Chapter 20 - Colossal Magneto-Resistance and Electron Spin Tunneling Heads

Abstract

Despite the overwhelming success and adoption of giant MR spin valve heads and the optimism that they will suffice up to areal densities close to 100 gigabit/in.2, research efforts continue to find even better reading heads, which fall into two areas: colossal magneto-resistance (CMR) and electron spin magnetic tunneling junction (MTJ) heads. This chapter provides the current (2001) status of these two topics. The CMR effect causes resistance changes of 127,000% at liquid nitrogen temperatures (77 K) and, more importantly, about 1300% at room temperature. The effect was, with considerable justification, termed the colossal MR effect. There still exists considerable uncertainty about the exact origin of the CMR. CMR in manganates appears to be accompanied by several drawbacks. First, in order to be useful in reading heads, the magnetic field required remains too high. Another troubling implication of the high magnetic field needed to cause the ferro- to paramagnetization phase transition is that the permeability is very low. The tunneling magneto-resistance ratio (TMR) is typically in the range of 20–40% at room temperature. It is realized that this is significantly higher than in current giant magneto-resistive (GMR) spin valves, and this is, of course, one of the principal motivations for investigating MTJs.