  Superconducting tips of YBa$_{2} $Cu$_{3}$O$_{7-x}$ were used to perform
point-contact Andreev reflection spectroscopy on half-metallic CrO$_{2}$ thin
films. At 4.2K, strong suppression of the d-wave Andreev reflection
characteristics was observed, consistent with the high spin polarization of
CrO$_{2}$. Our technique was validated by comparison with data taken on
non-magnetic Au films, and with data taken by superconducting Pb tips. The
point contacts were estimated to be $\lesssim$ 10nm in size, attesting to their
ballistic and microscopic nature. Our results demonstrate the feasibility of
using superconducting cuprate tips as spin-sensitive nanoprobes of
ferromagnets.
