arXiv subjects

S. V. Bhat

Publications and source records attributed to S. V. Bhat.

At least 19 recordsLinked to original sources

arXiv2019

Unexpected Effectiveness of Berezinskii-Kosterlitz-Thouless Description of Electron Paramagnetic Resonance Linewidth Behaviour in the 3-Dimensional Manganite Bi$_{0.5}$Sr$_{0.5}$Mn$_{0.9}$Cr$_{0.1}$O$_3$

We find that the temperature dependence of the Electron Paramagnetic Resonance (EPR) linewidth observed in Bi$_{0.5}$Sr$_{0.5}$Mn$_{0.9}$Cr$_{0.1}$O$_3$ (BSMCO), as well as in certain other 3-dimensional manganites undergoing antiferromagnetic transitions is satisfactorily described by the Berezinskii-Kosterlitz-Thouless (BKT) model. We understand this unexpected result in terms of an effective 2-dimensional XY easy plane anisotropy induced by the magnetic field applied in the EPR experiment. This conclusion is supported by the field dependence of the BKT correlations observed in the quasi two-dimensional antiferromagnetic compound BaNi$_{2}$V$_{2}$O$_{8}$.

cond-mat.str-el↗

arXiv2019

Occurrence of Mixed Phase in Bi0.5Sr0.5Mn0.9Cr0.1O3 bulk sample: Electron Paramagnetic Resonance and Magnetization Studies

We study the effects of 10% Cr substitution in Mn sites of Bi0.5Sr0.5MnO3 on the antiferromagnetic (AFM) (TN ~ 110 K) transition using structural, magnetic and electron paramagnetic resonance (EPR) techniques. Field cooled (FC) and zero field cooled (ZFC) magnetization measurements done from 400 K down to 4 K show that the compound is in the paramagnetic (PM) phase till 50 K where it undergoes a transition to a short range ferromagnetic phase (FM). Electron paramagnetic resonance measurements performed in the temperature range 300 K till 80 K conform with the magnetization measurements as symmetric signals are observed owing to the paramagnetic phase. Below 80 K, signals become asymmetric. Electron paramagnetic resonance intensity peaks at ~ 110 K, the decreasing intensity below this temperature confirming the presence of antiferromagnetism. We conclude that below 50 K the magnetization and EPR results are consistent with a cluster glass phase of BSMCO, where ferromagnetic clusters coexist with an antiferromagnetic background.

cond-mat.str-el↗

arXiv2014

Disappearance of Electron-Hole Asymmetry in Nanoparticles of Nd(1-x)CaxMnO3 (x = 0.6; 0.4): Magnetization and Electron Paramagnetic Resonance Evidence

We study and compare magnetic and electron paramagnetic resonance behaviors of bulk and nanoparticles of Nd(1-x)CaxMnO3 in hole doped (x = 0.4;NCMOH) and electron doped (x = 0.6;NCMOE) samples. NCMOH in bulk form shows a complex temperature dependence of magnetization M(T), with a charge ordering (CO) transition at around 250 K, an antiferromagnetic (AFM) transition at around 150 K and a transition to a canted AFM phase/mixed phase at around 80 K. Bulk NCMOE behaves quite differently with just a charge ordering transition at around 280 K, thus providing a striking example of the so called electron-hole asymmetry. While our magnetization data on bulk samples are consistent with the earlier reports, the new results on the nanoparticles bring out drastic effects of size reduction. They show that M(T) behaviors of the two nano samples are essentially similar in addition to the absence of the charge order in them thus providing strong evidence for vanishing of the electron-hole asymmetry in nanomanganites. This conclusion is further corroborated by electron paramagnetic resonance studies which show that the large difference in the g-values and their temperature dependence found for the two bulk samples disappears as they approach a common behavior in the corresponding nano samples.

cond-mat.str-el↗

arXiv2013

Magnetization in electron- and Mn- doped SrTiO3

Mn-doped SrTiO_3.0, when synthesized free of impurities, is a paramagnetic insulator with interesting dielectric properties. Since delocalized charge carriers are known to promote ferromagnetism in a large number of systems via diverse mechanisms, we have looked for the possibility of any intrinsic, spontaneous magnetization by simultaneous doping of Mn ions and electrons into SrTiO_3 via oxygen vacancies, thereby forming SrTi_(1-x)Mn_xO_(3-d), to the extent of making the doped system metallic. We find an absence of any enhancement of the magnetization in the metallic sample when compared with a similarly prepared Mn doped, however, insulating sample. Our results, thus, are not in agreement with a recent observation of a weak ferromagnetism in metallic Mn doped SrTiO_3 system.

cond-mat.mtrl-sci↗

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

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↗

arXiv2011

Tuning of dielectric properties and magnetism of SrTiO3 by site-specific doping of Mn

Combining experiments with first principles calculations, we show that site-specific doping of Mn into SrTiO3 has a decisive influence on the dielectric properties of these doped systems. We find that phonon contributions to the dielectric constant invariably decrease sharply on doping at any site. However, a sizable, random dipolar contribution only for Mn at the Sr site arises from a strong off-centric displacement of Mn in spite of Mn being in a non-d0 state; this leads to a large dielectric constant at higher temperatures and gives rise to a relaxor ferroelectric behavior at lower temperatures. We also investigate magnetic properties in detail and critically reevaluate the possibility of a true multi-glass state in such systems.

cond-mat.mtrl-sci↗

arXiv2010

Vanishing of Electron-Hole Asymmetry in Nano-Sized Charge Ordered Manganites Pr(1-x)Ca(x)MnO3 (x=0.36, 0.64)- EPR Studies

We observe a disappearance of electron-hole asymmetry in hole and electron doped manganites of composition Pr(1-x)Ca(x)MnO3 (x=0.36, 0.64), as reflected in the g-parameter values. The bulk property of opposite g-shifts in room temperature seems to have disappeared in nanoparticles. We analyze the results in the light of ferromagnetic fluctuations and melting of charge order in nanoparticles.

cond-mat.str-el↗

arXiv2009

Vitrification, relaxation and free volume in glycerol-water binary liquid mixture: Spin probe ESR studies

Glass transition and relaxation of the glycerol-water binary mixture system are studied over the glycerol concentration range of 5 - 85 mol% using the highly sensitive technique of spin probe ESR. For the water rich mixture the glass transition, sensed by the spin probe, arises from the vitrified mesoscopic portion of the binary system. The concentration dependence of the glass transition temperature manifests a closely related molecular level cooperativity in the system. A drastic change in the mesoscopic structure of the system at the critical concentration of 40 mol% is confirmed by an estimation of the spin probe effective volume in a temperature range where the tracer reorientation is strongly coupled to the system dynamics.

cond-mat.soft↗

arXiv2008

Spin Probe ESR Signature of Freezing in Water: Is it Global or Local?

First systematic spin probe ESR study of water freezing has been conducted using TEMPOL and TEMPO as the probes. The spin probe signature of the water freezing has been described in terms of the collapse of narrow triplet spectrum into a single broad line. This spin probe signature of freezing has been observed at an anomalously low temperature when a milimoler solution of TEMPOL is slowly cooled from room temperature. A systematic observation has revealed a spin probe concentration dependence of these freezing and respective melting points. These results can be explained in terms of localization of spin probe and liquid water, most probably in the interstices of ice grains, in an ice matrix. The lowering of spin probe freezing point, along with the secondary evidences, like spin probe concentration dependence of peak-to-peak width in frozen limit signal, indicates a possible size dependence of these localizations/entrapments with spin probe concentration. A weak concentration dependence of spin probe assisted freezing and melting points, which has been observed for TEMPO in comparison to TEMPOL, indicates different natures of interactions with water of these two probes. This view is also supported by the relaxation behavior of the two probes.

cond-mat.soft↗

arXiv2007

Temperature dependence of magnetic anisotropy in bulk and nanoparticles of Pr_0.5Sr_0.5MnO_3

Nanoparticles (size 20, 40 and 60 nm) of Pr_0.5Sr_0.5MnO_3 are prepared by sol-gel technique and their magnetic properties are studied using ferromagnetic resonance and magnetization measurements. A comparison with the properties of the bulk material shows that the ferromagnetic transition at 265 K remains unaffected but the anti-ferromagnetic transition at T_N = 150 K disappears in the nanoparticles. Further, the temperature dependence of magnetic anisotropy shows a complex behavior, being higher in the nanoparticles at high temperatures and lower at lower temperatures in comparison with the bulk.

cond-mat.str-el↗

arXiv2007

Non-resonant microwave absorption studies on MgB2 thin film

We report on the presence of weak links in the intermetallic superconductor MgB2 from the non-resonant microwave absorption (NRMA) studies on the MgB2 thin film. The strong low field signal, which is the characteristics of weak links and the presence of hysteresis, which arises due to flux trapping, are observed. The studies are carried out as a function of temperature, microwave power, magnitude of modulation field and the range of field scan. Presence of weak links is discussed in terms of increase in the oxygen content of grain boundaries in the thin film and because of disorder effects arises due to the aging of the sample. The pinning of flux lines with the changes in sweeping direction leads to hysteresis in the NRMA signal and shows the contribution of flux line motion in the power absorption.

cond-mat.supr-con↗

arXiv2006

Complete melting of charge order in hydrothermally grown Pr_0.57 Ca_0.41 Ba_0.02 MnO_3 nanowires

Nanowires of Pr_0.57 Ca_0.41 Ba_0.02 MnO_3 (PCBM) (diameter ~ 80-90 nm and length ~ 3.5 mm) were synthesized by a low reaction temperature hydrothermal method. Single-phase nature of the sample was confirmed by XRD experiments. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology and microstructures of the nanowires. While the bulk PCBM is known to exhibit charge order (CO) below 230 K along with a ferromagnetic transition at 110 K, SQUID measurements on the nanowires of PCBM show that the charge order is completely absent and a ferromagnetic transition occurs at 115 K. However, the magnetization in the nanowires is observed to be less compared to that in the bulk. This observation of the complete melting of the charge order in the PCBM nanowires is particularly significant in view of the observation of only a weakening of the CO in the nanowires of Pr_0.5 Ca_0.5 MnO_3. Electron paramagnetic resonance experiments were also carried out on the PCBM nanowires using an X-band EPR spectrometer. Characteristic differences were observed in the line width of nanowires when compared with that of the bulk.

cond-mat.str-el↗

arXiv2006

Equipartition of Current in Parallel Conductors on Cooling Through the Superconducting Transition

Our experiments show that for two or more pieces of a wire, of different lengths in general, combined in parallel and connected to a dc source, the current ratio evolves towards unity as the combination is cooled to the superconducting transition temperature Tc, and remains pinned at that value below it. This re-distribution of the total current towards equipartition without external fine tuning is a surprise. It can be physically understood in terms of a mechanism that involves the flux-flow resistance associated with the transport current in a wire of type-II superconducting material. It is the fact that the flux-flow resistance increases with current that drives the current division towards equipartition.

cond-mat.supr-con↗

arXiv2005

Vitrification and Glass Transition of Water: Insights from Spin Probe ESR

Three long standing problems related to the physics of water viz, the possibility of vitrifying bulk water by rapid quenching, its glass transition, and the supposed impossibility of obtaining supercooled water between 150 and 233 K, the so-called 'no man's land'of its phase diagram, are studied using the highly sensitive technique of spin probe ESR. Our results suggest that water can indeed be vitrified by rapid quenching, it undergoes a glass transition at \~ 135 K, and the relaxation behavior studied using this method between 165 K and 233 K closely follows the predictions of the Adam-Gibbs model.

cond-mat.mtrl-sci↗

arXiv2005

Non-resonant radio-frequency response in superconducting MgB2

Non-resonant radio frequency absorption (NRRA) of superconducting MgB2 pellet is investigated as a function of frequency, temperature, pressure, rf power, magnetic field, sample size and grain size. The NRRA signals of granular MgB2 samples show three separate and distinguish temperature dependent phase reversals, one of them is similar to the phase reversal occurs very often in high Tc superconductors at temperature few degrees below Tc, the other two are anomalous and occur at lower temperatures. Interpretation based on the assumptions in the line shape model indicates that the Josephson junctions are weakly coupled in MgB2 than other high temperature superconductors like YBa2Cu3O7, La1.9Sr0.1CuO4 and Bi2Sr2CaCu2O8. The samples show a significant magnetic field sweep hysteresis, which is a signature of trapped flux. The hysteresis behavior suggests that the vortices in the MgB2 are rather rigid. The effects of passing dc current through the sample on the NRRA signal patterns are discussed. It is seen that the transition temperature Tc measured using the NRRA technique decreases with decreasing sample size, whereas the Tc measured using ac susceptibility and temperature dependent resistivity is independent of the sample size. The results are discussed in terms of resistively shunted junction, flux flow and bond percolation models. Effect of MgO doping is analyzed and discussed. The intergranular coupling energy and the activation energy of MgB2 are calculated and compared with other high temperature superconductors. The effects of sintering on the weak links in MgB2 polycrystalline pellet are discussed.

cond-mat.supr-con↗

arXiv2005

AC loss in granular superconducting MgB2 at radio frequencies

AC losses of granular superconducting MgB2 were measured using non-resonant rf power absorption techniques. The presence of two band-gaps makes the temperature dependent ac loss pattern of MgB2 different from other high Tc superconductors like YBa2Cu3O and BSCCO-2223. Josephson junction decoupling plays a major role in MgB2 polycrystalline samples, both in the presence and absence of magnetic field. We studied the ac loss of MgB2 samples with different crystalline properties, grain sizes and pressures and sintered under different physical and chemical conditions. The effects of frequency and amplitude of ac current on the ac loss are discussed in detail.

cond-mat.supr-con↗