Kuldeep Chand Verma

Magneto-electric/dielectric and fluorescence effects in multiferroic xBaTiO3–(1 − x)ZnFe2O4 nanostructures

Multiferroic composites of xBaTiO3–(1 − x)ZnFe2O4 (BTZF) [x = 0.25 (BTZF2575), 0.35 (BTZF3565), 0.45 (BTZF4555), 0.50 (BTZF5050) and 0.75 (BTZF7525)] nanostructure have been synthesized by a sol–gel method. Different types of nanostructural shapes and sizes have been obtained by the effect of ionic radii, surface energy and poly vinyl alcohol, which enhances the magneto-electric/dielectric interaction between BT/ZF phases. The crystalline phases of BTZF composite are confirmed by X-ray diffraction, and nanostructural dimensions and shape by transmission electron microscopy. The improvement in magnetization of BTZF is dependent upon the size and shape of the nanostructure, stoichiometric ratio and strength of occupation of cations at octahedral and tetrahedral sites. The chemical states of Fe in BTZF are analyzed by X-ray photoelectron spectroscopy. The ferroelectric property is explained by the nano size effect, 1D nanostructure shape, lattice distortion and epitaxial strain between two phases. The magnetoelectric coefficient is measured at room temperature under an applied dc magnetizing field and show different types of behavior in each sample. The magnetocapacitance is measured and explained on the basis of Maxwell–Wagner space charge and magnetoresistance, and relates to theoretical investigation, which proves that the enhancement not only depends on the size/shape of nanostructure but also the strain-induced phase transition where out-of-plane polarization appears in the composite. The photoemission of BTZF is observed by fluorescence spectroscopy.

Multiferroic approach for Cr,Mn,Fe,Co,Ni,Cu substituted BaTiO3 nanoparticles

Multiferroic magnetoelectric (ME) at room temperature is significant for new design nano-scale spintronic devices. We have given a comparative study to report multiferroicity in BaTM0.01Ti0.99O3 [TM = Cr,Mn,Fe,Co,Ni,Cu (1 mol% each) substituted BaTiO3 (BTO)] nanoparticles. The TM ions influenced both nano-size and lattice distortion of Ti–O6 octahedra to the BTO. X ray diffraction study indicates that the dopant TM could influence lattice constants, distortion, tetragonal splitting of diffraction peaks (002/200) as well as peak shifting of diffraction angle in the BTO lattice. This can induce lattice strain which responsible to oxygen defects formation to mediate ferromagnetism. Also, the lattice strain effect could responsible to reduce the depolarization field of ferroelectricity and provide piezoelectric and magnetostrictive strains to enhance ME coupling. The size of BTO nanoparticles is varied in 13–51 nm with TM doping. The room temperature magnetic measurement indicates antiferromagnetic exchange interactions in BTO lattice with TM ions. The zero-field cooling and field cooling magnetic measurement at 500 Oe indicates antiferromagnetic to ferromagnetic transition. It also confirms that the substitution of Cr, Fe and Co into BTO could induce strong antiferromagnetic behavior. However, the substitutions of Mn, Ni and Cu have weak antiferromagnetic character. The temperature dependent dielectric measurements indicates polarization enhancement that influenced with both nano-size as well TM ions and exhibits ferroelectric phase transition with relaxor-like characteristics. Dynamic ME coupling is investigated, and the longitudinal ME voltage coefficient, α ME is equivalent to linear ME coupling coefficient, is also calculated.

Tailoring the multiferroic behavior in BiFeO3 nanostructures by Pb doping

The weak and deficient manipulation of charge–spin coupling in multiferroic BiFeO3 (BFO) notoriously limits device applications. To mould the spontaneous charge and the spin orientation synergistically in BFO, in this paper Pb2+ substitution for Bi3+ could induce lattice distortions and structural phase transitions to tune the lone-pair activity (6s2) for ferroelectricity and neutralized oxygen vacancies to valence Fe2+/Fe3+ superexchange for ferromagnetism. Multiferroic Bi1−xPbxFeO3 [x = 0, 0.05, 0.075 and 0.1] nanostructures were synthesized by a chemical combustion process. X-Ray diffraction confirms the distorted rhombohedral BFO structure and the lattice expansion with Pb doping. The Pb ions also modified the shape of the BFO nanostructures. The observed ferroelectric behavior depends upon lattice distortion, reduction in oxygen vacancies to induce low leakage current and the shape/size effect in BFO nanostructures. The zero field (ZFC) and field cooling (FC) SQUID measurement confirm the strength of antiferromagnetism in BFO with Pb2+ ions. The cusp in ZFC magnetization is studied by ac magnetic susceptibility measurements that include spin-glass and superparamagnetic interactions in antiferromagnetism at low temperature. The oxidation states in BFO suggest oxygen vacancies that are reduced with Pb doping and maintain Fe2+/Fe3+ valences. The dielectric permittivity changes with applied dc magnetic field, which could induce a magnetodielectric effect due to spin pair correlation of neighboring spins and the coupling constant. Furthermore, significant dielectric anomalies appear near both the ferroelectric phase transition, and the Neel temperature of BFO implies the magnetoelectric coupling.

Vacancies driven magnetic ordering in ZnO nanoparticles due to low concentrated Co ions

The lattice defects due to oxygen vacancies in ZnO nanoparticles with low doping of Co ions are investigated. The low concentrated Co ions in ZnO are responsible to the free charge carriers and oxygen vacancies to induce long-range ferromagnetic ordering. We have synthesized Zn1-xCoxO [x = 0.002, 0.004, 0.006 and 0.008] nanoparticles by a sol-gel process. X-ray fluorescence analysis detects the chemical composition of Zn, Co and O atoms. Rietveld refinement of x-ray diffraction pattern could confirm the wurtzite ZnO structure and the lattice constants with Co doping. The nanoparticles dimensions as well lattice spacing of ZnO are enhanced with Co substitution. Fourier transform infrared vibrational modes involve some organic groups to induce lattice defects and the ionic coordination among Zn, Co and O atoms. The room temperature Raman active mode E-2 indicates frequency shifting with Co to induce stress in the wurtzite lattice. Photoluminescence spectra have a strong near-band-edge emission due to band gap energy and defects related to oxygen vacancies. X-ray photoelectron spectra confirm that the low dopant Co ions in ZnO lattice occupied Zn atoms by introducing oxygen vacancies and the valance states Zn2+, Co-2,Co-3+. The zero-field and field cooling magnetic measurement at 500 Oe in Co:ZnO samples indicate long-range ferromagnetism that is enhanced at 10 K due to antiferromagnetic-ferromagnetic ordering. The lattice defects/vacancies due to oxygen act as the medium of magnetic interactions which is explained by the bound magnetic polaron model.