Ashok Kumar

Bio: Ashok Kumar is an academic researcher from Council of Scientific and Industrial Research. The author has contributed to research in topics: Ferroelectricity & Dielectric. The author has an hindex of 31, co-authored 173 publications receiving 3534 citations. Previous affiliations of Ashok Kumar include National Physical Laboratory & University of Puerto Rico, Río Piedras.

Impedance spectroscopy of multiferroic Pb Zr x Ti 1 − x O 3 ∕ Co Fe 2 O 4 layered thin films

Abstract: The electrical properties of ferroelectric $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3}$ (PZT) and ferromagnetic $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ (CFO) thin film multilayers (MLs) fabricated by pulsed laser deposition technique has been studied by impedance and modulus spectroscopy. The effect of various PZT/CFO configurations having three, five, and nine layers has been systematically investigated. The transmission electron microscopy images revealed that the ML structures were at least partially diffused near the interface. Diffraction patterns indicate clear PZT and CFO crystal structures in the interior and at the interface of the ML structure. Room temperature micro-Raman spectra indicate separate PZT and CFO phases in ML structure without any impurity phase. We studied frequency and temperature dependencies of impedance, electric modulus, and ac conductivity of ML thin films in the ranges of $100\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}\char21{}1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ and $200\char21{}650\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively. We observed two distinct electrical responses in all the investigated ML films at low temperature $(l400\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and at elevated temperature $(g400\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. We attributed these contributions to the grain effects at low temperature and grain boundary effects at high temperature. We explained this electrical behavior by Maxwell-Wagner-type contributions arising from the interfacial charge at the interface of the ML structure. Master modulus spectra indicate that the magnitude of grain boundary compared to grain becomes more prominent with the increase in the number of layer. The frequency dependent conductivity results well fitted with the double power law, $\ensuremath{\sigma}(\ensuremath{\omega})=\ensuremath{\sigma}(0)+{A}_{1}{\ensuremath{\omega}}^{{n}_{1}}+{A}_{2}{\ensuremath{\omega}}^{{n}_{2}}$, and the results showed evidence of three types of conduction process at elevated temperature: (i) low frequency $(l1\phantom{\rule{0.3em}{0ex}}\mathrm{kHz})$ conductivity is due to long-range ordering (frequency independent), (ii) midfrequency conductivity $(l10\phantom{\rule{0.3em}{0ex}}\mathrm{kHz})$ may be due to the short-range hopping, and (iii) high frequency $(l1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz})$ conduction is due to the localized relaxation hopping mechanism.

Multifunctional magnetoelectric materials for device applications.

Abstract: Over the past decade magnetoelectric (ME) mutiferroic (MF) materials and their devices are one of the highest priority research topics that has been investigated by the scientific ferroics community to develop the next generation of novel multifunctional materials. These systems show the simultaneous existence of two or more ferroic orders, and cross-coupling between them, such as magnetic spin, polarisation, ferroelastic ordering, and ferrotoroidicity. Based on the type of ordering and coupling, they have drawn increasing interest for a variety of device applications, such as magnetic field sensors, nonvolatile memory elements, ferroelectric photovoltaics, nano-electronics etc. Since single-phase materials exist rarely in nature with strong cross-coupling properties, intensive research activity is being pursued towards the discovery of new single-phase multiferroic materials and the design of new engineered materials with strong magneto-electric (ME) coupling. This review article summarises the development of different kinds of multiferroic material: single-phase and composite ceramic, laminated composite and nanostructured thin films. Thin-film nanostructures have higher magnitude direct ME coupling values and clear evidence of indirect ME coupling compared with bulk materials. Promising ME coupling coefficients have been reported in laminated composite materials in which the signal to noise ratio is good for device fabrication. We describe the possible applications of these materials.

Barium zirconate-titanate/barium calcium-titanate ceramics via sol?gel process: novel high-energy-density capacitors

Abstract: Lead-free barium zirconate-titanate/barium calcium-titanate, [(BaZr0.2Ti0.80)O3]1?x?[(Ba0.70Ca0.30)TiO3]x (x = 0.10, 0.15, 0.20) (BZT?BCT) ceramics with high dielectric constant, low dielectric loss and moderate electric breakdown field were prepared by the sol?gel synthesis technique. X-ray diffraction patterns revealed tetragonal crystal structure and this was further confirmed by Raman spectra. Well-behaved ferroelectric hysteresis loops and moderate polarizations (spontaneous polarization, Ps ~ 3?6??C?cm?2) were obtained in these BZT?BCT ceramics. Frequency-dependent dielectric spectra confirmed that ferroelectric diffuse phase transition (DPT) exists near room temperature. Scanning electron microscope images revealed monolithic grain growth in samples sintered at 1280??C. 1000/? versus (T) plots revealed ferroelectric DPT behaviour with estimated ? values of ~1.52, 1.51 and 1.88, respectively, for the studied BZT?BCT compositions. All three compositions showed packing-limited breakdown fields of ~47?73?kV?cm?1 with an energy density of 0.05?0.6?J?cm?3 for thick ceramics (>1?mm). Therefore these compositions might be useful in Y5V-type capacitor applications.

Magnetic switching of ferroelectric domains at room temperature in multiferroic PZTFT

Abstract: Single-phase magnetoelectric multiferroics are ferroelectric materials that display some form of magnetism. In addition, magnetic and ferroelectric order parameters are not independent of one another. Thus, the application of either an electric or magnetic field simultaneously alters both the electrical dipole configuration and the magnetic state of the material. The technological possibilities that could arise from magnetoelectric multiferroics are considerable and a range of functional devices has already been envisioned. Realising these devices, however, requires coupling effects to be significant and to occur at room temperature. Although such characteristics can be created in piezoelectric-magnetostrictive composites, to date they have only been weakly evident in single-phase multiferroics. Here in a newly discovered room temperature multiferroic, we demonstrate significant room temperature coupling by monitoring changes in ferroelectric domain patterns induced by magnetic fields. An order of magnitude estimate of the effective coupling coefficient suggests a value of ~1 × 10(-7) sm(-1).

Relaxor-ferroelectric superlattices: high energy density capacitors

Abstract: We report the breakdown electric field and energy density of laser ablated BaTiO3/Ba(1−x)SrxTiO3 (x = 0.7) (BT/BST) relaxor-ferroelectric superlattices (SLs) grown on (100) MgO single crystal substrates. The dielectric constant shows a frequency dispersion below the dielectric maximum temperature (Tm) with a merger above Tm behaving similarly to relaxors. It also follows the basic criteria of relaxor ferroelectrics such as low dielectric loss over wide temperature and frequency, and 50 K shift in Tm with change in probe frequency; the loss peaks follow a similar trend to the dielectric constant except that they increase with increase in frequency (∼40 kHz), and satisfy the nonlinear Vogel–Fulcher relation. Well-saturated ferroelectric hysteresis and 50–80% dielectric saturation are observed under high electric field (∼1.65 MV cm−1). The superlattices demonstrate an ‘in-built’ field in as grown samples at low probe frequency ( 1 kHz) which rules out the effect of any space charge and interfacial polarization. The P–E loops show around 12.24 J cm−3 energy density within the experimental limit, but extrapolation of this data suggests that the potential energy density could reach 46 J cm−3. The current density versus applied electric field indicates an exceptionally high breakdown field (5.8–6.0 MV cm−1) and low current density (∼10–25 mA cm−2) near the breakdown voltage. The current–voltage characteristics reveal that the space charge limited conduction mechanism prevails at very high voltage.

Physics and Applications of Bismuth Ferrite

Abstract: BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Organic–Inorganic Perovskites: Structural Versatility for Functional Materials Design

Abstract: Although known since the late 19th century, organic–inorganic perovskites have recently received extraordinary research community attention because of their unique physical properties, which make them promising candidates for application in photovoltaic (PV) and related optoelectronic devices. This review will explore beyond the current focus on three-dimensional (3-D) lead(II) halide perovskites, to highlight the great chemical flexibility and outstanding potential of the broader class of 3-D and lower dimensional organic-based perovskite family for electronic, optical, and energy-based applications as well as fundamental research. The concept of a multifunctional organic–inorganic hybrid, in which the organic and inorganic structural components provide intentional, unique, and hopefully synergistic features to the compound, represents an important contemporary target.

Recent Progress in Multiferroic Magnetoelectric Composites: from Bulk to Thin Films

Abstract: Multiferroic magnetoelectric composite systems such as ferromagnetic-ferroelectric heterostructures have recently attracted an ever-increasing interest and provoked a great number of research activities, driven by profound physics from coupling between ferroelectric and magnetic orders, as well as potential applications in novel multifunctional devices, such as sensors, transducers, memories, and spintronics. In this Review, we try to summarize what remarkable progress in multiferroic magnetoelectric composite systems has been achieved in most recent few years, with emphasis on thin films; and to describe unsolved issues and new device applications which can be controlled both electrically and magnetically.

Fullerene Derivative‐Doped Zinc Oxide Nanofilm as the Cathode of Inverted Polymer Solar Cells with Low‐Bandgap Polymer (PTB7‐Th) for High Performance

Abstract: Modification of a ZnO cathode by doping it with a hydroxyl-containing derivative - giving a ZnO-C60 cathode - provides a fullerene-derivative-rich surface and enhanced electron conduction. Inverted polymer solar cells with the ZnO-C60 cathode display markedly improved power conversion efficiency compared to those with a pristine ZnO cathode, especially when the active layer includes the low-bandgap polymer PTB7-Th.

Domain wall nanoelectronics

Abstract: Domains in ferroelectrics were considered to be well understood by the middle of the last century: They were generally rectilinear, and their walls were Ising-like. Their simplicity stood in stark contrast to the more complex Bloch walls or N\'eel walls in magnets. Only within the past decade and with the introduction of atomic-resolution studies via transmission electron microscopy, electron holography, and atomic force microscopy with polarization sensitivity has their real complexity been revealed. Additional phenomena appear in recent studies, especially of magnetoelectric materials, where functional properties inside domain walls are being directly measured. In this paper these studies are reviewed, focusing attention on ferroelectrics and multiferroics but making comparisons where possible with magnetic domains and domain walls. An important part of this review will concern device applications, with the spotlight on a new paradigm of ferroic devices where the domain walls, rather than the domains, are the active element. Here magnetic wall microelectronics is already in full swing, owing largely to the work of Cowburn and of Parkin and their colleagues. These devices exploit the high domain wall mobilities in magnets and their resulting high velocities, which can be supersonic, as shown by Kreines' and co-workers 30 years ago. By comparison, nanoelectronic devices employing ferroelectric domain walls often have slower domain wall speeds, but may exploit their smaller size as well as their different functional properties. These include domain wall conductivity (metallic or even superconducting in bulk insulating or semiconducting oxides) and the fact that domain walls can be ferromagnetic while the surrounding domains are not.