Venkata Ramana Mudinepalli

Effect of grain size on dielectric and ferroelectric properties of nanostructured Ba 0.8 Sr 0.2 TiO 3 ceramics

Barium strontium titanate (Ba0.8Sr0.2TiO3, BST) nanocrystalline ceramics have been synthesized by high energy ball milling. As the sintering temperature increases from 1200 °C to 1350 °C, the average grain size of BST ceramics increases from 86 nm to 123 nm. The X-ray diffraction (XRD) studies show that these ceramics are tetragonal. The phase and grain size of the sintered pellets have been estimated from the XRD patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. The effect of grain size on dielectric and ferroelectric properties is studied. The dielectric and piezoelectric parameters are greatly improved at room temperature with increase in grain size. The Curie transition temperature is found to shift slightly towards higher temperatures as the grain increases from 86 nm to 123 nm. The coercive field decreases and the remnant polarization and spontaneous polarization increase as the grain size of BST nano ceramics increases. These ceramics are promising materials for tunable capacitor device applications.

Hydrogen-mediated long-range magnetic ordering in Pd-rich alloy film

The effect of hydrogenation on a 14 nm Co14Pd86/Al2O3(0001) thin film was investigated on the basis of the magnetooptical Kerr effect. After exposure to H2 gas, the squareness of the hysteresis loop showed a large transition from approximately 10% to 100% and the saturation Kerr signal was reduced to nearly 30% of the pristine value. The reversibility of the transition was verified and the response time was within 2–3 s. These observations indicate that the hydride formation transformed the short-range coupled and disordered magnetic state of the Co14Pd86 film to a long-range-ordered ferromagnetic state and induced appreciable decrease in the magnetic moment. The enhanced long-range-ordering and the reduction of the magnetic moment were attributed to the change of electronic structure in Co14Pd86 with hydrogen uptake.

Voltage induced reversible and irreversible change of magnetic coercivity in Co/ZnO heterostructure

In this study, the application of bias voltage to 4–8 nm Co/275 nm ZnO heterostructures changed the magnetic behavior reversibly or irreversibly, depending on the different voltage-induced mechanisms. The magnetic coercivity (Hc) monotonically decreased 20% when the small voltages of 0–8 V were applied. The Hc reduction was symmetric with the voltage polarity, and the reversibility was demonstrated by cyclically switching the bias voltage between 0 and 7 V. While a large voltage up to 40 V was applied to the Co/ZnO junction, the current heating effect became considerable and the Co-oxide was formed, as confirmed by depth-profiling X-ray photoemission spectroscopy analysis. The presence of Co-oxide in the Co films induced the irreversible reduction of the Kerr signal and Hc at room temperature. The considerable Hc enhancement at 130 K also indicates the exchange bias coupling effect from the antiferromagnetic Co-oxide.

Preparation and characterisation of fine-grained barium lead titanate ceramics by spark plasma sintering technique

Barium lead titanate ferroelectric ceramic is a promising material for electronic devices at high frequency and high temperatures. These fine-grained ceramics were obtained at a low sintering temperature of 900°C within a short sintering time by means of a new sintering method – spark plasma sintering (SPS). The ceramics are highly densified to >99% of the theoretical X-ray density with homogeneous microstructures, signifying that the SPS process is effective for the densifying of compacts and the inhibition of exaggerated grain growth. Temperature dependence of dielectric constant of SPS pellets shows three times higher than the conventional sintering (CS) Ba0.8Pb0.2TiO3 (BPT) pellet. The variations in properties between SPS and CS materials are mainly because of the differences of the resultant microstructures, especially grain size, of the ceramics.