Evidence by spectroscopic ellipsometry of optical property change in pulsed laser deposited NiO films when heated in air at Neel temperature

A spectroscopic ellipsometry (SE) study of Nickel oxide (NiO) films, an anti-ferromagnetic insulator, deposited by pulsed laser ablation technique (PLD) on quartz substrate has been undertaken in the region of its Neel temperature (TN=523 K), when its structure changes from rock-salt to a rhombohedral one, owing to a slight contraction along the 111 direction. The films were grown under optimal conditions in a MECA 2000 process chamber so as to reduce surface roughness to a minimum as shown by Atomic Force Microscope (AFM) and SE characterization. The optical properties have been investigated as a function of temperature in an ultra high vacuum (UHV) process chamber (in situ) and in air, under standard atmospheric conditions (ex situ). We report in this study a change in the ellipsometric parameters of NiO in the visible spectral range from 550 nm to 700 nm when the film is heated in air in the region of TN differently from that observed in ultrahigh vacuum. On the contrary, as expected, SmFeO3 studied under the same conditions in the same temperature range in air shows no such change in ellipsometric parameters in this spectral region which corresponds to the spectral signature of NiO. Results are discussed in terms of spectral lines involving vibrational states of NiO molecule, when transitions occur between its electronic ground state X3Σ- and the excited states [16.0]3Σ0- and [16.0]3Σ1-. These results suggest that NiO ought to be classed as a ‘charge-transfer’ insulator rather than a Mott–Hubbard one.