Magnetoresistance effects in SrFeO3-δ:: Dependence on phase composition and relation to magnetic and charge order

Single crystals of iron(IV) rich oxides SrFeO3-delta with controlled oxygen content (0 <=delta <= 0.31) have been studied by Mossbauer 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 SrFeO3 (delta=0) a large negative MR of 25% at 9 T is associated with a hitherto unknown 60 K magnetic transition and a subsequent drop in resistivity. The 60 K transition appears in addition to the onset of helical ordering at similar to 130 K. In crystals with vacancy-ordered tetragonal SrFeO3-delta as the majority phase (delta similar to 0.15) a coincident charge-antiferromagnetic ordering transition near 70 K is considered as the origin of a negative giant MR effect of 90% at 9 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 SrFeO3-delta differ from those in manganites as no ferromagnetic states are involved.