Saibal Roy - Academia.edu (original) (raw)

Papers by Saibal Roy

Advanced Micro and Nanosystems, 2015

Physical review letters, Jan 6, 2015

Thick hard magnetic material Pulse-reverse plating technique High energy product a b s t r a c t

Ni nanoparticles ($32 nm particle diameter) have been synthesized on the walls of microporous ($1... more Ni nanoparticles ($32 nm particle diameter) have been synthesized on the walls of microporous ($1 nm pore diameter) silica spheres ($2.6 mm sphere diameter) and characterised magnetically to potentially produce a new class of core (silica micro-spheres)-shell (nanometallic)-type nanocomposite material. These magnetic nanocomposite materials display a characteristic increase in coercivity with reducing temperature. The average particle size has been used to calculate the anisotropy constant for the system, K. The discussion postulates the potential mechanisms contributing to the difference between the calculated K value and the magnetocrystalline anisotropy constant of bulk Ni. Various factors such as surface anisotropy and interparticle interactions are discussed as possible contributing factors to the anisotropy values calculated in the paper.

Micro-inductors, with an electrodeposited nickel-iron core have been fabricated on silicon substr... more Micro-inductors, with an electrodeposited nickel-iron core have been fabricated on silicon substrates, and have been characterized in the frequency range up to 100 MHz. The core consists of a nickel iron, 45:55 alloy which is deposited using pulse-reverse electroplating in the presence of a magnetic field to control anisotropy. The operation of the inductors with low loss at high-frequency critically depends on core thickness, which is used to control eddy-current loss as the frequency is increased. However, it is shown that the permeability of NiFe 45/55 has a dependency on thickness, and decreases with increasing thickness. For example, the permeability is measured to be approximately 1000 for a 1-mmthick film, decreasing to approximately 400 for a 5-mm-thick film. In order to correctly design microinductors for operation up to 100 MHz, it is important that this characteristic of the material is taken into account.

ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 2, 2007

ABSTRACT This work presents the thermal property study of single wall and multi wall carbon nanot... more ABSTRACT This work presents the thermal property study of single wall and multi wall carbon nanotubes (SWCNT and MWCNT) both in their purified and unpurified forms introduced to silicone elastomer to enhance the thermal diffusivity of this industrial polymer. An increase in thermal diffusivity was observed for incremental loading of both purified and unpurified single wall and multiwall CNT in epoxy at different percentages. An increase of thermal diffusivity as high as 130% was achieved for ∼2 wt% loading of both single wall and multi wall nanotubes. Electrical conductivity measurements showed a percolation threshold for 2% loading of multiwall CNT, below which the nanotube-epoxy composite behaved as an insulator — this is a key property for applications where electrical isolation is required. For single wall CNT-epoxy composite all the samples showed high resistance to the conduction of current. Thermal impedance measurements showed a strong dependency of contact resistance with percentage loading. Finally, the feasibility of deploying carbon nanotube-polymer composites as practical thermal interface materials for electronics thermal management is discussed.

Applied Physics Letters, 2010

Epl, 2012

We demonstrate semi-metallic transport in graphene oxide layers treated with an organic acid thro... more We demonstrate semi-metallic transport in graphene oxide layers treated with an organic acid through a nearly linear current-vs.-voltage relationship and the weak temperature dependence of resistance from high temperatures down to 20 K. Additionally an energy gap was observed below 17 K due to the formation of local barriers by residual oxygen groups and disorder in reduced graphene oxide (RGO) sheets. At higher temperatures resistance shows a negative T 2 temperature dependence. Temperature dependent magnetization measurements showed a phase transition from diamagnetic to ferromagnetic at around 10 K, in agreement with the electronic transport properties of the RGO films.

Ieee Transactions on Magnetics, 2013

In an integrated power microinductor, the size and shape of the magnetic material will have a rel... more In an integrated power microinductor, the size and shape of the magnetic material will have a relationship to the permeability and inductance of the device. To demonstrate these effects, a set of inductors with closed Ni Fe films were fabricated having similar structures but different footprint sizes and aspect ratios (ARs). Magnetic and electrical characterization was performed on the devices to determine magnetic properties, and in both measurements the same relationship between film shape and magnetic anisotropy is observed. Micromagnetic shape anisotropy simulations were used to predict this behavior and correlate with the experimentally determined parameters. The AR of the film is determined to have a strong influence on the anisotropy and permeability of the magnetic film via shape demagnetization effects which is shown to be a significant variation from the as-deposited magnetic material parameters.

Scientific Reports, 2013

We demonstrate a new type of transistors, the electrical/optical ''dual-function redox-potential ... more We demonstrate a new type of transistors, the electrical/optical ''dual-function redox-potential transistors'', which is solution processable and environmentally stable. This device consists of vertically staked electrodes that act as gate, emitter and collector. It can perform as a normal transistor, whilst one electrode which is sensitised by dye enables to generate photocurrent when illuminated. Solution processable oxide-nanoparticles were used to form various functional layers, which allow an electrolyte to penetrate through and, consequently, the current between emitter and collector can be controlled by the gate potential modulated distribution of ions. The result here shows that the device performs with high ON-current under low driving voltage (,1 V), while the transistor performance can readily be controlled by photo-illumination. Such device with combined optical and electrical functionalities allows single device to perform the tasks that are usually done by a circuit/system with multiple optical and electrical components, and it is promising for various applications.

We report a nonlinear stretching-based electromagnetic energy harvester using FR4 as a vibrating ... more We report a nonlinear stretching-based electromagnetic energy harvester using FR4 as a vibrating spring material due to its low Young's modulus. We show analytically that the nonlinearity is caused by the stretching, in addition to the bending, of the specially designed spring arms; this gives rise to a wider half-power bandwidth of 10 Hz at 1 g acceleration, which is almost 5 times higher than that of a comparable linear counterpart. The output spectra show the first reported experimental evidence of a symmetry broken nonlinear secondary peak in a single potential well system at frequencies close to the nonlinear jump frequency, which may appear to be due to the dynamic symmetry breaking of the oscillator or to the inherent asymmetry of the built prototype. The presence of this secondary peak is useful in generating a significant amount of power compared to the symmetric states, producing ∼3 times more power at the secondary peak than the nearby symmetric states. 110% of the peak power obtained for 0.5 g acceleration is achieved at the secondary peak during the frequency up-sweep. The experimental results are compared with a deterministic numerical model based on the Duffing oscillator, and we include a qualitative discussion on the influence of noise in an experimental energy harvesting system.

2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC), 2012

This paper details the design, fabrication, and characterization of silicon integrated micro-tran... more This paper details the design, fabrication, and characterization of silicon integrated micro-transformers. Two types of race-track shaped micro-transformers, single copper winding or single layer metal (SLM) and double copper winding or double layer metal (DLM) were designed and fabricated using standard CMOS processing. The DLM devices have higher inductance density than SLM devices realized within similar footprint area. The design study showed that the efficiency of micro-transformers increased from 37% for SLM designs to over 75% for DLM transformers at 20 MHz.

Physical Review B, 2014

ABSTRACT We report the results of direct measurement of remanent hysteresis loops on nanochains o... more ABSTRACT We report the results of direct measurement of remanent hysteresis loops on nanochains of BiFeO$_3$ at room temperature under zero and sim\simsim20 kOe magnetic field. We noticed a suppression of remanent polarization by nearly sim\simsim40\% under the magnetic field. The powder neutron diffraction data reveal significant ion displacements under a magnetic field which seems to be the origin of the suppression of polarization. The isolated nanoparticles, comprising the chains, exhibit evolution of ferroelectric domains under dc electric field and complete 180$^o$ switching in switching-spectroscopy piezoresponse force microscopy. They also exhibit stronger ferromagnetism with nearly an order of magnitude higher saturation magnetization than that of the bulk sample. These results show that the nanoscale BiFeO$_3$ exhibits coexistence of ferroelectric and ferromagnetic order and a strong magnetoelectric multiferroic coupling at room temperature comparable to what some of the type-II multiferroics show at a very low temperature.

Physical Review B, 2014

We show from detailed magnetometry across 2-300 K that the BiFeO 3 -Bi 2 Fe 4 O 9 nanocomposite o... more We show from detailed magnetometry across 2-300 K that the BiFeO 3 -Bi 2 Fe 4 O 9 nanocomposite offers a unique spin morphology where superspin glass (SSG) and dilute antiferromagnet in a field (DAFF) coexist at the interface between ferromagnetic Bi 2 Fe 4 O 9 and antiferromagnetic BiFeO 3 . The coexisting SSG and DAFF combine to form a local spin texture, which gives rise to a path-dependent exchange bias below the spin freezing temperature (∼29 K). The exchange bias varies depending on the protocol or path followed in tracing the hysteresis loop. The exchange bias has been observed below the blocking temperature (T B ) 60 K of the superparamagnetic Bi 2 Fe 4 O 9 . The conventional exchange bias (CEB) increases nonmonotonically as temperature decreases. The magnitude of both exchange bias (H E ) and coercivity (H C ) increase with decrease in temperature and are found to be asymmetric below 20 K depending on the path followed in tracing the hysteresis loop and bias field. The local spin texture at the interface between ferromagnetic and antiferromagnetic particles generates a nonswitchable unidirectional anisotropy along the negative direction of the applied field. The influence of this texture also shows up in "asymmetric" jumps in the hysteresis loop at 2 K, which smears off at higher temperature. The role of the interface spin texture in yielding the path dependency of exchange bias is thus clearly delineated.

Journal of the American Chemical Society, 2010

A supercritical fluid technique was used to prepare hexagonal nanoplatelets of magnetite. Ferroce... more A supercritical fluid technique was used to prepare hexagonal nanoplatelets of magnetite. Ferrocene was used as the Fe source, and sc-CO(2) acted as both a solvent and oxygen source in the process. Powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and magnetic measurements were used to characterize the products. It was found that the morphology and structure of the product strongly depended on the reaction conditions.

Journal of the American Ceramic Society, 2013

Journal of Physics D: Applied Physics, 2009

The investigation addresses the effect of magnetizing field on the magnetic properties of melt sp... more The investigation addresses the effect of magnetizing field on the magnetic properties of melt spun Ni 52.84 Mn 19.6 Ga 27.56 (at%) alloy ribbons. Magnetization behaviour at different fields was observed using a superconducting quantum interference device magnetometer for heating and cooling cycles. The plots showed distinct changes in magnetization around the characteristic temperatures at austenitic start and finish (A S , A F ), martensitic start and finish (M S , M F ). With increasing field A S , M F were unaffected. In the range of martensitic start and its finish temperature, the zero field cooled and field cooled measurements indicated magnetization drops indicating antiferromagnetic interactions, which is characteristic of the martensitic phase formation. It was shown from x-ray diffraction analysis that the low martensitic fraction in the majority austenite phase induced the splitting in the L2 1 austenitic ordering. This was further corroborated by the evidence of a few martensitic plates around grain boundaries at room temperature which is close to martensitic start temperature.

Recently, a number of attempts have been made to increase the operational bandwidth of the energy... more Recently, a number of attempts have been made to increase the operational bandwidth of the energy harvesting devices. Nonlinear mechanisms are one of them. In this paper, we report design and analytical formulation of stretching strain of an electromagnetic energy harvester on FR4 material under large deformation of the spring arms. It is found that nonlinearity has an inverse square dependence on thickness of the arms. Numerical solution of a monostable Duffing oscillator that governs the dynamics of such a large deformed nonlinear energy harvester showed that with decrease of load resistance, the average power output increases, where the output response depends strongly on the input force. For small input acceleration, the desired large amplitude vibration does not come into play and the response becomes linear. However, for higher input acceleration nonlinearity appears and the operational bandwidth increases, at the same time, output power level also increases.

Advanced Micro and Nanosystems, 2015

Physical review letters, Jan 6, 2015

Thick hard magnetic material Pulse-reverse plating technique High energy product a b s t r a c t

Ni nanoparticles ($32 nm particle diameter) have been synthesized on the walls of microporous ($1... more Ni nanoparticles ($32 nm particle diameter) have been synthesized on the walls of microporous ($1 nm pore diameter) silica spheres ($2.6 mm sphere diameter) and characterised magnetically to potentially produce a new class of core (silica micro-spheres)-shell (nanometallic)-type nanocomposite material. These magnetic nanocomposite materials display a characteristic increase in coercivity with reducing temperature. The average particle size has been used to calculate the anisotropy constant for the system, K. The discussion postulates the potential mechanisms contributing to the difference between the calculated K value and the magnetocrystalline anisotropy constant of bulk Ni. Various factors such as surface anisotropy and interparticle interactions are discussed as possible contributing factors to the anisotropy values calculated in the paper.

Micro-inductors, with an electrodeposited nickel-iron core have been fabricated on silicon substr... more Micro-inductors, with an electrodeposited nickel-iron core have been fabricated on silicon substrates, and have been characterized in the frequency range up to 100 MHz. The core consists of a nickel iron, 45:55 alloy which is deposited using pulse-reverse electroplating in the presence of a magnetic field to control anisotropy. The operation of the inductors with low loss at high-frequency critically depends on core thickness, which is used to control eddy-current loss as the frequency is increased. However, it is shown that the permeability of NiFe 45/55 has a dependency on thickness, and decreases with increasing thickness. For example, the permeability is measured to be approximately 1000 for a 1-mmthick film, decreasing to approximately 400 for a 5-mm-thick film. In order to correctly design microinductors for operation up to 100 MHz, it is important that this characteristic of the material is taken into account.

ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 2, 2007

ABSTRACT This work presents the thermal property study of single wall and multi wall carbon nanot... more ABSTRACT This work presents the thermal property study of single wall and multi wall carbon nanotubes (SWCNT and MWCNT) both in their purified and unpurified forms introduced to silicone elastomer to enhance the thermal diffusivity of this industrial polymer. An increase in thermal diffusivity was observed for incremental loading of both purified and unpurified single wall and multiwall CNT in epoxy at different percentages. An increase of thermal diffusivity as high as 130% was achieved for ∼2 wt% loading of both single wall and multi wall nanotubes. Electrical conductivity measurements showed a percolation threshold for 2% loading of multiwall CNT, below which the nanotube-epoxy composite behaved as an insulator — this is a key property for applications where electrical isolation is required. For single wall CNT-epoxy composite all the samples showed high resistance to the conduction of current. Thermal impedance measurements showed a strong dependency of contact resistance with percentage loading. Finally, the feasibility of deploying carbon nanotube-polymer composites as practical thermal interface materials for electronics thermal management is discussed.

Applied Physics Letters, 2010

Epl, 2012

We demonstrate semi-metallic transport in graphene oxide layers treated with an organic acid thro... more We demonstrate semi-metallic transport in graphene oxide layers treated with an organic acid through a nearly linear current-vs.-voltage relationship and the weak temperature dependence of resistance from high temperatures down to 20 K. Additionally an energy gap was observed below 17 K due to the formation of local barriers by residual oxygen groups and disorder in reduced graphene oxide (RGO) sheets. At higher temperatures resistance shows a negative T 2 temperature dependence. Temperature dependent magnetization measurements showed a phase transition from diamagnetic to ferromagnetic at around 10 K, in agreement with the electronic transport properties of the RGO films.

Ieee Transactions on Magnetics, 2013

In an integrated power microinductor, the size and shape of the magnetic material will have a rel... more In an integrated power microinductor, the size and shape of the magnetic material will have a relationship to the permeability and inductance of the device. To demonstrate these effects, a set of inductors with closed Ni Fe films were fabricated having similar structures but different footprint sizes and aspect ratios (ARs). Magnetic and electrical characterization was performed on the devices to determine magnetic properties, and in both measurements the same relationship between film shape and magnetic anisotropy is observed. Micromagnetic shape anisotropy simulations were used to predict this behavior and correlate with the experimentally determined parameters. The AR of the film is determined to have a strong influence on the anisotropy and permeability of the magnetic film via shape demagnetization effects which is shown to be a significant variation from the as-deposited magnetic material parameters.

Scientific Reports, 2013

We demonstrate a new type of transistors, the electrical/optical ''dual-function redox-potential ... more We demonstrate a new type of transistors, the electrical/optical ''dual-function redox-potential transistors'', which is solution processable and environmentally stable. This device consists of vertically staked electrodes that act as gate, emitter and collector. It can perform as a normal transistor, whilst one electrode which is sensitised by dye enables to generate photocurrent when illuminated. Solution processable oxide-nanoparticles were used to form various functional layers, which allow an electrolyte to penetrate through and, consequently, the current between emitter and collector can be controlled by the gate potential modulated distribution of ions. The result here shows that the device performs with high ON-current under low driving voltage (,1 V), while the transistor performance can readily be controlled by photo-illumination. Such device with combined optical and electrical functionalities allows single device to perform the tasks that are usually done by a circuit/system with multiple optical and electrical components, and it is promising for various applications.

We report a nonlinear stretching-based electromagnetic energy harvester using FR4 as a vibrating ... more We report a nonlinear stretching-based electromagnetic energy harvester using FR4 as a vibrating spring material due to its low Young's modulus. We show analytically that the nonlinearity is caused by the stretching, in addition to the bending, of the specially designed spring arms; this gives rise to a wider half-power bandwidth of 10 Hz at 1 g acceleration, which is almost 5 times higher than that of a comparable linear counterpart. The output spectra show the first reported experimental evidence of a symmetry broken nonlinear secondary peak in a single potential well system at frequencies close to the nonlinear jump frequency, which may appear to be due to the dynamic symmetry breaking of the oscillator or to the inherent asymmetry of the built prototype. The presence of this secondary peak is useful in generating a significant amount of power compared to the symmetric states, producing ∼3 times more power at the secondary peak than the nearby symmetric states. 110% of the peak power obtained for 0.5 g acceleration is achieved at the secondary peak during the frequency up-sweep. The experimental results are compared with a deterministic numerical model based on the Duffing oscillator, and we include a qualitative discussion on the influence of noise in an experimental energy harvesting system.

2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC), 2012

This paper details the design, fabrication, and characterization of silicon integrated micro-tran... more This paper details the design, fabrication, and characterization of silicon integrated micro-transformers. Two types of race-track shaped micro-transformers, single copper winding or single layer metal (SLM) and double copper winding or double layer metal (DLM) were designed and fabricated using standard CMOS processing. The DLM devices have higher inductance density than SLM devices realized within similar footprint area. The design study showed that the efficiency of micro-transformers increased from 37% for SLM designs to over 75% for DLM transformers at 20 MHz.

Physical Review B, 2014

ABSTRACT We report the results of direct measurement of remanent hysteresis loops on nanochains o... more ABSTRACT We report the results of direct measurement of remanent hysteresis loops on nanochains of BiFeO$_3$ at room temperature under zero and sim\simsim20 kOe magnetic field. We noticed a suppression of remanent polarization by nearly sim\simsim40\% under the magnetic field. The powder neutron diffraction data reveal significant ion displacements under a magnetic field which seems to be the origin of the suppression of polarization. The isolated nanoparticles, comprising the chains, exhibit evolution of ferroelectric domains under dc electric field and complete 180$^o$ switching in switching-spectroscopy piezoresponse force microscopy. They also exhibit stronger ferromagnetism with nearly an order of magnitude higher saturation magnetization than that of the bulk sample. These results show that the nanoscale BiFeO$_3$ exhibits coexistence of ferroelectric and ferromagnetic order and a strong magnetoelectric multiferroic coupling at room temperature comparable to what some of the type-II multiferroics show at a very low temperature.

Physical Review B, 2014

We show from detailed magnetometry across 2-300 K that the BiFeO 3 -Bi 2 Fe 4 O 9 nanocomposite o... more We show from detailed magnetometry across 2-300 K that the BiFeO 3 -Bi 2 Fe 4 O 9 nanocomposite offers a unique spin morphology where superspin glass (SSG) and dilute antiferromagnet in a field (DAFF) coexist at the interface between ferromagnetic Bi 2 Fe 4 O 9 and antiferromagnetic BiFeO 3 . The coexisting SSG and DAFF combine to form a local spin texture, which gives rise to a path-dependent exchange bias below the spin freezing temperature (∼29 K). The exchange bias varies depending on the protocol or path followed in tracing the hysteresis loop. The exchange bias has been observed below the blocking temperature (T B ) 60 K of the superparamagnetic Bi 2 Fe 4 O 9 . The conventional exchange bias (CEB) increases nonmonotonically as temperature decreases. The magnitude of both exchange bias (H E ) and coercivity (H C ) increase with decrease in temperature and are found to be asymmetric below 20 K depending on the path followed in tracing the hysteresis loop and bias field. The local spin texture at the interface between ferromagnetic and antiferromagnetic particles generates a nonswitchable unidirectional anisotropy along the negative direction of the applied field. The influence of this texture also shows up in "asymmetric" jumps in the hysteresis loop at 2 K, which smears off at higher temperature. The role of the interface spin texture in yielding the path dependency of exchange bias is thus clearly delineated.

Journal of the American Chemical Society, 2010

A supercritical fluid technique was used to prepare hexagonal nanoplatelets of magnetite. Ferroce... more A supercritical fluid technique was used to prepare hexagonal nanoplatelets of magnetite. Ferrocene was used as the Fe source, and sc-CO(2) acted as both a solvent and oxygen source in the process. Powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and magnetic measurements were used to characterize the products. It was found that the morphology and structure of the product strongly depended on the reaction conditions.

Journal of the American Ceramic Society, 2013

Journal of Physics D: Applied Physics, 2009

The investigation addresses the effect of magnetizing field on the magnetic properties of melt sp... more The investigation addresses the effect of magnetizing field on the magnetic properties of melt spun Ni 52.84 Mn 19.6 Ga 27.56 (at%) alloy ribbons. Magnetization behaviour at different fields was observed using a superconducting quantum interference device magnetometer for heating and cooling cycles. The plots showed distinct changes in magnetization around the characteristic temperatures at austenitic start and finish (A S , A F ), martensitic start and finish (M S , M F ). With increasing field A S , M F were unaffected. In the range of martensitic start and its finish temperature, the zero field cooled and field cooled measurements indicated magnetization drops indicating antiferromagnetic interactions, which is characteristic of the martensitic phase formation. It was shown from x-ray diffraction analysis that the low martensitic fraction in the majority austenite phase induced the splitting in the L2 1 austenitic ordering. This was further corroborated by the evidence of a few martensitic plates around grain boundaries at room temperature which is close to martensitic start temperature.

Recently, a number of attempts have been made to increase the operational bandwidth of the energy... more Recently, a number of attempts have been made to increase the operational bandwidth of the energy harvesting devices. Nonlinear mechanisms are one of them. In this paper, we report design and analytical formulation of stretching strain of an electromagnetic energy harvester on FR4 material under large deformation of the spring arms. It is found that nonlinearity has an inverse square dependence on thickness of the arms. Numerical solution of a monostable Duffing oscillator that governs the dynamics of such a large deformed nonlinear energy harvester showed that with decrease of load resistance, the average power output increases, where the output response depends strongly on the input force. For small input acceleration, the desired large amplitude vibration does not come into play and the response becomes linear. However, for higher input acceleration nonlinearity appears and the operational bandwidth increases, at the same time, output power level also increases.