Spin-Polarized Tunnelling Magnetoresistance Effects in La_0.833K_0.167MnO₃/SrTiO₃ Polycrystalline Perovskite Manganites

A sample of La_0.833K_0.167MnO₃-SrTiO₃(LKMO/STO) is fabricated by the sol-gel method. The microstructure, magnetic and transportation properties have been studied. X-ray diffraction patterns indicate that the structure of LKMO/STO is a homogeneous solid solution phase. The resistivity of LKMO/STO shows the insulator behaviour, which is different from La_0.833K_0.167MnO₃ (LKMO) whose resistivity shows metal--insulator transition with decreasing temperature. The low-field (mLa_0.833K_0.167MnO₃/SrTiO₀ H=0.02T) magnetoresistance decreases from 11% to 0.2% with the increasing temperature from 4K to 220K for the LKMO/STO sample. The magnitude of magnetoresistance in a strong field (La_0.833K_0.167MnO₃/SrTiO₀H=5.5T) almost increases linearly with decreasing temperature and reaches the maximum of 65% at the low temperature of 4.2K, which is much higher than that of LKMO (40%). The enhanced low-field magnetoresistance effects are quantitatively explained by the spin-polarized tunnelling at grain boundaries.