Size-dependent magnetic properties of nickel nanochains

Magnetic properties with three different sizes of Ni nanochains, synthesized by a technique of wet chemical solution, have been investigated experimentally. The sample sizes (average diameter of the nano-particles) are 50, 75, 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 a one-dimensional (1D) chain-like structure. Magnetic properties have been investigated by temperature dependent magnetization M(T) and field dependent magnetization M(H) measurements. The results are explained within the context of the core-shell model. First, the freezing of disordered spins in the shell layer has resulted in a peak structure on the zero-field-cooled (ZFC) M(T) curve. The peak position is identified as the freezing temperature T-F. It is well described by the de Almeida-Thouless (AT) equation for the surface spin glass state. Second, the shape anisotropy of the 1D structure has caused a wide separation between the field-cooled (FC) and ZFC M(T) curves. This is mainly attributed to the blocking of the core magnetism by an anisotropy barrier, E-A. Third, by the M( H) measurement in the low field region, the open hysteresis loop measured at T = 5 K < TF is significantly enlarged in comparison with that taken at T > T-F. This indicates that a significant part of the contribution to the magnetic irreversibility at T < T-F is arising from the disordered spins in the shell layer. Last, with the reduced sample size, the coercivity, H-C, increases whereas the saturation magnetization goes down substantially. 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.