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要旨:
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.
