Intraparticle Magnetic Properties of Co₃O₄ Nanocrystals

We have prepared a diluted system of Co 3 O 4 nanocrystals dispersed in nonmagnetic SiO 2 by heating an MCM-41 molecular sieve soaked in 0.075 mol/L CoCl 2 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 Co 3 O 4 crystal distribute randomly in amorphous SiO 2 . 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 ≈10 3 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 Co 3 O 4 nanocrystals is a new and ideally suitable material for the study of macroscopic magnetic quantum effects.