Spin-valve-type magnetoresistance: a generic feature of ferromagnetic double perovskites

An unusual magnetoresistance (MR) behavior in Sr2FeMoO6, recently termed as spin-valve-type MR (SVMR), presents several anomalies that are little understood so far. The formation of a magnetically distorted skin layer around every soft, ferromagnetic grain seems to be the reason behind such an unprecedented tunneling MR (TMR) response. However, initially it appeared that Sr2FeMoO6 is an exclusive TMR material showing an unusual SVMR effect, while other well-known magnetoresistive compounds do not exhibit such behavior. Instead, in this present work, we show that SVMR is a generic feature of the extremely important class of the metallic, ferromagnetic double perovskite family as a whole. Our detailed magnetic study also offers a serious correction to our previous SVMR model, where the strong exchange coupling between the ferromagnetic core and the magnetically frustrated skin of each grain, generating the famous 'exchange bias' effect and consequently the 'valve' for SVMR, needs to be considered.