Bulk-like structural, magnetic and optical properties of (111)- and (001)-NiO thin films

Abstract

We have grown (111)- and (001)-oriented NiO thin films on (0001)-Sapphire and (001)-MgO substrates
using pulsed laser deposition (PLD), respectively. DC magnetic susceptibility measurements underline
that the Néel temperatures of the samples are beyond room-temperature. This is further confirmed by
the presence of two-magnon Raman scattering modes in these films in ambient conditions. Moreover,
relative intensity of the two magnon-mode with respect to a neighboring phonon mode in the films, at
least down to 30 nm thickness, is comparable to the same for bulk NiO. UV-vis spectroscopy
and spectroscopic ellipsometry determined that the bandgap of the films is 3.6 eV which is well within
the range for bulk NiO. Thus, these indicate that the thin films are bulk-like. Further, photoluminescence
measurements on (111)-NiO films obtained two-radiative transitions at 385 and 405 nm. The linewidth
of the latter broadens towards low temperatures, indicating a plausible exciton-magnon coupling.
Overall, these PLD-grown oxide films hold significant technological importance due to their optical
transparency and their capacity to host robust magnons at room temperature.

Keywords: Bulk-like properties; Magnons; Nickel oxide; Optical properties; films.