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Topological surface states reveal proximity-induced ferromagnetism with perpendicular anisotropy persisting above 400 K.

We describe the synthesis of novel monocrystalline FeCo nanowires encapsulated inside multiwalled carbon nanotubes (MWNTs). These FeCo nanowires exhibit homogeneous Fe and Co concentrations and do not contain an external oxide layer due to the presence of insulating nanotube layers. The method involves the aerosol thermolysis of toluene-ferrocene-cobaltocene solutions in inert atmospheres. The materials have been carefully characterized using state-of-the-art high-resolution transmission electron microscopy (HRTEM), electron-energy-loss spectroscopy (EELS), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), electron diffraction, HREELS-STM elemental mapping, X-ray powder diffraction, and SQUID magnetometry. We noted that the formation of FeCo alloys occurs at relatively low pyrolytic temperatures (e.g., 650-750 degrees C). These single-crystal nanowires, which have not been reported hitherto, always exhibit the FeCo (110) plane parallel to the carbon nanotube axis. The FeCo nanomaterials have shown large coercive fields at room temperature (e.g., 900 Oe). We envisage that these aligned ferromagnetic nanowires could be used in the fabrication of high-density magnetic storage devices and magnetic composites.

Electron holography provides a unique phase-imaging approach for characterizing nanoscale electrostatic and magnetic fields. From the relative phase shifts of the electron wave that has passed through the sample, quantitative field measurements that can be related back to specific features of the object can be made. The basic theory and experimental geometry for the off-axis mode of electron holography are first presented. Representative studies over a wide range of materials are then described. Applications involving electrostatic fields include p-n junctions and dopant profiles, piezoelectric fields and ferroelectrics, and charged defects and boundaries. Applications involving magnetic materials include hard magnets, thin films, and nanostructures, both man-made and naturally occurring. Finally, prospects for future developments and applications are briefly discussed.