N. Rama

Study of magnetic properties of A2B'NbO6 (A=Ba, Sr, BaSr; and B'=Fe and mn) double perovskites

We have studied the magnetic properties of Ba2FeNbO6 and Ba2MnNbO6. It is seen that Ba2FeNbO6 is an antiferromagnet with a weak ferromagnetic behavior at 5 K while Ba2MnNbO6 shows two magnetic transitions, one at 45 K and the other at 12 K. Electron spin resonance measurements at room temperature show that the Mn compound does not show any Jahn-Teller distortion. It is also seen that the Neel temperatures of the A2FeNbO6 (A=Ba,Sr,BaSr) compounds do not vary significantly. However variations in the average A-site ionic radius influence the formation of short range correlations that persist above TN.

Correlation between electrical transport, optical, and magnetic properties of transition metal ion doped ZnO

We report on the transport and optical properties of transition metal ions, namely Ti, V, Fe, Co and Ni doped ZnO samples. Diffuse reflectance spectroscopy of doped ZnO showed the existence of absorption bands which were attributed to the d‐d transitions of respective dopants. Resistivity was found to decrease in the case of Ti, V, Fe, and Ni dopings. Correlation between the electrical resistivity and magnetic properties of the samples has been explained on the basis of impurity d-band splitting model.

Role of double exchange interaction on the magnetic and electrical properties of Pr0.8Sr0.2MnO3 ferromagnetic insulating manganite

In this work the ferromagnetic state in a ferromagnetic insulating manganite viz. Pr0.8Sr0.2MnO3 was analyzed. The temperature variation of ac susceptibility is seen to show two transitions with a high temperature transition occurring at 158K and a low temperature cusp (Tf) around 90K. The high temperature transition (TC) was found to be independent of frequency while the low temperature transition shifts with frequency indicating a frustrated magnetic state. The relaxation time observed from the critical slowing down spin analysis indicates that this state is that of a cluster glass. Temperature variation of resistivity shows an insulating behavior with two distinct humps corresponding to TC and Tf. Temperature variation of thermopower exhibits a peak at TC and decreases below TC which is attributed to the sudden release of holes from traps, similar to a ferromagnetic metallic manganite. Magnetoresistance is seen to show a change in curvature upon cooling the sample across TC which in conjunction with th...

Study of pulsed laser deposited ZnGa2O4 : Mn phosphor thin films in an oxygen controlled environment

Photoluminescent properties of ZnGa2O4 : Mn phosphor thin films grown on Si and quartz substrate using the pulsed laser deposition technique under different deposition conditions (i.e. oxygen partial pressure and substrate temperature) are reported. The charge transfer band (283 nm) excitation of the phosphor exhibited green emission (504 nm) due to electronic transition from 4G(4T1)–6S(6A1) of 3d5 Mn2+ ions. The SEM image with elemental composition analysis shows a change in the film porosity and the Ga/Zn ratio with respect to variation in the oxygen partial pressure during the growth of the thin films at a constant temperature (650 °C). The changes in the emission intensity of the films are attributed to the variation in oxygen and Zn content (low vapour pressure) with respect to the change in O2 partial pressure.

Effect of substrate on the electrical transport property of Ba2FeNbO6 double perovskite thin films

Ba2FeNbO6 (BFNO) is an antiferromagnetic insulator with a N?el temperature TN of 25?K. We have systematically studied the effect of substrate on the electrical transport of this compound under different growth conditions. We find that while the bulk and thin films grown on LaAlO3 and NdGaO3 substrates are insulating, the film grown on SrTiO3 substrate shows metallic behaviour. The anomalous behaviour of this compound on SrTiO3 is attributed to the defect conduction of SrTiO3 induced by the film growth conditions.

A-site dependent percolative thermopower and Griffiths phase in Pr(0.7−x)HoxSr0.3MnO3 (x=0.0, 0.04, 0.08, and 0.1)

We report our analysis of thermopower S(T) of Pr(0.7−x)HoxSr0.3MnO3 (x=0, 0.04, 0.08, and 0.1) compounds within a percolative framework and correlate the results with the existence of a Griffiths phase. The Griffiths temperature, at which ferromagnetic metallic clusters nucleate above TC, is evaluated. We further show that the Griffiths phase extends over a higher temperature range above TC with increase in the variance of ionic radii of the A site.

Electrical transport and morphological study of PLD-grown nanostructured amorphous carbon thin films

Nanostructured carbon thin films have been actively investigated recently for their electroresistance (ER) properties. Furthermore, carbon films with nonlinear current?voltage (I?V) characteristics have potential application in field-emission devices. This has motivated us to study the effect of various growth parameters on the physical and morphological properties of carbon films grown by pulsed laser deposition (PLD). Carbon films have been deposited using a graphite target at different partial pressures of argon. The morphology of film surfaces deposited at various growth conditions was monitored using an atomic force microscope (AFM). AFM studies showed nanostructured grain growth with average grain size of about 80?90?nm. As the deposition time was decreased down to 1?min, the grain size was also found to decrease correspondingly. From Raman spectroscopic measurements an increase in the I(D)/I(G) ratio and a decrease in FWHM (G) clearly revealed the promotion of sp2 hybridization as the substrate temperature increased. All the films show semiconducting behaviour with the dominant conduction process being the three-dimensional (3D) variable range hopping (VRH) mechanism. Nonlinear I?V curves were obtained for carbon films deposited on p-type Si indicating diode-like behaviour. The most significant result of this study was the observation of a large electroresistance value.

PLD Growth of CNTs using a Nanostuctured Ni Buffer Layer: Dependence of H2 partial Pressure

The effect of hydrogen partial pressure on the growth of CNT thin films using on-axis PLD has been studied. Using nickel as buffer layer, carbon nanotubes can be produced with good yield in Hydrogen atmosphere. We have found strong visible-near infrared (NIR) photoluminescence (PL) from carbon nanotube thin films synthesized under various hydrogen partial pressures. The present result shows that bright, narrow PL from CNTs can be obtained without any post growth processing.

Transport Studies of Transition Metal Ion Doped ZnO: Bulk and Thin Films

The effect of doping of transition metal ions (Fe and Co) on transport properties of ZnO has been studied in both bulk and thin films. The solubility limit of these ions have been found to be higher in thin films compared to bulk. Optical measurements reveal the presence of Fe in both 2+ and 3+ state. Co is believed to be in 2+ states. Electrical resistivity measurements show that while for bulk Fe doped ZnO samples there is a decrease in resistivity compared to undoped ZnO, it increases for bulk Co doped ZnO samples. However, thin film samples of both types of doped compounds show a decrease in resistivity compared to undoped ZnO. This difference in bulk and thin film behaviour has been explained on the basis of experimental results.