Effect of an in-plane field on microwave dynamics in point contact spin valve structures combining inplane and out-of-plane magnetic layers

Point contact spin transfer devices combining out-of-plane and in-plane magnetized layers are promising candidates for spin transfer oscillators, in that they can achieve high-power and close to zero field operation [1][2]. We present recent experimental results on spin transfer torque oscillators combining both out-of-plane (CoFe/Pd) and in-plane (CoFe) magnetized layers. The device structure is such that the in-plane layer is lithographically patterned (to serve as a nanopillar/point contact) whereas the out-ofplane layer is extended. We find that, unlike previously reported point-contact devices where both ferromagnetic layers were extended [2], microwave oscillations are excited at zero applied field. We investigate how the high frequency dynamics are affected as a function of in-plane applied magnetic fields. As the in-plane field is increased, the current frequency dependence changes from negative (red-shift) to positive (blue-shift) slope. This occurs as the magnetisation direction of the out-of-plane layer nears its anisotropy field which is determined from MR curves. High frequency oscillations persist beyond the anisotropy field of the out-of-plane magnetised layer.
In order to gain insight to the observed response of the system, we calculated current-field phase diagrams for both layers by numerical integration of the LLG equation, taking into account the Slonczewski spin Transfer torque including the spin transfer asymmetry characterized by the coefficient λ [3]. According to These calculations, dynamics in the in-plane layer are only expected when the initial magnetisation is aligned antiparallel to the applied field, while oscillations of the out-of-plane layer are not expected with an in-plane applied field above the anisotropy field. Since the observed microwave spectra seem not to be adequately explained by individual layer dynamics it may be necessary to account for dynamics of both layers simultaneously.

A.M.D. and C.F. acknowledge financial support from the Swiss National Foundation Ambizione grant (PZ00P2_131808).
References:
[1] Houssameddine, D. et al., Nature Mat. 6, 447 (2007).
[2] Rippard, W.H., et al., Phys. Rev. B. 81, 014426 (2010).
[3] Slonczewski, J.C., J. Magn. Magn. Mater. 247, 324 (2002).