  We present in this paper the structural and magnetic properties of high
aspect ratio Co nanoparticles (~10) at high temperatures (up to 623 K) using in
situ X ray diffraction (XRD) and SQUID characterizations. We show that the
anisotropic shapes, the structural and texture properties are preserved up to
500 K. The coercivity can be modelled by u0Hc=2(Kmc+Kshape)/Ms with Kmc the
magnetocrystalline anisotropy constant, Kshape the shape anisotropy constant
and Ms the saturation magnetization. Hc decreases linearly when the temperature
is increased due to the loss of the Co magnetocrystalline anisotropy
contribution. At 500K, 50% of the room temperature coercivity is preserved
corresponding to the shape anisotropy contribution only. We show that the
coercivity drop is reversible in the range 300 - 500 K in good agreement with
the absence of particle alteration. Above 525 K, the magnetic properties are
irreversibly altered either by sintering or by oxidation.
