The Study Of The Effect Of The Magnetic And Electrical Properties In Mn-site Doped In La_0.7Ca_0.3MnO₃ CMR Material

Since the discovery of colossal magnetoresistance (CMR) in perovskite manganese oxides R_1-xA_xMnO₃ (where R is a trivalent rare-earth element and A is a divalent metal element such as Ca, Sr, Ba, or Pb), much theoretical and experimental work has been done to investigate the physical mechanism of the CMR effect because of its physical interest and application potential. In previous work, large resistance changes are achieved only in a strong magnetic field of Tesla range and in a narrow temperature range around Curie temperature, severely limiting their practical applications. Thus, reducing the field scale and increasing the operation temperature are the key points to be studied widely. One way to promote CMR effect at room temperature is to choose the parent materials with high Curie temperature. By tuning the composition of these compounds, e.g, doping ions into the A or B sites with different ionic sizes as well as adjusting the appropriate dopant amount, the CMR effect can be improved a lot. As one of magnetism properties in the magnetic materials, magnetoresistance (MR) effect shows the magnitude about resistivity dependence of the magnetic field.In this thesis, it concluded mainly two parts, experimental study on CMR material and theoretical study on CMR effect. In the first part of the experimental study, we use perovskite manganese oxides La_0.7Ca_0.3MnO₃ as matrix for our study, the MR materials were synthesized by using sol-gel technology, and mainly investigate the effect on the MR properties using Cu and V as doped elements.The experiment results on the Cu doping are that, the magnetoelectric character of the samples changed prominently, in low dopant, the temperature dependence of the resistivity exhibited M-I transition, but in high dopant, it only exhibit insulator behavior. Corresponding to the magnetoresistance contains both Tunneling MR and Percolative MR in low dopant, but only Tunneling MR in high dopant. The experiment results on the V doping are described as follows: with doping of V element, the doped system presents relatively high magnetoresistance. The electronic properties of the doped system are, the temperature dependences of the resistivity exhibited distinct double peak in low dopant, and with the increase of the dopant it exhibited only single resistivity peak. In this part of...