The spin dynamics of high-aspect-ratio nickel nanorings in a longitudinal magnetic field have been investigated by Brillouin spectroscopy and the results are compared with a macroscopic theory and three-dimensional micromagnetic simulations. Good agreement is found between the measured and calculated magnetic field dependence of the spin wave frequency. Simulations show that as the field decreases from saturation, the rings switch from a "bamboo" to a novel "twisted bamboo" state at a certain critical field, and predict a corresponding dip in the dependence of the spin wave frequency on the magnetic field.
Brillouin light scattering has been employed to study the magnetic-field dependence of the discrete spin waves in permalloy Ni 80 Fe 20 nanowires. When a small magnetic field is applied transverse to the nanowire, the results reveal a low-frequency mode, which is absent in the longitudinal case. A Hamiltonian-based microscopic theory for nanowires with inhomogeneous magnetization shows that the appearance of this mode, along with its higher-field behavior, is a significant consequence of a small easy-plane single-ion anisotropy at the nanowire surface. The calculations provide a good description of experimental data for both the transverse and longitudinal cases.
We report here the potential competency of MnCO 3 versus MnO 2 for supercapacitor applications. MnCO 3 was synthesized by a hydrothermal method using KMnO 4 as a manganese source and either sugar or pyrrole as carbon source. MnCO 3 synthesized using sugar and pyrrole as carbon source is referred hereafter as MnCO 3 (s) and MnCO 3 (p), respectively. The synthesized products were characterized by powder X-ray diffraction, scanning electron microscopic and transmission electron microscopic studies. Microscopic studies revealed that MnO 2 possesses micro-flower-like morphology constructed by self-assembled nano-petals. While the morphology of MnCO 3 (s) is sub-micron size particles of different shape, the morphology of MnCO 3 (p) is crystalline particles of 10-20 nm dia. The capacitive characteristics of MnO 2 , MnCO 3 (s) and MnCO 3 (p) were evaluated in aqueous 0.1 M Mg(ClO 4 ) 2 electrolyte between 0 and 1 V using cyclic voltammetry and galvanostatic charge/discharge cycling. Specific capacitance (SC) values of 216 and 296 F g −1 obtained for MnCO 3 (s) and MnCO 3 (p) are 35 and 85 % higher than SC value of 160 F g −1 obtained for MnO 2 , respectively. Besides better capacitive storage characteristics, MnCO 3 (s) and MnCO 3 (p) have also exhibited better rate capability and cycle life than MnO 2 .
Nanocrystalline MnCO 3 is synthesized by hydrothermal reduction of KMnO 4 using different amounts of pyrrole. The effect of molar ratio of KMnO 4 :pyrrole on the phase purity, size of the particle, textural and capacitance properties of MnCO 3 is studied systematically using various physico-chemical and electrochemical techniques. While X-ray diffraction studies confirm decline in the phase purity of MnCO 3 , FTIR, Raman spectroscopic and thermogravimetric studies reveal an increase in the amount of adsorbed water and residual carbon content on increasing the pyrrole concentration during the synthesis. An increase in the size of the particles and reduction in the number of mesopores are observed from the morphological and sorption studies on increasing the pyrrole concentration during the synthesis. A highest specific capacitance value of 296 F g −1 is obtained at a current density of 0.16 A g −1 for the nanocrystalline MnCO 3 , and this capacitance value found to decrease on increasing the concentration of pyrrole during the synthesis of nanocrystalline MnCO 3 (166 and 140 F g −1 for the KMnO 4 :pyrrole ratio of 1:1 and 1:2, respectively).
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