Magnetic Chains Created by Polymer-Induced Assembly of Hollow Cobalt   Nanoparticles
  Magnetic chains of hollow cobalt nanoparticles (450-750 nm) have been synthesized by using poly(vinyl pyrrolidone) (PVP) as a template in an ethylene glycol solution. In this process, CoCl2.6H2O was reduced by N2H4.H2O in the presence of PVP. All of the Co nanoparticles are hollow with a shell of 40 nm and self-assembled into a chainlike structure that is as long as ~10 micron. At 300 K, the branched hollow Co nanoparticle chains exhibit a saturation magnetization of 37.5 emu/g, a remnant magnetization of approximately 1.55 emu/g, and a coercivity of about 66 Oe, which is more than an order of magnitude larger than that of the bulk 
Lin Guo, Fang Liang, Chen Min Liu, Hui Bin Xu, Qun Peng Zhong,   Xiaogang Wen, Shihe Yang, Wangzhi Zheng, Chinping Chen