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

TL;DR

This study demonstrates that (111)- and (001)-oriented NiO thin films grown on sapphire and MgO substrates exhibit bulk-like magnetic, optical, and structural properties, including high Néel temperatures and stable magnon modes at room temperature.

Contribution

It provides evidence that NiO thin films grown by pulsed laser deposition retain bulk-like properties, including magnetic transition temperatures and optical bandgap, across various orientations and thicknesses.

Findings

Néel temperatures are above room temperature.

Magnon modes are comparable to bulk NiO.

Bandgap is approximately 3.6 eV, similar to bulk NiO.

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 N\'eel 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.