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Three powder samples of Ni nanochains formed of polycrystalline Ni nanoparticles with an estimated diameter of about 30 nm have been synthesized by a wet chemical method using different organic surfactants. These samples, having magnetically/structurally core-shell structures, all with a ferromagnetic Ni core, are Ni@Ni3C nanochains, Ni@NiSG nanochains with a spin glass (SG) surface layer, and Ni@NiNM nanochains with a nonmagnetic (NM) surface layer. The average thickness of the shell for these three samples is determined as about 2 nm. Magnetic properties tailored by the different surface magnetism are studied. In particular, suppression in saturation magnetization usually observed with magnetic nanoparticles is revealed to arise from the surface magnetic states with the present samples., Comment: 23 pages, 6 figures included

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., Comment: 27 pages, 6 figures, 1 table, changed content, 1 Fig. replaced