Biennial Report 2005-2007 - Saha Institute of Nuclear Physics

106 Biennial Report 2005-07chain compound viz. BaCu 2 V 2 O 8 using 51 V NMR. Shift, K% and the spin lattice relaxation rate,1/T 1 have been performed in a polycrystalline sample in the temperature range 4-380 K at 7.04tesla. Behaviour of K% for the two types of V sites, suggests that at low temperature, Cu 2+ iongradually approaches towards a nonmagnetic state with S=0. Existence of a non zero shift ofsmaller magnitude for the V2 site, (connecting inter-chain Cu atoms) in comparison to that of V1(connecting Cu atoms within a chain), indicate the existence of an inter-chain exchange. Thusthe importance of the inter-chain interaction in the magnetic properties of this system can not beruled out. The temperature dependence of 1/T 1 clearly shows the opening of a single spin gap(=380 K) in the excitations spectrum. The magnitude is significantly higher than that obtainedfrom shift data (∆=230K). The large discrepancy between these two values is yet to be explained.Importance of diffusion, if any, in the relaxation process would be studied using very low field NMRmeasurement. We also propose to extend this work using Cu NMR, which should reveal furtherdetails of the excitation spectra.K Ghoshray, B Pahari, B Bandyopadhyay, R Sarkar, A GhoshrayECMP4.1.1.7 Magnetization and NMR studies on granular FeCu alloyFeCu granular alloys (Fe≈1-20 %) have been prepared by chemical reduction and characterizedby X-ray diffraction, energy dispersive spectroscopy (EDS) and transmission electron microscopy(TEM). Magnetic properties have been studied macroscopically and also using 63 Cu nuclear magneticresonance (NMR) as a local probe. Study of ZFC-FC magnetization yields blocking temperaturedistribution of magnetic fine particles system in all samples. At low Fe concentration, themagnetizations of the samples combine superparamagnetism (SPM) and paramagnetism (PM) nearroom temperature, and a ferromagnetic (FM) component is also displayed at 5 K. High Fe contentalloys are almost entirely ferromagnetic even at 300 K. However, in all these samples the observationof the 63 Cu NMR signal confirms the existence of a paramagnetic component that remainsundiminished from 300 K down to 4 K. The NMR results are compared with those in bulk FeCualloys studied earlier, and the comparison yields that the paramagnetic component of the FeCualloy contains about 0.02 to 0.04 % Fe, and seems to exhibit a Kondo temperature much higherthan that observed in bulk alloys of similar composition.B Bandyopadhyay, B Pahari, K GhoshrayECMP4.1.1.8 Ballistic spin dependent transportWe have shown that the ballistic channels in MTJs are not only magnetoresistive, it, in fact,can cause inverse tunneling magnetoresistance. Inverse magnetoresistance has been observed inmagnetic tunnel junctions with pinhole nanocontacts over a broad temperature range. The tunnelmagnetoresistance undergoes a change of sign at higher bias and temperature. This is the firstdirect experimental evidence that pinhole shorts through the insulating spacer in a magnetic tunneljunction can cause inverse tunneling magnetoresistance when the transmission probability is close tounity. The relative contributions from the conduction channels due to elastic tunneling and ballisticspin conserved transport through the pinholes can be changed by proper adjustment of the biasand temperature, which can even result in the change of sign of the tunneling magnetoresistance.