Magnetic and transport properties of SrFeOx

We report the results from magnetic and transport measurements on SrFeOx where we focus on oxygen contents about the Sr4Fe4O11 phase (x=2.75) that antiferromagnetically orders at similar to 230 K. We find an exchange-bias-like effect in Sr4Fe4O11 that commences near the Fe(2) moment Neel temperature (similar to 230 K). While there is a weak spin-glass component, possibly arising from spin-frustrated moments on the Fe(1) sites, the spin-glass temperature is significantly less than 230 K, and hence the exchange-bias-like effect is unlikely to be due to a spin glass. The exchange-bias-like effect clearly arises from an exchange interaction term that includes the Fe(2) spins but its exact nature is not clear and awaits a detailed theoretical study. Room-temperature thermopower measurements show a large change in sign for oxygen contents on either side of the Sr4Fe4O11 phase, which we show can be qualitatively interpreted by assuming that the Fermi level is within a narrow band. High-field resistivity measurements show a large magnetoresistance that commences near the Sr4Fe4O11 Neel temperature.