• K BALAMURUGAN

      Articles written in Bulletin of Materials Science

    • Effect of heat treatment on the optical properties of layered muscovite single crystal sheets

      M KIRUBANITHY N SIVANANTHAM N GOPALAKRISHNAN K BALAMURUGAN

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      Naturally, thin sheets of layered, dioctahedral muscovite single crystals are transparent to visible light. Upon heat treatments performed up to 900$^{\circ}$C, the single crystal sheets of thickness $\sim$200 $\mu$m exhibited changes in its optical transparent character. The muscovite sheets exhibited either translucent or nontransparent, dark-brown, gold or copper like luster, varying optical band gap (from $\sim$3.7 to 2.7 eV) and photoluminescence (PL) properties, which depend on the temperature of the heat treatment. A comprehensive investigation has been made using X-ray diffraction, thermal analyses, energy dispersive X-ray and Raman spectroscopies to understand how/why the optical properties of the muscovite sheets changed up on heat treatments. Besides, no phase changes were found in the layered, lamellar crystal structure, evidences for the known dehydroxylation of muscovite were clearly seen from the thermal analyses data. Further, the Raman spectrum of the muscovite sheet heat treated at 900$^{\circ}$C showed no evidence for the presence of hydroxyl group (OH$^−$), which confirmed the completion of the dehydroxylation. The dihydroxylation-induced changes in the structural-elements (for example, Al-(O,OH)$_6$ partly or fully changing to Al-O$_5$/O$_6$ and its effects on the lattice constants, and also the defect levels introduced by the heat treatments are responsible for the changes in the optical transparency, colour of appearance, band gap and PL of the muscovite natural single crystal sheets.

    • Easy axis orientation dependence of the micromagnetic properties of CrO$_2$ nanodiscs

      K BALAMURUGAN G RAVI

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      Micromagnetic simulations were performed on seven isolated CrO$_2$ discs, each having different orientations of easy axis of magnetization (EAM), but, same 100 nm diameter and 50 nm thickness. The simulation results showed that for an external magnetic field applied along the x-axis, there was no emergence of vortex states corresponding to the relative orientations of the easy axis along [1 0 0], [1 1 0], [1 0 1] and [1 1 1]. Whereas, magnetic vortex states emerged for the relative orientations of the easy axis along [0 1 0], [0 0 1] and [0 1 1]. In another words, for the external field applied along the $x$-axis, if the relative orientation of the EAM is at $\theta = 0^{\circ}$ or any proximity of 45$^{\circ}$ (i.e., $0\geq \theta\leq 55^{\circ}$), the vortex states did not emerge; but, for any orthogonal orientations ($\theta = 90^{\circ}$), magnetic vortex states emerged. For the easy axis orientation along [0 1 0] and [0 1 1], out-of-plane vortex states with its core magnetization pointing along the normal to the plane of the disc emerged. However, for the easy axis orientation along [0 0 1], in-plane magnetic vortex states emerged, with its core pointing along the applied magnetic field direction. Further, depending on the relative orientation ofthe applied magnetic field and its strength, various other magnetization configurations, such as C, S and O states, wereobtained in the simulations.

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