Probing ground state in circular magnetic dots: Single vs. double magnetic vortex

We investigate static magnetization as a function of a magnetic field during nucleation of the magnetic vortex state in Py dots with diameter of 1000 nm and with four different thicknesses varying between 15 and 50 nm. For the 50 nm thick Py dots we observe direct nucleation of the single vortex state from the saturated state, while the dots with thickness equal or below 25 nm reveal formation of intermediate metastable double vortex state before entering into the single vortex ground state at small negative fields. This scenario is confirmed by micromagnetic simulations and variable field magnetic force microscopy imaging. Our simulations investigate stability of the double vortex metastable state as a function of temperature and external microwave pulses. Magnetic force microscopy was found to be a invasive imaging technique to study this kind of samples. For 20 nm thick Py dots only the single vortex state is observed with commercial tips, meanwhile home made tips with lower magnetic moment were found to destroy the double vortex state during imaging process.