Biennial Report 2005-2007 - Saha Institute of Nuclear Physics

Material Physics 151magnetization data, extracted from polarized neutron reflectivity curves, exhibit a continuous increasein in-plane Bohr-magneton value per gadolinium-ion with temperature with well definedmaxima at around 100mK and data in spin-flip channel do not show corresponding decrease below100mK. This result makes the Berezinskii-Kostrerliz-Thouless transition insignificant here and isconsistent with the model proposed earlier that suggested interplay between in-pane exchange termand magnetocrystalline anisotropic interactions induced by out-of-plane easy axis.Sirshendu Gayen, Milan K Sanyal, Max Wolff†, Hartmut Zabel†SP5.1.2 Soft Matter5.1.2.1 Au-thiols monolayer on solid substrates: structure and stabilityNearly monodisperse and stable thiol encapsulated Au (Au-thiols) nanoparticles can be synthesizedchemically, which show interesting properties. However, when such system is transferred orplaced on solid substrate, reorganization can take place depending upon film-substrate interaction.Time-evolution XRR measurements and derived EDP indicate that the monolayer structure of theAu-thiols film is not very stable on Si substrate. Increase in thickness as well as roughness is evidentfrom the analysis. As there is no chemical interaction between the substrate and nanoparticles,dewetting seems to play a dominant role. Initial dewetting probably starts due to water evaporation,which persists for about 10-20 hours. However, even after water evaporates, the nanoparticleshave a tendency to reorganize themselves to form interesting structures, such as disks, rings orbroken chains as have been observed by AFM.R Banerjee, S Hazra, MK SanyalSP5.1.2.2 Relaxation study of Langmuir monolayerLangmuir monolayer is a fascinating example of 2-dimensional system, which is however, thermodynamicallymetastable. The study of relaxation, combining macroscopic and microscopic information,can elucidate the processes involved in such system. The pressure relaxation (i.e. π − tcurves for constant area) and the area relaxation (i.e. A − t curves for constant pressure) of preformedfatty acid salt, namely ferric stearate show the presence of three distinct relaxation time.XRR analysis of the transfer monolayer, on the other hand, suggest that the first two relaxationtime (a few minutes and tens of minutes) can be associated with the reorganization of molecules inLangmuir monolayer, while the last one (nearly thousands of minutes) is due to the global materialloss or desorption. It seems that the molecules move from the lower molecular layer to the uppermolecular layer with changing ‘asymmetric’ to ‘symmetric’ molecular configuration under area relaxation.While in the pressure relaxation the reverse process takes place, i.e., the molecules fromthe upper molecular layer come down to the lower molecular layer with changing their molecularconfiguration.Sarathi Kundu, A Datta, S HazraSP