Anisotropy and magnetization reversal with chains of submicron-sized Co hollow spheres

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 20-35 nm. The blocking temperature determined by the zero-field-cooling M-ZFC(T) measurement at H=90 Oe is about 325 K. The corresponding effective anisotropy is determined as K-eff similar to 4.6x10(4) J/m(3). In addition, the blocking temperature and the effective anisotropy determined by the analysis on H-C(T) are 395 K and 5.7x10(4) J/m(3), respectively. The experimentally determined anisotropy is smaller by 1 order of magnitude than the magnetocrystalline anisotropy of the bulk hcp Co, which is about (3-5)x10(5) J/m(3). A further analysis on H-C(T) shows that the magnetization reversal follows a nucleation rotational mode with an effective switching volume V*similar to 2.3x10(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.