Growth of the Cr oxides via activated oxygen reactive molecular beam epitaxy: Comparison of the Mo and W oxides

The realization of spin polarized tunnel devices made with CrO2, a theorized half-metallic ferromagnet, requires stringent control of surface and interface quality ideally obtainable via molecular beam epitaxy (MBE) growth. We have studied the MBE growth of all the di- and tri-oxides of the group VIB transition metals (Cr, Mo, and W), with the aid of a high flux atomic oxygen source and detection scheme, to help understand which oxidation states are reachable. We find that even though we can reach the +6 oxidation state of Cr (CrO3) we are unable to obtain single phase CrO2, the +4 oxidation state. One interpretation of our results is that the physical effect of pressure, not solely the oxidation potential, is important to the growth of single phase CrO2. (C) 2001 American Institute of Physics.