Spin-torque driven magnetization dynamics in a nanocontact setup for low external fields: Numerical simulation study

We present numerical simulation studies of the steady-state magnetization dynamics driven by a spin-polarized current in a point-contact geometry for the case of a relatively large contact diameter (D-c=80 nm) and small external field (H=30 Oe). We show that under these conditions the magnetization dynamics is qualitatively different from the dynamics observed for small contacts in large external fields. In particular, the bullet mode with a relatively homogeneous mode core, which was the dominating localized mode for small contacts, is not found here. Instead, all localized oscillation modes observed in simulations correspond to different motion kinds of vortex-antivortex (V-AV) pairs. These modes include rotation of pairs with the V-AV distance d similar to D-c and creation/annihilation of much smaller (satellite) V-AV pairs. We also show that for our geometry the Oersted field has a qualitative effect on the magnetization dynamics of a free layer. This effect offers (in principle) a possibility to control magnetization dynamics by a suitable electric contact setup, adjusted to produce a desired Oersted field. Finally, we demonstrate that when the magnetization dynamics of the fixed layer-induced by the magnetodipolar interaction with the ''free'' layer-is taken into account, the threshold current for the oscillation onset is drastically reduced and new types of localized modes appear. In conclusion, we show that our simulations reproduce semiquantitatively several important features of the magnetization dynamics in a point-contact system for low external fields reported experimentally.