Magnetization reversal and anisotropy with chains of superparamagnetic Ni nanoparticles

Chains of superparamagnetic (SPM) Ni nanoparticles of 20 and 33 nm in diameter are synthesized by a facile, template-free, wet chemical method in magnetic field. The blocking temperature for the sample of 20 nm appears at T(B) approximately 255 K, above which the sample exhibits SPM behaviors by a M(H) measurement. At T < T(B), the temperature dependent coercivity, Hc(T), is beyond the description by the simple two-well model proposed by Neel and Brown in the early day. In contrast, for the Ni nanochains of diameter larger than the coherence length, Lcoh approximately 25 nm, the magnetization reversal properties are well described by this model. The magnetization reversal behavior of the SPM Ni nanochains with the diameter much smaller than the coherence length is one of the interesting points to study. Possible transition from the localized magnetization reversal mode in the high temperature end to the coherently rotational mode at low temperature is discussed.