Superparamagnetism in polycrystalline Li0.5Ni0.5O samples:: Low-field susceptibility measurements

A frequency-dependent sharp peak in ac susceptibility (chi(ac)) was found in Li0.5Ni0.5O around 110 K having an unusual magnetic-field (H) dependence. Temperature variation of first- and third-order chi(ac) have shown good agreement with Wohlfarth's model (WM) for superparamagnetism (SP) but with an unusually low value of anisotropy constant to saturation magnetization ratio (2R/M-S). A simple model is proposed using Neel's relaxation approach to explain the unusual field dependence of chi(ac). The data fit well to this model and gives a similar 2K/M-S value found from WM. This model can be used to quantitatively explain the field and temperature dependence as well as to extract various important physical parameters of other SP systems from a simple experiment. Existence of dipolar interactions among the superparamagnetic particles is concluded from history and time dependence of de magnetization measurements. The correlation of ac and de susceptibility also brings out some interesting features.