Unusual Current Perpendicular-to-Plane (CPP) Magnetoresistance (MR) for Thick Co Layers—Difference in MR for fcc and hcp Co

We report an unexpected difference in behavior of the CPP-MRs of double exchange-biased spin-valves, involving Permalloy (Py), Co, and Cu, when the Co layer is thin (t_Co ¿ 20 nm) or thick (t_Co ¿ 50 nm). Following Hehn et al., we attribute this difference to a change in direction of magnetization of the Co with increasing t_Co, with the magnetization lying in-plane for thin Co layers (t_Co ¿ 20 nm), but tilting partially out of plane for thick Co layers (t_Co ¿ 50 nm) by adopting a multidomain structure containing equal areas of "up" and "down" domains. If our attribution is correct, this is the first example of a qualitative change in giant magnetoresistance due to a change in Co structure from mainly fcc to mainly hcp.