Description
The dynamic response under spin transfer torque depends on the magnetization configuration of the polarizer and the free layer. At SPINTEC we have focused on two configurations which are (i) a perpendicular polarizer or (ii) a synthetic antiferromagnetic free layer.

Partners
LETI/DCOS (Marie Claire Cyrille)
CEA/SPEC Saclay (Olivier Klein)
CNRS/THALES (Vincent Cros, Julien Kermorvant)

Financing
ANR-05-NANO-44 MagIC
ANR-09-NANO-037 Milestone (2010-2013)
ANR-2011-NANO-016 SPINNOVA (2011-2015)
FP7-PEOPLE-2009-IEF MILESTONE No 252067 (2010-2012)

Objectives
Different oscillator configurations lead to different excitation modes with different spectral characteristics such as the frequency tuning with current and field, the linewidth, the power, but also the non-linear magnetization dynamics properties. In order to meet specifications for applications, it will be important to ‘tailor’ the spectral characteristics via the magnetization configuration to obtain (i) large oscillation amplitudes, (ii) robust oscillations and (iii) appropriate non-linear parameters. In this respect the influence of the coupling between different layers (such as exchange coupling in SAFs) on the spin torque driven dynamics is still relatively unexplored.

Latest results
We have developed a spin valve oscillator with a perpendicular polarizer and an in-plane free layer, confirming experimentally micromagnetics simulations that predict (i) that the only dynamic solutions are out of-plane trajectories characterized by a frequency increase with current, followed by a saturation ; (ii) a maximum precession amplitude with a dynamic magneto-resistance close to the static magneto-resistance.
For a standard spin valve with a free layer and a synthetic antiferromagnetic (SAF) pinned layer, we have shown steady oscillations in the SAF characterized by a change of slope of frequency vs current from redshift (low field) to blueshift (high field) attributed to the dynamic exchange coupling.

Staff
Ursula Ebels (SPINTEC), Marie Claire Cyrille (LETI), Elmer Monteblanco (SPINTEC), Alex Jenkins (SPINTEC), Christophe Dieudonné (LETI & SPINTEC), Jiafeng Feng (SPINTEC & LETI), Juan Sierra (SPINTEC), Dimitri Houssemeddine (SPINTEC), Bernard Rodmacq (SPINTEC), Bernard Dieny (SPINTEC), Bertrand Delaët (LETI), Cécile Manoury (LETI), Marité Delaye (LETI)

Related content
[1] Appl. Phys. Lett. 96, 072511 (2010), D. Houssameddine, et al;
[2] Phys. Rev. B 79, 104406 (2009), D. Gusakova et al;
[3] Phys. Rev. B 78, 024436 (2008), U. Ebels et al;
[4] Nature Materials 6, 447 (2007), D. Houssameddine et al;
[5] PhD thesis D Houssameddine

Patents
US Patent No. 6,532,164 B2 (2003), O. Redon, B. Dieny et al;

Figure 1

Perpendicular polarizer stack (left), excitation spectra as a function of current for close to zero effective field (middle) and frequency vs. current (right). The saturation of the frequency is explained through numerical simulations as the nucleation of an inhomogeneous magnetization configuration

Figure 2

Frequency vs field for different applied currents ratios (normalized to the critical current). At low fields the frequency decreases upon increasing the current (redshift) while at higher fields (below the spin flow field) the frequency increases with current (blueshift)