Valmir Chitta - Academia.edu (original) (raw)

Papers by Valmir Chitta

Research paper thumbnail of Spin-flip scattering contribution to resonant-tunneling current in semimagnetic semiconductor heterostructures

Brazilian Journal of Physics, 1999

We calculate the characteristic current-voltage curve of a tunneling device based on semimagnetic... more We calculate the characteristic current-voltage curve of a tunneling device based on semimagnetic semiconductor materials. The device is a heterostructure with layers of Cd1,xMnxTe in which the magnetic ions Mn 2+ interact strongly with the conducting electrons via the s-d exchange interaction. Thermal uctuations of Mn 2+ magnetic moments cause spin-dependent electron scattering that modi es the characteristic current-voltage curve. Our calculation shows how this electron-ion scattering is expected to a ect the spin dynamics in transport measurements.

Research paper thumbnail of Resonant tunnelling at far infra-red frequencies

The influence of far infra-red (FIR) radiation on the tunnel current of GaAs/GaAlAs double-barrie... more The influence of far infra-red (FIR) radiation on the tunnel current of GaAs/GaAlAs double-barrier resonant tunnelling structures is investigated both experimentally and theoretically. For a tunnelling process characterized by a transmission coefficient with a full width at half maximum Gamma c which is smaller than the photon energy h(cross) omega , a theoretical FIR response which depends on the radiation

Research paper thumbnail of Magnetic resonant x-ray diffraction study of europium telluride

Here we use magnetic resonant x-ray diffraction to study the magnetic order in a 1.5 m EuTe film ... more Here we use magnetic resonant x-ray diffraction to study the magnetic order in a 1.5 m EuTe film grown on ͑111͒ BaF 2 by molecular-beam epitaxy. At Eu L II and L III absorption edges, a resonant enhancement of more than two orders was observed for the → Ј diffracted intensity at half-order reciprocal-lattice points, consistent with the magnetic character of the scattering. We studied the evolution of the ͑

Research paper thumbnail of Spin-flip scattering contribution to resonant-tunneling current in semimagnetic semiconductor heterostructures

Brazilian Journal of Physics, 1999

We calculate the characteristic current-voltage curve of a tunneling device based on semimagnetic... more We calculate the characteristic current-voltage curve of a tunneling device based on semimagnetic semiconductor materials. The device is a heterostructure with layers of Cd1,xMnxTe in which the magnetic ions Mn 2+ interact strongly with the conducting electrons via the s-d exchange interaction. Thermal uctuations of Mn 2+ magnetic moments cause spin-dependent electron scattering that modi es the characteristic current-voltage curve. Our calculation shows how this electron-ion scattering is expected to a ect the spin dynamics in transport measurements.

Research paper thumbnail of Evidence of Cr 3+ and Cr 4+ Coexistence in Chromium-Doped SnO 2 Nanoparticles: A Structural and Magnetic Study

The present study reports the successful synthesis of Sn 1−x Cr x O 2 nanoparticles with doping c... more The present study reports the successful synthesis of Sn 1−x Cr x O 2 nanoparticles with doping content (x) ranging from 0 to 0.20. Samples were synthesized by a polymer precursor method using SnCl 2 ·H 2 O and Cr(NO 3) 3 ·5H 2 O as metal ion suppliers. In all samples X-ray diffraction data show one single-phase formation (rutile-type), with crystalline size (crystal strain) decreasing (increasing) monotonically while increasing the x content, which are assigned to substitutional solution of Cr and Sn ions in the crystalline structure. In addition to a weak magnetic ordering observed in a few samples (x = 0.01, 0.02, 0.03, and 0.05), paramagnetism is the main magnetic contribution in all synthesized samples, which is due to the presence of two chromium ions (Cr 3+ and Cr 4+). X-ray photoelectron spectroscopy measurements confirm the coexistence of Cr 3+ and Cr 4+ ions in excellent agreement with the monotonic decrease of the [Cr 3+ ]/[Cr 4+ ] versus doping content (x), assessed from the fitting of the susceptibility versus temperature data using the Curie−Weiss law.

Research paper thumbnail of Distant exchange interactions in Cd 1-x Mn x S from magnetization steps method.

Three new distant neighbor (DN) antiferromagnetic exchange constants have been measured in wurtzi... more Three new distant neighbor (DN) antiferromagnetic exchange constants have been measured in wurtzite Cd 1–x Mn x S (x = 0.0087 and x = 0.0278) using the magnetization steps method in the millikelvin regime. The second (J (2)), third (J (3)), and fourth (J (4)) largest exchange constants (after the two first neighbor exchange interactions J 1 and J ' 1) have been measured. The results are J (2) = 300 +/- 10 mK, J (3) = 173 +/- 10 mK, and J (4) = 55 +/- 10 mK. No significant change of the exchange constant values has been observed as a function of the Mn concentration. The mapping of the DN exchange interactions has been investigated within a sphere of radius 2 times the nearest neighbors distance. The only way to identify the J constants was using the number coordination Zn of the different DN classes: J (2) is associated to DN classes with Z n= 6, J (3) with Z n = 12, and J (4) is ascribed to a group of three DN classes with J values around J (4). The magnitude of the J constants is not in agreement with theoretical predictions of the monotonic decrease of the exchange constant with increasing distance. The present results are similar to those obtained for Cd 1–x Mn x Se but are different from the ones obtained for Zn 1–x Mn x O. The present work confirms the long-range character of the DN exchange interactions in the wurtzite type Mn-based II-VI diluted magnetic semiconductors. Published by AIP Publishing. [http://dx.

Research paper thumbnail of Magnetite Nanoparticles Encapsulated with PCL and Poloxamer by Nano Spray Drying Technique

Polymeric particles loaded with biocompatible magnetic nanoparticles are promising platforms to p... more Polymeric particles loaded with biocompatible magnetic nanoparticles are promising platforms to perform therapy and diagnostic of diseases, acting as the so called theranostic agents. In this work, magnetite nanoparticles were synthesized by coprecipitation method and encapsulated with Polycaprolactone (PCL) and Poloxamer 188 polymers through the innovative Nano Spray Drying technique. X-Ray Diffraction and Infrared Spectroscopy confirmed that the formed particles are composed by PCL, Poloxamer and magnetite, with amount of magnetite determined as 3% by Thermogravimetric Analysis. Particles present a superparamagnetic-like behaviour at 300 K, having negligible coercivity. Dynamic Light Scattering and Scanning Electron Microscopy indicated that the hydrodynamic diameters are of 49 nm for magnetite nanoparticles and of approximately 600 nm for encapsulated particles, with aspect analogous to a " nanocookie " of a PCL/Poloxamer blend with magnetite nanoparticles embedded in it. Cytotoxicity assays presented IC 50 superior to 2.5 mg/mL for healthy cells and a higher viability of cancer cells in comparison to control, suggesting that the particles may have affinity for tumours, which can be useful for the intended biological applications.

Research paper thumbnail of Structural and magnetic characterization of colloidal CdMnS

Recibido el 25 de junio de 2010; aceptado el 6 de octubre de 2010 This paper reports on the synth... more Recibido el 25 de junio de 2010; aceptado el 6 de octubre de 2010 This paper reports on the synthesis (chemical co-precipitation reaction) and characterization (X-ray diffraction, magnetization, and electron paramagnetic resonance) of nanosized Cd 1−x Mn x S particles with manganese concentration up to x = 0.73. Though the literature reports that nanosized (bulk) CdS can incorporate as much as 30% (50%) of manganese ion within its crystal structure we found manganese segregation at the nanoparticle surface at doping levels as low as 14%. We found that both XRD and magnetization data support the presence of the Mn 3 O 4 phase (observed spin-glass transition around 43 K) at the high manganese doping levels whereas the EPR data strongly suggest preferential incorporation of manganese at the nanoparticle's surface, even at low manganese doping levels. Analyses of the experimental data strongly suggest the preparation of well-defined core/shell (Cd 1−x Mn x S/Mn 3 O 4) structures at ...

Research paper thumbnail of EPR study of local symmetry sites of Ce3+ in Pb1−xCexA (A=S, Se, and Te)

Physical Review B, 2009

... SA Nemov, TA Gavrikova, VA Zykov, PA Osipov, and VI Proshin, Semiconductors 32, 689 (1998) [S... more ... SA Nemov, TA Gavrikova, VA Zykov, PA Osipov, and VI Proshin, Semiconductors 32, 689 (1998) [SPIN]. O. Nugraha, W. Tamura, O. Itoh, T. Amemiya, K. Suto, and J. Nishizawa, J. Cryst. ... GT Alekseeva, MV Vedernikov, EA Gurieva, PP Konstantinov, LV Prokof'eva, and Yu. ...

Research paper thumbnail of A luminescent supramolecular assembly composed of a single-walled carbon nanotube and a molecular magnet precursor

Journal of Nanoparticle Research, 2013

Magnetism of supramolecular assemblies of single-walled carbon nanotubes (SWCNTS) with a magnetic... more Magnetism of supramolecular assemblies of single-walled carbon nanotubes (SWCNTS) with a magnetic dinuclear molecule is investigated. Raman, optical absorption and confocal fluorescence images are used to probe the interaction of the dinuclear compound and the SWCNT. The supramolecular assembly shows antiferromagnetism, on the contrary to the case when strong electronic doping of the SWCNT occurs, yielding a spin-glass system, and contrary to the case of the dinuclear molecular crystal, which is ferromagnetic. The SWCNT imposes the antiferromagnetic order to the dinuclear molecule, corroborating recent findings that antiferromagnetism is present in pure SWCNTs. Two theoretical models are used to fit the data, both yielding good fitting results. The nanoparticle size range is around 2-10 nm.

Research paper thumbnail of Spin-flip scattering contribution to resonant-tunneling current in semimagnetic semiconductor heterostructures

Brazilian Journal of Physics, 1999

We calculate the characteristic current-voltage curve of a tunneling device based on semimagnetic... more We calculate the characteristic current-voltage curve of a tunneling device based on semimagnetic semiconductor materials. The device is a heterostructure with layers of Cd1,xMnxTe in which the magnetic ions Mn 2+ interact strongly with the conducting electrons via the s-d exchange interaction. Thermal uctuations of Mn 2+ magnetic moments cause spin-dependent electron scattering that modi es the characteristic current-voltage curve. Our calculation shows how this electron-ion scattering is expected to a ect the spin dynamics in transport measurements.

Research paper thumbnail of Resonant tunnelling at far infra-red frequencies

The influence of far infra-red (FIR) radiation on the tunnel current of GaAs/GaAlAs double-barrie... more The influence of far infra-red (FIR) radiation on the tunnel current of GaAs/GaAlAs double-barrier resonant tunnelling structures is investigated both experimentally and theoretically. For a tunnelling process characterized by a transmission coefficient with a full width at half maximum Gamma c which is smaller than the photon energy h(cross) omega , a theoretical FIR response which depends on the radiation

Research paper thumbnail of Magnetic resonant x-ray diffraction study of europium telluride

Here we use magnetic resonant x-ray diffraction to study the magnetic order in a 1.5 m EuTe film ... more Here we use magnetic resonant x-ray diffraction to study the magnetic order in a 1.5 m EuTe film grown on ͑111͒ BaF 2 by molecular-beam epitaxy. At Eu L II and L III absorption edges, a resonant enhancement of more than two orders was observed for the → Ј diffracted intensity at half-order reciprocal-lattice points, consistent with the magnetic character of the scattering. We studied the evolution of the ͑

Research paper thumbnail of Spin-flip scattering contribution to resonant-tunneling current in semimagnetic semiconductor heterostructures

Brazilian Journal of Physics, 1999

We calculate the characteristic current-voltage curve of a tunneling device based on semimagnetic... more We calculate the characteristic current-voltage curve of a tunneling device based on semimagnetic semiconductor materials. The device is a heterostructure with layers of Cd1,xMnxTe in which the magnetic ions Mn 2+ interact strongly with the conducting electrons via the s-d exchange interaction. Thermal uctuations of Mn 2+ magnetic moments cause spin-dependent electron scattering that modi es the characteristic current-voltage curve. Our calculation shows how this electron-ion scattering is expected to a ect the spin dynamics in transport measurements.

Research paper thumbnail of Evidence of Cr 3+ and Cr 4+ Coexistence in Chromium-Doped SnO 2 Nanoparticles: A Structural and Magnetic Study

The present study reports the successful synthesis of Sn 1−x Cr x O 2 nanoparticles with doping c... more The present study reports the successful synthesis of Sn 1−x Cr x O 2 nanoparticles with doping content (x) ranging from 0 to 0.20. Samples were synthesized by a polymer precursor method using SnCl 2 ·H 2 O and Cr(NO 3) 3 ·5H 2 O as metal ion suppliers. In all samples X-ray diffraction data show one single-phase formation (rutile-type), with crystalline size (crystal strain) decreasing (increasing) monotonically while increasing the x content, which are assigned to substitutional solution of Cr and Sn ions in the crystalline structure. In addition to a weak magnetic ordering observed in a few samples (x = 0.01, 0.02, 0.03, and 0.05), paramagnetism is the main magnetic contribution in all synthesized samples, which is due to the presence of two chromium ions (Cr 3+ and Cr 4+). X-ray photoelectron spectroscopy measurements confirm the coexistence of Cr 3+ and Cr 4+ ions in excellent agreement with the monotonic decrease of the [Cr 3+ ]/[Cr 4+ ] versus doping content (x), assessed from the fitting of the susceptibility versus temperature data using the Curie−Weiss law.

Research paper thumbnail of Distant exchange interactions in Cd 1-x Mn x S from magnetization steps method.

Three new distant neighbor (DN) antiferromagnetic exchange constants have been measured in wurtzi... more Three new distant neighbor (DN) antiferromagnetic exchange constants have been measured in wurtzite Cd 1–x Mn x S (x = 0.0087 and x = 0.0278) using the magnetization steps method in the millikelvin regime. The second (J (2)), third (J (3)), and fourth (J (4)) largest exchange constants (after the two first neighbor exchange interactions J 1 and J ' 1) have been measured. The results are J (2) = 300 +/- 10 mK, J (3) = 173 +/- 10 mK, and J (4) = 55 +/- 10 mK. No significant change of the exchange constant values has been observed as a function of the Mn concentration. The mapping of the DN exchange interactions has been investigated within a sphere of radius 2 times the nearest neighbors distance. The only way to identify the J constants was using the number coordination Zn of the different DN classes: J (2) is associated to DN classes with Z n= 6, J (3) with Z n = 12, and J (4) is ascribed to a group of three DN classes with J values around J (4). The magnitude of the J constants is not in agreement with theoretical predictions of the monotonic decrease of the exchange constant with increasing distance. The present results are similar to those obtained for Cd 1–x Mn x Se but are different from the ones obtained for Zn 1–x Mn x O. The present work confirms the long-range character of the DN exchange interactions in the wurtzite type Mn-based II-VI diluted magnetic semiconductors. Published by AIP Publishing. [http://dx.

Research paper thumbnail of Magnetite Nanoparticles Encapsulated with PCL and Poloxamer by Nano Spray Drying Technique

Polymeric particles loaded with biocompatible magnetic nanoparticles are promising platforms to p... more Polymeric particles loaded with biocompatible magnetic nanoparticles are promising platforms to perform therapy and diagnostic of diseases, acting as the so called theranostic agents. In this work, magnetite nanoparticles were synthesized by coprecipitation method and encapsulated with Polycaprolactone (PCL) and Poloxamer 188 polymers through the innovative Nano Spray Drying technique. X-Ray Diffraction and Infrared Spectroscopy confirmed that the formed particles are composed by PCL, Poloxamer and magnetite, with amount of magnetite determined as 3% by Thermogravimetric Analysis. Particles present a superparamagnetic-like behaviour at 300 K, having negligible coercivity. Dynamic Light Scattering and Scanning Electron Microscopy indicated that the hydrodynamic diameters are of 49 nm for magnetite nanoparticles and of approximately 600 nm for encapsulated particles, with aspect analogous to a " nanocookie " of a PCL/Poloxamer blend with magnetite nanoparticles embedded in it. Cytotoxicity assays presented IC 50 superior to 2.5 mg/mL for healthy cells and a higher viability of cancer cells in comparison to control, suggesting that the particles may have affinity for tumours, which can be useful for the intended biological applications.

Research paper thumbnail of Structural and magnetic characterization of colloidal CdMnS

Recibido el 25 de junio de 2010; aceptado el 6 de octubre de 2010 This paper reports on the synth... more Recibido el 25 de junio de 2010; aceptado el 6 de octubre de 2010 This paper reports on the synthesis (chemical co-precipitation reaction) and characterization (X-ray diffraction, magnetization, and electron paramagnetic resonance) of nanosized Cd 1−x Mn x S particles with manganese concentration up to x = 0.73. Though the literature reports that nanosized (bulk) CdS can incorporate as much as 30% (50%) of manganese ion within its crystal structure we found manganese segregation at the nanoparticle surface at doping levels as low as 14%. We found that both XRD and magnetization data support the presence of the Mn 3 O 4 phase (observed spin-glass transition around 43 K) at the high manganese doping levels whereas the EPR data strongly suggest preferential incorporation of manganese at the nanoparticle's surface, even at low manganese doping levels. Analyses of the experimental data strongly suggest the preparation of well-defined core/shell (Cd 1−x Mn x S/Mn 3 O 4) structures at ...

Research paper thumbnail of EPR study of local symmetry sites of Ce3+ in Pb1−xCexA (A=S, Se, and Te)

Physical Review B, 2009

... SA Nemov, TA Gavrikova, VA Zykov, PA Osipov, and VI Proshin, Semiconductors 32, 689 (1998) [S... more ... SA Nemov, TA Gavrikova, VA Zykov, PA Osipov, and VI Proshin, Semiconductors 32, 689 (1998) [SPIN]. O. Nugraha, W. Tamura, O. Itoh, T. Amemiya, K. Suto, and J. Nishizawa, J. Cryst. ... GT Alekseeva, MV Vedernikov, EA Gurieva, PP Konstantinov, LV Prokof'eva, and Yu. ...

Research paper thumbnail of A luminescent supramolecular assembly composed of a single-walled carbon nanotube and a molecular magnet precursor

Journal of Nanoparticle Research, 2013

Magnetism of supramolecular assemblies of single-walled carbon nanotubes (SWCNTS) with a magnetic... more Magnetism of supramolecular assemblies of single-walled carbon nanotubes (SWCNTS) with a magnetic dinuclear molecule is investigated. Raman, optical absorption and confocal fluorescence images are used to probe the interaction of the dinuclear compound and the SWCNT. The supramolecular assembly shows antiferromagnetism, on the contrary to the case when strong electronic doping of the SWCNT occurs, yielding a spin-glass system, and contrary to the case of the dinuclear molecular crystal, which is ferromagnetic. The SWCNT imposes the antiferromagnetic order to the dinuclear molecule, corroborating recent findings that antiferromagnetism is present in pure SWCNTs. Two theoretical models are used to fit the data, both yielding good fitting results. The nanoparticle size range is around 2-10 nm.