CRYSTAL-STRUCTURE AND MAGNETORESISTANCE OF LA(2-X) 3NDX/3CA1/3MNO3 - LOCAL LATTICE DISTORTION EFFECT/

The crystal structure, low field ac susceptibility, magnetization, and magnetoresistance behavior of La-(2-x/3)Ndx/3Ca1/3MnO3 (x=0-2) compou nds are investigated. The crystal structure of the compounds belongs t o an orthorhombically distorted perovskite structure. The lattice dist ortion and the bending of Mn-O-Mn bond increase with the increase of N d content, ac susceptibility and magnetization measurements show that the magnetic transition temperature decreases with the increase of Nd content, and two evident magnetic transitions are observed for x=0.75- 1.0. The magnetization isotherms exhibit a field-induced magnetic tran sition in the sample. Large magnetoresistance (MR) effects are observe d in bulk La-(2-x/3)Ndx/3Ca1/3MnO3 at low field. For bulk La1/3Nd1/3Ca 1/3MnO3 a MR ratio as high as -96% is achieved in a field of 0.67 T. T he maximum MR ratio in a field of 0.67 T increases linearly with the d ecrease of peak temperature of zero-field resistivity. Based upon the discussion on local distortions of the lattice and measurements of ac susceptibility, magnetization, and resistivity, a magnetic inhomogenei ty model, i.e., simultaneous occurrence of metallic La-rich ferromagne tic domains and semiconducting Nd-rich domains, is proposed for the ma gnetic structure of La-(2-x/3)Ndx/3Ca1/3MnO3 compounds. Within the fra mework of the magnetic inhomogeneity model, it is argued that the occu rrence of resistivity peak could result from a competition of opposite temperature dependence of resistivities in La-rich phase and in Nd-ri ch phase, thus the peak temperature of resistivity does not necessaril y correspond to a magnetic transition temperature.