Magnetoresistance peak in the vicinity of the charge disproportionation/ordering transition in the R_1/3Sr_2/3FeO_3-δ (R=La, Pr) perovskite

In this letter, we present evidence that charge disproportionation (CD) and/or charge ordering (CO) states have influence upon magnetotransport in both compounds, R_1/3Sr_2/3FeO_3-δ (R=La and Pr). Field-dependent resistivity versus temperature measurements reveal the existence of a magnetoresistance (MR) peak in the temperature regime where the CD/CO transition takes place. Further increased MR of up to 6.5% is observed at temperatures as low as 25 K. These compounds are basically semiconductors which below the CD/CO temperature follow the variable range hopping process down to around 50 K and at even lower temperatures again turn to moderately doped semiconductors. Magnetization measurements show that the CD/CO is accompanied by an antiferromagnetic spin ordering of Fe ions with a further weak ferromagnetic component at low temperatures. While an ordering of Fe(III) and Fe(V) species seems to cause the MR peak near the CD/CO temperature, the appearance of some ferromagnetic component within an antiferrognatically ordered state may be largely responsible for the MR at low temperatures.