P. Murugavel

Magnetoelectric effects of nanoparticulate Pb(Zr0.52Ti0.48)O3–NiFe2O4 composite films

The authors fabricated Pb(Zr0.52Ti0.48)O3–NiFe2O4 composite films consisting of randomly dispersed NiFe2O4 nanoparticles in the Pb(Zr0.52Ti0.48)O3 matrix. The structural analysis revealed that the crystal axes of the NiFe2O4 nanoparticles are aligned with those of the ferroelectric matrix. The composite has good ferroelectric and magnetic properties. The authors measured the transverse and longitudinal components of the magnetoelectric voltage coefficient, which supports the postulate that the magnetoelectric effect comes from direct stress coupling between magnetostrictive NiFe2O4 and piezoelectric Pb(Zr0.52Ti0.48)O3 grains.

The role of ferroelectric-ferromagnetic layers on the properties of superlattice-based multiferroics

A series of superlattices and trilayers composed of ferromagnetic and ferroelectric or paraelectric layers were grown on (100) SrTiO3 by the pulsed-laser deposition technique. Their structural and magnetoelectric properties were examined. The superlattices made of ferromagnetic Pr0.85Ca0.15MnO3 (PCMO) and a ferroelectric, namely, Ba0.6Sr0.4TiO3 (BST) or BaTiO3, showed enhanced magnetoresistance (MR) at high applied magnetic field, whereas such enhancement was absent in Pr0.85Ca0.15MnO3∕SrTiO3 superlattices, which clearly demonstrates the preponderant role of the ferroelectric layers in this enhanced MR. Furthermore, the absence of enhanced MR in trilayers of PCMO∕BST indicates that the magnetoelectric coupling which is responsible for MR in these systems is stronger in multilayers than in their trilayer counterparts.

Role of oxygen vacancy and Fe–O–Fe bond angle in compositional, magnetic, and dielectric relaxation on Eu-substituted BiFeO3 nanoparticles

The influence of oxygen vacancies on the dielectric relaxation behavior of pure and Eu-substituted BiFeO3 nanoparticles synthesized by a sol-gel technique has been studied using impedance spectroscopy in the temperature range of 90 °C to 180 °C. The electric relaxation time and activation energy of the oxygen vacancies can be calculated from the Arrhenius equation, and found to be 1.26 eV and 1.76 eV for pure and Eu-substituted BiFeO3, respectively. Substitution induces structural disorder and changes in the Fe-O-Fe bond angle, leading to alteration of the magnetic properties, observed from magnetic studies and evaluated using Rietveld refinement of the XRD patterns. X-ray photoelectron spectroscopy (XPS) confirms the shifting of the binding energy of the Bi 4f orbital, establishing Eu substitution at the Bi site. Calculation of the area under the Fe(2+)/Fe(3+) (2p) and O (1s) XPS spectra gives approximate values of the oxygen vacancies.

Tailoring of ferromagnetic Pr0.85Ca0.15MnO3/ferroelectric Ba0.6Sr0.4TiO3 superlattices for multiferroic properties

Superlattices composed of ferromagnetic Pr0.85Ca0.15MnO3 and ferroelectric Ba0.6Sr0.4TiO3 layers were fabricated on (100) SrTiO3 substrates by a pulsed-laser deposition method. The capacitance and resistive parts of the samples were analyzed from the complex impedance measurements. The superlattice with larger ferroelectric thickness shows unique characteristics which are not present in the parent ferromagnetic thin film. The superlattice shows both ferromagnetic and ferroelectric transitions which is an evidence for the coexistence of both the properties. The high magnetoresistance (40% at 80K) shown by the superlattice can be attributed to the coupling between ferromagnetic and ferroelectric layers, i.e., to the magnetoelectric effect.

Multiferroic properties of epitaxially stabilized hexagonal DyMnO3 thin films

The authors fabricated epitaxial thin films of hexagonal DyMnO3, which otherwise form in a bulk perovskite structure, via deposition on Pt(111)‖Al2O3(0001) and YSZ(111) substrates, each of which has in-plane hexagonal symmetry. The polarization hysteresis loop demonstrated the existence of ferroelectricity in our hexagonal DyMnO3 films at least below 70K. The observed 2.2μC∕cm2 remnant polarization at 25K corresponded to a polarization enhancement by a factor of 10 compared to that of the bulk orthorhombic DyMnO3. Interestingly, this system showed an antiferroelectriclike feature in its hysteresis loop. Our hexagonal DyMnO3 films showed an antiferromagnetic Neel temperature around 60K and a spin reorientation transition around 40K. The authors also found spin-glass-like behavior, which was likely to arise from the geometric frustration of antiferromagnetically coupled Mn spins with an edge-sharing triangular lattice.

Origin of metal–insulator transition temperature enhancement in underdoped lanthanum manganite films

The possibility of controlling transport properties of colossal magnetoresistance manganite films using substrate-induced strain has attracted great interest. We have investigated transport properties of La0.9Ca0.1MnO3, La0.92Ba0.08MnO3, La0.8Ba0.2MnO3, and LaMnO3 films. When the films were post-annealed at proper conditions, all of them showed metal–insulator transitions. Their transition temperatures TMI were much higher than the corresponding bulk values, irrespective of the type of substrate-induced biaxial strain. This surprising fact demonstrated that strain could not be the main origin of the TMI enhancement observed in the underdoped (dopant concentration x<0.3) manganite films. We suggested that TMI enhancements should be attributed mostly to the cationic vacancies in the post-annealed films.

Electronic structures of hexagonal R Mn O 3 ( R = Gd , Tb, Dy, and Ho) thin films: Optical spectroscopy and first-principles calculations

We investigated the electronic structure of multiferroic hexagonal RMnO3 (R = Gd, Tb, Dy, and Ho) thin films using both optical spectroscopy and first-principles calculations. Using artificially stabilized hexagonal RMnO3, we extended the optical spectroscopic studies on the hexagonal multiferroic manganite system. We observed two optical transitions located near 1.7 eV and 2.3 eV, in addition to the predominant absorption above 5 eV. With the help of first-principles calculations, we attribute the low-lying optical absorption peaks to inter-site transitions from the oxygen states hybridized strongly with different Mn orbital symmetries to the Mn 3d3z2-r2 state. As the ionic radius of the rare earth ion increased, the lowest peak showed a systematic increase in its peak position. We explained this systematic change in terms of a flattening of the MnO5 triangular bipyramid.

Sub-micrometre spherical particles of TiO2, ZrO2, and PZT by nebulized spray pyrolysis of metal-organic precursors

Nebulized spray pyrolysis of metal-organic precursors in methanol solution has been employed to prepare powders of TiO2, ZrO2 and PbZr0.5Ti0.5O3 (PZT). This process ensures complete decomposition of the precursors at relatively low temperatures. The particles have been examined by scanning and transmission electron microscopy as well as X-ray diffraction. As prepared, the particles are hollow agglomerates of diameter 0.1-1.6 mu m, but after heating to higher temperatures the ultimate size of the particles comprising the agglomerates are considerably smaller (0.1 mu m or less in diameter) and crystalline.

Enhanced magnetoresistance in ferromagnetic Pr0.85Ca0.15MnO3∕ferroelectric Ba0.6Sr0.4TiO3 superlattice films

Artificial superlattices designed with a ferromagnetic Pr0.85Ca0.15MnO3 insulating layer and ferroelectric Ba0.6Sr0.4TiO3 layer were grown on (100) SrTiO3 substrates. The magnetotransport properties were measured with the current perpendicular to the plane geometry. An increase in magnetoresistance (MR), with no significant low-field effect, was observed as the amount of ferroelectric Ba0.6Sr0.4TiO3 layer thickness increases even up to 9 unit cells. For example, the superlattice [(Pr0.85Ca0.15MnO3)10(Ba0.6Sr0.4TiO3)9]25 shows a 35% MR at 100K, though the Pr0.85Ca0.15MnO3 film was a robust insulator with negligible MR even at a high applied magnetic field. Similar superlattice structure, (Pr0.85Ca0.15MnO3)∕(SrTiO3) did not show any enhancement in MR. We discussed that the enhanced MR could be due to the ferroelectric spacer layer and the associated magnetoelectric coupling.

Epitaxial stabilization of artificial hexagonal GdMnO3 thin films and their magnetic properties

The authors investigated the role of oxygen partial pressure on the epitaxial growth of an artificial hexagonal GdMnO3 phase, which should exist in an orthorhombic structure in bulk. The hexagonal GdMnO3 film showed diverse, but obvious, magnetic phase transitions with highly enhanced ferromagnetic properties. Its remnant magnetization at 4.2K is higher than those of other hexagonal RMnO3 (R=Ho, Er, and Yb) compounds, and the Curie temperature increases by around 25K. The results demonstrate that the epitaxial stabilization technique is a promising method for fabricating an artificial material with enhanced magnetic properties.The authors investigated the role of oxygen partial pressure on the epitaxial growth of an artificial hexagonal GdMnO3 phase, which should exist in an orthorhombic structure in bulk. The hexagonal GdMnO3 film showed diverse, but obvious, magnetic phase transitions with highly enhanced ferromagnetic properties. Its remnant magnetization at 4.2K is higher than those of other hexagonal RMnO3 (R=Ho, Er, and Yb) compounds, and the Curie temperature increases by around 25K. The results demonstrate that the epitaxial stabilization technique is a promising method for fabricating an artificial material with enhanced magnetic properties.