Half-metallic ferromagnets exhibit the property of 100% spin polarization of the electrons at the Fermi level.  One of the main applications for these materials is tunneling magnetoresistance devices, which can be used for non-volatile magnetic random access memory.  Tunneling magnetoresistance devices based on half-metallic ferromagnetic materials should provide a much larger magnetoresistive response than with conventional ferromagnetic materials.  Several classes of materials are predicted to have this half-metallic ferromagnetic property, including the Heusler alloys, magnitites, Fe₃O₄, and CrO₂.  However, there is very little direct experimental evidence that the surface region of any of these materials is half-metallic.

Of all of the half-metallic ferromagnetic materials, CrO₂ is the only simple oxide, which should minimize problems with surface stoichiometry and reconstruction that can occur complicated materials such as the Heusler alloys and magnetites.  However, attempts to make devices based on CrO₂ have yielded very low efficiencies.  Photoemission studies of CrO₂ have found a very high spin polarization of electrons below the Fermi level but with no intensity at the Fermi level.  This non-metallic behavior has previously been attributed to the formation of Cr₂O₃ at the surface.  With the recent development of techniques to grow of epitaxial CrO₂ films, surface stoichiometry problems can be excluded as the cause of this non-metallic behavior.  An overview of recent photoemission results for epitaxial CrO₂(100) and CrO₂(110) films, which provide evidence that neither surface is half metallic, will be presented.