arXiv subjects

A. K. Raychaudhuri

Publications and source records attributed to A. K. Raychaudhuri.

At least 37 records · Page 2Linked to original sources

arXiv2011

Voltage bias induced modification of all oxide Pr0.5Ca0.5MnO3/SrTi0.95Nb.05O3 junctions

In this paper we report what happens to a pristine oxide junction Pr0.5Ca0.5MnO3/SrTi0.95Nb.05O3 (PCMO/Nb:STO), when it is subjected to cycling of voltage bias of moderate value ({\pm}4V). It is found that the initial cycling leads to formation of a permanent state of lower resistance where the lower resistance arises predominantly due to development of a shunt across the device film (PCMO). On successive voltage cycling with increasing magnitude, this state transforms into states of successive lower resistance that can be transformed back to the initial stable state on cycling to below a certain bias. A simple model based on p-n junction with shunt has been used to obtain information on the change of the junction on voltage cycling. It has been shown that the observation can be explained if the voltage cycling leads to lowering of barrier at the interface and also reduction in series resistance. It is suggested that this lowering can be related to the migration of oxygen ions and vacancies at the junction region. Cross-sectional imaging of the junction shows formation of permanent filamentary bridges across the thickness of the PCMO after the pristine p-n junction is first taken through a voltage cycle, which would explain appearance of a finite shunt across the p-n junction.

cond-mat.mtrl-sci↗

arXiv2011

Effect of size reduction on the ferromagnetism of the manganite La1-xCaxMnO3 (x = 0.33)

In this paper we report an investigation on the ferromagnetic state and the nature of ferromagnetic transition of nanoparticles of $\mathrm{La_{0.67}Ca_{0.33}MnO_3}$ using magnetic measurements and neutron diffraction. The investigation was made on nanoparticles with crystal size down to $15$ nm. The neutron data show that even down to a size of $15$ nm the nanoparticles show finite spontaneous magnetization ($M_S$) although the value is much reduced compared to the bulk sample. We observed a non-monotonic variation of the ferromagnetic to paramagnetic transition temperature $T_C$ with size $d$ and found that $T_C$ initially enhances on size reduction, but for $d < 50$ nm it decreases again. The initial enhancement in $T_C$ was related to an increase in the bandwidth that occured due to a compaction of the Mn-O bond length and a straightening of the Mn-O-Mn bond angle, as determined form the neutron data. The size reduction also changes the nature of the ferromagnetic to paramagnetic transition from first order to second order with critical exponents approaching mean field values. This was explained as arising from a truncation of the coherence length by the finite sample size.

cond-mat.str-el↗

arXiv2010

Single step precursor free synthesis and characterisation of stable Au nanochains by laser ablation

In this paper we report a simple one step and one-pot synthesis of stable assembly of Au nanoparticles (diameter 8-10nm) into chains in an Ethylene Glycol medium, using only a solid metallic Au target and a pulsed excimer laser (λ=248nm). The process reported does not use any precursor, reducing agent or surfactant and thus can be described as chemistry free synthesis route. The Au nanoparticle-ethylene glycol nanochains (with unbroken lengths often more than few microns) formed in liquid medium are mechanically as well as thermally stable and can be transferred unchanged into a solid substrate which can span a large surface area. The nanochains show a broad optical absorption covering almost the complete visible spectrum. A hybrid consisting of Au nanochains and separated nanoparticles can be formed by the same method using a proper choice of the laser fluence and Ethylene Glycol /DI water concentration. The Au nanochain - Ethylene Glycol hybrid material formed by the above method shows enhanced low frequency dielectric constant (one order more than the Ethylene Glycol) and enhanced electrical conductivity even with low Au fill fraction can show enhancement up to two orders. Based on 2D-NMR experiments we suggest that the quasi-1D chain like structure forms due to the formation of dimer and trimers of Ethylene Glycol molecules that attach to the Au nanoparticles formed by the ablation process and facilitate the chain formation.

cond-mat.mtrl-sci↗

arXiv2009

Resistance fluctuations and 1/f noise in single crystalline Ni nanowires

We measured the low frequency (10mHz < f < 10Hz) resistance fluctuations (Noise) in single crystalline ferromagnetic Ni nanowires (diameter ~35nm) in the temperature range 80K-300K. The noise spectral power shows 1/f dependence. The nanowires in an applied magnetic field show negative magetoresistance that saturates for H <= HC. The noise spectral power shows a reduction in low applied field and becomes field independent for H >= HC. This indicates that a part of the observed 1/f noise arises from magnetic origin. The magnetic part is associated with thermally activated domain wall fluctuations that couples to the resistance fluctuations.

cond-mat.mes-hall↗

arXiv2009

Temperature dependent electrical resistivity of a single strand of ferromagnetic single crystalline nanowire

We have measured the electrical resistivity of a single strand of a ferromagnetic Ni nanowire of diameter 55 nm using a 4-probe method in the temperature range 3 K-300 K. The wire used is chemically pure and is a high quality oriented single crystalline sample in which the temperature independent residual resistivity is determined predominantly by surface scattering. Precise evaluation of the temperature dependent resistivity ($ρ$) allowed us to identify quantitatively the electron-phonon contribution (characterized by a Debye temperature $θ_R$) as well as the spin-wave contribution which is significantly suppressed upon size reduction.

cond-mat.mes-hall↗

arXiv2009

Field induced reversible control of visible luminescence in ZnO nanostructures

In this work, a reversible control over the visible luminescence of phosphor ZnO is achieved by the application of a few volts (<5V) to nanostructured ZnO film sandwiched between ITO and LiClO4/PEO solid electrolyte. Since ZnO is a good absorbing material but ITO-Glass substrate is transparent in the range of 320-375 nm, the ZnO-ITO interface has been illuminated by a 345 nm light passing through the ITO glass substrate and the emitted light near 545 nm has been collected in reflection mode. When the solid polymer electrolyte is negatively biased, up to 107 percent enhancement in the visible luminescence has been observed, whereas a complete quenching (87 percent reduction) of the visible emission has been seen when it is positively biased. The sharp near band edge (NBE) emission in the ultraviolet region however has no dependence within the voltage range we are working. The luminescence output follows variations in the applied voltage up to maximum frequency limited by response time of the device which lies within the range of 20-30 sec. The upward band bending created by application of negative bias to the conductive polymer populates increasing number of defect levels above the Fermi level which enhances the visible photoluminescence and vice versa.

cond-mat.mtrl-sci↗

arXiv2009

Frequency dependence of dielectric anomaly around Neel temperature in bilayer manganite Pr(Sr0.1Ca0.9)2Mn2O7

A novel frequency dependence of anomaly in dielectric constant versus temperature plot, around the Neel temperature T_N (~150 K), has been observed in a single crystal of bilayer manganite Pr(Sr0.1Ca0.9)2Mn2O7. The anomaly in the permittivity (epsilon'||c) occurs at a temperature T_f which moves within a temperature window (delT_f) of ~40 K around T_N for a frequency range 50 kHz-5 MHz. The capacitive component Cp of the dielectric response exhibits a clear yet broad feature around T_N which establishes the intrinsic capacitive nature of the anomaly.

cond-mat.str-el↗

arXiv2008

Electrical resistance of Ni nanowires (diameter greater than or equal to 20 nm) near the Curie Temperatures

In this letter we report electrical transport measurements on nickel nanowires of diameters down to 20 nm in the region close to its paramagnetic-ferromagnetic transition temperature T_C (reduced temperature|t| less than or equal to 0.001). The data analysis done in the frame work of critical behavior of resistance near TC shows that the critical behavior persists even down to the lowest diameter wire of 20 nm. However, there is a suppression of the critical behavior of the resistivity as measured by the critical exponents and the parameters quantifying the anomaly. The spin system shows approach to a quasi one-dimensional spin system.

cond-mat.mtrl-sci↗

arXiv2008

Effect of intrinsic instability of cantilevers on static mode Atomic Force Spectroscopy

We show that the static force spectroscopy curve is significantly modified due to presence of intrinsic cantilever instability. This instability acts in tandem with such instabilities like water bridge or molecular bond rupture and makes the static force spectroscopy curve (including "jump-off-contact") dependent on the step-size of the movement of sample stage. A model has been proposed to explain the data. This has been further validated by applying an electric field between tip and substrate which modifies the tip-substrate interaction.

cond-mat.mtrl-sci↗

arXiv2008

A method to quantitatively evaluate Hamaker constant using the jump-into-contact effect in Atomic Force microscopy

We find that the jump-into-contact of the cantilever in the atomic force microscope (AFM) is caused by an inherent instability in the motion of the AFM cantilever. The analysis is based on a simple model of the cantilever moving in a nonlinear force field. We show that the jump-into-contact distance can be used to find the interaction of the cantilever tip with the surface. In the specific context of the attractive van der Waals interaction, this method can be realized as a new method of measuring the Hamaker constant for materials. The Hamaker constant is determined from the deflection of the cantilever at the jump-into-contact using the force constant of the cantilever and the tip radius of curvature, all of which can be obtained by measurements. The results have been verified experimentally on a sample of cleaved mica, a sample of Si wafer with natural oxide and a silver film, using a number of cantilevers with different spring constants. We emphasize that the method described here is applicable only to surfaces that have van der Waals interaction as the tip-sample interaction. We also find that the tip to sample separation at the jump-into-contact is simply related to the cantilever deflection at this point, and this provides a method to exactly locate the surface.

cond-mat.mtrl-sci↗

arXiv2008

Ionic environment control of visible photo-luminescence from ZnO nanoparticles

We report a novel effect that the visible photoluminescence (in the blue-green band) from ZnO anoparticles can be controlled by changing the ionic or polar nature of the medium in which the nanoparticles are dispersed. We find that the presence of sufficient amount of electrolytes can even quench the emission. We propose an explanation based on surface charge of the ZnO nanoparticles which control the band bending in the depletion layer at the surface of the anoparticles. The band bending in turn, decides the predominant nature of the visible emission. The explanation is validated by establishing a direct correlation between the visible emission and the zeta potential.

cond-mat.mtrl-sci↗

arXiv2008

Shape transition in ZnO nanostructures and its effect on blue-green photoluminescence

We report that ZnO nanostructures synthesized by chemical route undergo a shape transition at ~ 20 nm from spherical to hexagonal morphology thereby changing the spectral components of the blue-green emission. Spherically shaped nanocrystals (size range 11 -18 nm) show emission in the range of 555-564 nm and the emission shifts to the longer wavelength as the size increases. On the other hand, rods and hexagonal platelets (size range 20-85 nm), which is the equilibrium morphology after the shape transition, show emission near 465-500 nm and it shifts to shorter wavelength as the size increases. The shape transition also leads to relaxation of microstrain in the system. Our analysis shows that the visible emission originates from a defect layer on the nanostructure surface which is affected by the shape transition. The change in the spectral component of the blue green emission on change of shape has been explained as arising from band bending due to depletion layer in smaller spherical particles which is absent in the larger particles with flat faces.

cond-mat.mtrl-sci↗

arXiv2008

Hot electron effects and non-linear transport in hole doped manganites

We show that strong non--linear electron transport in the ferromagnetic insulating (FMI) state of manganites, responsible for phenomena such as colossal electroresistance and current induced resistance switching, can occur due to a hot electron effect. In the FMI state, which we show is an insulator with a Coulomb gap, the temperature of the electron and lattice baths can decouple at high input power levels, leading to heating of the electron bath. Parameters of the hot electron effect model were independently determined via time dependence experiments and are in good agreement with the experimental values.

cond-mat.str-el↗

arXiv2008

Size induced change-over from first to second order magnetic phase transition in La0.67Ca0.33MnO3 nanoparticles

In this report we show that in the perovskite manganite La_{1-x}Ca_{x}MnO_3 for a fixed x \approx 0.33, the magnetic transition changes over from first order to second order on reducing the particle size to nearly few tens of a nanometer. The change-over is brought about only by reducing the size and with no change in the stoichiometry. The size reduction to an average size of about 15 nm retains the ferromagnetic state albeit with somewhat smaller saturation magnetization and the ferromagnetic transition temperature T_{C} is suppressed by a small amount (4%). The magnetization of the nanoparticles near T_{C} follow the scaling equation $M/|ε|^β= f_\pm(H/|ε|^{γ+β})$, where, $ε= |T-T_C|/T_C$. The critical exponents, associated with the transition have been obtained from modified Arrott plots and they are found to be $β=0.47\pm 0.01$ and $γ=1.06\pm 0.03$. From a plot of M vs H at T_{C} we find the exponent $δ=3.10 \pm 0.13$. All the exponents are close to the mean field values. The change-over of the order of the transition has been attributed to a lowering of the value of the derivative dT_{C}/dP due to an increased pressure in the nanoparticles arising due to size reduction. This effect acts in tandem with the rounding off effect due to random strain in the nanoparticles.

cond-mat.str-el↗

arXiv2008

Crystal structure and physical properties of half-doped manganite nanocrystals with size < 100nm

In this paper we report the structural and property (magnetic and electrical transport) measurements of nanocrystals of half-doped $\mathrm{La_{0.5}Ca_{0.5}MnO_3}$(LCMO) synthesized by chemical route, having particle size down to an average diameter of 15nm. It was observed that the size reduction leads to change in crystal structure and the room temperature structure is arrested so that the structure does not evolve on cooling unlike bulk samples. The structural change mainly affects the orthorhombic distortion of the lattice. By making comparison with observed crystal structure data under hydrostatic pressure it is suggested that the change in the crystal structure of the nanocrystals occurs due to an effective hydrostatic pressure created by the surface pressure on size reduction. This not only changes the structure but also causes the room temperature structure to freeze-in. The size reduction also does not allow the long supercell modulation needed for the Charge Ordering, characteristic of this half-doped manganite, to set-in. The magnetic and transport measurements also show that the Charge Ordering (CO) does not occur when the size is reduced below a critical size. Instead, the nanocrystals show ferromagnetic ordering down to the lowest temperatures along with metallic type conductivity. Our investigation establishes a structural basis for the destabilization of CO state observed in half-doped manganite nanocrystals.

cond-mat.str-el↗

arXiv2007

Localized reversible nanoscale phase separation in Pr_0.63Ca_0.37MnO_3 single crystal using a scanning tunneling microscope tip

We report the destabilization of the charge ordered insulating (COI) state in a localized region of Pr_0.63Ca_0.37MnO_3 single crystal by current injection using a scanning tunneling microscope tip. This leads to controlled phase separation and formation of localized metallic nanoislands in the COI matrix which have been detected by local tunneling conductance mapping. The metallic regions thus created persist even after reducing the injected current to lower values. The original conductance state can be restored by injecting a current of similar magnitude but of opposite polarity. We thus achieve reversible nanoscale phase separation that gives rise to the possibility to "write, read, and erase" nanosized conducting regions in an insulating matrix with high spatial resolution.

cond-mat.mtrl-sci↗

arXiv2007

Scanning Thermal Microscope Study of a Metal Film Under Current Stressing: Role of Temperature Inhomogeneity in Damage Process

We report direct observation of the evolution of local temperature inhomogeneity and the resulting atomic migration in a metal film (Ag on Si) stressed by a current by using a Scanning Thermal Microscope that allows simultaneous temperature mapping and topography imaging. The experimental observation is analyzed using a model based simulation. The experimental observation and the simulation show that due to current stressing the temperature of the film becomes significantly inhomogeneous over time (with local temperature deviating strongly from the mean). This creates local stress as well as local temperature gradient that lead to mass migration in addition to the electromigration. We show that the local temperature inhomogeneity serves as one of the main agents for local atomic migration which leads to change in film microstructure. The migration leads to damage and eventual failure as simultaneously monitored by in-situ resistance measurement.

cond-mat.other↗

arXiv2007

Colossal electroresistance in ferromagnetic insulating state of single crystal Nd$_0.7$Pb$_0.3$MnO$_3$

Colossal electroresistance (CER) has been observed in the ferromagnetic insulating (FMI) state of a manganite. Notably, the CER in the FMI state occurs in the absence of magnetoresistance (MR). Measurements of electroresistance (ER) and current induced resistivity switching have been performed in the ferromagnetic insulating state of a single crystal manganite of composition Nd$_0.7$Pb$_0.3$MnO$_3$ (NPMO30). The sample has a paramagnetic to ferromagnetic (Curie) transition temperature, Tc = 150 K and the ferromagnetic insulating state is realized for temperatures, T <~ 130 K. The colossal electroresistance, arising from a strongly nonlinear dependence of resistivity ($ρ$) on current density (j), attains a large value ($\approx 100%$) in the ferromagnetic insulating state. The severity of this nonlinear behavior of resistivity at high current densities is progressively enhanced with decreasing temperature, resulting ultimately, in a regime of negative differential resistivity (NDR, d$ρ$/dj < 0) for temperatures <~ 25 K. Concomitant with the build-up of the ER however, is a collapse of the MR to a small value (< 20%) even in magnetic field, H = 7 T. This demonstrates that the mechanisms that give rise to ER and MR are effectively decoupled in the ferromagnetic insulating phase of manganites. We establish that, the behavior of ferromagnetic insulating phase is distinct from the ferromagnetic metallic (FMM) phase as well as the charge ordered insulating (COI) phase, which are the two commonly realized ground state phases of manganites.

cond-mat.str-el↗