Influence of ferromagnetic current screen layer on current perpendicularto the plane-giant magnetoresistance

We investigated the influence of the ferromagnetic current screen layer on a current perpendicular to the plane-giant magnetoresistance (CPP-GMR). We prepared a bottom-type synthetic-pinned spin-valve film with a ferromagnetic current screen layer of naturally oxidized CoFe, which was separated magnetically from the free and pinned layers. When the CoFe layer thickness increased to above 1.0 nm before oxidation, we observed a magnetic moment corresponding to the unoxidized CoFe. We also found that the MR ratio abruptly increased from 1.6% to 7.2% with the increase in the CoFe layer thickness from 1.0 to 3.7 nm before oxidation, which was the same tendency observed in the magnetic moment. The MR ratio seemed to be strongly dependent on the magnetic moment of the current screen layer. The MR curve with the high MR ratio had two peaks near the zero magnetic fields. Based on the analysis of the magnetization configuration of the free, pinned, and current screen layers, we found that the ferromagnetic properties of the current screen layer contributed to the high MR ratio. From transmission electron microscopy observation and magnetic properties of the current screen layer, we estimated that the metal CoFe bridge exists in the current screen layer. We believe that the metal bridge made from the ferromagnetic metal in the current screen layer is important for enhancing the MR ratio of CPP-GMR for both current screening and spin dependent scattering.