Study On The Colossal Magnetoresistant Effect In Perovskite-like Manganese Oxide

The colossal magnetoresistant (CMR) effect has become an important research subject in the fields of material science and condensed matter physics since the recent years due to its potential sensitive application value and extremely complex magnetic interaction mechanism involved. In this work, Mn-based perovskite-like oxide AMnO₃ where A is substituted by three elements were obtained by so-called Solid Phase Reaction Method, and their magnetism, electrical conductivity and magnetoresistant effect were investigated systematically. All the new phenomena observed experimentally and new opinions put forward subsequently enriched the knowledge about CMR.The relationship between the CMR effect as well as properties concerned and stoichiometry along with the preparation conditions of sample such as sintering temperature, sintering period and grinding times was researched on the Vibrating Sample Magnetometer (VSM) for the (La, Nd)_1-xCa_xMnO₃(x=0.21, 0.33, 0.41). The results show that the magnetic and transporting properties is quite sensitive to solid phase reaction procedure concluded by the fact that the change of either sintering temperature or grinding times, which reflect the preparation procedure, will affect both the Curie temperature T_C, electrical resistivity transitional temperature T_P and CMR effect, witch characterize the properties of materials. With the increase of either sintering temperature or grinding times, the CMR effect increases generally and both the Curie temperature T_C and electrical resistivity transitional temperature T_P decrease monotonously. Alkali earth element doping will change the average electronic valence of Mn ion, the average ion diameter and accordingly the distortion degree of perovskite structure that will lead to varying the magnetoresistivity. There is an optimizing peak for CMR and T_P when the doping amount is between 0.21 and 0.41(near 0.33).Then, the same work as done above for (La, Nd)_1-xCa_xMnO₃(x=0.21, 0.33, 0.41) is done for La_1/3Nd_1/3B_1/3MnO₃(B=Ca, Ba, Sr, Pb). All samples show soft ferromagnetism at low temperature, and the fact that their ferromagnetism except Sr doped disappears at room temperature indicates that ferromagnetism-paramagnetism phase transition has happened. While La_1/3Nd_1/3B_1/3MnO₃(B=Ba, Pb) shows interesting so-called Parasitic Ferro- magnetism at room temperature, only La_1/3Nd_1/3Ca_1/3MnO₃ is complete paramagnetism. And the curve of temperature dependence of electrical resistivity for La_1/3Nd_1/3Ba_1/3MnO₃ shows another peek sensitive to magnetic field, near 230K above T_P. The extra peak is restrained strongly by applied field. This is very likely aroused by double magnetic phases, which cause two critical conductive transitions. The reason for parasitic ferromagnetism and double peak of electrical resistivity transition was reduced to ferromagnetism-antiferromagnetism conflict that exists...