How to stabilize an ascillation around the energy maximum ?
Why a perpendicular polarizer ? First experiments of spin torque driven excitations using an in-plane magnetized polarizer (in a standard spin valve structures) [Cornell, NIST] showed (i) that a non-zero external magnetic field is required to induce auto-oscillations and (ii) that the oscillation amplitude makes use of only about 30% of the total magneto-resistance. In other words the magnetization does oscillate only in a restricted cone-angle around the static equilibrium (in-plane energy minimum), see Fig. 1.
Here SPINTEC has proposed [1, 2] a perpendicular polarizer to induce large angle magnetization oscillations where the magnetization describes a full 360° trajectory (almost) parallel to the film plane.
This difference in the precession trajectory is due to the different symmetries between the spin transfer torque and the trajectory [2, 3]. In fact the action of the spin transfer torque is such as to align the free layer magnetization parallel or antiparallel to the polarizer magnetization. A perpendicular polarizer has thus the tendency to push the magnetization out of the film plane. In this configuration in-plane trajectories are unstable and the only dynamic solutions are out of-plane trajectories where the magnetization oscillates around the out-of-plane energy maximum [2, 3] and thus between the parallel and antiparallel configuration.
Experiments and Simulations : This type of oscillator has been realized using a polarizer based on a (Co/Pt)5 multilayer (to produce a stable out-of-plane orientation) combined with a thin Co layer laminated by a Cu interlayer (to obtain a strong spin polarization). The free layer (here Permalloy) is separated by a Cu spacer from the polarizer. In order to read out the magneto-resistive voltage signal of the free layer, a third magnetic layer is required, called the analyzer, see, Fig. 2.
We have validated for this oscillator configuration  the out-of-plane trajectories characterized by excitation frequencies that increase with increasing current (see Fig. 2) and that can be obtained in zero effective bias field. In particular the dynamic magneto-resistance in low current is close to the static magneto-resistance (i.e. 100%), confirming the 360° oscillation of the free layer magnetization with respect to the in-plane magnetized polarizer, see Fig. 3.
The experimental results  have been well reproduced by macrospin and micromagnetics simulations [3, 5, 6].
 US Patent No. 6,532,164 B2 (2003), O. Redon, B. Dieny et al ;  Appl. Phys. Lett. 86, 022505 (2005), K. J. Lee et al;  Phys. Rev. B 78, 024436 (2008), U. Ebels et al;  Nature Materials 6, 447 (2007), D. Houssameddine et al;  Phys. Rev. B, 78, 024437 (2008), I. Firastrau et al;  J. Mag. Magn. Mat. 310, 2029 (2007), I. Firastrau et al;
- Bernard Rodmacq for materials development,
- Bertrand Delaët for nanofabrication,
- Dimitri Houssameddine, PhD microwave characterization,
- Ioana Firastrau and Liliana Buda-Prejbeanu for numerical simulations,
- Bernard Dieny
- Ursula Ebels
[Cornell] Nature 425, 380-383 (2003), Kiselev et al;
[NIST] Phys. Rev. Lett. 92, 27201-27204 (2004), Rippard et al;