| Nanophase materials are one of the most frontier subjects in recent years. Many interesting phenomena have been found during the research of fine grain systems and most of them have the potential technological applications in the future. In this dissertation manganite perovskite samples with nominal composition La2/3Ca1/3MnO3 are studied. Two different ways (sol-gel and solid-state reaction methods) were used to prepare the samples. The difference in the polycrystalline single phase samples is the size of the grains. The order of the ferromagnetic phase transition, heat capacity and ac susceptibility are carried out on two kinds samples. From these experiments some results are given as fellows: 1. The grains in the sample prepared by solid-state reaction method is much bigger than that in the sample parpared by sol-gel mothod. And the grain size increase with the sintering temperature increase. 2. A first order ferromagnetic phase transition is observed in the sample prepared by solid-state reaction method and sintered in 1450°C, however, a second order ferromagnetic phase transition is observed in the sample prepared by sol-gel method and sintered in 1100°C , in which the critical exponent a=0.61,?=0.3226,?=1.38 and d=4.92 respectively. 3. In the sample prepared by sol-gel method and sintered in 1100°C, strong effects about grain size and surface are observed. Because of the small volume of the grains, the structures of the grains are single domain and the high level of spin disorder occurs on the grain boundary. The resistivity of the bulk material is relatively high and the smallest resistance value is observed in the low temperature area. The spin disorder and spin frustration that come from the grain boundary decrease the interaction between the grains. The big volume of the grain boundary makes surface effect more obvious and it leads a relatively small value of the imaginary part of the ac susceptibility. The level of spin disorder on the grain boundary is related to the volume of the grain size, the smaller grain size corresponds to the higher level of spin disorder. Thus a lower value of the saturated magnetization of the sample is observed under 5 Tesla at 10K. According to the real part of the ac susceptibility in high temperature area we calculate the effective spin quantum number, which is smaller than the theoretical value. This result suggests that the spins on the grain boundary take an important role in the total spin and the total moment of the grain can not be treated as a super spin. 4. The property of the sample prepared by solid-state reaction method and sintered in 1450°C is similar to single crystal due to large volume of the grain, in which there are many domains with random directions. The interaction in the domains leads a metamagnetic behavior just above the Curie temperature and relatively high critical field in a low temperature area. The large value of imaginary part of ac susceptibility shows the strong interaction between the grains and domains. According to the real part of ac susceptibility we get the value of effective spin quantum number, which is much larger than the theoretical value. In this case, the total moment of a grain in the sample can be treated as a super spin. 5. The larger size of the grains, the higher level of homogeneity, the stronger interaction. The high level of spin disorder on the grain boundary due to small grain size decreases the level of homogeneity of the sample, results in weakening of the interaction. This suggests strong interaction maybe the reason of the first order of ferromagnetic phase transition in this compound and the ferromagnetic phase transition may become a second order due to the effects of disorder. The similar results were reported by A. Moreo, M. Mayr and J. Burgy.
|
PDF Full Text Request