Study On Modification Of Grain Boundaries And Magnetoresistance Properties Of Provskite Manganites

Peroveskite Manganites are the subject of intense study for the fields of condensed physics and materials physics in recent years due to the discovery of colossal manganetoresistance (CMR) effect and 100% spin polarization, as well as the complex interaction between electrons,lattices and spins in these compounds. However, the intrinsic CMR can be observed only at a high magnetic field (~ several Teslas) and in a narrow temperature window near Curie temperature, which is not appealing for practical application. For polycrystalline manganites, although a higher magnetoresistance is observed at a lower magnetic field in the temperature much lower than the Curie temperature, the low field magnetoresistance decreases rapidly with increasing temperature. Thus, there are many technological problems as the manganese oxides are used to the electronic devices. In this thesis, we base on the polycrystalline manganese oxides of La2/3Ca1/3MnO3 (LCMO) and La2/3Sr1/3MnO3 (LSMO), modify the grain boundaries in these manganites by the introdunction of second phases with different properties to grain boundary regions. The electrical and magnetic transport properties of the composites have been investigated, which provides experimental and theoretic basis for the enhancement of low-field magnetoresistance. The main investigations are shown as followed:1. An introduction of spintronics has been presented. More attentions have been paid on the structure, electrical transport behavior, and magnetic properties of perovskite manganites, which are considered as the most promising magneto electronics materials. On the basis of the review of the intrinsic CMR effect and extrinsic low-field magnatoresistance (LFMR) in perovskite manganites, the basic thinking of subject selection and investigation significance is put forward.2. Based on the different properties of the second phases, the special experimental methods have been designed respectively. In this way, the second phases successfully disperse at the grain boundaries of ferromagnetic metallic granular systems and their distribution can be controlled effectively. Therefore, the grain boundaries of the ferromagnetic metallic granular systems can be modified purposely, which has a key role on the electrical and magnetic transport behavior of the composites.3. Through the fabrication of (1-x)LCMO/xCuO composites, the LCMO ferromagnetic particles are successfully coated with some paramagnetic Cu dependent materials. In this way, Cu2+ ion, carrying an extra electron with 1/2 spin is introduced to the grain boundary regions of LCMO. The effects of sintering temperature on the structure, electrical and magnetic transport for the composites have been investigated and the proper heat treatment conditions are obtained. It is found that with low CuO addition level, the temperature of insulator-metal transition (Tp) shifts downwards with the increase of x. But at high CuO addition level, the Tp is almost independent on x. Moreover, the LFMR is enhanced substantially in the composites and the MR value at 0.3 T can reach as high as ~90 %.4. The electrical and magnetic transport behavior for the composite of (1-x)LCMO/xCuO (x=20 %) at the cooling and warming process has been investigated systematically. An abnormal magnetic and thermal hysteritic behavior is observed at the vicinity of Tp, where substantial magnetoresistance effect also appears. We suggest that they may originate from the same physical underlying, namely, the spin disordering at grain boundaries induced by the paramagnetic Cu dependent materials. The iso-temperature of magnetic field dependence of resistivity and magnetic intensity for the composite has been investigated, as well as the relaxation behavior of resistivity. The composite exhibits substantial hysteresis behavior especially at the temperature of Tp, which indicates that the spin disordering is the most pronounced at Tp.5. The composites of (1-x)LCMO/xSb2O5, (1-x)LSMO/xSb2O5 were fabricated, in which the insulator Sb2O5 is introduced to the grain boundary regions suscessfully. With low Sb2O5 addition level, Sb2O5 mostly disperses at the grain boundaries of LCMO or LSMO, Tp of the composites shifts towards low temperature. With high Sb2O5 additon level, Sb2O5 forms clusters in the background of manganites, and Tp of the composites shifts upwards. Comparing to pure LCMO, the LFMR effect is enhanced in the composites of LCMO/Sb2O5 especially at the temperature region near Tp, but such enhancement in magnetoresistance is not as remarkable as that observed in LCMO/CuO composites. For the composites of LSMO/Sb2O5, the MR effect is enhanced at a wide temperature region. Especially, the room temperature MR effect is enhanced substantially. The experimental results are interpretted on the basis of spin polarized tunneling.6. The antiferromagnetic insulator CuMn2O4 is introduced into the LCMO matrix to fabricate LCMO/xCuMn2O4 composites. Due to the ferromagnetic and antiferromagnetic coupling at grain boundaries, the magnetoresistance of the composites is enhanced at a wide temperature window. There is a MR value of ~ 45 % observed in the x=30 % sample at 3 T through very low temperature till 220 K.7. The composites of (1-x)LSMO/xCuO have been investigated on the basis of different fabrication methods. Although the magnetoresistance for the composites is enhanced, it is not as large as that observed in LCMO/CuO composites. As a conclusion, the LFMR can be substantially enhanced in the perovskite manganites-based composites, in which the grain boundaries are modified by the second phases with different properties, such as paramagnetic Cu dependent materials, non-magnetic insulator, anti-ferromagnetic insulator. Such a process presents a new way to control LFMR effects in manganites, which would be beneficial to the practical application.