  We propose an accurate clocked single-spin source for ac-spintronic
applications. Our device consists of a superconducting island covered by a
ferromagnetic insulator layer through which it is coupled to superconducting
leads. Single-particle transfer relies on the energy gaps and the island's
charging energy, and is enabled by a bias and a time-periodic gate voltage.
Accurate spin transfer is achieved by the ferromagnetic insulator layer which
polarizes the island, provides spin-selective tunneling barriers and improves
the precision by suppressing Andreev reflection. We analyze realistic material
combinations and experimental requirements which allow for a clocked spin
current in the MHz regime.
