Crystal structure and magnetoresistance ofLa(2−x)/3Ndx/3Ca1/3MnO3:Local lattice distortion effect

Abstract

The crystal structure, low field ac susceptibility, magnetization, and magnetoresistance behavior of ${\mathrm{La}}_{(2\mathrm{\ensuremath{-}}\mathrm{x})\mathrm{/}3}$${\mathrm{Nd}}_{\mathrm{x}\mathrm{/}3}$${\mathrm{Ca}}_{1\mathrm{/}3}$${\mathrm{MnO}}_{3}$ (x=0--2) compounds are investigated. The crystal structure of the compounds belongs to an orthorhombically distorted perovskite structure. The lattice distortion and the bending of Mn-O-Mn bond increase with the increase of Nd content. ac susceptibility and magnetization measurements show that the magnetic transition temperature decreases with the increase of Nd content, and two evident magnetic transitions are observed for x=0.75--1.0. The magnetization isotherms exhibit a field-induced magnetic transition in the sample. Large magnetoresistance (MR) effects are observed in bulk ${\mathrm{La}}_{(2\mathrm{\ensuremath{-}}\mathrm{x})\mathrm{/}3}$${\mathrm{Nd}}_{\mathrm{x}\mathrm{/}3}$${\mathrm{Ca}}_{1\mathrm{/}3}$${\mathrm{MnO}}_{3}$ at low field. For bulk ${\mathrm{La}}_{1\mathrm{/}3}$${\mathrm{Nd}}_{1\mathrm{/}3}$${\mathrm{Ca}}_{1\mathrm{/}3}$${\mathrm{MnO}}_{3}$ a MR ratio as high as -96% is achieved in a field of 0.67 T. The maximum MR ratio in a field of 0.67 T increases linearly with the decrease of peak temperature of zero-field resistivity. Based upon the discussion on local distortions of the lattice and measurements of ac susceptibility, magnetization, and resistivity, a magnetic inhomogeneity model, i.e., simultaneous occurrence of metallic La-rich ferromagnetic domains and semiconducting Nd-rich domains, is proposed for the magnetic structure of ${\mathrm{La}}_{(2\mathrm{\ensuremath{-}}\mathrm{x})\mathrm{/}3}$${\mathrm{Nd}}_{\mathrm{x}\mathrm{/}3}$${\mathrm{Ca}}_{1\mathrm{/}3}$${\mathrm{MnO}}_{3}$ compounds. Within the framework of the magnetic inhomogeneity model, it is argued that the occurrence of resistivity peak could result from a competition of opposite temperature dependence of resistivities in La-rich phase and in Nd-rich phase, thus the peak temperature of resistivity does not necessarily correspond to a magnetic transition temperature.