Quantum depinning of the magnetic vortex core in micron-size permalloy disks

The vortex state, characterized by an in-plane closed flux domain structure and an out-of-plane magnetization at its center (the vortex core), is one of the magnetic equilibria of thin soft ferromagnetic micron-size dots. In the past two decades many groups have been working on the dynamics of the magnetic moment in nanomagnetic materials at low temperatures, giving rise to the observation of quantum relaxations and quantum hysteresis cycles. We report experimental evidence of quantum dynamics of the vortex core of micron-size permalloy (Fe19Ni81) disks induced by the application of an in-plane magnetic field. It is attributed to the quantum tunneling of the vortex core through pinning barriers, which are associated with structural defects in the dots, toward its equilibrium position.