Magnetoresistance and phase composition of La-Sn-Mn-O systems

The transport properties of the manganites La1 - xSnxMnO3 + delta with x = 0.1-0.5 and of Fe-doped samples have been comprehensively studied using mag netoresistance measurements, Fe-57 and Sn-119 Mossbauer spectroscopy, and x -ray diffraction. At the Sn concentration x = 0.5, La0.5Sn0.5MnO3 + delta, is a single phase. The manganites La1 - x SnxMnO3 + delta with x<0.5 consis t of two phases, ABO(3) and A(2) B2O7; the chemical formulas are LaMnO3 + d elta and (La0.5Sn0.5)(2)Mn2O7, respectively. The colossal magnetoresistance (CMR) in La1 - xSnxMnO3 + delta appears to have its origin in deficient La and/or Mn ions in the ABO(3) phase. Mossbauer spectra show that a superpar amagnetic doublet exists from T=250-77 K for the ABO(3) phase in the Fe-dop ed sample and may have superparamagneticlike spin clusters that lead to a l ow saturation field. Consequently, the CMR reaches 75% at an applied field of only 20 kOe for the Fe-doped sample; however, the same CMR requires fiel ds as high as 70 kOe in the undoped La0.7Sn0.3MnO3 + delta sample.