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

The transport properties of the manganites ${\mathrm{La}}_{1-x}{\mathrm{Sn}}_x{\mathrm{MnO}}_{3+\delta }$ with $x=0.1\mathrm{}–0.5$ and of Fe-doped samples have been comprehensively studied using magnetoresistance measurements, ${}^{57}\mathrm{Fe}$ and ${}^{119}\mathrm{Sn}$ Mössbauer spectroscopy, and x-ray diffraction. At the Sn concentration $x=0.5,$ ${\mathrm{La}}_{0.5}{\mathrm{Sn}}_{0.5}{\mathrm{MnO}}_{3+\delta },$ is a single phase. The manganites ${\mathrm{La}}_{1-x}{\mathrm{Sn}}_x{\mathrm{MnO}}_{3+\delta }$ with $x<\mathrm{}0.5\mathrm{}$ consist of two phases, $\mathrm{AB}{\mathrm{O}}_3$ and $A_2{B}_2{\mathrm{O}}_7$; the chemical formulas are ${\mathrm{LaMnO}}_{3+\delta }$ and $({\mathrm{La}}_{0.5}{\mathrm{Sn}}_{0.5}{)}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$, respectively. The colossal magnetoresistance (CMR) in ${\mathrm{La}}_{1-x}{\mathrm{Sn}}_x{\mathrm{MnO}}_{3+\delta }$ appears to have its origin in deficient La and/or Mn ions in the $\mathrm{AB}{\mathrm{O}}_3$ phase. Mössbauer spectra show that a superparamagnetic doublet exists from $T=250\mathrm{}–77\hspace{0.5em}\mathrm{K}$ for the $\mathrm{AB}{\mathrm{O}}_3$ phase in the Fe-doped sample and may have superparamagneticlike spin clusters that lead to a low 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 fields as high as 70 kOe in the undoped ${\mathrm{La}}_{0.7}{\mathrm{Sn}}_{0.3}{\mathrm{MnO}}_{3+\delta }$ sample.