Are the surfaces of CrO₂metallic?

Abstract

Previous photoelectron spectroscopy studies of CrO2 have found either no density of states or a very low density of states at the Fermi level, suggesting that CrO2 is a semiconductor or a semi-metal. This is in contradiction to calculations that predict that CrO2 should be a half-metallic ferromagnet. Recently, techniques have been developed to grow high-quality epitaxial films of CrO2 on TiO2 substrates by chemical vapour deposition. We present photoelectron spectroscopy measurements of epitaxial CrO2(110)/TiO2(110) and CrO2(100)/TiO2(100) grown using a CrO3 precursor. In addition, measurements of epitaxial Cr2O3(0001)/Pt(111) films grown by thermal evaporation of Cr in an oxygen atmosphere are presented as a reference for reduced CrO2 films. The measurements of the CrO2 surfaces show no emission at the Fermi level after sputtering and annealing the surfaces in oxygen, even though our soft core photoemission data and low-energy electron diffraction measurements provide evidence that stoichiometric CrO2 is present. The consequence of this is that neither surface of CrO2 is metallic. This behaviour could result from a metal to semiconductor transition at the (110) and (100) surfaces.

Origins of Thermal Spin Depolarization in Half-Metallic Ferromagnet CrO2

“…[30] If the sample surface is completely covered by the contaminants (mainly Cr2O3), the photoemission spectrum has a peak located at 2 eV without the Fermi edge. [30] However, no clear structure at 2 eV was observed in our measurements. Based on these facts, our sample can be regarded as a high-quality sample enough to investigate the intrinsic electronic states of CrO2 by the photoemission method using the 7-eV laser.…”

Interface effects in spin-polarized metal/insulator layered structures

“…For example, CrO 2 has been predicted [168] and shown experimentally, using both Andreev reflection [178] and MeserveyTedrow type measurements [179], to be a half metal (although it may not be metallic but rather a semi-metal with high polarization [217]). CrO 2 was used as electrodes in CrO 2 /RuO 2 /CrO 2 spin valves, where RuO 2 is a metal which has the same rutile structure and very closely matched lattice constant [218].…”

Intrinsic spin polarized electronic structure of CrO2 epitaxial film revealed by bulk-sensitive spin-resolved photoemission spectroscopy

“…It is well known that the surface of CrO 2 easily transforms into antiferromagnetic insulator Cr 2 O 3 , 9 whose energy position in photoemission spectra was reported to be $2 eV. 10,11 In the previous spin-resolved PES study, 6 the maximum spin polarization was observed after removing the surface layer by moderate sputtering for certain optimum periods, and further sputtering reduced the spin polarization. Thus, it is likely that the Cr 2 O 3 formed on the surface and sputtering procedure significantly influenced the results of the previous spin-resolved PES studies because of the surface sensitivity of the detected photoelectrons excited by He Ia.…”