  We report high-temperature (300-1120 K) magnetization data of Fe and Fe3O4
nanoparticles embedded in multi-walled carbon nanotubes. The magnetic impurity
concerntations are precisely determined by both high-energy synchrotron x-ray
diffractometer and inductively coupled plasma mass spectrometer. We
unambiguously show that the magnetic moments of Fe and Fe3O4 nanoparticles are
enhanced by a factor of about 3 compared with what they would be expected to
have for free (unembedded) magnetic nanoparticles. The magnetization
enhancement factor is nearly independent of the applied magnetic field but
depends significantly on the cooling rate. What is more intriguing is that the
enhanced moments were completely lost when the sample was heated up to 1120 K
and the lost moments at 1120 K were completely recovered through several
thermal cycles below 1020 K. Furthermore, there is a rapid increase or decrease
in the magnetization below about 60 K. The anomalous magnetic properties cannot
be explained by existing physics models except for the paramagnetic Meissner
effect due to the existence of ultrahigh temperature superconductivity in the
multi-walled carbon nanotubes.
