Crystal Structure, Magnetic Properties In Cr Doped La_1-x Ca_1-xMnO₃

Perovskite manganites have attracted enormous attention in recent times due to their colossal magnetoresistance (CMR) property and potential applications in magnetic information store and low-field magnetic sensors. Nowadays, it has been extensively accepted that the double exchange interaction plays a key role in perovskite manganites. Therefore, the structure and magnetic properties of perovskite manganites have been studied by doping on Mn sites in this thesis, the main results are as following:The structure and magnetic properties have shown that the substitution of Cr for Mn in La_2/3Ca_1/3Mn_1-xCr_xO₃ and La_0.8Ca_0.2Mn_1-xCr_xO₃ decrease the Curie temperature. The unit cell volume and the average angler Mn-O-Mn of the sample decrease as a function of the Cr content. There is a general tnedencey for the distortion of MnO₆ octahedra to increase as the Cr content increases. With increasing Cr content, ferromagnetic metal-insulator(M-I) transition and ferromagnetic-antiferromagnetic (FM-AFM) transition were observed in the magnetic phase diagram. The above results support the double exchange model in manganite.We have also studied the spin glass (SG) behavior of La_0.8Ca_0.2Mn_1-xCr_xO₃ in definite conditions, which has been investigated from different aspects and differentmethods(ZFC-FC, ac susceptibility). Using the concept of spin glass and thedouble-potential-well model, we have plotted the relation between the freezing temperature and the relaxation of M time, and calculated the characteristic time constant of the spin glass.