arXiv subjects

S. Narayana Jammalamadaka

Publications and source records attributed to S. Narayana Jammalamadaka.

At least 19 recordsLinked to original sources

arXiv2020

Graphene oxide based synaptic memristor device for neuromorphic computing

Brain-inspired neuromorphic computing which consist neurons and synapses, with an ability to perform complex information processing has unfolded a new paradigm of computing to overcome the von Neumann bottleneck. Electronic synaptic memristor devices which can compete with the biological synapses are indeed significant for neuromorphic computing. In this work, we demonstrate our efforts to develop and realize the graphene oxide (GO) based memristor device as a synaptic device, which mimic as a biological synapse. Indeed, this device exhibits the essential synaptic learning behavior including analog memory characteristics, potentiation and depression. Furthermore, spike-timing-dependent-plasticity learning rule is mimicked by engineering the pre- and post-synaptic spikes. In addition, non-volatile properties such as endurance, retentivity, multilevel switching of the device are explored. These results suggest that Ag/GO/FTO memristor device would indeed be a potential candidate for future neuromorphic computing applications. Keywords: RRAM, Graphene oxide, neuromorphic computing, synaptic device, potentiation, depression

cs.ET↗

arXiv2017

Magnetic and Hyperfine interactions in HoFe1-xCrxO3 compounds

We report on the magnetic and Mossbauer properties of polycrystalline HoFe1-xCrxO3 compounds. Magnetization data reveals the continuous tailoring of magnetic transition due to weakening of Ho3+ Fe3+ and Fe3+ Fe3+ interactions in the entire temperature range by replacing the Fe3+ ions with Cr3+ ions. The observed decrease in Neel temperature (TN) and increase in spin re-orientation transition temperature (TSR) with the replacement of Fe3+ with Cr3+ is ascribed to the weakening of Fe(Cr) O Fe(Cr) antiferromagnetic exchange interaction. In addition, we also attribute such a change in TN to the enhancement of ferromagnetic interaction of adjacent Cr3+ moments through t e hybridization as a result of the structural distortion. The decrease in isomer shift (IS) suggests enhancement of the interaction between nuclear charge with the 3s electrons as a result of decrease in radial part of 3d wave function with Cr addition. In this paper we also discuss about the variation of quadrupole splitting (QS) and hyperfine fields (Hhf) with Cr addition in HoFe1-xCrxO3 compounds.

cond-mat.mtrl-sci↗

arXiv2017

Spin orbit coupling and Lorentz force enhanced efficiency of TiO2 based dye sensitized solar cells

We report on the effect of the strong spin orbit coupling and the Lorentz force on the efficiency of TiO2 based dye sensitized solar cells. Upon inclusion of Ho2O3, due to the strong spin orbit coupling of the rare earth Ho3+ ion, we do see 13 percent enhancement in the efficiency. We attribute such an enhancement in power conversion efficiency to the increased lifetime of the photo-excited excitons. Essentially, a Ho3+ ion accelerates the phenomenon of the spin rephasing or the intersystem crossing of the excitons in a photosensitizer. Increase in the absorbance and decrease in the photoluminescence intensity suggests a decrease in the recombination rate, hinting an enhanced charge transport and is in accordance with our electrochemical impedance spectra and the J V characteristics. From the above we strongly believe that enhanced efficiency of the device is due to increased intersystem crossing which would accelerate the exciton dissociation. On top of spin orbit interaction, a configuration where the electric and magnetic fields are perpendicular to each other helped in enhancing the efficiency by 16 percent, suggesting that the Lorentz force also plays a dominant role in controlling the charge transport of the photo-generated charge carriers. We strongly believe that this simple and novel strategy of improving the efficiency may pave the way for realizing higher efficiency dye sensitized solar cells.

cond-mat.mtrl-sci↗

arXiv2016

Magnetostrictive Fe73Ga27 nanocontacts for low field conductance switching

The electrical conductance G of magnetostrictive nanocontacts made from Galfenol Fe73Ga27 can be reproducibly switched between on and off states in a low magnetic field of 20 to 30 mT at 10 K. The switching behavior is in agreement with the magnetic field dependence of the magnetostriction inferred from the magnetization behavior, causing a positive magnetostrictive strain along the magnetic field. The repeated magneticfield cycling leads to a stable contact geometry and to a robust contact configuration with a very low hysteresis of 1 mT between opening and closing the contact due to a training effect. Nonintegral multiples of the conductance quantum G0 observed for G is greater than G0 are attributed to electron backscattering at defect sites in the electrodes near the contact interface. When the contact is closed either mechanically or by magnetic field, the conductance shows an exponential behavior below G0 due to electron tunneling. This allows to estimate the magnetostriction at 10 K. The results demonstrate that such magnetostrictive devices are suitable for the remote control of the conductance by low magnetic fields in future nanotechnology applications.

cond-mat.mes-hall↗

arXiv2016

Magnetocaloric effect and nature of magnetic transition in low dimensional DyCu2

In this manuscript, we propose a method to prepare small flakes of DyCu2. On top of that we also report on the magnetocaloric effect and nature of magnetic transition of a strongly anisotropic DyCu2 in its low dimension. Magnetization measurements were carried out in the temperature range of 5 to 100 K and up to the maximum magnetic field strength of 50 kOe. Magnetic entropy change delta SM is estimated using the well known Maxwells equations and is found to be 4.31 J per kg K. Indeed, the delta SM peak broadened marginally compared with its bulk DyCu2 and such a broadening can be attributed to significant increase in the total grain boundary volume. As these small flakes consists larger delta SM values at temperatures higher than the Neel temperature TN, one can use them as a magnetic refrigerant material in a broad temperature range. We also plotted the M2 vs H by M in order to find the nature of magnetic transition. Arrott plots infer that indeed there exists nonlinearity in M2 vs H by M behavior and such nonlinear behavior is ascribed to the random anisotropy or a random field that is present in the system.

cond-mat.dis-nn↗

arXiv2016

Enhanced spin reorientation temperature and origin of magnetocapacitance in HoFeO3

We report on the increase in the spin reorientation temperature in HoFe0.5Cr0.5O3 compound by isovalent substitution Cr3+ at the Fe-site and the magnetocapacitance in the HoFeO3 compound. Spin reorientation transition is evident around 50 K and 150 K for the x = 0 and x = 0.5 compounds respectively. The increase in the spin reorientation transition temperature in case of x = 0.5 compound can be attributed to the domination of the Ho3+ to Fe3+ interaction over the Fe3+ to Fe3+ interaction. Decrease in Neel temperature from 643 K (x = 0) to 273 K (x = 0.5) can be ascribed to the decrease in the interaction between antiferromagnetically aligned Fe3+ moments as a result of the dilution of the Fe3+ moments with the Cr3+ addition. From the magnetization M vs magnetic field H variation it is evident that the coercivity, HC decreases for x = 0.5 compound, hinting the magnetic softening of the HoFeO3 compound. Observed magnetocapacitance could be due to lossy dielectric mechanism in the present compound. Indeed, present results would be helpful in understanding the physics behind rare- earth orthoferrites.

cond-mat.mtrl-sci↗

arXiv2016

Band gap tuning and orbital mediated electron phonon coupling in $HoFe_{1-x}Cr_xO_3$

We report on the evidenced orbital mediated electron phonon coupling and band gap tuning in HoFe1-xCrxO3 compounds. From the room temperature Raman scattering, it is apparent that the electron-phonon coupling is sensitive to the presence of both the Fe and Cr at the B-site. Essentially, an Ag like local oxygen breathing mode is activated due to the charge transfer between Fe3+ - Cr3+ at around 670 cm-1, this observation is explained on the basis of Franck-Condon (FC) mechanism. Optical absorption studies infer that there exists a direct band gap in the HoFe1-xCrxO3 compounds. Decrease in band gap until x = 0.5 is ascribed to the broadening of the oxygen p orbitals as a result of the induced spin disorder due to Fe3+ and Cr3+ at B site. In contrast, the increase in band gap above x = 0.5 is explained on the basis of the reduction in the available unoccupied d - orbitals of Fe3+ at the conduction band. We believe that above results would be helpful for the development of the optoelectronic devices based on the ortho-ferrites.

cond-mat.mtrl-sci↗

arXiv2014

Resistive switching in ultra-thin La0.7Sr0.3MnO3 / SrRuO3 superlattices

Superlattices may play an important role in next generation electronic and spintronic devices if the key-challenge of the reading and writing data can be solved. This challenge emerges from the coupling of low dimensional individual layers with macroscopic world. Here we report the study of the resistive switching characteristics of a of hybrid structure made out of a superlattice with ultrathin layers of two ferromagnetic metallic oxides, La0.7Sr0.3MnO3 (LSMO) and SrRuO3 (SRO). Bipolar resistive switching memory effects are measured on these LSMO/SRO superlattices, and the observed switching is explainable by ohmic and space charge-limited conduction laws. It is evident from the endurance characteristics that the on/off memory window of the cell is greater than 14, which indicates that this cell can reliably distinguish the stored information between high and low resistance states. The findings may pave a way to the construction of devices based on nonvolatile resistive memory effects.

cond-mat.mes-hall↗

arXiv2012

Magnetocaloric effect and nature of magnetic transition in nanoscale Pr0.5Ca0.5MnO3

Systematic measurements pertinent to the magnetocaloric effect and nature of magnetic transition around the transition temperature are performed in the 10 nm Pr0.5Ca0.5MnO3 nanoparticles (PCMO10) . Maxwell relation is employed to estimate the change in magnetic entropy. At Curie temperature TC, 83.5 K, the change in magnetic entropy discloses a typical variation with a value 0.57 J/kg K, and is found to be magnetic field dependent. From the area under the curve Delta S vs T, the refrigeration capacity is calculated at TC, 83.5 K and it is found to be 7.01 J/kg. Arrott plots infer that due to the competition between the ferromagnetic and anti ferromagnetic interactions, the magnetic phase transition in PCMO10 is broadly spread over both in temperature as well as in magnetic field coordinates. Upon tuning the particle size, size distribution, morphology, and relative fraction of magnetic phases, it may be possible to enhance the magnetocalorific effect further in PCMO10.

cond-mat.mes-hall↗

arXiv2012

Ferromagnetism in graphene nanoribbons: split versus oxidative unzipped ribbons

Two types of graphene nanoribbons: (a) potassium-split graphene nanoribbons (GNRs), and (b) oxidative unzipped and chemically converted graphene nanoribbons (CCGNRs) were investigated for their magnetic properties using the combination of static magnetization and electron spin resonance measurements. The two types of ribbons possess remarkably different magnetic properties. While the low temperature ferromagnet-like feature is observed in both types of ribbons, such room temperature feature persists only in potassium-split ribbons. The GNRs show negative exchange bias, but the CCGNRs exhibit a 'positive exchange bias'. Electron spin resonance measurements infer that the carbon related defects may responsible for the observed magnetic behaviour in both types of ribbons. Furthermore, proton hyperfine coupling strength has been obtained from hyperfine sublevel correlation experiments performed on the GNRs. Electron spin resonance provides no indications for the presence of potassium (cluster) related signals, emphasizing the intrinsic magnetic nature of the ribbons. Our combined experimental results may infer the coexistence of ferromagnetic clusters with anti-ferromagnetic regions leading to disordered magnetic phase. We discuss the origin of the observed contrast in the magnetic behaviours of these two types of ribbons.

cond-mat.mes-hall↗

arXiv2012

Oscillatory exchange bias and training effects in nanocrystalline Pr0.5Ca0.5MnO3

We report on exchange bias effects in 10 nm particles of Pr0.5Ca0.5MnO3 which appear as a result of competing interactions between the ferromagnetic (FM)/anti-ferromagnetic (AFM) phases. The fascinating new observation is the demonstration of the temperature dependence of oscillatory exchange bias (OEB) and is tunable as a function of cooling field strength below the SG phase, may be attributable to the presence of charge/spin density wave (CDW/SDW) in the AFM core of PCMO10. The pronounced training effect is noticed at 5 K from the variation of the EB field as a function of number of field cycles (n) upon the field cooling (FC) process. For n > 1, power-law behavior describes the experimental data well; however, the breakdown of spin configuration model is noticed at n \geq 1.

cond-mat.mes-hall↗

arXiv2012

Martensite-like transition and spin-glass behavior in nanocrystalline Pr0.5Ca0.5MnO3

We report on isothermal pulsed (20 ms) field magnetization, temperature dependent AC - susceptibility, and the static low magnetic field measurements carried out on 10 nm sized Pr0.5Ca0.5MnO3 nanoparticles (PCMO10). The saturation field for the magnetization of PCMO10 (~ 250 kOe) is found to be reduced in comparison with that of bulk PCMO (~300 kOe). With increasing temperature, the critical magnetic field required to 'melt' the residual charge-ordered phase decays exponentially while the field transition range broadens, which is indicative of a Martensite-like transition. The AC - susceptibility data indicate the presence of a frequency-dependent freezing temperature, satisfying the conventional Vogel-Fulcher and power laws, pointing to the existence of a spin-glass-like disordered magnetic phase. The present results lead to a better understanding of manganite physics and might prove helpful for practical applications.

cond-mat.mtrl-sci↗

arXiv2012

Exchange bias and training effects in antiferromagnetically coupled La0.7Sr0.3MnO3 / SrRuO3 superlattices

Exchange bias (EB) and the training effects (TE) in an antiferromagnetically coupled La0.7Sr0.3MnO3 / SrRuO3 superlattices were studied in the temperature range 1.8 - 150 K. Strong antiferromagnetic (AFM) interlayer coupling is evidenced from AC - susceptibility measurements. Below 100 K, vertical magnetization shifts are present due to the two remanent states corresponding to the two ferromagnetic (FM) layers at FM and AFM coupling condition. After field cooling (FC), significant decrease in the exchange bias field (HEB) is observed when cycling the system through several consecutive hysteresis loops. Quantitative analysis for the variation of HEB vs. number of field cycles (n) indicates an excellent agreement between the theory, based on triggered relaxation phenomena, and our experimental observations. Nevertheless, the crucial fitting parameter K indicates smooth training effect upon repeated field cycling, in accordance with our observation.

cond-mat.mtrl-sci↗

arXiv2009

Magnetic ordering in the fine particles of some bulk Pauli-paramgnets

Fine particles of some of the extensively studied 'enhanced' Pauli paramagnets - LuCo2, ScCo2, ZrCo2 and CeRu2Si2, have been prepared by high-energy ball-milling and investigated for their magnetic behavior. In the case of Co systems, these particles show ferromagnetic features even at room temperature. In the case of CeRu2Si2, a well-known Kondo lattice which is non-magnetic in the bulk form, features attributable to the onset of magnetic order below about 8 K could be seen in the fine particles. The magnitudes of the magnetic moment of these small particles at high fields are sufficiently large to conclude that the observed magnetic ordering is not just a surface effect. This transformation to magnetic ordering in small particles could be widespread among enhanced Pauli paramagnets.

cond-mat.str-el↗

arXiv2009

Insensitivity of magnetic anomalies in Sr3NiPtO6 to positive and negative pressures

The compound, Sr3NiPtO6, belonging to a K4CdCl6-type rhombohedral structure, has been reported not to exhibit magnetic ordering at least down to 1.8 K, despite a relatively large value of paramagnetic Curie temperature. This is attributable to geometrical frustration. Here we report the results of our efforts to gradually replace Sr by Ba and to probe the influence of positive (external) and negative (chemical) pressure on the magnetic behavior of this compound. In the Ba substituted series, single phase is formed up to x= 1.0 with Ba substituting for Sr. The magnetic properties of the parent compound in the entire temperature range of investigation are not influenced at all in any of the compositions studied as well as under external pressure (investigated up to 10 kbar). Spin-liquid-like heat-capacity behavior (finite linear term) is observed even in Ba substituted specimens. Thus, the magnetic anomalies of this compound are quite robust.

cond-mat.str-el↗

arXiv2009

Enhancement of positive magnetoresistance following a magnetic-field-induced ferromagnetic transition in an intermetallic compound, Tb5Si3

We report the existence of a field-induced ferromagnetic transition in the magnetically ordered state (<69 K) of an intermetallic compound, Tb5Si3, and this transition is distinctly first-order at 1.8 K (near 60 kOe), whereas it appears to become second order near 20 K. The finding we stress is that the electrical resistivity becomes suddenly large in the high-field state after this transition and this is observed in the entire temperature range in the magnetically ordered state. Such an enhancement of 'positive' magnetoresistance (below 100 kOe) at the metamagnetic transition field is unexpected on the basis that the application of magnetic field should favor a low-resistive state due to alignment of spins.

cond-mat.str-el↗

arXiv2009

Magnetic anomalies in Gd6Co1.67Si3 and Tb6Co1.67Si3

The compounds, Gd6Co1.67Si3 and Tb6Co1.67Si3, recently reported to form in a Ce6Ni2Si3-derived hexagonal structure (space group: P6_3 / m) and to order magnetically below 295 and 190 K respectively, have been investigated by detailed magnetization (M) studies in the temperature interval 1.8-330 K as a function of magnetic field (H). The points of emphasis are: We observe multiple steps in the M(H) curve for the Tb compound at 1.8 K while increasing H, but these steps do not appear in the reverse cycle of H. At higher temperatures, such steps are absent. However, this 'staircase' behavior of M(H) is not observed for the Gd compound at any temperature and the isothermal magnetization is not hysteretic unlike in Tb compound. From the M(H) data measured at close intervals of temperature, we have derived isothermal entropy change (Delta S) and it is found that Delta S follows a theoretically predicted H^2/3-dependence.

cond-mat.str-el↗

arXiv2009

Magnetic anomalies in Nd6Co(1.67)Si3: Surprising first order transitions in the low-temperature isothermal magnetization

We present the results of magnetic measurements on Nd6Co(1.67)Si3, a compound recently reported to crystallize in a hexagonal structure (space group P6_3/m) and to undergo long range magnetic ordering below 84 K. The results reveal that the magnetism of this compound is quite complex with additional magnetic anomalies near 50 and 20 K. There are qualitative changes in the isothermal magnetization behavior with the variation of temperature. Notably, there is a field-induced spin reorientation as the temperature is lowered below 20 K. A finding we stress is that this transition is discontinuous for 1.8K in the virgin curve, but the first order character appears only after a field-cycling for a narrow higher temperature range near 5 K. Thus, this compound serves as an example for the stabilisation of first-order transition induced by magnetic-field-cycling. The issues of 'Phase co-existence' and 'meta-stability' after a field-cycling at low temperatures in this compound are also addressed.

cond-mat.str-el↗