Magnetoresistance Effects in SrFeO(3-x): Dependence on Phase Composition and Relation to Magnetic and Charge Order

TL;DR

This study investigates how magnetoresistance effects in SrFeO(3-x) vary with phase composition, magnetic, and charge order, revealing three distinct MR phenomena linked to specific structural and magnetic transitions.

Contribution

It provides a detailed correlation between phase composition, magnetic and charge order, and magnetoresistance effects in SrFeO(3-x), identifying three different types of MR effects and their underlying mechanisms.

Findings

Large negative MR of 25% at 9 T in cubic SrFeO3 associated with a 60 K magnetic transition.

Giant negative MR of 90% at 9 T in vacancy-ordered tetragonal SrFeO(3-x) near 70 K.

Positive MR effects in more oxygen-deficient, insulating tetragonal and orthorhombic phases.

Abstract

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.