arXiv subjects

C. V. Tomy

Publications and source records attributed to C. V. Tomy.

At least 37 records · Page 2Linked to original sources

arXiv2015

Improvement in thermoelectric properties by tailoring at In and Te site in In2Te5

We study role of site substitutions at In and Te site in In2Te5 on the thermoelectric behavior. Single crystals with compositions In2(Te1-xSex)5 (x = 0, 0.05, 0.10) and Fe0.05In1.95(Te0.90Se0.10)5 were prepared using modified Bridgman-Stockbarger technique. Electrical and thermal transport properties of these single crystals were measured in the temperature range 6 - 395 K. A substantial decrease in thermal conductivity is observed in Fe substituted samples attributed to the enhanced phonon point-defect scattering. Marked enhancement in Seebeck coefficient S along with a concomitant suppression of electrical resistivity \r{ho} is observed in Se substituted single crystals. An overall enhancement of thermoelectric figure of merit (zT) by a factor of 310 is observed in single crystals of Fe0.05In1.95(Te0.90Se0.10)5 compared to the parent In2Te5 single crystals.

cond-mat.mtrl-sci↗

arXiv2015

Growth and angular dependent resistivity of Nb2Pd0.73S5.7 superconducting single crystals fiber

We report the growth of Nb2Pd0.73S5.7 superconducting single crystal fibers via slow cooling solid state reaction method. Superconducting transition temperature (Tc ~ 6.5K) is confirmed from magnetization and transport measurements. A comparative study is performed for determination of superconducting anisotropy, Γ, via conventional method (by taking ration of two superconducting parameters) and scaling approach method. Scaling approach, defined within the framework of the Ginzburg-Landau theory is applied to the angular dependent resistivity measurements to estimate the anisotropy. The value of Γ close to Tc from scaling approach is found to be ~ 2.5 that is slight higher compare to conventional approach (~2.2). Further, variation of anisotropy with temperature suggests that it is a type of multi-band superconductor.

cond-mat.supr-con↗

arXiv2015

Effect of Sb deficiency on the thermoelectric properties of Zn4Sb3

We have investigated the effect of Sb-deficiency on the thermoelectric figure of merit (zT) of Zn4Sb3 prepared by solid state reaction route. At high temperatures, the Seebeck coefficient (S) and electrical conductivity (σ) increase with increase in Sb deficiency whereas the thermal conductivity (\k{appa}) decreases giving rise to an increase in the overall zT value. The observations suggest that creation of vacancies could be an effective route in improving the thermoelectric properties of Zn4Sb3 system. This coupled to nanostructuring strategy could lead to the ultimate maximum value of zT in this system for high temperature thermoelectric applications.

cond-mat.mtrl-sci↗

arXiv2015

Enhancement in thermoelectric properties of FeSb2 by Sb site deficiency

We report a strategy based on introduction of point defects for improving the thermoelectric properties of FeSb2, a promising candidate for low temperature applications. Introduction of Sb deficiency to the tune of 20% leads to enhancement in the values of electrical conductivity (σ) and Seebeck coefficient (S) accompanied with a concomitant suppression in lattice thermal conductivity (\k{appa}lat) values in samples prepared using conventional solid state reaction route. These observations in polycrystalline FeSb2-x provides ample motivation for a dedicated exploration of thermoelectric behavior of the corresponding single crystalline as well as hot-pressed polycrystalline counterparts.

cond-mat.mtrl-sci↗

arXiv2015

Investigation of Magnetic Domain Structure in Bi0.7Dy0.3FeO3 thin films integrated with ZnO film

The magnetic domain structure of the multiferroic Bi1-xDyxFeO3 (BDFO) deposited on ZnO at the macroscopic level has been demonstrated in this paper. Magnetic properties are observed by saturated magnetic and ferroelectric hysteresis loops at room temperature. The coupling behaviour and magnetic transition are verified using Multimode Atomic Force Microscope by applying bias between sample and MFM tip. ZnO thin film of 300 nm was deposited by dielectric sputtering using a ZnO target on Si p-type conducting (.0001-0005 Ω cm) substrate. BDFO thin films of 300 nm were deposited on Si/ZnO using PLD (Pulsed laser deposition) technique. It is observed that BDFO thin films integrated with ZnO film shows the coexistence of ferromagnetic and ferroelectric ordering with significant coupling at room temperature. Integration of BDFO films with ZnO piezoelectric thin film shows the potential in MEMS applications as well as in Memory devices and strong history dependent systems.

cond-mat.mes-hall↗

arXiv2015

Unveiling of Bragg glass to vortex glass transition by an ac driving force in a single crystal of Yb3Rh4Sn13

We present here some striking discrepancies in the results of ac and dc magnetization measurements performed in a single crystal of low Tc superconductor, Yb3Rh4Sn13. Fingerprint of a transition from an ordered vortex lattice a la Bragg glass (BG) phase to a partially-disordered vortex glass (VG) like phase gets unearthed under the influence of an ac driving force present inevitably in the isothermal ac susceptibility measurements. In contrast to its well-known effect of improving the state of spatial order in the vortex matter, the ac drive is surprisingly found to promote disorder by assisting the BG to VG transition to occur at a lower field value in this compound. On the other hand, the isothermal dc magnetization (M-H) scans, devoid of such a driving force, do not reveal this transition; they instead yield signature of another order-disorder transition at elevated fields, viz., peak effect (PE), located substantially above the BG to VG transition observed in the isothermal ac susceptibility measurements. Further, the evolution of PE feature with increasing field as observed in isofield ac susceptibility plots indicates emergence of an ordered vortex configuration (BG) from a disordered phase above a certain field, H* (~ 4 kOe). Below H*, the vortex matter created via field-cooling (FC) is found to be better spatially ordered than that prepared in zero field-cooled (ZFC) mode. This is contrary to the usual behavior anticipated near the high-field order-disorder transition (PE) wherein a FC state is supposed to be a supercooled disordered phase and the ZFC state is comparatively better ordered.

cond-mat.supr-con↗

arXiv2015

Paramagnetic magnetization signals and curious metastable behaviour in field-cooled magnetization of a single crystal of superconductor 2H-NbSe2

We present here some newer characteristics pertaining to paramagnetic Meissner effect like response in a single crystal of the low Tc superconducting compound 2H-NbSe2 via a detailed study of effects of perturbation on the field-cooled magnetization response. In the temperature range, where an anomalous paramagnetic magnetization occurs, the field-cooled magnetization response is found to be highly metastable: it displays a curious tendency to switch randomly from a given paramagnetic value to a diamagnetic or to a different paramagnetic value, when the system is perturbed by an impulse of an externally applied ac field. The new facets revealed in a single crystal of 2H-NbSe2 surprisingly bear a marked resemblance with the characteristics of magnetization behaviour anticipated for the giant vortex states with multiple flux quanta predicted to occur in mesoscopic-sized superconducting specimen and possible transitions amongst such states.

cond-mat.supr-con↗

arXiv2014

Sign Reversal of anisotropic magnetoresistance in La0.7Ca0.3MnO3/STO ultrathin films

We present the observation of strain induced sign reversal of anisotropic magnetoresistance (AMR) in LCMO (LCMO) ultrathin films (thickness 4 nm) deposited on STO (001) substrate (STO). We have also observed unusually large AMR in LCMO/STO thin films with thickness of 6 nm below but close to its Curie temperature (TC) which decrease as the film thickness increases. The sign reversal of AMR (with a maximum value of - 6) with magnetic field or temperature for the 4 nm thin film may be attributed to the increase in tensile strain in the plane of the thin film which in turn facilitates the rotation of the magnetization easy axis.

cond-mat.mes-hall↗

arXiv2014

Magneto-Seebeck effect in spin-valve with in-plane thermal gradient

We present measurements of magneto-Seebeck effect on a spin valve with in-plane thermal gradient. We measured open circuit voltage and short circuit current by applying a temperature gradient across a spin valve stack, where one of the ferromagnetic layers is pinned. We found a clear hysteresis in these two quantities as a function of magnetic field. From these measurements, the magneto-Seebeck effect was found to be 0.82%.

cond-mat.mes-hall↗

arXiv2014

Evidence of surface superconductivity and multi-quanta vortex states in a weakly-pinned single crystal of Ca3Ir4Sn13

We report here the observation of anomalous paramagnetic signal(s) in the isofield field-cooled cool-down magnetization scans (MFCC(T)) recorded for a single crystal of a low Tc superconductor Ca3Ir4Sn13. Novel features emanating from the MFCC(T) response include an oscillatory magnetization behaviour below Tc and a rich multiplicity (non-uniqueness) in magnetization ranging from diamagnetism to paramagnetism at a given H, T value. The metastability in MFCC(T) has been ascribed to non-unique coexistence of multi-quanta vortex states and single quantum (Abrikosov) vortices. Additionally, the isothermal M(H) scans recorded across a short window of temperature just below Tc show evidence for only the multi-quanta vortex states in the domain of surface superconductivity, with no fingerprint(s) of pinned Abrikosov lattice.

cond-mat.supr-con↗

arXiv2014

Dual role of an ac driving force and the underlying two distinct order-disorder transitions in the vortex phase diagram of Ca3Ir4Sn13

We present distinct demarcation of the Bragg glass (BG) to multi-domain vortex glass (VG) transition line and the eventual amorphization of the VG phase in a weakly pinned single crystal of the superconducting compound Ca3Ir4Sn13 on the basis of comprehension of the different yields about the second magnetization peak (SMP) anomaly in the dc magnetization and the corresponding anomalous feature in the ac susceptibility measurements. The shaking by a small ac magnetic field, inevitably present in the ac susceptibility measurements, is seen to result in contrasting responses in two different portions of the field-temperature (H, T) phase space of the multi-domain VG. In one of the portions, embracing the BG to VG transition across the onset of the SMP anomaly, the ac drive is surprisingly seen to assist the transformation of the well ordered BG phase to a lesser ordered VG phase. The BG phase exists as a superheated state over a small portion of the VG space and this attests to the first order nature of the BG to VG transition.

cond-mat.supr-con↗

arXiv2013

Spintronic Oscillator Based on Magnetic Field Feedback

We present a circuit design of a spintronic oscillator based on magnetic tunnel junction. In this design, a dc current is passed through a magnetic tunnel junction which is connected to a feed-back wire below it. Any fluctuation in the magnetization direction of the free layer of MTJ, drives a fluctuating current through the feed-back wire, which exerts a magnetic field on the free layer. This in turn can amplify the magnetization fluctuations of the free layer. If the dc current passing through the MTJ is more than a critical value, continuous precessing states of the magnetization are possible.

cond-mat.mes-hall↗

arXiv2013

Multiple charge density wave transitions in Lu_{2} Ir_{3} Si_{5} single crystal

The physical properties of the single-crystalline samples of Lu_{2} Ir_{3} Si_{5} have been investigated by magnetic susceptibility, resistivity and heat capacity studies. We observed multiple charge density wave (CDW) transitions in all the measurements. A strong thermal hysteresis at these transitions suggests a possible first order CDW ordering. In addition, the first order nature is ascertained by a very narrow and a huge cusp (62 J/mol K) in the zero field specific heat data which also suggests a strong interchain coupling. By applying a field of 9T in the specific heat measurement, one of the CDW transitions is suppressed.

cond-mat.str-el↗

arXiv2013

Growth, Characterization, Vortex Pinning and Vortex Flow Properties of Single Crystals of Iron Chalcogenide Superconductor FeCr$_{0.02}$Se

We report the growth and characterization of single crystals of iron chalcogenide superconductor FeCr$_{0.02}$Se. There is an enhancement of the superconducting transition temperature (T$_{\rm c}$) as compared to the T$_{\rm c}$ of the single crystals of the parent compound Fe$_{1+x}$Se by about 25%. The superconducting parameters such as the critical fields, coherence length, penetration depth and the Ginzburg-Landau parameter have been estimated for these single crystals. Analysis of the critical current data suggests a fluctuation in electronic mean free path induced ($δl$) pinning mechanism in this material. Thermally activated transport across the superconducting transition in the presence of external magnetic fields suggests a crossover from a single vortex pinning regime at low fields to a collective flux creep regime at higher magnetic fields. The nature of charge carriers in the normal state estimated from the Hall effect and thermal transport measurements could provide crucial information on the mechanism of superconductivity in Fe-based materials.

cond-mat.supr-con↗

arXiv2012

Nanostructured Zinc Oxide as a Prospective Room Temperature Thermoelectric Material

Nanostructured Zinc oxide (ZnO) was synthesized via a ball milling for 10 hours using high energy planetary ball mill. Phase purity and homogeneity of all the samples have been investigated by X-ray diffraction (XRD) and Field Emission Scanning Electron Microscopy (FE-SEM). All the diffraction peaks can be indexed to the hexagonal phase ZnO with hexagonal symmetry (space group P63mc). Average crystallite size was observed to be 20 nm. There was a remarkable suppression in thermal conductivity (κ) compared to the bulk values by a factor of ~50 at room temperature. This suggests to the possibility of using nanostructured ZnO as a prospective room temperature thermoelectric material.

cond-mat.mtrl-sci↗

arXiv2011

Magnetization hysteresis and time decay measurements in FeSe$_{0.50}$Te$_{0.50}$ : Evidence for fluctuation in mean free path induced pinning

We present results of magnetic measurements relating to vortex phase diagram in a single crystal of FeSe$_{0.5}$Te$_{0.5}$ which displays second magnetization peak anomaly for $H \parallel c$. The possible role of the crystalline anisotropy on vortex pinning is explored via magnetic torque magnetometry. We present evidence in favor of pinning related to spatial variations of the charge carrier mean free path leading to small bundle vortex pinning by randomly distributed (weak) pinning centers for both $H \parallel c$ and $H \perp c$. This is further corroborated using magnetization data for $H \parallel c$ in a single crystal of FeSe$_{0.35}$Te$_{0.65}$. Dynamical response across second magnetization peak (SMP) anomaly in FeSe$_{0.5}$Te$_{0.5}$ has been compared with that across the well researched phenomenon of peak effect (PE) in a single crystal of CeRu$_2$.

cond-mat.supr-con↗

arXiv2011

Crossover from paramagnetic compressed flux regime to diamagnetic pinned vortex lattice in a single crystal of cubic Ca(3)Rh(4)Sn(13)

We report the observation of positive magnetization on field cooling (PMFC) in low applied magnetic fields (H < 100 Oe) in a single crystal of Ca(3)Rh(4)Sn(13) near its superconducting transition temperature (T(c) approx 8.35 K). For 30 Oe < H < 100 Oe, the PMFC response crosses over to a diamagnetic response as the temperature is lowered below 8 K. For 100 Oe < H < 300 Oe, the diamagnetic response undergoes an unexpected reversal in its field dependence above a characteristic temperature (designated as T*(VL) = 7.9 K), where the field-cooled cool-down magnetization curves intersect. The in-phase and out-of-phase ac susceptibility data confirm the change in the superconducting state across T*(VL). We ascribe the PMFC response to a compression of magnetic flux caused by the nucleation of superconductivity at the surface of the sample. In very low fields (H < 20 Oe), the PMFC response has an interesting oscillatory behaviour which persists up to about 7 K. The oscillatory nature underlines the interplay between competing responses contributing to the magnetization signal in PMFC regime. We believe that the (i) counterintuitive field dependence of the diamagnetic response for H > 100 Oe and above T*(VL) (lasting up to T(c), (ii) the oscillatory character in PMFC response at low fields and (iii) the PMFC peaks near 8.2 K in 30 Oe <= H <= 100 Oe provide support in favour of a theoretical scenario based on the Ginzburg-Landau equations. The scenario predicts the possibility of complex magnetic fluctuations associated with transformation between different metastable giant vortex states prior to transforming into the conventional vortex state as the sample is cooled below T*(VL).

cond-mat.supr-con↗

arXiv2011

Multiferroic properties and magnetic structure of Sm1-xYxMnO3

We have successfully induced multiferroic behavior in the A-type antiferromagnet SmMnO3 by the substitution of Y at the Sm site. A magnetic transition develops at 24 K for Sm1-xYxMnO3 (x=0.4,0.5) which is not present in the parent compound. This transition coincides with the onset of electric order, with an electric polarization measured along the c axis. It is proposed that the effect of Y doping is to bring about a subtle distortion of the MnO6 octahedra, causing a magnetic ordering of the Mn3+ moments similar to that reported for the well studied multiferroic TbMnO3. Following on from our previous study on polycrystalline samples, we present measurements of the magnetic and electric properties of single crystal Sm0.6Y0.4MnO3 and Sm0.5Y0.5MnO3. The data are summarized in a phase diagram for each of the principal crystallographic axes for the x=0.5 compound. Powder neutron diffraction experiments on SmMnO3 and Sm0.6Y0.4MnO3 show that the Y substitution causes a change in the Mn-O-Mn bond angle towards the value found for TbMnO3. The magnetic structure of Sm0.6Y0.4MnO3 has been shown to consist of two phases: a sinusoidal ordering of the Mn3+ moments below 50 K and a cycloidal ordering below 27 K. The cycloidal ordering occurs at the same temperature as the previously observed ferroelectric polarization, suggesting a similar multiferroic mechanism to that found in TbMnO3.

cond-mat.str-el↗