Fabrication Of La_2/3Ca_1/3MnO₃/Cu Composite Systems And Their Magnetotransport Properties

Manganese oxides with peroveskite structure have been the subject of intense study in recent years due to the complex interaction between electrons,lattices and spins. Specially, the potential technological application related to the colossal manganetoresistance (CMR) effect has activated the interesting on the materials. However, for a single crystal of a pure manganese oxide, the intrinsic CMR can be observed only at a high magnetic field (~ several Teslas) and in a narrow temperature window near Curie temperature. For a crystalline, although a higher magnetoresistance was observed at a lower magnetic field in the temperature much lower than the Curie temperature, the low field magnetoresistance decreased rapidly with increasing in the temperature. Thus, there are many technological problems as the manganese oxides were used to the electronic devices. The material which has the structure "FM/ML/FM" shows the low field MR(LFMR),where FM and ML represents the ferromagnetic metal and middle layer,respectively.So,we try to optimize the structure to enhance the LFMR and the sensitivity to the magnetic field.For the ferromagnetic metals,MR is higher with the higher spin polarization,so we try to bring Cu into the grain- boundaries of La2/3Ca1/3MnO3,changing the its electric and magnetic properties,to enhance the MR.In this thesis, our main investigation including:(1)A summary of the formation and its physical problems of the structure, properties and magnetoresistance in the manganese oxides has been given. We summarize the research work related to the FM/ML/FM material,and the fundamental of theory and experiment of the thesis has put forward on the basis of it. (2) The influences of different preparation procedures are compared and optimal one is chosen.We use the sol-gel to produce the manganese oxides. The electric ,magnetic properties and magnetoresistance of La2/3Ca1/3MnO3 are investigated.(3) For the different preparation procedures of composite La2/3Ca1/3MnO3/Cu,we observed that sintering temperature evidently affectes the results by changing the grain size of La2/3Ca1/3MnO3 ,the larger and smaller size both go against the effect.(4)A composite containing La2/3Ca1/3MnO3(LCMO) and Cu dependent material is investigated.Compared to LCMO,the composite has a lower insulator-metal transition temperature(TP) and a substantial reduction in the resistivity over the whole temperature range studied.For a low applied magnetic field(0.3T),the composite displays a high magnetoresistance (MR) peak near (TP) and a temperature-independent MR at low temperatures, a behavior diffenrent form LCMO that shows a monotonical increase in MR with cooling. The MR peak is different from the intrinsic CMR of LCMO,Our observation may reveal the special features of MR due to the introduction of Cu dependent material into the grain-boundary of LCMO,which is the extrinsic CMR. It is also found that the behavior of resistivity minimum appears at low temperature in the zero-field, but suppressed gradually in the applied field.We consider it from the spin-polarized scattering.