Magnetic and transport properties ofSrFeOx

Abstract

We report the results from magnetic and transport measurements on ${\text{SrFeO}}_{x}$ where we focus on oxygen contents about the ${\text{Sr}}_{4}{\text{Fe}}_{4}{\text{O}}_{11}$ phase $(x=2.75)$ that antiferromagnetically orders at $\ensuremath{\sim}230\text{ }\text{K}$. We find an exchange-bias-like effect in ${\text{Sr}}_{4}{\text{Fe}}_{4}{\text{O}}_{11}$ that commences near the Fe(2) moment N\'eel temperature $(\ensuremath{\sim}230\text{ }\text{K})$. 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 ${\text{Sr}}_{4}{\text{Fe}}_{4}{\text{O}}_{11}$ 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 ${\text{Sr}}_{4}{\text{Fe}}_{4}{\text{O}}_{11}$ N\'eel temperature.