Review Articles | Insight

The interdependence between magnetization and charge transport is not a new story. For instance, anisotropic magnetoresistance (AMR), which links the value of the resistance to the respective orientation of magnetization and current, was first observed in 1856 by William Thomson, but its amplitude is weak (up to a few per cent variation in resistance on changing the relative orientation of magnetization and current). Nevertheless, the introduction by IBM in 1991 of a magnetoresistive read head based on AMR was a major technological step forward1. In hard disk drives (HDD), the head flies at constant height above the magnetic domains that define the ‘bits’ in the recording medium, and senses the spatial variations of the stray magnetic field of these domains. The old ring-shaped magnetic head, invented in 1933 by Eduard Schuller (AEG) for tape recording, measured a magnetic flux, and so was approaching its sensitivity limit with the reduction of the head dimensions. In the IBM head (Fig. 1), the AMR sensor is combined with a ‘ring’ element still used for writing, and directly senses the magnetic field through its influence on the magnetization orientation in the head. The AMR head had a relative ‘magnetoresistance’ (hereafter referred to as ΔR/R = (Rmax – Rmin)/Rmin) of the order of only 1%, but this was enough to increase the growth rate of HDD storage areal density from 25% per year, its value since nearly the introduction of HDD in 1957, up to 60% per year. Fundamentals oF spin eleCtroniCs