Interference effect between superparamagnetic and spin glass correlated moments in a system of dispersed Co3O4 nanocrystallites

An inhomogeneous system of aggregates of Co3O4 nanocrystallites dispersed in an amorphous SiO2 matrix has been studied. X-ray diffraction and atomic force microscopy reveal a bimodal distribution of crystallite sizes, smaller nanocrystallites with dimension below 10 nm and larger nanocrystallites of about 20 nm. The Co3O4 nanocrystallites enter the composition of nanograins with dimension 20-60 nm. The nanograins build aggregates with dimension 200-500 nm. A large value of the effective magnetic moment per Co2+ ion obtained from the high-temperature susceptibility measurements indicates possible disturbance of the normal spinel structure in which a fraction of Co3+ ions also possesses magnetic moment. An analysis based on the temperature dependence of the coercive field has shown that the smaller nanocrystallites behave as superparamagnetic particles with a blocking temperature of about 10 K. Simultaneous existence of two relaxation processes is observed in the frequency dependence of the imaginary part of the ac magnetic susceptibility in the vicinity of T = 15.8 K. The temperature dependence of the width of the distribution function of relaxation times obtained from the Cole-Cole diagrams exhibits behaviour characteristic for spin glass dynamics in a temperature range above 17.6 K and is temperature independent below 15.8 K, which is a property of superparamagnetic particles. The variation of the width of the distribution function between 17.6 and 15.8 K indicates that interference of the superparamagnetic and spin glass dynamics occurs. It has been found that average relaxation time increases with decreasing temperature from tau(c) < 10(-4) s at 17.6 K to 1.5 x 10(-1) s at 15 K. The increase of the average relaxation time with decreasing temperature, the observed blocking temperature of the superparamagnetic moments at about 10 K and interference appearing between the two spin dynamics suggest that the magnetic moments in the smaller as well as in the larger nanocrystallites are subject to a thermally activated blocking process at low temperatures.