We report muon-spin-rotation studies of the magnetic-field distribution n(B) in the vortex state in both underdoped and strongly overdoped Bi2Sr2CaCu208+z single crystals. With increasing magnetic field we observe sudden changes in n(B) that are indicative of a reduction in the dimensionality of the vortex system.The crossover field B* depends strongly on the doping state and the corresponding anisotropy y of the system.From magnetization measurements we find evidence that the second peak in the M-0 loops is related to the crossover in the vortex structure in B*.Due to their layered structure the cuprate high-T, superconductors exhibit very anisotropic electronic and magnetic properties. Numerous experiments indicate that only the blocks of CuOz layers are intrinsically superconducting while the layers of thickness s which separate them act mainly as insulating, or sometimes metallic, blocking layers and charge reservoirs. In the mixed state, e.g. , when the ex-

Iron sand is one of the natural resources in West Sumatra that has not been optimally utilized. One of the potential locations to get this iron sand deposits is the coast of Kata. This study aims to analyze the content and size of the iron grains found on the coast of Kata Padang Beach, West Sumatera Province. Coastal sand samples are extracted using permanent magnets to separate magnetic and non-magnetic materials. Characterization of iron sand using Scanning Electron Microscopy with Energy Dispersive X-ray (SEM-EDX) and Vibrating Sample Magnetometer (VSM). The result of characterization using SEM-EDX shows that iron sand samples contain Fe and O elements derived from Phase Magnetite (Fe3O4).  Keywords: iron sand, magnetic materials, SEM-EDX, VSM

Magnetite (Fe3O4) nanostructures and their modifications with other materials show proper characteristics to be implemented as a sensing material. This paper provides a brief review of the application of the Fe3O4 nanostructures and their modifications as sensitive material for pollutant gas sensors. Several studies were highlighted to explain the past-to-present progress of materials development. Various synthesis procedures of the materials were also clearly explained. The application of pure Fe3O4 nanostructures and their modification as sensitive materials in gas sensor devices to detect toxic gases is the main section of this paper. Last, the future prospects section summarized the materials’ development and provided a suggestion for future development.

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