Anisotropic ferromagnetism in carbon-doped zinc oxide from first-principles studies
A density functional theory study of substitutional carbon impurities in ZnO has been performed, using both the generalized gradient approximation (GGA) and a hybrid functional (HSE06) as exchange-correlation functional. It is found that the nonspinpolarized C_Zn impurity is under almost all conditions thermodynamically more stable than the CO impurity which has a magnetic moment of 2μB, with the exception of very O-poor and C-rich conditions. This explains the experimental difficulties in sample preparation in order to realize d0 ferromagnetism in C-doped ZnO. From GGA calculations with large 96-atom supercells, we conclude that two C_O-C_O impurities in ZnO interact ferromagnetically, but the interaction is found to be short-ranged and anisotropic, much stronger within the hexagonal ab plane of wurtzite ZnO than along the c axis. This layered ferromagnetism is attributed to the anisotropy of the dispersion of carbon impurity bands near the Fermi level for C_O impurities in ZnO. From the calculated results, we derive that a C_O concentration between 2% and 6% should be optimal to achieve d0-ferromagnetism in C-doped ZnO.
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