Magnetic properties of iron-doped manganites

Polycrystalline samples of ${\mathrm{La}}_{1-x}{\mathrm{Pb}}_x{\mathrm{Mn}}_{1-y}{}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}_y^{}{}_{}{}^{}{\mathrm{O}}_3$ with $0.20<x<\mathrm{}0.45\mathrm{}$ and $0.03<~y<~0.17$ have been made and characterized by x-ray and wet chemical analysis. Earlier, it had been established that manganites with $y=0\mathrm{}$ and $0.25<x<\mathrm{}0.45\mathrm{}$ are ferromagnets with a narrow double-exchange band providing the dominant coupling. The introduction of iron reduces the magnetization and the Curie temperature and also changes the shape of the magnetization versus temperature curve. Mössbauer spectra with and without an external magnetic field of 50 kOe imply that an iron ion with only Mn nearest neighbors has its spin almost antiparallel to the manganese magnetization. However, when two or more iron ions are adjacent, their spins make angles of about 55° to the direction of the net magnetization. From the isomer shift, it follows that the iron ions are in the high-spin ${\mathrm{Fe}}^{3+}$ state. The data are interpreted in terms of a model that considers the antiferromagnetic Fe-O-Mn and Fe-O-Fe superexchange interactions in addition to the ferromagnetic ${\mathrm{Mn}}^{3+}$-${\mathrm{Mn}}^{4+}$ double-exchange interaction.