  Magnetic properties with 3 different sizes of Ni nanochains, synthesized by a
technique of wet chemical solution, have been investigated experimentally. The
sample sizes are 50 nm, 75 nm, and 150 nm with a typical length of a few
microns. The characterizations by XRD and TEM reveal that the samples consist
of Ni nano-particles forming one dimensional (1D) chain-like structure. The
magnetic properties have been investigated by FC, ZFC measurements and M-H
measurements. The results can be well explained within the context of
core-shell model. First of all, the freezing of disordered spins in the shell
layer have resulted in a peak-like structure on the ZFC curve. The peak
position occurs around TF ~ 13 K. With the 50 nm sample, the field dependent
behavior of TF(H) has been investigated in detail. It is well described by the
de Almeida-Thouless (AT) equation for the surface spin glass state. Secondly,
the shape anisotropy of 1D structure has caused a wide separation between the
FC and ZFC curves. This is mainly attributed to the blocking of the core
magnetism. Thirdly, by the M-H measurement in the low field region, the open
hysteresis loop measured at 5 K < TF is significantly enlarged in comparison
with that taken at T > TF. This indicates that a significant part of the
contribution to the magnetic irreversibility at T < TF is coming from the
disordered spins in the shell layer. Lastly, with reducing sample size, the
coercivity, HC, increases, whereas the saturation magnetization goes down
dramatically. These imply that, as the sample size reduces, the effect of shape
anisotropy becomes larger in the magnetization reversal process and the
contribution to the magnetism from the ferromagnetically ordered core becomes
smaller.
