Showing papers by "Ripandeep Singh published in 2014"

Effect of BaTiO 3 addition on structural, multiferroic and magneto-dielectric properties of 0.3CoFe 2 O 4 -0.7BiFeO 3 ceramics

Abstract: This study reports the various physical properties of (1�x)(0.3CoFe2O4-0.7BiFeO3)-xBaTiO3 composites (equivalently denoted as 0.3CFO-0.7BFO/BT) with the compositions x=0, 0.30, 0.35, 0.40 and 1.0. The composites are synthesized through a hybrid processing technique in which 0.3CFO–0.7BFO is prepared through a sol-gel process, and BT is processed through a solid state reaction method. Subsequently, the effects of the addition of BT on the structural, dielectric, magnetic and magneto-dielectric properties of 0.3CFO–0.7BFO have been investigated for various BT concentrations. The Rietveld refinement analysis of x-ray diffraction patterns reveals the structural distortion in the BFO phase with the addition of BT, while no such distortion has been observed for the CFO phase. Energy dispersive spectroscopy confirms the presence of two types of grains that correspond to the 0.3CFO–0.7BFO and BT phases in field emission scanning electron micrographs of the composites. Improved dielectric properties have been observed, which are associated with the improved density of composites with the addition of BT. Measurements of the magnetic and ferroelectric hysteresis loops at room temperature indicate that the composites exhibit ferroelectricity and ferromagnetism simultaneously at room temperature. An increase of the electric polarization has been observed due to structural distortion arising with the addition of BT. The significant dependence of the dielectric constant on the magnetic field has been observed in the prepared composites. The highest value of the magneto-dielectric response (3.2%) has been observed for a 40 mol% addition of BT.

Crystal structure and magnetic properties of Bi0.8A0.2FeO3 (A = La, Ca, Sr, Ba) multiferroics using neutron diffraction and Mossbauer spectroscopy

Abstract: Bi0.8A0.2FeO3 (A = La, Ca, Sr, Ba) multiferroics were studied using x-ray, neutron diffraction and magnetization techniques. All the samples crystallized in rhombohedral structure with space group R3c. The compounds exhibit antiferromagnetic (AFM) ordering at 300 K and no evidence of further structural or magnetic transition was observed on lowering of temperature below it. The magnetic structure of these substituted compounds are found to be collinear G-type AFM structure as against the non collinear incommensurate magnetic structure reported in the case of parent compound. The moments on Fe at 6 K are aligned along the a-axis in the case of Ca-doped sample. With increase in the ionic radii of dopant, the moments are found to be aligned in the ac plane and the angle of tilt away from the a-axis increases. The observed change in the magnetic structure with substitution is attributed to the intrinsic structural distortion as evidenced by the change in the bond angle (Fe-O-Fe) and bond distances (Bi-O, Fe-O). It has been found that heterovalent substitution A2+ results in the formation of oxygen vacancies in the parent lattices as the possibility of Fe4+ ruled out by Mossbauer spectra recorded at room temperature. Higher value of remnant magnetization (0.4187 emu/g) and coercivity (4.7554kOe) is observed in Bi0.8Ba0.2FeO3 sample in comparison to other substituted samples revealing a strong correlation between ionic radii and magnetization.

Spin-phonon coupling, high-pressure phase transitions, and thermal expansion of multiferroic GaFeO 3 : A combined first principles and inelastic neutron scattering study

Abstract: We have carried out an extensive phonon study on multiferroic ${\mathrm{GaFeO}}_{3}$ to elucidate its dynamical behavior. Inelastic neutron scattering measurements are performed over a wide temperature range, 150 to 1198 K. First principles lattice dynamical calculations are done for the sake of the analysis and interpretation of the observations. The comparison of the phonon spectra from magnetic and nonmagnetic calculations highlights pronounced differences. The energy range of the vibrational atomistic contributions of the Fe and O ions are found to differ significantly in the two calculation types. Therefore, magnetism induced by the active spin degrees of freedom of Fe cations plays a key role in stabilizing the structure and dynamics of ${\mathrm{GaFeO}}_{3}$. Moreover, the computed enthalpy in various phases of ${\mathrm{GaFeO}}_{3}$ is used to gain deeper insights into the high-pressure phase stability of this material. Further, the volume dependence of the phonon spectra is used to determine its thermal expansion behavior.

Chemical Pressure effect at the boundary of Mott insulator and itinerant electron limit of Spinel Vanadates

Abstract: The chemical pressure effect on the structural, transport, magnetic and electronic properties (by measuring X-ray photoemission spectroscopy) of ZnV2O4 has been investigated by doping Mn and Co on the Zinc site of ZnV2O4 With Mn doping the V-V distance increases and with Co doping it decreases The resistivity and thermoelectric power data indicate that as the V-V distance decreases the system moves towards Quantum Phase Transition The transport data also indicate that the conduction is due to the small polaron hopping The chemical pressure shows the non-monotonous behaviour of charge gap and activation energy The XPS study also supports the observation that with decrease of the V-V separation the system moves towards Quantum Phase Transition On the other hand when Ti is doped on the V-site of ZnV2O4 the metal-metal distance decreases and at the same time the TN also increases

Transport, magnetic and structural properties of Mott insulator MnV2O4 at the boundary between localized and itinerant electron limit

Abstract: The effect of Zn and Cr doping on the transport and magnetic properties of MnV2O4 have been investigated using resistivity, thermoelectric power (TEP), magnetization, neutron diffraction and X-ray diffraction techniques. It is observed, that with increase in Zn substitution the non-collinear orientation of Mn spins with the V spins decreases which effectively leads to the decrease of structural transition temperature more rapidly than Curie temperature. Investigations also show that with Zn doping both the Curie temperature (T C) and structural transition temperature (T S) decrease, while the gap between them increases rapidly. On the other hand, with Cr doping on the V site the T C remains almost constant but T S decreases rapidly. Moreover, with Zn doping both resistivity and TEP decrease, whereas with 10 % Cr doping the TEP decreases and a change of sign occurs indicating an increase in the band gap. This leads to the decrease of the mobility of the polaronic holes than the mobility of the electronic polarons at low temperature.