Nanotube magnetism

  • Sui Y
  • Skomski R
  • Sorge K
 et al. 
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Abstract

FePt and Fe3O4 nanotubes are produced by hydrogen reduction in nanochannels
of porous alumina templates and investigated by electron microscopy,
x-ray diffraction analysis, and magnetic measurements. Loading the
templates with a Fe chloride and Pt chloride mixture followed by
hydrogen reduction at 560 °C leads to the formation of ferromagnetic
FePt nanotubes in the alumina pores. Using a Fe nitrate solution,
thermally decomposed at 250 °C and reduced in hydrogen for 2.5 h
at the same temperature, yields Fe3O4 tubes. The length of the nanotubes
is about 50 µm and their diameters range from about 150 to 220 nm,
depending on the thickness of the template film and the pore diameter
distribution. Reflecting the different magnetocrystalline anisotropies
of the compounds, the coercivities range from 0.61 kOe for Fe3O4
to 20.9 kOe for FePt. The hysteresis is explained in terms of a tubular
random-anisotropy model, which yields a diameter and anisotropy dependent
transition from a curling-type mode (Fe3O4) to a localized mode (FePt).

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