S. Angappane

Experimental studies of strong dipolar interparticle interaction in monodisperse Fe3O4 nanoparticles

Interparticle interaction of monodisperse Fe3O4 nanoparticles has been experimentally investigated by dispersing the nanoparticles in solvents. With increasing the interparticle distances to larger than 100nm in a controlled manner, the authors found that the blocking temperature (TB) of the nanoparticles drops continuously and eventually gets saturated with a total drop in TB of 7–17K observed for 3, 5, and 7nm samples, compared with their respective nanopowder samples. By carefully studying the dependence of TB on the interparticle distance, the authors could demonstrate that the experimental dependence of TB follows the theoretical curve of the dipole-dipole interaction.

Complex ferromagnetic state and magnetocaloric effect in single crystalline Nd0.7Sr0.3MnO3

The magnetocaloric effect in single crystalline Nd_{0.7}Sr_{0.3}MnO_{3} is investigated by measuring the field-induced adiabatic change in temperature which reveals a single negative peak around 130 K well below the Curie temperature (T_C=203 K). In order to understand this unusual magnetocaloric effect, we invoke the reported {55}^Mn spin-echo nuclear magnetic resonance, electron magnetic resonance and polarized Raman scattering measurements on Nd_{0.7}Sr_{0.3}MnO_{3}. We show that this effect is a manifestation of a competition between the double exchange mechanism and correlations arising from coupled spin and lattice degrees of freedom which results in a complex ferromagnetic state. The critical behavior of Nd_{0.7}Sr_{0.3}MnO_{3} near Curie temperature is investigated to study the influence of the coupled degrees of freedom. We find a complicated behavior at low fields in which the order of the transition could not be fixed and a second-order-like behavior at high fields.

PAni-PMMA blend/metal Schottky barriers

Abstract We report here the fabrication of Schottky barriers using polyaniline and insulating polymers like polymethylmethacrylate (PAni–PMMA) blends. The Schottky barriers were prepared by the thermal evaporation of gold metal electrodes on to free-standing films of the blends. The electrical characteristics of the barriers are found to depend on the work function of the PAni–PMMA blend, which in turn depend on the volume fraction of PAni. Current–voltage ( I – V ) and capacitance–voltage ( C – V ) measurements were used to characterize these devices. The values of various junction parameters such as ideality factor, barrier heights and rectification ratio were calculated.

Magnetic clusters in Nd1−xSrxMnO3 (0.3⩽x⩽0.5): An electron-spin resonance study

The paramagnetic electron-spin resonance (ESR) linewidth of polycrystalline Nd1−xSrxMnO3 (x=0.3, 0.33, 0.4, and 0.5) increases in a quasilinear manner up to 400 K due to the formation of paramagnetic clusters, which is also confirmed by the observation of an activated behavior of the ESR intensity above TC. Upon cooling, a pair of ferromagnetic resonance lines appear well above the magnetic transition temperature (TC, TN) in all cases. This suggests the presence of ferromagnetic clusters at a temperature T*>TC. The two-line structure below T* is discussed in terms of phase separation and formation of magnetic clusters and their coexistence with a charge-ordered phase for x>0.4.

Tricritical point and magnetocaloric effect of Nd1−xSrxMnO3

A tricritical point is observed in the Nd1−xSrxMnO3 (NSMO) (x=0.3, 0.33, and 0.4) manganites at x=0.33 which separates the first-order transition in NSMO-0.3 and second order transition in NSMO-0.4. The ferromagnetic transition of these compounds is further investigated by measuring magnetocaloric effect (MCE) and by applying a theoretical model based on Landau theory of phase transitions. Results indicate that the contributions to the free energy from the presence of correlated clusters are strongly influencing the MCE by coupling with the order parameter around the Curie temperature.

Magnetic Pd nanoparticles: effects of surface atoms

We have investigated the magnetic properties of trioctylphosphine (TOP)-stabilized monodisperse palladium nanoparticles of 2, 3, 5 and 10 nm in size, in order to study the possible effects of surface Pd atoms. These nanoparticles display clear signatures of ferromagnetism such as hysteresis and saturation magnetization over the entire temperature range studied here from 2 to 380 K. The magnetization of the nanoparticles increases with decreasing particle size, indicating a possibly important role played by Pd atoms on the surface of the nanoparticles. More importantly, we also found that the magnetization of our TOP-stabilized Pd nanoparticles is one order of magnitude smaller than those of other Pd nanoparticles reported so far, which is most likely to be due to the weak nature of interface interaction between TOP ligands and Pd nanoparticles compared to other ligands. This observation is consistent with the view that the magnetism of Pd nanoparticles is strongly influenced by the interaction of surface atoms with the ligands. We discuss our experimental findings in terms of a charge transfer mechanism due to a covalent bond of Pd atoms with the protective TOP ligand, which would increase the 4d density of states of Pd atoms due to localization by the bonded P atoms.

Electrical and magnetoresistivity studies in chemical solution deposited La(1−x)CaxMnO3 thin films

High quality magnetoresistive La(1−x)CaxMnO3 thin films have been prepared by the chemical solution deposition technique. A solution of propionate precursors of lanthanum, calcium, and manganese in propionic acid was used for this purpose. Films of varying compositions (x varying from 0.1 to 0.4) were spin coated on to LaAlO3(100) and SrTiO3(100) substrates at room temperature and pyrolyzed in the temperature range 600–850 °C. For fixed compositions, annealing at higher temperatures shifts the insulator–metal transition temperature (TI–M) to higher values accompanied by a reduction in the resistivity values. The TI–M variation for different x values was found to be less pronounced in the compositions x=0.2, 0.3, and 0.4. Typical TI–M values of 283 K and 290 K were obtained for La0.7Ca0.3MnO3 coated on LaAlO3 and SrTiO3 substrates, respectively, when annealed at 850 °C. The substrate effect was found to be more pronounced for the x value 0.1 which showed two peaks (one at 271 K and another at 122 K) in the...

ESR study of spin–lattice correlated clusters in single crystalline Nd0.7Sr0.3MnO3

We report electron spin resonance measurements in single crystalline Nd0.7Sr0.3MnO3 that provide evidence for the existence of spin–lattice correlated clusters above and below TC (= 205 K). The linewidth of the paramagnetic spectrum indicates the presence of strong electron–phonon interaction rather than the spin only interaction seen in other manganites. The gradual increase observed in the g value above TC is attributed to the presence of orbital correlations. The observation of ferromagnetic resonance (FMR) spectra only below 185 K (TC = 205 K) and noisy features in the FMR spectra above 140 K are ascribed to the strong competition between localization due to lattice distortions and delocalization of charge carriers. The magnetocrystalline anisotropy of the sample splits the FMR line into two below 185 K. An additional splitting observed in the FMR line is attributed to the existence of spin–lattice correlated (insulating) clusters within the ferromagnetic (metallic) phase. The influence of these clusters on the spin dynamics above and below TC is discussed.

Electron spin resonance study of Nd0.7Sr0.3MnO3 single crystals

Electron spin resonance (ESR) measurements in Nd0.7Sr0.3MnO3 single crystals provide evidence for the existence of spin and lattice correlated clusters above and below TC=205K. The ESR linewidth deviates from the quasilinear “universal” temperature dependence of the linewidth observed in several manganites. A linear temperature dependence of the linewidth is attributed to the spin-phonon interaction. Thus the linewidth of the paramagnetic resonance line indicates the presence of a strong spin-phonon interaction rather than the spin-spin interaction seen in other manganites. The gradual increase observed in the g value above TC is attributed to the presence of orbital correlations. The split in the ESR spectra only below 180K (TC=205K) and more noise in the spectra between 150 and 180K are ascribed to the strong competition between localization due to lattice distortions and delocalization of charge carriers.

Electrical conduction in polyaniline-PMMA blends and their use as cryomagnetic temperature sensors

Electrical conduction in polymeric conductors is obscured by the presence of strong disorder. Localization and Coulomb Interactions play a dominant role. The temperature dependence of the reduced activation energy, W = dln(σ)/dln(T), shows that PAni-PMMA blends are on the metallic side of the metal-insulator transition. The temperature and field dependences of conductivity of PAni - PMMA blends are discussed. ESR line shape changes from a Dysonian with A/B ratio of about 2.5 to a Lorentzian, mirroring the temperature dependence of electrical conductivity and hence of the skin depth. Comparison with the characteristics of commercial semiconductor thermometers show that these blends can be a viable alternative to them as cryomagnetic temperature sensors.