Magnetic vortex excitation dependence on the magnetic free layer and size of spin-valve nanocontacts

In this Letter we report on the spin-torque-driven magnetic switching and dynamic excitations in spin-valve, circular nanocontacts, incorporating an unpinned CoFe/Ru/CoFe artificial antiferromagnet as polarizer. Our investigation concentrates on the influence of the magnetic free layer moment and of the nanocontact size on these spin-torque related phenomena. Two multilayers are therefore investigated, one with an amorphous CoFeB and the other with a CoFe/CoFeB free layer. The nanocontact radii range from 40-130 nm. Both multilayers show clear current-induced magnetic switching for small in-plane magnetic fields. On the other hand, sub-gigahertz dynamic spectra, related to magnetic vortex precession, are only observed for multilayers employing the CoFeB free layer and only for contact radii below 100 nm. The current and frequency dependence of the oscillation on the point contact radius and magnetic field is discussed. Multiple excitation modes are shown to exist at zero or small magnetic fields as a result of a highly inhomogeneous magnetic state below the nanocontact. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) Spin-torque-driven vortex-oscillators can be applied as nano-sized microwave sources, ultra-compact magnetic field sensors or in fast and non-volatile magnetic memories. This Letter demonstrates the dependence of vortex dynamics on the free layer and the size of relatively large spin-valve nanocontacts, incorporating an unpinned artificial antiferromagnet. Nucleation current density, multiple mode excitation and magnetic field dependence are specifically addressed. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.