability.
Materials Science and Engineering: C
Materials Science and Engineering: C
Materials Science and Engineering: C
Journal of Alloys and Compounds
Journal of Alloys and Compounds
Journal of Alloys and Compounds
Journal of Applied Physics
ABSTRACT High resolution imaging and electron diffraction confirm that in the as-quenched state t... more ABSTRACT High resolution imaging and electron diffraction confirm that in the as-quenched state the structure of Fe79.7−xCrx Nb 0.3B20 (x = 11-13 at. %) melt-spun ribbons is completely amorphous, independent of the Cr content. Energy-dispersive X-ray spectroscopy mapping emphasizes clearly the presence of Fe and Cr clusters varying from approximately 1 to 2-3 nm in size with the increase of Cr content from 11 to 13 at. %. The Fe and Cr atoms segregate the atomic scale to form nanometer sized clusters, influencing strongly the macroscopic magnetic behavior. The Curie temperature of the system, TC system, confirmed by the magnetic susceptibility versus temperature measurements, gives the strength of the magnetic interactions between clusters. The inter-cluster interactions are much stronger for lower contents of Cr, the microstructure is less uniform, and TC system increases from 290 K for 13 at. % Cr to 330 K for 11.5 at. % Cr. The whole system transforms to a ferromagnetic state through interactions between the clusters. Zero-field cooling and field cooling curves confirm the cluster behavior with a blocking temperature, Tb, of about 250 K. Above Tb, the ribbons behave as a superferromagnetic system, whilst below the blocking temperature a classical ferromagnetic behavior is observed.
Journal of The Electrochemical Society, 2016
Journal of The Electrochemical Society, 2015
Applied Surface Science, 2015
ABSTRACT Nd–Fe–B films with good perpendicular magnetic anisotropy were obtained by using a Mo fi... more ABSTRACT Nd–Fe–B films with good perpendicular magnetic anisotropy were obtained by using a Mo film with the thickness of about 40 nm as buffer layer. Large out-of-plane coercivity (Hc⊥ = 1182 kA/m) and remanent ratio (remanent magnetization/saturation magnetization)⊥ = 0.99 are obtained by the stratification of the Nd–Fe–B magnetic film in three NdFeB(180 nm)/Mo(5 nm) bilayer sequences using Mo as interlayer. Using a Ni film, with a thickness of about 8 nm, alongside with a Mo film of 40 nm as buffer layer improves the adhesion to substrate and keeps the anisotropic hard magnetic performances of the Nd–Fe–B films unchanged. By increasing the total thickness of the Nd–Fe–B magnetic layer from 540 nm to 1620 nm the coercivity measured at high fields increases from about 1350 kA/m for Nd–Fe–B film with 3 NdFeB(180 nm)/Mo(5 nm) sequences to about 1640 kA/m for Nd–Fe–B film with 9 NdFeB(180 nm)/Mo(5 nm) sequences which can be ascribed to a stronger pinning effect as a result of the increased number of NdFeB/Mo interfaces.
Journal of Optoelectronics and Advanced Materials, 2007
This paper reports some results concerning the influence of the Cu, Nb, and NbCu as additions M a... more This paper reports some results concerning the influence of the Cu, Nb, and NbCu as additions M and Nb, Ta and NbCu as buffer and capping layers N on the microstructure and magnetic properties of multilayer N/[NdFeB/(M)]-n/N thin films. The materials used as additions were 'mixed' with NdFeB alloy either by stratification as multilayer system or by disposing of the additions on the target's surface and simultaneously by stratification as multilayer system. When the additions were introduced in NdFeB alloy by stratification, the thickness of individual NdFeB layer was held fixed at 180 nm, while the M spacer layer thickness was varied from 3 to 10 nm. The Si/NbCu/[NdFeBNbCu(180nm)/NbCu (3nm)]-3/NbCu and Si/Ta/NdFeBNbCu(540nm)/Ta systems exhibit an enhanced coercivity of about 1510 kA/m and 1580 kA/m, respectively, in comparison with the NdFeB single layer having the same thickness.
2018 IEEE International Magnetic Conference (INTERMAG), 2018
Fe-oxide nanoparticles are of considerable interest nowadays because of their unique characterist... more Fe-oxide nanoparticles are of considerable interest nowadays because of their unique characteristics, such as superparamagnetism, high saturation fields, and extra anisotropy contributions, which arise from the effects of finite size and large surface area. Usually they are obtained by chemical methods, but more recently some groups reported on their successful preparation by wet high-energy ball-milling. It is also well known that as the size of the nanoparticles decreases, surface effects would become more significant due to the increasing surface relative to their volume. We report here our recent results on the effect of ligands on the induced surface anisotropies and magnetic properties of Fe/Fe2O3 and Fe/Fe3O4 core-shell nanoparticles functionalized with 3-aminopropyltriethoxysilane (APTS) for biomedical applications (image contrast agents in magnetic resonance imaging (MRI) and magnetic carriers for drug delivery). Core-shell nanoparticles have been prepared by high-energy ba...
Journal of Biomedical Materials Research Part B: Applied Biomaterials, 2020
Magnetic nanoparticles (MNP) are intensely scrutinized for biomedical applications due to their e... more Magnetic nanoparticles (MNP) are intensely scrutinized for biomedical applications due to their excellent biocompatibility and adjustable magnetic field (MF) responsiveness. Three-dimensional spheroid culture of ADSC improves stem cell proliferation and differentiation, increasing their potential for clinical applications. In this study we aimed to detect if MF levitated culture of ADSC loaded with proprietary MNP maintain the properties of ADSC and improve their performances. Levitated ADSC-MNP formed aggregates with increased volume and reduced number compared to nonlevitated ones. ADSC-MNP from levitated spheroid displayed higher viability, proliferation and mobility compared to nonlevitated and 2D culture. Levitated and nonlevitated ADSC-MNP spheroids underwent three lineage differentiation, demonstrating preserved ADSC stemness. Quantitative osteogenesis showed similar values in MNP-loaded levitated and nonlevitated spheroids. Significant increases in adipogenic conversion was observed for all 3D formulation. Chondrogenic conversion in levitated and nonlevitated spheroids produced comparable ratio glucosaminoglycan (GAG)/DNA. Increased chondrogenesis could be observed for ADSC-MNP in both levitated and nonlevitated condition. Taken together, ADSC-MNP levitated spheroids retain stemness and display superior cell viability and migratory capabilities. Furthermore, the method consistently increases spheroid maneuverability, potentially facilitating large scale manufacturing and automation. Levitated spheroid culture of ADSC-MNP can be further tested for various application in regenerative medicine and organ modeling. K E Y W O R D S differentiation, human adipose derived mesenchymal cells 1 , levitation 5 , magnetic field 4 , magnetic nanoparticle 3 , proliferation 6 , spheroid 2
Fe-Nb-Cr-B rapidly quenched melt-spun ribbons (MSRs) show glassy structures in the as-quenched st... more Fe-Nb-Cr-B rapidly quenched melt-spun ribbons (MSRs) show glassy structures in the as-quenched state and Curie temperatures (T<sub>C</sub>) of 290-330 K depending on the Cr content [1]. The medical applications are usually requiring submicron powders, which can be easily targeted to the sick body organs by means of the external magnetic field [2].<br/>The present work focusses on the preparation of Fe<sub>67.2</sub>Nb<sub>0.3</sub>Cr<sub>12.5</sub>B<sub>20</sub> submicron powders by high-energy ball milling from MSRs precursors, using different milling agents: oleic acid (OA), sodium oleate (NAOL) and stearic acid (SA). The main challenge is to preserve the glassy structure from MSRs through the milling process, because only the glassy structure guarantees the low T<sub>C</sub>. MSRs were annealed at 400<sup>0</sup>C for 1 h to facilitate the milling. The ratio stainless steel balls:powder was ...
Journal of Optoelectronics and Advanced Materials, 2004
The granular metal - insulator films are composite materials consisting of metallic nanograins em... more The granular metal - insulator films are composite materials consisting of metallic nanograins embedded in an insulating matrix. The ferromagnetic metal - insulator thin films with very high electrical resistivity are an excelent candidate for high permeability cores in thin film inductors which operate at high frequencies. A source of losses are eddy currents in the magnetic layers. A possible way to reduce these losses is to fabricate the magnetic/dielectric multilayers. This paper reports some results concerning the influence of the FeCoB and SiO 2 layers composition and annealing temperature on the resistive, magnetic and microstructural properties of [FeCoB/(SiO 2 )]×n multilayer thin films. As compared to the well known FeCoB alloys, widely used in the electromagnetic micro-devices which can operate at high frequencies, the [FeCoB/(SiO 2 )]×n (n ≥ 30) thin films exhibit high resistivity over 8 x 10 - 3 mΩ.m while superior soft magnetic properties specific for FeCoB thin films ...
Journal of Magnetism and Magnetic Materials
Abstract Results on the magnetic and structural properties of Ce10+xFe78−xB6 (x = 0, 2, 4, 6) rib... more Abstract Results on the magnetic and structural properties of Ce10+xFe78−xB6 (x = 0, 2, 4, 6) ribbons with good potential for the manufacturing of permanent magnets are presented. Ce–Fe–B based ribbons have been prepared by melt-spinning technique. The as-spun ribbons reveal either amorphous or nanocrystalline structure depending on the velocity of the wheel. In order to obtain ribbons with optimum nanocrystalline structure and magnetic properties, the as-spun ribbons with amorphous structure were annealed for different periods of time at temperatures ranging between 650 and 750 °C. It was found that the nanocrystalline ribbons obtained by optimal annealing of amorphous precursors presented better magnetic properties compared to the nanocrystalline ribbons obtained directly by rapid quenching from the melt. The best magnetic properties have been obtained for the amorphous Ce14Fe80B6 ribbons after annealing at 700 °C for 20 min. when the coercivity was about 6.08 kOe, remanence about 7.7 kG and energy product of 8.21 MGOe.
Journal of Magnetism and Magnetic Materials
Abstract Hybrid magnets of Nd 12 Fe 82 B 6 (2:14:1-phase)/Nd 9.4 Fe 59 Co 25.3 Ti 6.3 (3:29-phase... more Abstract Hybrid magnets of Nd 12 Fe 82 B 6 (2:14:1-phase)/Nd 9.4 Fe 59 Co 25.3 Ti 6.3 (3:29-phase) and Nd 12 Fe 82 B 6 /Sm 11.1 Co 65.8 Fe 8.9 Cu 10.7 Zr 3.5 (2:17-phase) with different weight ratio have been prepared by spark plasma sintering pressing technique from ball-milled powders obtained from melt-spun ribbons. Influence of the ratio between the two phases on the magnetic properties and thermal stability of the hybrid magnets was studied. It has been found that the ratio has a remarkable influence, especially on the thermal stability of the bulk magnets. However, the magnetic properties of such type of hybrid magnets result not only from the type and ratio of components but also from the interaction between them. It was found that in NdFeB/3:29 hybrid magnets with 15% content of 3:29-phase, the temperature coefficients of remanence (α) and of coercivity (β) are improved from −0.095 to −0.082 (%/°C) and from −0.57 to −0.47 (%/°C), respectively, as compared to the Nd 2 Fe 14 B single-phase counterpart. While for the NdFeB/2:17 hybrid magnets the content of 2:17-phase is not significantly influencing the temperature coefficient of induction (α), the temperature coefficient of °C (β) increases up to −0.41 (%/°C) for 10% content of 2:17-phase. The increase in the reversible temperature coefficients of hybrid magnets indicate a remarkable improvement of their thermal stability.
Materials Science and Engineering: C
Materials Science and Engineering: C
Materials Science and Engineering: C
Journal of Alloys and Compounds
Journal of Alloys and Compounds
Journal of Alloys and Compounds
Journal of Applied Physics
ABSTRACT High resolution imaging and electron diffraction confirm that in the as-quenched state t... more ABSTRACT High resolution imaging and electron diffraction confirm that in the as-quenched state the structure of Fe79.7−xCrx Nb 0.3B20 (x = 11-13 at. %) melt-spun ribbons is completely amorphous, independent of the Cr content. Energy-dispersive X-ray spectroscopy mapping emphasizes clearly the presence of Fe and Cr clusters varying from approximately 1 to 2-3 nm in size with the increase of Cr content from 11 to 13 at. %. The Fe and Cr atoms segregate the atomic scale to form nanometer sized clusters, influencing strongly the macroscopic magnetic behavior. The Curie temperature of the system, TC system, confirmed by the magnetic susceptibility versus temperature measurements, gives the strength of the magnetic interactions between clusters. The inter-cluster interactions are much stronger for lower contents of Cr, the microstructure is less uniform, and TC system increases from 290 K for 13 at. % Cr to 330 K for 11.5 at. % Cr. The whole system transforms to a ferromagnetic state through interactions between the clusters. Zero-field cooling and field cooling curves confirm the cluster behavior with a blocking temperature, Tb, of about 250 K. Above Tb, the ribbons behave as a superferromagnetic system, whilst below the blocking temperature a classical ferromagnetic behavior is observed.
Journal of The Electrochemical Society, 2016
Journal of The Electrochemical Society, 2015
Applied Surface Science, 2015
ABSTRACT Nd–Fe–B films with good perpendicular magnetic anisotropy were obtained by using a Mo fi... more ABSTRACT Nd–Fe–B films with good perpendicular magnetic anisotropy were obtained by using a Mo film with the thickness of about 40 nm as buffer layer. Large out-of-plane coercivity (Hc⊥ = 1182 kA/m) and remanent ratio (remanent magnetization/saturation magnetization)⊥ = 0.99 are obtained by the stratification of the Nd–Fe–B magnetic film in three NdFeB(180 nm)/Mo(5 nm) bilayer sequences using Mo as interlayer. Using a Ni film, with a thickness of about 8 nm, alongside with a Mo film of 40 nm as buffer layer improves the adhesion to substrate and keeps the anisotropic hard magnetic performances of the Nd–Fe–B films unchanged. By increasing the total thickness of the Nd–Fe–B magnetic layer from 540 nm to 1620 nm the coercivity measured at high fields increases from about 1350 kA/m for Nd–Fe–B film with 3 NdFeB(180 nm)/Mo(5 nm) sequences to about 1640 kA/m for Nd–Fe–B film with 9 NdFeB(180 nm)/Mo(5 nm) sequences which can be ascribed to a stronger pinning effect as a result of the increased number of NdFeB/Mo interfaces.