Large Magnetoelectric Coupling in the Thin Film of Multiferroic CuO.

We report observation of large magnetoelectric coupling in an epitaxial thin film of multiferroic CuO grown on the (100)MgO substrate by the pulsed laser deposition technique. The film is characterized by X-ray diffraction, transmission electron microscopy, and Raman spectrometry. The crystallographic structure of the film turns out to be monoclinic (space group C2/c) with [111]CuO||[100]MgO "out-of-plane" epitaxy and "in-plane" domain structure. The lattice misfit strain is found to vary within ±1-3%. The dc resistivity, magnetization, dielectric spectroscopy, and remanent ferroeletric polarization have been measured across 80-300 K. The dielectric constant is found to decrease by >20% under a moderate magnetic field of ∼18 kOe while the remanent ferroelectric polarization, emerging at the onset of magnetic transition (T N ∼ 175 K), decreases by nearly 50% under ∼18 kOe field. These results could assume importance as the strain-free bulk CuO does not exhibit magnetoelectric coupling within such magnetic field regime. The strain-induced large magnetoelectric coupling in the CuO thin film would generate new possibility of further strain tuning to observe room-temperature magnetoelectric multiferroicity suitable for scores of applications such as memories, sensors, energy-harvesting devices, generators, amplifiers, and so forth.

Nonmonotonic particle-size-dependence of magnetoelectric coupling in strained nanosized particles of BiFeO3.

Using high resolution powder x-ray and neutron diffraction experiments, we determined the off-centered displacement of the ions within a unit cell and magnetoelectric coupling in nanoscale BiFeO3 (≈20-200 nm). We found that both the off-centered displacement of the ions and magnetoelectric coupling exhibit nonmonotonic variation with particle size. They increase as the particle size reduces from bulk and reach maximum around 30 nm. With further decrease in particle size, they decrease precipitously. The magnetoelectric coupling is determined by the anomaly in off-centering of ions around the magnetic transition temperature (T N ). The ions, in fact, exhibit large anomalous displacement around the T N which is analyzed using group theoretical approach. It underlies the nonmonotonic particle-size-dependence of off-centre displacement of ions and magnetoelectric coupling. The nonmonotonic variation of magnetoelectric coupling with particle size is further verified by direct electrical measurement of remanent ferroelectric hysteresis loops at room temperature under zero and ∼20 kOe magnetic field. Competition between enhanced lattice strain and compressive pressure appears to be causing the nonmonotonic particle-size-dependence of off-centre displacement while coupling between piezo and magnetostriction leads to nonmonotonicity in the variation of magnetoelectric coupling.

A training effect on electrical properties in nanoscale BiFeO3.

We report our observation of the training effect on dc electrical properties in a nanochain of BiFeO3 as a result of large scale migration of defects under the combined influence of electric field and Joule heating. We show that an optimum number of cycles of electric field within the range zero to ~1.0 MV cm(-1) across a temperature range 80-300 K helps in reaching the stable state via a glass-transition-like process in the defect structure. Further treatment does not give rise to any substantial modification. We conclude that such a training effect is ubiquitous in pristine nanowires or chains of oxides and needs to be addressed for applications in nanoelectronic devices.

Superspin glass mediated giant spontaneous exchange bias in a nanocomposite of BiFeO3-Bi2Fe4O9.

We observe an enormous spontaneous exchange bias (~300-600 Oe)--measured in an unmagnetized state following zero-field cooling--in a nanocomposite of BiFeO(3) (~94%)-Bi(2)Fe(4)O(9) (~6%) over a temperature range 5-300 K. Depending on the path followed in tracing the hysteresis loop--positive (p) or negative (n)--as well as the maximum field applied, the exchange bias (H(E)) varies significantly with | - H(Ep) | > | H(En) |. The temperature dependence of H(E) is nonmonotonic. It increases, initially, till ~150 K and then decreases as the blocking temperature T(B) is approached. All these rich features appear to be originating from the spontaneous symmetry breaking and consequent onset of unidirectional anisotropy driven by "superinteraction bias coupling" between the ferromagnetic core of Bi(2)Fe(4)O(9) (of average size ~19 nm) and the canted antiferromagnetic structure of BiFeO(3) (of average size ~112 nm) via superspin glass moments at the shell.

Stiffness- and wettability-dependent myoblast cell compatibility of transparent poly(vinyl alcohol) hydrogels.

This study reports the in vitro compatibility of muscle cells (C2C12 mouse myoblast cell line) with the transparent poly(vinyl alcohol) (PVA) hydrogels and the results are explained on the basis of surface wettability, crystallinity, and nanoscale elastic stiffness property. Nanoindentation was carried out with a maximum load of 100 µN for all the hydrogel compositions and the properties such as elastic stiffness, hardness and total work done during indentation were computed. The difference in cell viability as well as adhesion of cultured myoblast cells on the investigated hydrogel substrates were discussed in reference to the difference in the nanoscale elastic properties, crystallinity, and surface wettability. An important result has been that both elastic stiffness and surface wettability synergistically influence myoblast viability/adhesion on PVA hydrogels.

A combined effect of freeze--thaw cycles and polymer concentration on the structure and mechanical properties of transparent PVA gels.

Transparent poly(vinyl alcohol) (PVA) hydrogel films, derived from aqueous solutions of varying concentration, were synthesized by the cyclic freeze-thaw method (0°-37 °C). This study demonstrates a variation in the transparency, degree of crystallinity, wettability, swelling and mechanical properties of the hydrogels as a function of the solution concentration and the number of freeze-thaw cycles for a given average molecular weight (95,000 Da). The study manifests a strong control of the number of freeze-thaw cycles on the structure-property correlations of the synthesized transparent PVA hydrogels, revealing the possibility of obtaining a window of structural and process parameters for the physically cross-linked hydrogels, making them suitable for cell-gel interactions.