We present a detailed study of the magnetic properties of sol-gel prepared nickel oxide nanoparticles of different sizes. We report various measurements such as frequency, field and temperature dependence of ac susceptibility, temperature and field dependence of dc magnetization and time decay of thermoremanent magnetization. Our results and analysis show that the system behaves as a spin glass.

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Magnetization of nanoparticles of NiO are measured and analyzed taking into account a distribution in particle magnetic moment. We find that disregarding this distribution in the analysis is the reason for the many anomalous observations reported on this system in the literature.

We report the temperature and field dependence of dc magnetization on sol gel prepared nanoparticles of Ni(OH)2. At higher temperature the system is found to behave as a paramagnet while we find evidence for superparamagnetic blocking at low temperature. The system shows a paramagnetsuperparamagnet transition and we discuss the underlying mechanism.

Magnetic properties of β‐Co(OH)2 nanoparticles covering temperature range of 5–300 K in magnetic field upto 50 kG are studied. Zero‐field‐cooled and field‐cooled susceptibility as a function of temperature curves in different applied magnetic fields are seen to bifurcate at Tbf=11thinmathspacenormalK. Curie–Weiss fit of zero‐field‐cooled susceptibility curve well above Tbf yields θ=9thinmathspacenormalK and ionic magnetic moment μ=5.03thinmathspaceμB. Magnetization as a function of applied magnetic field curve below Tbf shows a two‐step transition to ferromagnetic state. Strength of three possible exchange interactions among Co2+ ions in β‐Co(OH)2 nanoparticles is also determined. We find that the present system orders antiferromagnetically at TN=11thinmathspacenormalK, with a dominant intralayer ferromagnetic coupling and a weaker interlayer antiferromagnetic coupling.