| Perovskite manganites have been gaining much attention since the discovery of the colossal magnetoresistance (CMR) in these materials. Recently, the investigation of CMR in Perovskite manganites is one of the important subjects. Considerable works have been done on the experimental and theoretical studies for two reasons. On the one hand, CMR effect possesses giant application potential in the magnetic memories and sensors. On the other hand, there are many rich physical contents in these perovskite manganites. However, a large magnetoresistance (MR) value is often obtained in a very large magnetic field (up to several tesla) and at low temperature, these conditions severely limit its application. From the viewpoint of the practical applications, a large MR obtained at low field and room temperature is desirable. So how to increase the field sensitivity and working temperature has become one of the important investigation subjects.Polycrystalline La0.67Ba0.33MnO3 (abbreviated to LBMO) synthesized by the sol-gel technique is selected as the parent material. On the one hand, its Curie temperature (TC) can be adjusted by ion substituting to room temperature, since its TC is higher than room temperature. On the other hand, the boundary effect of polycrystalline can be used to improve the low-field MR. At first, the substituting effects of A or B sites on the magnetism, electrical property, room-temperature and low-field MR are investigated, in which, the substituting amount is small, limited in 10% mole ratio, in order to adjust TC to room temperature. And then, some good conductive metals are composed with the perovskite manganites, in which TC are near room temperature. The good conductivity, even magnetism of the metal is applied into the composites. As a result, the room-temperature MR can be enhanced obviously. In fact, an extra-large room-temperature MR is obtained, which is the largest MR obtained at room temperature in all CMR materials so far.
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