Two modes behavior of vortex oscillations in spin-transfer nanocontacts subject to in-plane magneticfields

Vortex oscillations induced by dc currents (Idc) through a metallic nanocontact subject to in-plane magnetic fields (Hext) are studied by measuring voltage power spectra. Two oscillations modes exist: at large Idc the oscillation frequency (fosc) is substantially insensitive to Hext, whereas at low Idc, fosc decreases with Hext increasing. At intermediate Idc the two modes coexist. This behavior is ascribed to the magnetic states of the device ferromagnetic layers: in the first mode vortices are formed in both layers while in the second mode one layer is in a vortex state while the other is in a quasiuniform state