The study on a macroscopic quantum tunneling in antiferromagnetic

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We have prepared a diluted system of Co_3O_4 nanocrystals dispersed in nonmagnetic SiO_2. Magnetic measurements suggested the presence of quantum resonant spain 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. 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_3O_4 nanocrystals is a mew and ideally suitable material for the study of macroscopic magnetic quantum effects.La_<1-x>Sr_xMnO3 nanopariticles were prepared by calcinations of a molecular sieve MCM-41 soaked in precursor solution. Superparamagnetic behavior of the nanoparticles is observed both for the antiferromagnetic and ferromagnetic phases at low temperatures. For the antiferromagnetic phase, the superparamagnetic moment corresponds to the uncompensated moments in the antiferromagnetic particles. While for the superparamagnetism for the ferromagnetic phase is smaller than the superpramagnetism estimated for the single-domain ferromagnetic particles.