Spin transfer effects in exchange-biased spin-valves for current-perpendicular-to-plane magnetoresistive heads

Magnetoresistive heads used in computer disk drives are nowadays based on the giant magnetoresistance of spin-valves. In the present technology, the current is flowing parallel to the interfaces of the spin-valve stack (the so-called “current-in-plane geometry”). However, due to the steady increase in the storage areal density of disk drives, this technology is close to its physical limit. Consequently, more and more efforts are made aiming at using the current-perpendicular-to-plane geometry which offers larger signal amplitude and better spatial resolution. However, new physical effects appear when the current flows perpendicular to the plane in a magnetic multilayer. Of particular interest are the spin transfer effects which were predicted by Slonczewski and Berger in 1996. When they traverse the pinned layer of the spin-valve, the conduction electrons acquire a spin polarization. This spin current exerts a torque on the magnetization of the free layer which induces very peculiar dynamical phenomena. This spin torque can induce the switching of the magnetization of the free layer or the generation of magnetic excitations which contribute to the noise of the head. Therefore in the context of magnetoresistive heads, it is important to understand and master these effects which perturb the behavior of the heads. On the contrary, these effects can be used in a positive way as a new write scheme in non-volatile magnetic random access memories or to generate a steady precession of the magnetization in RF oscillators.