Magnetoresistance effects inSrFeO3−δ: Dependence on phase composition and relation to magnetic and charge order

Abstract

Single crystals of iron(IV) rich oxides $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3-\delta }$ with controlled oxygen content $(0⩽\delta ⩽0.31)$ have been studied by Mössbauer spectroscopy, magnetometry, magnetotransport measurements, Raman spectroscopy, and infrared ellipsometry in order to relate the large magnetoresistance (MR) effects in this system to phase composition and magnetic and charge order. It is shown that three different types of MR effects occur. In cubic $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_3$ $(\delta =0)$ a large negative MR of 25% at $9\mathrm{T}$ is associated with a hitherto unknown $60\mathrm{K}$ magnetic transition and a subsequent drop in resistivity. The $60\mathrm{K}$ transition appears in addition to the onset of helical ordering at $\sim 130\mathrm{K}$. In crystals with vacancy-ordered tetragonal $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3-\delta }$ as the majority phase $(\delta \sim 0.15)$ a coincident charge-antiferromagnetic ordering transition near $70\mathrm{K}$ is considered as the origin of a negative giant MR effect of 90% at $9\mathrm{T}$. A positive MR effect is observed in tetragonal and orthorhombic materials with increased oxygen deficiency $(\delta =0.19,0.23)$ which are insulating at low temperatures. Phase mixtures can result in a complex superposition of these different MR phenomena. The MR effects in $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3-\delta }$ differ from those in manganites as no ferromagnetic states are involved.