Influence of the interfaces on the anisotropic magnetoresistance of Ni/Co multilayers

All materials exhibit magnetoresistance ~MR!, that is, a change in resistance when a magnetic field is applied. This property is stronger in magnetic than in nonmagnetic materials. Magnetoresistance is a phenomenon of great interest due to its technological applications. Among the different types of MR observed in magnetic materials, the following three should be remarked: anisotropic magnetoresistance ~AMR!, giant magnetoresistance ~GMR!, and colossal magnetoresistance ~CMR!. AMR, which appears in traditional 3D magnetic materials is anisotropic with respect to field direction. MR is positive ~i.e., resistance grows with the applied magnetic field! when the applied magnetic field is parallel to the current, and negative when perpendicular. On the other hand, GMR and CMR, which have been discovered more recently in multiphase systems and perovskite oxides, respectively, are characterized by large MR factors and by their isotropy with respect to the field direction ~both are always negative!. Systems which exhibit GMR or CMR would seem to be the most promising for applications, as they show MR factors one or two orders of magnitude above those which solely display AMR, but in general, with high applied magnetic field and low temperature. Moreover, in AMR materials, a special configuration of the contacts makes use of the anisotropy of MR to extraordinarily enhance the change in the output signal at low field and room temperature, as recently published by the authors. AMR in Ni/Co multilayers is much larger than that of pure bulk Ni or Co, and similar in magnitude to that displayed in the best homogeneous materials ~alloys of Ni and Co!. Furthermore, Ni/Co multilayers for a wide range of thickness have low saturation field and high sensitivity; the required characteristics for applications. The very different macroscopic transport properties of AMR and GMR materials are a consequence of the different underlying microscopic physics. The intrinsic origin of AMR is the spin-orbit coupling, this changes the shape of the electron cloud and creates a local anisotropy in each domain, whereas GMR is due to spin-dependent scattering of electrons. Interfaces also play a key role in the magnetotransport properties of magnetic multilayers ~for GMR see for instance Ref. 11!. In order to study this effect in AMR, a Ni sample