Structural and magnetic properties ofLa0.7Pb0.3(Mn1−xFex)O3(0<~x<~0.3)giant magnetoresistance perovskites

Abstract

Neutron-diffraction, magnetic and transport measurements, and M\ossbauer spectroscopy have been used in order to study the structural and magnetic changes of the magnetoresistive ${\mathrm{La}}_{0.7}{\mathrm{Pb}}_{0.3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{3}$ $(x=0,$ 0.05, 0.1, 0.2, 0.3) perovskitelike compounds. Samples were prepared by the sol-gel low-temperature method. In all cases the exact stoichiometry is different from the nominal one probably due to the presence of cation vacancies. All the phases exhibit ferromagnetic behavior except the compositions with the highest Fe contents $(x=0.2,$ 0.3), which show spin-glass order. A 10% Fe contribution lowers the ${T}_{C}$ by about 130 K. The magnetic moment continuously decreases with the increasing amount of iron. The magnetoresistance $[\mathrm{MR}=\ensuremath{\Delta}R/R(0)],$ between 0 and 6 T applied fields, has been measured only in the $x=0.05$ and 0.1 compositions, being of about 65% around the magnetic transition in the case of the 10% Fe content sample. For higher Fe contents, the samples show clearly insulating trends. These results can be interpreted in terms of a hindering of the double-exchange mechanism by ${\mathrm{Fe}}^{3+}$ ions, which gives rise to the localization of the electronic carriers and to a subsequent frustration of the long-range magnetic order.