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I. Das

Publications and source records attributed to I. Das.

At least 37 records · Page 2Linked to original sources
arXiv2014

Direct experimental evidence of multiferroicity in a nanocrystalline Zener polaron ordered manganite

We discuss the particle size driven tunability of the coexistence of ferromagnetism and ferroelectricity in Pr0.67Ca0.33MnO3(PCMO) with the help of x ray diffraction, magnetization, impedance spectroscopy, and remanent polarization measurements. The remanent polarization measurements using the positive up negative down method clearly prove the existence of ferroelectricity in PCMO with phase separation between Zener polaron(ZP) ordered P21nm and disordered Pbnm structures. We also find that the ferroelectric response is enhanced in nanocrystalline samples so long as ZP ordering is not destroyed while the long-range antiferromagnetic ordering at low temperature in bulk system is replaced by ferromagnetic correlations in nanoparticles. The conclusion that by reducing the crystallite size it might be possible to make ferromagnetism and ferroelectricity coexist near room temperature should be generally applicable to all ZP ordered manganites.

cond-mat.str-el↗
arXiv2012

On generalized multinomial models and joint percentile estimation

This article proposes a family of link functions for the multinomial response model. The link family includes the multicategorical logistic link as one of its members. Conditions for the local orthogonality of the link and the regression parameters are given. It is shown that local orthogonality of the parameters in a neighbourhood makes the link family location and scale invariant. Confidence regions for jointly estimating the percentiles based on the parametric family of link functions are also determined. A numerical example based on a combination drug study is used to illustrate the proposed parametric link family and the confidence regions for joint percentile estimation.

math.ST↗
arXiv2010

Colossal enhancement of magnetoresistance in La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ / Pr$_{0.67}$Ca$_{0.33}$MnO$_{3}$ multilayers: reproducing the phase-separation scenario

Colossal enhancement of magnetoresistance has been achieved over a broad temperature range which extends upto the room temperature, in ferromagnetic metal-charge ordered insulator manganite multi-layers. The artificially created phase coexistence in the multilayers reproduce the characteristic signatures of metastability in the magnetotransport properties commonly observed in electronically phase-separated manganites.

cond-mat.mes-hall↗
arXiv2009

Inverse magnetocaloric effect in polycrystalline La$_{0.125}$Ca$_{0.875}$MnO$_{3}$

Recently the inverse magnetocaloric effect is observed for different compounds. However there is very rare for any manifestation of the effect to be seen in manganites. We have found inverse magnetocaloric effect in the case of polycrystalline La$_{0.125}$Ca$_{0.875}$MnO$_{3}$. Such phenomenon is attributed to the stabilization of antiferromagnetic state associated with inherent magnetic inhomogeneous phases for this compound.

cond-mat.mtrl-sci↗
arXiv2009

Magnetocaloric properties of nanocrystalline La$_{0.125}$Ca$_{0.875}$MnO$_{3}$

Some recent experimental studies show the invisibility of antiferromagnetic transition in the cases of manganites when their particle size is reduced to nanometer scale. In complete contrast to these cases, we have observed the signature of antiferromagnetic transition in the magnetocaloric properties of nanocrystalline La$_{0.125}$Ca$_{0.875}$MnO$_{3}$ of average particle size 70 and 60 nm similar to its polycrystalline bulk form. The system exhibit inverse magnetocaloric effect in its polycrystalline and nanocrystalline form. An extra ferromagnetic phase is stabilized at low temperature for the sample with particle size $\sim 60$ nm.

cond-mat.mtrl-sci↗
arXiv2007

Experimental observation of charge ordering in nanocrystalline Pr$_{0.65}$Ca$_{0.35}$MnO$_{3}$

Observation of charge ordering in single crystalline and bulk polycrystalline systems of various rare-earth based manganites is well documented. However, there is hardly any manifestation of the same when the grain size is reduced to nanoscale. We have observed charge ordering in case of nanocrystalline Pr$_{0.65}$Ca$_{0.35}$MnO$_{3}$ of average particle size 40 nm. This phenomenon is attributed to the primary role played by the martensitic character of the charge order transition in the material.

cond-mat.mtrl-sci↗
arXiv2007

Charge ordering in Nd1=2Sr1=2MnO3 Nanoparticles

Nanoparticles of Nd1=2Sr1=2MnO3 of average particle sizes 12 nm and 25 nm are prepared by sol-gel technique. Transport, magnetotransport, magnetization and I-V characteristics studies have been performed on the nanoparticles. Although the particle size is order of magnitude smaller than the correlation length of charge ordering of the bulk material,the existence of charge ordered state has been observed in the nanoparticles at low temperature.

cond-mat.mtrl-sci↗
arXiv2006

Influence of Domain Wall on Magnetocaloric Effect in GdPt$_{2}$

The resistivity, magnetoresistance and in-field heat capacity measurements were performed on GdPt$_{2}$ intermetallic compound. The magnetocaloric parameters $ΔT_{ad}$ and $-ΔS$ were derived from the in-field heat capacity data. Comparison has been made between the magnetocaloric effect $-ΔS$ and difference in resistivity $-Δρ$ $(=ρ(H)-ρ(0))$ as a function of temperature. There is distinct difference in the temperature dependence of $-ΔS$ and $-Δρ$ below the ferromagnetic transition temperature. However after removing the domain wall contribution from $-Δρ$, the nature of $-ΔS$ and $-Δρ$ dependence as a function of temperature are similar. Our observation indicates that the domain wall contribution in magnetocaloric effect is negligible in spite of the fact that it has significant contribution in magnetotransport.

cond-mat.mtrl-sci↗
arXiv2006

Pinhole and tunneling conduction channels superimposed in magnetic tunnel junction: results and inferences

The influence of ballistic channels superimposed on tunneling conduction channels in magnetic tunnel junctions has been studied in a manganese oxide based tunneling device. Inversion of magnetoresistance has been observed in magnetic tunnel junctions with pinhole nanocontacts over a broad temperature range. The tunnel magnetoresistance undergoes a change of sign at higher bias and temperature. This phenomenon is attributed to the parallel conduction channels consisting of spin conserved ballistic transport through the pinhole contact where the transmission probability is close to unity and spin polarized tunneling across the insulating spacer with weak transmittivity. The results seem to resolve a controversy regarding ballistic magnetoresistance in ferromagnetic nanocontacts and establishes that ballistic magnetoresistance do exist even if the previous results are attributed to magnetostriction and magnetostatic force related artifacts.

cond-mat.mes-hall↗
arXiv2006

Giant enhancement of room temperature magnetoresistance in La_{0.67}Sr_{0.33}MnO_{3}/Nd_{0.67}Sr_{0.33}MnO_{3} multilayers

The metal-insulator transition temperature in CMR manganites has been altered and brought close to the room temperature by preparing La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO)/ Nd$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (NSMO) multilayers with ultra thin individual layers of LSMO and NSMO. The LSMO/NSMO multilayers with ultra thin individual layers of thickness of about $10Å$ exhibits 150% magnetoresistance at 270 K whereas LSMO/NSMO multilayers with moderate individual layer thickness of about $40Å$ each exhibits a mere 15% magnetoresistance at the same temperature. We have shown that the reduction in thickness of the individual layers leads to increased spin fluctuation which results in the enhancement of magnetoresistance.

cond-mat.mes-hall↗
arXiv2006

Inversion of magnetoresistance in magnetic tunnel junctions : effect of pinhole nanocontacts

Inverse magnetoresistance has been observed in magnetic tunnel junctions with pinhole nanocontacts over a broad temperature range. The tunnel magnetoresistance undergoes a change of sign at higher bias and temperature. This phenomenon is attributed to the competition between the spin conserved ballistic transport through the pinhole contact where the transmission probability is close to unity and spin polarized tunneling across the insulating spacer with weak transmittivity.

cond-mat.mes-hall↗
arXiv2005

Modification of the Charge ordering in Pr$_{1/2}$Sr$_{1/2}$MnO$_{3}$ Nanoparticles

Transport and magnetic properties have been studied in two sets of sol-gel prepared Pr$_{1/2}$Sr$_{1/2}$MnO$_{3}$ nanoparticles having average particle size of 30 nm and 45 nm. Our measurements suggest that the formation of charge ordered state is largely affected due to lowering of particle size, but the ferromagnetic transition temperature ($T_{C}$) remains unaffected.

cond-mat.mtrl-sci↗
arXiv2005

Evidence of ratchet effect in nanowires of a conducting polymer

Ratchet effect, observed in many systems starting from living organism to artificially designed device, is a manifestation of motion in asymmetric potential. Here we report results of a conductivity study of Polypyrrole nanowires, which have been prepared by a simple method to generate a variation of doping concentration along the length. This variation gives rise to an asymmetric potential profile that hinders the symmetry of the hopping process of charges and hence the value of measured resistance of these nanowires become sensitive to the direction of current flow. The asymmetry in resistance was found to increase with decreasing nanowire diameter and increasing temperature. The observed phenomena could be explained with the assumption that the spatial extension of localized state involved in hopping process reduces as the doping concentration reduces along the length of the nanowires.

cond-mat.mes-hall↗
arXiv2005

Enhancement of low field magnetoresistance at room temperature in La$_{0.67}$Sr$_{0.33}$MnO$_{3}$/Al$_{2}$O$_{3}$ nanocomposite

Magnetotransport properties in a nanocrystalline La$_{0.67}$Sr$_{0.33}$MnO$_{3}$/micron sized Al$_{2}$O$_{3}$ granular composite with different concentrations of Al$_{2}$O$_{3}$ have been studied. The resistivity curves in absence of magnetic field and the various transport mechanisms which might account for the upturn in resistivity at low temperature, has been discussed. Enhancement of low field magnetoresistance at room temperature with the introduction of Al$_{2}$O$_{3}$ has been observed.

cond-mat.mtrl-sci↗
arXiv2005

Anomalous bias dependence of tunnel magnetoresistance in a magnetic tunnel junction

We have fabricated a spin-polarized tunneling device based on half metallic manganites incorporating $Ba_{2}LaNbO_{6}$ as insulating barrier. An anomalous bias dependence of tunnel magnetoresistance (TMR) has been observed, the first of its kind in a symmetric electrode tunnel junction with single insulating barrier. The bias dependence of TMR shows an extremely sharp zero bias anomaly, which can be considered as a demonstration of the drastic density of states variation around the Fermi level of the half metal. This serves as a strong evidence for the existence of minority spin tunneling states at the half-metal insulator interface.

cond-mat.mtrl-sci↗
arXiv2001

Thermal relaxation in charge ordered Pr$_{0.63}$ Ca$_{0.37}$ MnO$_3$ in presence of a magnetic field

We report observation of substantial thermal relaxation in single crystal of charge ordered system Pr$_{0.63}$Ca$_{0.37}$MnO$_3$ in an applied magnetic field of H = 8T. The relaxation is observed when the temperature is scanned in presence of a magnetic field in the temperature interval $T_{MH}<T<T_{CO}$ where $T_{CO}$ is the charge ordering temperature and $T_{MH}$ is charge melting temperature in a field. In this temperature range the system has coexisting charged ordered insulator (COI) and ferromagnetic metallic (FMM) phases. No such relaxation is observed in the COI state in H = 0T or in the FMM phase at $T < T_{MH}$ in presence of a magnetic field. We conclude that the thermal relaxation is due to two coexisting phases with nearly same free energies but separated by a potential barrier. This barrier makes the transformation from one phase to the other time-dependent in the scale of the specific heat experiment and gives rise to the thermal relaxation.

cond-mat.str-el↗
arXiv2001

Specific heat study of single crystalline Pr$_{0.63}$ Ca$_{0.37}$ MnO$_{3}$ in presence of a magnetic field

We present the results of a study of specific heat on a single crystal of Pr$_{0.63}$Ca$_{0.37}$MnO$_3$ performed over a temperature range 3K-300K in presence of 0 and 8T magnetic fields. An estimate of the entropy and latent heat in a magnetic field at the first order charge ordering (CO) transition is presented. The total entropy change at the CO transition which is $\approx$ 1.8 J/mol K at 0T, decreases to $\sim$ 1.5 J/mol K in presence of 8T magnetic field. Our measurements enable us to estimate the latent heat $L_{CO}$ $\approx$ 235 J/mol involved in the CO transition. Since the entropy of the ferromagnetic metallic (FMM) state is comparable to that of the charge-ordered insulating (COI) state, a subtle change in entropy stabilises either of these two states. Our low temperature specific heat measurements reveal that the linear term is absent in 0T and surprisingly not seen even in the metallic FMM state.

cond-mat.str-el↗