  Magnetic properties with chains of hcp Co hollow spheres have been studied.
The diameter of the spheres ranges from 500 to 800 nm, with a typical shell
thickness of about 60 nm. The shell is polycrystalline with an average
crystallite size of 20 to 35 nm. The blocking temperature determined by the
zero-field-cooling MZFC(T) measurement at H = 90 Oe is about 325 K. The
corresponding effective anisotropy is determined as, Keff = 4.6*10^4 J/m^3. In
addition, the blocking temperature and the effective anisotropy determined by
the analysis on HC(T) are 395 K and 5.7*10^4 J/m^3, respectively. The
experimentally determined anisotropy is smaller by one order of magnitude than
the magnetocrystalline anisotropy of the bulk hcp Co, which is about 3 to
5*10^5 J/m^3. A further analysis on HC(T) shows that the magnetization reversal
follows a nucleation rotational mode with an effective switching volume, V* =
2.3*10^3 nm^3. The corresponding effective diameter is calculated as 16.4 nm.
It is slightly larger than the coherence length of Co, about 15 nm. The
possible reason for the much reduced magnetic anisotropy is discussed briefly.
