P Murugavel

Microwave dielectric properties of dielectric resonators (BaNd2(1 - x)Sm2xTi5O14) with high dielectric constant (εr), low loss (high Q) and low temperature coefficient of resonant frequency (τf) have been studied by adjusting the Nd/Sm ratio. The composition with x = 0.7 gives high εr, high Q and low τf which is highly suitable for dielectric resonators (DR). © 1997 Elsevier Science S.A.

We report the transition from robust ferromagnetism to a spin-glass state in nanoparticulate La0.7Sr0.3MnO3 through solid solution with BaTiO3. The field- and temperature-dependent magnetization and the frequency-dependent ac magnetic susceptibility measurements strongly indicate the existence of a spin-glass state in the system, which is further confirmed from memory effect measurements. The breaking of long-range ordering into short-range magnetic domains is further investigated using density-functional calculations. We show that Ti ions remain magnetically inactive due to insufficient electron leakage from La0.7Sr0.3MnO3 to the otherwise unoccupied Ti-d states. This results in the absence of a Mn-Ti-Mn spin exchange interaction and hence the breaking of the long-range ordering. Total-energy calculations suggest that the segregation of nonmagnetic Ti ions leads to the formation of short-range ferromagnetic Mn domains.

BiFeO3 is a promising multifunctional material in terms of its intriguing physics and diverse application potential. In this work, self-powered UV-visible photodetector characteristics of the polycrystalline BiFeO3 thin film exhibiting pronounced photo-response under both UV and visible light are demonstrated. Interestingly, the film displays switchable photodetector characteristics in accordance with the polarization switching properties and thereby confirms the role of polarization driven depolarization field on the observed properties. Importantly, the photodetector performance parameters such as responsivity and external quantum efficiency (5.3 %) observed under UV light are 2 to 4 order of magnitude higher than the values reported in similar oxide ferroelectric systems. These studies establish the application of polycrystalline BiFeO3 film as a self-powered UV-visible photodetector.

An epitaxial lead free ferroelectric 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) thin film is fabricated on a (001)Nb:SrTiO3 single crystalline substrate by the pulsed laser deposition method. The 2.3% lattice mismatch between the BZT-BCT and substrate suggests that the film is grown under a compressively strained state by leaving a large strain gradient near the interface. Polarization versus electric field measurement reveals that the film exhibits a ferroelectric hysteresis character with a large imprint effect indicating the existence of an internal electric field. The origin of the internal electric field is correlated with the strain gradient induced flexoelectric effect and the interfacial built-in field. Consequently, the resultant internal electric field could lead to a self-polarized non-switchable layer at the interface. The evidence for the envisaged self-polarization effect is indeed shown by the piezo force microscopic measurements. Importantly, photovoltaic studies performed on the film display an open circuit voltage of 1.1 V, which is higher than the values reported for many ferroelectric films. The observed photovoltaic response is correlated with the depolarization field and the self-polarization effect. The demonstrated large photo-response illustrates the application potential of the BZT-BCT system in photovoltaic devices.

Single phase Bi(1-x)EuxFeO3 multiferroic nanoparticles were synthesized by tartaric acid assisted sol-gel method and the effect of Eu substitution in Bismuth ferrite (BFO) was investigated on the structural, optical and magnetic properties. It was observed a gradual structural transition from rhombohedral R3c to orthorhombic Pbnm with the increase of Eu content resulted from the lattice distortion in BFO system due to the smaller radius of Eu3+ ions. The optical band gap of BFO was altered with respect to the change of particle sizes with the increment of Eu content. The magnetic property of Bi(1-x)EuxFeO 3 system was found to be increased with the increase of Eu substitution. This might be attributed to the substitution-induced suppression of spiral spin modulation caused by an appropriate substitution of Eu in the BFO system. © 2013 American Institute of Physics.

We investigated the effects of temperature and magnetic field on the electronic structure of hexagonal R MnO3 (R=Gd, Tb, Dy, and Ho) thin films using optical spectroscopy. As the magnetic ordering of the system was disturbed, a systematic change in the electronic structure was identified in this series. The optical-absorption peak near 1.7 eV showed an unexpectedly large shift of more than 150 meV from 300 to 15 K, accompanied by an anomaly of the shift at the Néel temperature. The magnetic-field dependent measurement clearly revealed a sizable shift of the corresponding peak when a high magnetic field was applied. Our findings indicated strong coupling between the magnetic ordering and the electronic structure in the multiferroic hexagonal R MnO3 compounds. © 2008 The American Physical Society.

The authors investigated the magnetic and ferroelectric properties of hexagonally grown HoMn O3 thin films. The magnetic measurements revealed bulklike magnetic phase transitions with an additional spin-glass-like behavior feature below the Náel temperature. The ferroelectricity in the films was distinctly different from the suggested bulk behavior. Below 40 K, the HoMn O3 films showed typical ferroelectric character: their remnant polarization and coercive field values at 20 K were 3.7 μC cm2 and 0.69 MVcm. Above 40 K, however, the films exhibited an unusual antiferroelectriclike behavior, with more pronounced features appearing at higher temperatures. These intriguing physical properties make HoMn O3 films a potential candidate material for numerous future applications. © 2007 American Institute of Physics.

Ferroelectric bismuth ferrite is an attractive candidate for switchable devices. The effect of dipole pinning due to the oxygen vacancy layer on the switching behavior of the BiFeO3 thin film fabricated by the chemical solution deposition method was studied after annealing under air, O2, and N2 environment. The air annealed film showed well defined and dense grains leading to a lower leakage current and superior electrical properties compared to the other two films. The photovoltage and transient photocurrent measured under positive and negative poling elucidated the switching nature of the films. Though the air and O2 annealed films showed a switchable photovoltaic response, the response was severely affected by oxygen vacancies in the N2 annealed film. In addition, the open circuit voltage was found to be mostly dependent on the polarization of BiFeO3 rather than the Schottky barriers at the interface. This work provides an important insight into the effect of dipole pinning caused by oxygen vacancies on the switchable photovoltaic effect of BiFeO3 thin films along with the importance of stoichiometric, defect free, and phase pure samples to facilitate meaningful practical applications.

The effect of the cationic redistribution on the complex spinel structure and magnetic properties were investigated in Zn0.7Cu0.3Fe2O4 ferrite. X-ray photoelectron spectroscopy and x-ray diffraction studies revealed that the system exhibits a mixed spinel structure with Fe3+, Zn2+, and Cu2+ occupying both tetrahedral and octahedral sublattices. The DC magnetization results revealed the absence of long-range magnetic order in the system. Furthermore, the AC susceptibility data analysis using dynamic scaling laws suggests that the system exhibits magnetic relaxation below two different temperatures: (i) a spin-glass-like transition at low temperature (∼49.2K) with critical exponent 10.3 and spin-flip time ∼10-11s, and (ii) a cluster-glass-like transition at higher temperature (∼317K) with critical exponent 4.6 and spin-flip time ∼10-10s. The existence of glassy behavior and magnetic memory effects below the spin-glass transition temperature proves that the system is in nonequilibrium dynamical state. The coexistence of spin-glass and cluster-glass along with the thermal hysteresis between these two transitions could widen the technological applications of these systems.

The bulk photovoltaic (PV) effect exhibited by non-centrosymmetric systems gained research interest due to the observed large open-circuit voltage. Ferroelectric systems exhibiting anomalous photovoltaic effects are mostly crystallized with multi-phase coexistence. Hence, the computational difficulty in building a multi-phase system restricts the detailed photovoltaic studies through phenomenological and shift current theory. In this work, ferroelectric Ba1-x(Bi0.5K0.5)xTiO3 (BBKT) oxide is designed to crystallize in single-phase tetragonal symmetry with improved polarization characteristics, and it is found to exhibit large PV response. Both experimental and theoretical studies on BBKT samples reveal ∼18% reduction in bandgap compared to the parent BaTiO3. Short-circuit current measured as a function of light intensity and light polarization angle reveal linear and sinusoidal response, respectively. The observed features are in accordance with phenomenological theory. Remarkably, the x = 0.125 sample displays ∼8 times higher open-circuit voltage (7.39 V) than the parent compound. The enhanced PV effect is attributed to the large shift current along the z direction as evidenced by the additional charge-center shift of the valence band occupied by the O-2p orbital and the conduction band occupied by the Bi-6p orbital. Notably, the degenerate Bi-pz state at the conduction band minimum in BBKT favors the large shift current response in the z direction.