Frustrated Spin Ice Nanomagnets Probed by Superconducting Vortex Dynamics.*

We have studied the superconducting vortex dynamics on hybrid superconducting/magnetic samples. The samples are connected Co nanomagnets with honeycomb structure which are embedded in Nb superconducting film. The samples are patterned in a cross-shaped bridge for magnetotransport measurements. Superconducting vortex lattice motion detects distinct magnetic states of the honeycomb array. These states are governed by the topology of the Co honeycomb which frustrates in-plane magnetic configurations which must follow spin ice rules. Furthermore, in the honeycomb array, each vertex consists of two charged Néel walls. This hybrid system can be set easily in magnetic configuration corresponding to Ice II or Ice I states. We will show that the superconducting vortex flow is sensitive to subtle changes in the magnetic state of the spin ice and remarkably the as-grown state is not fully demagnetized; that is, this spin ice presents remanent magnetization in all the possible magnetic configurations.