Static and Dynamic Properties of Magnetic Antivortices in Asteroid-Shaped Permalloy Nanomagnets

Patterned nanomagnets display unconventional spin configurations like vortex, anti-vortex, bubble, which have unique static and dynamic properties. Such micro-magnetic structures are potentially applicable to ultrafast memory, rf oscillators and detectors. Studies on magnetic thin films containing vortex structures exhibit interesting behavior under external field and/or current bias like polarity switching, core displacement and core gyration with high frequencies inside the nanomagnet. In this study, we report on our investigation of stable anti-vortex formation conditions and the subsequent magnetic field/dc current driven excitations in 2x2$\mu$m$^2$ Permalloy based asteroid geometry devices which exhibit an anti-vortex pair nucleation at the center. The Magnetic Force Microscopy images show that the antivortex pair can be rotated around the center by an external magnetic field. We obtain a high frequency (GHz) signal measured via anisotropic magneto-resistance effect (AMR) under constant dc current-bias which triggers antivortex pair gyration around the center of the device through spin transfer torque. We study the dynamic response of the structure as a function of current and field to assess utilization of the device as a practical on-chip microwave oscillator.