Ultra-efficient Cooling in Ferromagnet-Superconductor Microrefrigerators

A promising scheme for electron microrefrigeration based on ferromagnet-superconductor contacts is presented. In this setup, cooling power densities up to 600 nW/$\mu$m${}^2$ can be achieved leading to electronic temperature reductions largely exceeding those obtained with existing superconductor-normal metal tunnel contacts. Half-metallic CrO${}_2$/Al bilayers are indicated as ideal candidates for the implementation of the device.