Dipten Bhattacharya

Ferromagnetism in nanoscale BiFeO3

A remarkably high saturation magnetization of ∼0.4μB∕Fe along with room temperature ferromagnetic hysteresis loop has been observed in nanoscale (4–40nm) multiferroic BiFeO3 which in bulk form exhibits weak magnetization (∼0.02μB∕Fe) and an antiferromagnetic order. The magnetic hysteresis loops exhibit exchange bias and vertical asymmetry which could be because of spin pinning at the boundaries between ferromagnetic and antiferromagnetic domains. Interestingly, both the calorimetric and dielectric permittivity data in nanoscale BiFeO3 exhibit characteristic features at the magnetic transition point. These features establish the formation of a true ferromagnetic-ferroelectric system with a coupling between the respective order parameters in nanoscale BiFeO3.

Particle size dependence of magnetization and phase transition near TN in multiferroic BiFeO3

We report results of a comprehensive study of the phase transition at TN (∼643K) as a function of particle size in multiferroic BiFeO3 system. We employed electrical, thermal, and temperature dependent x-ray diffraction studies in order to characterize the transition in a host of samples. We also carried out detailed magnetic measurements over a temperature regime of 2–300K under a magnetic field of 100–10000Oe both on bulk and nanocrystalline systems. While in the bulk system a sharp endothermic peak at TN together with a broad feature, ranging over nearly ∼100K (ΔT), could be observed in calorimetry, the nanoscale systems exhibit only the broad feature. The characteristic dielectric anomaly, expected at TN, is found to occur both at TO and TN across ΔT in the bulk sample. The Maxwell-Wagner component due to interfaces between heterogenous regions with different conductivities is also present. The magnetic properties, measured at lower temperature, corroborate our observations in calorimetry. The metasta...

Multiferroic coupling in nanoscale BiFeO3

Using the results of x-ray and neutron diffraction experiments, we show that the ferroelectric polarization, in ∼22 nm particles of BiFeO3, exhibits a jump by ∼30% around the magnetic transition point TN (∼635 K) and a suppression by ∼7% under 5T magnetic field at room temperature (≪TN). These results confirm the presence of strong multiferroic coupling even in nanoscale BiFeO3 and thus could prove to be quite useful for applications based on nanosized devices of BiFeO3.

Particle size dependence of magnetization and noncentrosymmetry in nanoscale BiFeO3

The saturation magnetization (MS), antiferromagnetic transition point (TN), and the off-center displacements of Bi and Fe ions have been measured as a function of particle size in nanoscale BiFeO3. TN decreases down to ∼550 K for particles of size ∼5 nm from ∼653 K in bulk, while MS rises by more than an order of magnitude. Analysis of crystallographic structure from Rietveld refinement of x-ray diffraction patterns shows significant rise in off-center displacements of Bi (δBi) and Fe (δFe) ions within a unit cell with the decrease in particle size. The net unit-cell polarization PS too, is found to be larger in nanoscale regime.

Mechanism of dc conduction in ferric chloride doped poly(3-methyl thiophene)

Abstract Poly(3-methyl thiophene) has been synthesized by chemical oxidation method in an inert atmosphere using ferric chloride as a dopant. The doping level varies from 0.1 to 2.0 M. The dc conductivity has been measured in the temperature range 20–300 K. The observed dc conductivity data has been analyzed in the light of existing theoretical models. Different Mott’s parameters such as characteristic temperature ( T 0 ), density of states at Fermi level [ N ( E F )], average hopping energy ( W ) and the average hopping distance ( R ) have been evaluated that agree well with the values reported earlier for the other conjugated polymers. A detailed analysis of the mechanism of charge transport in this system has been reported.

Room temperature multiferroicity in orthorhombic LuFeO3

From the measurement of dielectric, ferroelectric, and magnetic properties, we observe simultaneous ferroelectric and magnetic transitions around ∼600 K in orthorhombic LuFeO3. We also observe suppression of the remanent polarization by ∼95% under a magnetic field of ∼15 kOe at room temperature. The extent of suppression of the polarization under magnetic field increases monotonically with the field. These results show that even the orthorhombic LuFeO3 is a room temperature multiferroic of type-II variety exhibiting quite a strong coupling between magnetization and polarization.

Particle-size dependence of orbital order-disorder transition in LaMnO3

The latent heat (L) associated with the orbital order-disorder transition at TJT is found to depend significantly on the average particle size (d) of LaMnO3. It rises slowly with the decrease in d down to ∼100 nm and then jumps by more than an order of magnitude in between d∼100 nm and ∼30 nm. Finally, L falls sharply to zero at a critical particle size dc≈19 nm. The transition temperature TJT also exhibits an almost similar trend of variation with the particle size, near d∼30 nm and below, even though the extent of variation is relatively small. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization versus temperature study over a temperature range 10–300 K reveals that the antiferromagnetic transition temperature decreases with d while the temperature range, over which the ZFC and FC data diverge, increases with the drop in d. The FC magnetization also is found to increase sharply with the drop in particle size. A conjecture of nonmonotonic variation in orbital domain structure with decrease in particle size—from smaller domains with large number of boundaries to larger domains with small number of boundaries due to lesser lattice defects and, finally, down to even finer domain structures with higher degree of metastability—along with increase in surface area in core-shell structure, could possibly rationalize the observed L versus d and TJT versus d patterns. Transmission electron microscopy data provide evidence for presence of core-shell structure as well as for increase in lattice defects in finer particles.

Ultrasonic cavitation induced water in vegetable oil emulsion droplets – A simple and easy technique to synthesize manganese zinc ferrite nanocrystals with improved magnetization

In the present investigation, synthesis of manganese zinc ferrite (Mn(0.5)Zn(0.5)Fe(2)O(4)) nanoparticles with narrow size distribution have been prepared using ultrasound assisted emulsion (consisting of rapeseed oil as an oil phase and aqueous solution of Mn(2+), Zn(2+) and Fe(2+) acetates) and evaporation processes. The as-prepared ferrite was nanocrystalline. In order to remove the small amount of oil present on the surface of the ferrite, it was subjected to heat treatment at 300 °C for 3h. Both the as-prepared and heat treated ferrites have been characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), TGA/DTA, transmission electron microscopy (TEM) and energy dispersion X-ray spectroscopy (EDS) techniques. As-prepared ferrite is of 20 nm, whereas the heat treated ferrite shows the size of 33 nm. In addition, magnetic properties of the as-prepared as well as the heat treated ferrites have also been carried out and the results of which show that the spontaneous magnetization (σ(s)) of the heat treated sample (24.1 emu/g) is significantly higher than that of the as-synthesized sample (1.81 emu/g). The key features of this method are avoiding (a) the cumbersome conditions that exist in the conventional methods; (b) usage of necessary additive components (stabilizers or surfactants, precipitants) and (c) calcination requirements. In addition, rapeseed oil as an oil phase has been used for the first time, replacing the toxic and troublesome organic nonpolar solvents. As a whole, this simple straightforward sonochemical approach results in more phase pure system with improved magnetization.

Critical current in electrophoretically deposited thick films of YBCO superconductor

Abstract Thick films of YB 2 Cu 3 O 7 − δ (YBCO) superconductor have been prepared by the electrophoretic deposition technique using flexible silver strip as the substrate. Effects of several deposition parameters, e.g., time of deposition, particle size of the powder, number of layers deposited, etc., on the various properties of the films have been studied in detail. Microstructural investigation of the films indicates extensive grain growth and recrystallization phenomena particularly in films of large thickness. The microstructural morphology changes significantly as the thickness decreases. The transport J c (at 77 K, B =0) increases from ≈450 A/cm 2 for a film thickness of ∼ 65 μ m to > 4000 A/cm 2 for a reduced thickness of ∼ 3 μ m. Possible reasons for this dimensional dependence are discussed.

Magnetoelastic effects in multiferroic YMnO3

We have investigated magnetoelastic effects in multiferroic YMnO(3) below the antiferromagnetic phase transition, T(N) ≈ 70 K, using neutron powder diffraction. The a lattice parameter of the hexagonal unit cell of YMnO(3) decreases normally above T(N), but decreases anomalously below T(N), whereas the c lattice parameter increases with decreasing temperature and then increases anomalously below T(N). The unit cell volume also undergoes an anomalous contraction below T(N). By fitting the background thermal expansion for a non-magnetic lattice with the Einstein-Grüneisen equation, we determined the lattice strains Δa, Δc and ΔV due to the magnetoelastic effects as a function of temperature. We have also determined the temperature variation of the ordered magnetic moment of the Mn ion by fitting the measured Bragg intensities of the nuclear and magnetic reflections with the known crystal and magnetic structure models and have established that the lattice strain due to the magnetoelastic effect in YMnO(3) couples with the square of the ordered magnetic moment or the square of the order parameter of the antiferromagnetic phase transition.