High frequency nano-oscillators based on magnetic vortices – VOICE

Spin Transfer induced Dynamics of magnetic vortices : A vortex is a configuration that can exist in many forms in nature (e.g, the vortex of a tornado or microscopic vortex currents in some supraconductors etc..) that is formed in the case of a magnetic vortex by a magnetization rotating in plane around a center of rotation that is called the vortex core in which the magnetization is pointing out of plane. Despite their apparent complexity, systems based vortex can be considered as model system to understand very complex nonlinear dynamic effects. In the project "VOICE", we have investigated how the spin transfer phenomena can excite, in spintronic nanostructure some large amplitude oscillations of the core of a magnetic vortex in a frequency range of a few hundred MHz, but also how such sustained vortex precession can be efficiently converted into a rf signal through magnetoresistive effects.
These devices, called spin transfer oscillators (STNOs) have interesting characteristics as a new generation of non-linear microwave sources with extremely large frequency accordability and a large level of integration. However, several technological barriers had not yet been raised, in particular, low emitted power and large spectral linewidths obtained for STNO. In the VOICE project, our goal was to address these issues through an innovative approach using vortex-induced oscillations as a source of microwave power.

Detection the gyrotropic motion of a vortex core :
In the VOICE project, we performed a fundamental study on sustained oscillations by spin transfer in systems of single magnetic vortex or assemblies of vortices in a synchronized state. An important effort has been made on the material parameters (magnetization, anisotropy, structural properties, magnetic damping etc...) which should allow us to directly tune the frequencies of such vortex modes but also to improve the emitted power as well as the quality factor of our oscillators. An original approach of the project was to combine high-resolution magnetic imaging techniques (MFM X-PEEM STXM) with some local spectroscopy (MRFM) to characterize in detail the static and dynamic vortex modes. These characterizations through advanced imaging techniques were combined with transport measurements both in the frequency and time domain. Moreover, all these experimental studies have been compared to numerical simulations (micromagnetism and spin transport) and original analytical developments.