DST Unit for Nanosciences

Salt Lake, India

DST Unit for Nanosciences

Salt Lake, India

Time filter

Source Type

Venkata Kamalakar M.,DST Unit for Nanosciences | Raychaudhuri A.K.,DST Unit for Nanosciences
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

In this paper we report the study of the resistance anomaly close to the ferromagnetic phase transition temperature TC in Ni nanowires. The electrical resistance measurements on well characterized Ni nanowires of diameters down to 20 nm were carried out over an extensive temperature range (4-675 K), which encompasses the region close to TC (reduced temperature |t| ≤ 10⊃-2). The wires were grown in porous alumina templates by electrodeposition and the experiments were carried out by retaining the wires in the templates. The strain that is generated by retaining the wires in the templates was estimated and its effect was considered in the analysis. The data analysis done in the framework of critical behavior of resistance near TC reveals 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 characterized by the critical exponent α and the amplitude of the derivative dρ (T) dT at TC. We observed a considerable suppression of TC (as determined from resistance data). The shift in TC as well as the suppression of dρ (T) dT are explained as arising from finite size effects. © 2010 The American Physical Society.


Sarkar T.,DST Unit for NanoSciences | Sarkar T.,CNRS Crystallography and Material Science Laboratory | Raychaudhuri A.K.,DST Unit for NanoSciences | Raychaudhuri A.K.,CNRS Crystallography and Material Science Laboratory | And 2 more authors.
New Journal of Physics | Year: 2010

In this paper, we report an investigation of the ferromagnetic state and the nature of ferromagnetic transition of nanoparticles of La 0.67Ca0.33MnO3 using magnetic measurements and neutron diffraction. The investigation was performed 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 (MS) although the value is much reduced compared to the bulk sample. We observed a non-monotonic variation of the ferromagnetic-to-paramagnetic transition temperature TC with size d and found that TC initially enhances upon size reduction, but for d < 50 nm it decreases again. The initial enhancement in TC was related to an increase in the bandwidth that occurred due to a compaction of the Mn-O bond length and a straightening of the Mn-O-Mn bond angle, as determined from 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. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.


Biswas S.,DST Unit for NanoSciences | Raychaudhuri A.K.,DST Unit for NanoSciences | Sreeram P.A.,Indian Institute of Science | Dietzel D.,University of Munster | Dietzel D.,Karlsruhe Institute of Technology
Ultramicroscopy | Year: 2012

We have investigated experimentally the role of cantilever instabilities in determination of the static mode force-distance curves in presence of a dc electric field. The electric field has been applied between the tip and the sample in an atomic force microscope working in ultra-high vacuum. We have shown how an electric field modifies the observed force (or cantilever deflection)-vs-distance curves, commonly referred to as the static mode force spectroscopy curves, taken using an atomic force microscope. The electric field induced instabilities shift the jump-into-contact and jump-off-contact points and also the deflection at these instability points. We explained the experimental results using a model of the tip-sample interaction and quantitatively established a relation between the observed static mode force spectroscopy curves and the applied electric field which modifies the effective tip-sample interaction in a controlled manner. The investigation establishes a way to quantitatively evaluate the electrostatic force in an atomic force microscope using the static mode force spectroscopy curves. © 2012 Elsevier B.V.


Kamalakar M.V.,DST Unit for NanoSciences | Kamalakar M.V.,CNRS Institute of Genetics and of Molecular and Cellular Biology | Raychaudhuri A.K.,DST Unit for NanoSciences
New Journal of Physics | Year: 2012

We have studied the temperature-dependent (3-300 K) electrical resistance of metal nanowires and nanotubes of the same diameter with the specific aim to understand the changes in electrical transport brought about by a change in the geometry of a nanowire to a nanotube. Single crystalline nanowires and nanotubes of copper were synthesized by electrodeposition in nanoporous alumina templates. The temperature-dependent resistivity data have been analysed using the Bloch-Grüneisen function for the lattice contribution to resistivity, and the characteristic Debye temperature θ R was determined along with the residual resistivity ρ 0. Substantial size effects were observed in both the parameters ρ 0 and θ R, where the former is enhanced and the latter is suppressed from bulk to nanowires and further to nanotubes. It has been observed that the transport parameters in the nanotubes with wall thickness t are similar to those of a nanowire with diameter d, where d ≈ 2t in the specific size range used in this work. It is suggested that appreciable size effects in the electrical transport parameters occur due to the extra surface in the nanotube. In both nanotubes and nanowires, the single parameter that determines the size effect is the surface area to volume ratio. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.


Sarkar T.,DST Unit for NanoSciences | Sarkar T.,CNRS Crystallography and Material Science Laboratory | Kamalakar M.V.,DST Unit for NanoSciences | Raychaudhuri A.K.,DST Unit for NanoSciences
New Journal of Physics | Year: 2012

We report a comprehensive study of the electrical and magnetotransport properties of nanocrystals of La 0.67Ca 0.33MnO 3 (LCMO) (with size down to 15 nm) and La 0.5Sr 0.5CoO 3 (LSCO) (with size down to 35 nm) in the temperature range 0.3-5K and magnetic fields up to 14 T. The transport, magneto-transport and nonlinear conduction (I -V curves) were analysed using the concept of spin-polarized tunnelling in the presence of Coulomb blockade. The activation energy of transport, Δ, was used to estimate the tunnelling distances and the inverse decay length of the tunnelling wave function (χ) and the height of the tunnelling barrier (φ B). The magneto-transport data were used to find the magnetic field dependences of these tunnelling parameters. The data taken over a large magnetic field range allowed us to separate out the magnetoresistance (MR) contributions at low temperatures arising from tunnelling into two distinct contributions. In LCMO, at low magnetic field, the transport and MR are dominated by the spin polarization, while at higher magnetic field the MR arises from the lowering of the tunnel barrier by the magnetic field, leading to an MR that does not saturate even at 14 T. In contrast, in LSCO, which does not have substantial spin polarization, the first contribution at low field is absent, while the second contribution related to barrier height persists. The idea of intergrain tunnelling has been validated by direct measurements of the nonlinear I -V data in this temperature range, and the I -V data were found to be strongly dependent on magnetic field. We made the important observation that a gaplike feature (with magnitude ∼E C, the Coulomb charging energy) shows up in the conductance g(V) at low bias for the systems with the smallest nanocrystal size at the lowest temperatures (T ≤ 0.7 K). The gap closes when the magnetic field and temperature are increased. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.


Das S.,DST Unit for Nanosciences | Raychaudhuri A.K.,DST Unit for Nanosciences | Sreeram P.A.,Indian Institute of Science | Dietzel D.,University of Munster | Dietzel D.,Karlsruhe Institute of Technology
Nanotechnology | Year: 2010

We show that the static force spectroscopy curve taken in an atomic force microscope is significantly modified due to presence of intrinsic cantilever instability which occurs as a result of its movement in a nonlinear force field. This instability acts in tandem with such instabilities as water bridge or molecular bond rupture and makes the static force spectroscopy curve (including 'jump-off-contact') dependent on the step size of data collection. A theoretical model has been proposed to explain the data. We emphasize the necessity of taking care of this fundamental instability of the microcantilever in calculating the adhesive force and also in the interpretation of data taken using an atomic force microscope. © 2010 IOP Publishing Ltd.


We have investigated experimentally the role of cantilever instabilities in determination of the static mode force-distance curves in presence of a dc electric field. The electric field has been applied between the tip and the sample in an atomic force microscope working in ultra-high vacuum. We have shown how an electric field modifies the observed force (or cantilever deflection)-vs-distance curves, commonly referred to as the static mode force spectroscopy curves, taken using an atomic force microscope. The electric field induced instabilities shift the jump-into-contact and jump-off-contact points and also the deflection at these instability points. We explained the experimental results using a model of the tip-sample interaction and quantitatively established a relation between the observed static mode force spectroscopy curves and the applied electric field which modifies the effective tip-sample interaction in a controlled manner. The investigation establishes a way to quantitatively evaluate the electrostatic force in an atomic force microscope using the static mode force spectroscopy curves.

Loading DST Unit for Nanosciences collaborators
Loading DST Unit for Nanosciences collaborators