2.6 Spin-transfer Effect and Current-induced Magnetization Switching
It has been recently shown, both theoretically and experimentally, that a spin-polarized current when passing through a small magnetic conductor can affect its magnetization state. The interaction between spin polarized current and magnetization in small ferromagnetic bodies can produce steady state precessional magnetization dynamics, that is self-oscillating behavior, or even the switching of magnetization direction -. Both types of dynamical behavior have potential applications in magnetic storage technology and spintronics. In this respect, it was predicted, and later confirmed, that spin-polarized current can lead to current-controlled hysteretic switching in magnetic nanostructures. This kind of behavior may become very important for applications such as current-controlled switching of magnetic random access memory elements and stabilization of magnetic hard-disk read heads. Steady precessional oscillations of magnetization due to spin polarized currents have also interesting potential applications for the realization of current-controlled microwave oscillators integrable with semiconductor electronics. This kind of oscillators could be used to realize a very new design of clocks for synchronization of electronic devices. Here spin-polarized current induced dynamics is studied in the case of a uniformly magnetized ferromagnetic thin film . Magnetization dynamics is described by the
Landau-Lifshitz-Gilbert equation and the effect of spin-polarized currents is taken into account through the additional torque term derived by Slonczewski in Ref. .
This model can be applied to describe the magnetization dynamics in the free layer of trilayers structures constituted by two ferromagnetic layers separated by nonmagnetic metal layer (typically the system is a Co-Cu-Co trilayers as sketched in Fig. 2.20).
One of the magnetic layer is ``fixed'', namely has a given and constant value of magnetization (indicated with in Fig. 2.20) while the second magnetic layer is a
thin film where the magnetization is ``free'' to change and where dynamics takes place. This kind of structure is traversed by an electric current whose direction is normal to the plane of the layer (generally this configuration in called ``current perpendicular to plane (CPP) geometry''). The fixed layer is instrumental to provide a controlled polarization (on the average parallel to the fixed magnetization direction) of the electron spins which travel across the trilayers, from the fixed to the free layer.
Figure 2.20: Sketch of Trilayers Co-Cu-Co structure.
It important to underline that the effect of spin induced torque is predominant on the effect of the magnetic field generated by the current itself for structures which have small enough transversal dimensions. By using reasonably estimate it has been predicted and then verified experimentally that the effect of the current generated magnetic field can be considered negligible for transversal dimension as small as
; 2.6.1 Landau-Lifshitz-Gilbert equation with Slonczewski
spin-transfer torque term
; 2.6.2 Discussion about units and typical values of parameters
o 18.104.22.168 Fundamental constants and value of characteristic current
o 22.214.171.124 Discussion about the function
; 2.6.3 Analytical investigation of self-oscillating behavior
and current-induced switching
o 126.96.36.199 Perturbative technique
o 188.8.131.52 Current-driven switching experiment. Analytical and numerical