  Single crystals of iron(IV) rich oxides SrFeO(3-x) with controlled oxygen
content have been studied by Moessbauer spectroscopy, magnetometry,
magnetotransport measurements, Raman spectroscopy, and infrared ellipsometry in
order to relate the large magnetoresistance (MR) effects in this system to
phase composition, magnetic and charge order. It is shown that three different
types of MR effects occur. In cubic SrFeO3 (x = 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 ~130 K. In crystals with vacancy-ordered
tetragonal SrFeO(3-x) as majority phase (x ~0.15) a coincident
charge/antiferromagnetic ordering transition near 70 K gives rise to 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 (x = 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
SrFeO(3-x) differ from those in manganites as no ferromagnetic states are
involved.
