Intraparticle Magnetic Properties of Co₃O₄ Nanocrystals

Abstract

We have prepared a diluted system of Co3O4 nanocrystals dispersed in nonmagnetic SiO2 by heating an MCM-41 molecular sieve soaked in 0.075 mol/L CoCl2 aqueous solution in flowing oxygen at 300 °C for 3 h. Transmission electron microscopy showed that fine particles of about 3 nm exhibiting the (220) and (331) lattice fringes of the Co3O4 crystal distribute randomly in amorphous SiO2. Magnetic measurements using a SQUID magnetometer suggested the presence of quantum resonant spin tunneling of intraparticle superparamagnetic moments below the blocking temperature. The maximum points in the ac susceptibility shifted toward higher temperature from ≈3 to ≈5 K with increasing the frequency from ≈10-2 to ≈103 Hz. There were no divergent peaks above 1.7 K in the nonlinear susceptibility. The field dependent magnetization from 0 to ≈20 kG showed a small hysteresis loop at 2 K, whereas there were no hysteresis loops at both 5 and 10 K. The diluted Co3O4 nanocrystals is a new and ideally suitable material for the study of macroscopic magnetic quantum effects.