Chapter 2 - Spin valves

Abstract

This chapter provides an overview of spin valves representing a particular class of systems exhibiting giant magnetoresistance (GMR) at low fields. The differences between the magnetotransport properties in “current-in-plane” (CIP) and “current-perpendicular-to-plane” (CPP) geometry are outlined in the chapter. The implementation of spin valves in CIP and CPP heads and the present trends in magnetoresistive heads are also discussed. The simplest spin valves comprise two ferromagnetic layers separated by a nonmagnetic (NM) spacer layer. The magnetization of the second ferromagnetic layer is pinned by coupling with an adjacent antiferromagnetic layer using the so-called exchange anisotropy phenomenon. In spin valves, the interlayer coupling between the free and the pinned layers causes a shift in the hysteresis loop of the free layer. There are three main contributions to this interlayer coupling. First, at very low thickness, direct coupling through pinholes in the nonmagnetic spacer layer may occur. Another source of ferromagnetic coupling which is often encountered in multilayers, and particularly spin valves of average crystallographic quality, is caused by the orange peel mechanism. A third contribution to the interlayer coupling observable in spin valves of high structural quality is the oscillatory Ruderman–Kittel–Kasuya–Yosida (RKKY) coupling. In order to increase the GMR amplitude in spin valves, the spacer layer thickness must be reduced as much as possible. However, due to the increasing parallel coupling between the free and pinned layers at low spacer layer thickness, a compromise must be found between large GMR amplitude and a coupling that is too large.