The study on a macroscopic quantum tunneling in antiferromagnetic

制备了分散在非磁性SiO_2中的Co_3O_4纳米晶体稀释体系。磁测量表明，在阻滞温度以下存在粒子内超顺磁矩的量子共振西班牙隧穿。随着频率从10^2到10^3 Hz的增加，交流磁化率的最大值从3到5 K向更高的温度移动。在0 ~ 20 kG范围内，磁场相关磁化强度在2 K时出现了小的磁滞回线，而在5和10 K时则没有磁滞回线。稀释后的Co_3O_4纳米晶体是研究宏观磁量子效应的理想材料。用MCM-41分子筛浸泡在前驱体溶液中，煅烧制备了La_<1-x>Sr_xMnO3纳米颗粒。在低温下观察到纳米颗粒的反铁磁相和铁磁相的超顺磁行为。对于反铁磁相，超顺磁矩对应于反铁磁粒子中的未补偿矩。而铁磁相的超顺磁性则小于单畴铁磁粒子的超顺磁性。

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.