  NiO thin films with various strains were grown on SrTiO3 (STO) and MgO
substrates using a pulsed laser deposition technique. The films were
characterized using an x-ray diffractometer, atomic force microscopy, and
infrared reflectance spectroscopy. The films grown on STO (001) substrate show
a compressive in-plane strain which increases as the film thickness is reduced,
resulting in an increase of the NiO phonon frequency. On the other hand, a
tensile strain was detected in the NiO film grown on MgO (001) substrate which
induces a softening of the phonon frequency. Overall, the variation of in-plane
strain from -0.36% to +0.48% yields the decrease of the phonon frequency from
409.6 cm-1 to 377.5 cm-1 which occurs due to the ~1% change of the inter-atomic
distances. The magnetic exchange -driven phonon splitting Delta(W) in three
different sample, with relaxed (i.e. zero) strain, 0.36% compressive and 0.48%
tensile strain was measured as a function of temperature. The Delta(W)
increases on cooling in NiO relaxed film as in the previously published work on
a bulk crystal. The splitting increases on cooling also in 0.48% tensile
strained film, but Delta(W) is systematically 3-4 cm-1 smaller than in relaxed
film. Since the phonon splitting is proportional to the non-dominant magnetic
exchange interaction J1, the reduction of phonon splitting in tensile-strained
film was explained by a diminishing J1 with lattice expansion. Increase of
Delta(W) on cooling can be also explained by rising of J1 with reduced
temperature.
