Hysteresis and control of ferromagnetic resonances in rings

The spin dynamics in narrow ferromagnetic rings is studied in the frequency range from 45 MHz to 20 GHz at room temperature. Our broadband spectrometer allows us to monitor the ferromagnetic resonance of characteristic spin configurations as a function of an external field mu H-0. We observe hysteresis and irreversible jumps of the resonance frequencies which we attribute to onion-to-vortex and vortex-to-reversed-onion transitions. (C) 2005 American Institute of Physics.