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Superparamagnetism in polycrystalline Li 0.5 Ni 0.5 O samples: Low-field susceptibility measurements

2000; American Physical Society; Volume: 62; Issue: 13 Linguagem: Inglês

10.1103/physrevb.62.8996

1095-3795

A. Bajpai, A. Banerjee,

Magnetic Properties and Synthesis of Ferrites

A frequency-dependent sharp peak in ac susceptibility $({\ensuremath{\chi}}_{\mathrm{ac}})$ was found in ${\mathrm{Li}}_{0.5}{\mathrm{Ni}}_{0.5}\mathrm{O}$ around 110 K having an unusual magnetic-field (H) dependence. Temperature variation of first- and third-order ${\ensuremath{\chi}}_{\mathrm{ac}}$ 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 ${(2K/M}_{S}).$ A simple model is proposed using N\'eel's relaxation approach to explain the unusual field dependence of ${\ensuremath{\chi}}_{\mathrm{ac}}.$ 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 dc magnetization measurements. The correlation of ac and dc susceptibility also brings out some interesting features.