Strain analysis from M-edge resonant inelastic X-ray scattering of nickel oxide films

Electronic structure modifications due to strain are an effective method for tailoring nano-scale functional materials. Demonstrated on nickel oxide (NiO) thin films, Resonant Inelastic X-ray Scattering (RIXS) at the transition-metal M 2,3 -edge is shown to be a powerful tool for measuring the electronic structure modification due to strain in the near-surface region. Analyses from the M 2,3 -edge RIXS in comparison with dedicated crystal field multiplet calculations show distortions in 40 nm NiO grown on a magnesium oxide (MgO) substrate (NiO/MgO) similar to those caused by surface relaxation of bulk NiO. The films of 20 and 10 nm NiO/MgO show slightly larger differences from bulk NiO. Quantitatively, the NiO/MgO samples all are distorted from perfect octahedral ( O h ) symmetry with a tetragonal parameter Ds of about −0.1 eV, very close to the Ds distortion from octahedral ( O h ) symmetry parameter of −0.11 eV obtained for the surface-near region from a bulk NiO crystal. Comparing the spectra of a 20 nm film of NiO grown on a 20 nm magnetite (Fe 3 O 4 ) film on a MgO substrate (NiO/Fe 3 O 4 /MgO) with the calculated multiplet analyses, the distortion parameter Ds appears to be closer to zero, showing that the surface-near region of this templated film is less distorted from O h symmetry than the surface-near region in bulk NiO. Finally, the potential of M 2,3 -edge RIXS for other investigations of strain on electronic structure is discussed. 3d-Metal M-edge RIXS quantitatively measures the electronic structure distortion due to strain with help of crystal field multiplet calculations.