Vilas Shelke

Nanoscale Switching Characteristics of Nearly Tetragonal BiFeO3 Thin Films

We have investigated the nanoscale switching properties of strain-engineered BiFeO(3) thin films deposited on LaAlO(3) substrates using a combination of scanning probe techniques. Polarized Raman spectral analysis indicates that the nearly tetragonal films have monoclinic (Cc) rather than P4mm tetragonal symmetry. Through local switching-spectroscopy measurements and piezoresponse force microscopy, we provide clear evidence of ferroelectric switching of the tetragonal phase, but the polarization direction, and therefore its switching, deviates strongly from the expected (001) tetragonal axis. We also demonstrate a large and reversible, electrically driven structural phase transition from the tetragonal to the rhombohedral polymorph in this material, which is promising for a plethora of applications.

Reduced Coercive Field in BiFeO3 Thin Films through Domain Engineering

The bismuth ferrite (BiFeO3) material offers a comprehensive package of multifunctionality. In addition to the multiferroic behavior, i.e. coexistence of electric and magnetic orderings,[1] it also exhibits photovoltaic effect, [2] metal-insulator transition,[3] electric modulation of conduction,[4] and terahertz radiation emission.[5] The possibility of joint control of electric, magnetic, and optical properties provides several degrees of freedom to design exotic devices. It is a green energy material in the sense that it is lead-free and energy-efficient due to cryogen-less functionality. Therefore, a wide variety of applications in terms of sensors, memories and spintronic devices are foreseen.[6] However, the incipient lower value of magneto-electric coupling has raised skepticism regarding its multiferroic capabilities and allied applications.[6] Nevertheless, the highest value of ferroelectric polarization (~ 100 C.cm-2) is very promising for next generation ferroelectric random access memory devices.

Effect of kinetic growth parameters on leakage current and ferroelectric behavior of BiFeO3 thin films

Epitaxial BiFeO3 thin films have been grown on (100)-oriented SrTiO3 and Nb-doped SrTiO3 substrates using the pulsed laser deposition technique under identical thermodynamic and variable kinetic conditions. The variation of growth kinetics through laser fluence and pulse repetition rate had minimal effect on the structure and magnetic properties of films. However, large changes were observed in the microstructure, with initial island growth mode approaching toward step-flow type growth and roughness reducing from 12.5 to 1.8 nm for 50 nm thick film. Correspondingly, the leakage current density at room temperature dropped consistently by almost four orders of magnitude. The dominant mechanism in low leakage current films was space-charge-limited conduction. These findings suggest that the issue of leakage current can be dealt favorably by controlling kinetic growth parameters. The application of high electric field and observation of maximum polarization value up to 103 μC/cm2 could be possible in these sa...

Ferroelectric domain scaling and switching in ultrathin BiFeO3 films deposited on vicinal substrates

We report on electrically switchable polarization and ferroelectric domain scaling over a thickness range of 5-100nm in BiFeO3 films deposited on (110) vicinal substrates. The BiFeO3 films of variable thickness were deposited with SrRuO3 bottom layer using the pulsed laser deposition technique. The domains are engineered into preferentially oriented patterns due to substrate vicinality along the (110) direction. The domain width scales closely with the square root of film thickness, in agreement with the Landau-Lifschitz-Kittel (LLK) law. Switching spectroscopy piezo-response force microscopy provides clear evidence for the ferroelectric switching behavior in all the films.

The role of SrRuO3 bottom layer in strain relaxation of BiFeO3 thin films deposited on lattice mismatched substrates

The present study deals with BiFeO3 and SrRuO3 thin films deposited on large lattice mismatched substrates like LaAlO3, MgO, and SrTiO3. The 80 nm thickness BiFeO3 films deposited directly on LaAlO3 substrate are highly constrained, while those on SrTiO3 substrate are only somewhat constrained. The BiFeO3 films deposited with 50 nm SrRuO3 bottom layer are fully relaxed on all the three substrates. The separate SrRuO3 layers deposited under identical conditions on these substrates are also found to be relaxed. These films exhibit different morphological features in accordance with strain relaxation process. The BiFeO3 thin films show characteristic low magnetic moment resulting from antiferromagnetic ordering irrespective of substrate induced strain.

X-band frequency response and electromagnetic interference shielding in multiferroic BiFeO3 nanomaterials

The presence of electric dipoles, magnetic dipoles and mobile charges is a prerequisite for electromagnetic interference (EMI) shielding materials. Here, we demonstrate that multiferroic compound with incipient ensemble of electric and magnetic dipoles can perform as an EMI shielding material. We synthesized single phase BiFeO3 nanomaterial and studied complex electromagnetic properties in an X-band frequency region. A shielding effectiveness up to 11 dB with a major contribution from absorption was observed in the BiFeO3 nanomaterials. An auxiliary functionality of radiation shielding is revealed in the multiferroic BiFeO3 compound.

Confirmation of enhanced magnetic moment in interface-engineered BiFeO3-LaMnO3 composites

We have confirmed the observation of a higher magnetic moment in the bulk composite of two distinct antiferromagnetic materials. The low temperature neutron diffraction technique was used for the structure and magnetic ordering study of polycrystalline samples with compositions (1-x)BiFeO3-xLaMnO3. The magnetic moment increased from 3.9 μB/f.u. for x = 0 to 4.3 μB/f.u. for x = 0.2 samples. The Fourier Transform Infra Red study indicated the structural invariance of the composite phases. The possibility of magnetic contribution from the multiple valencies of magnetic ions or oxygen vacancies was overruled on the basis of the X-ray Photoelectron Spectra analysis.

Investigation on gas sensing properties of Ag doped BiFeO3

Bismuth ferrite (BFO) and Ag substituted Bismuth ferrite with perovskite structure have been synthesized using sol-gel method and investigated for their gas sensing properties. Single phase and rhombohedral crystal structure of the samples were confirmed from XRD pattern. Oxidation state of the elements is confirmed using X-Ray Photoelectron Spectroscopy (XPS). Since Ag substituted Bismuth ferrite exhibited pronounced response to H2S gas as compared to other gases, H2S gas sensing properties of Bismuth ferrite (BFO) and Ag substituted Bismuth ferrite were investigated in detail. Bi0.9Ag0.1FeO3 (BAFO) exhibits enhanced sensitivity, quick response and selectivity towards H2S as compared to BFO.Bismuth ferrite (BFO) and Ag substituted Bismuth ferrite with perovskite structure have been synthesized using sol-gel method and investigated for their gas sensing properties. Single phase and rhombohedral crystal structure of the samples were confirmed from XRD pattern. Oxidation state of the elements is confirmed using X-Ray Photoelectron Spectroscopy (XPS). Since Ag substituted Bismuth ferrite exhibited pronounced response to H2S gas as compared to other gases, H2S gas sensing properties of Bismuth ferrite (BFO) and Ag substituted Bismuth ferrite were investigated in detail. Bi0.9Ag0.1FeO3 (BAFO) exhibits enhanced sensitivity, quick response and selectivity towards H2S as compared to BFO.

Synthesis of ZnSnO3 nanostructure by sol gel method

Zinc Stannate (ZST) with composition ZnSnO3 is known for high electron mobility, optical, piezoelectric and charge storage properties. ZST crystalizes in different lattice structures, which allows a wide range of tunablity. We demonstrate successful synthesis of ZnSnO3 nanomaterial by sol-gel method. ZnSnO3 nanomaterials were calcined and sintered at different temperatures. Powder X-ray diffraction confirmed the single phase of the nanomaterial with rhombohedral R-3 space group. The Rietveld refinement of diffraction pattern yielded lattice parameter values a=5.26A, c=14.09A. Raman spectroscopy revealed higher activity towards higher wavenumbers. Raman shift around 530cm−1 was found to be highly structure dependent, most probably due to anharmonic atomic vibrations in ZnO6/SnO6 octahedra around center of mass. Sharp Peak around 650cm−1 is characteristic of ZnSnO3 molecule.