Reversible Dielectric Phase of Highly Crystalline NiFe₂O₄(111) Thin Films with Temperature in X-Ray Photoelectron Spectroscopy (XPS)

Recently, we observed that CoFe₂O₄ and NiCo₂O₄ thin film surfaces undergo non-metal to metal phase transition with temperature [1]. These non-metal to metal phase transitions were not observed to be reversible with the temperature. Since the non-metal to metal phase transitions observed for CoFe₂O₄ and NiCo₂O₄ thin film surfaces are mediated by oxygen vacancies, voltage control of non-volatile states may be possible, making such materials electronically attractive especially if the phase transition is reversible. We have observed reversible dielectric (non-metallic) phase change for the NiFe₂O₄(111) thin film surface region, with temperature in X-ray photoelectron spectroscopy (XPS). Energy shifts of 5 eV, to higher binding energies relative to expected values, were observed in Ni 2p_3/2, Fe 2p_3/2, and O 1s XPS core level spectra at room temperature due to surface photovoltage charging during photoemission. This indicated that the surface of the NiFe₂O₄(111) thin film was highly dielectric (non-metallic) at room temperature. The core level binding energy shifts decreased when the thin film was annealed in vacuum, making the NiFe₂O₄(111) thin film less dielectric or more metallic at higher temperatures. At the temperature of 410 K or above, the observed Ni 2p_3/2, Fe 2p_3/2, and O 1s core level binding energies were found to have negligible or no binding energy shifts from the expected values, thereby establishing much diminished dielectric character or much enhanced metallic character at the surface of the thin film at elevated temperatures. When the thin film sample was cooled down to room temperature, core level binding energy shifts of 5 eV were again observed. Thus, NiFe₂O₄(111) thin films exhibit reversible surface photovoltage charging, and thus are distinct from the very similar inverse spinel thin film oxides CoFe₂O₄ and NiCo₂O₄. Such a reversible dielectric phase transition, for the surface of NiFe₂O₄(111) thin film, was further supported by the reversible Fermi level edge shift of the thin film sample with respect to the Fermi level of the spectrometer. The temperature dependent XPS core level binding energies were found to closely follow a new modified Arrhenius-type model [1], recently proposed. This study also showed that a large number of oxygen vacancies at the surface of the thin film might affect the reversible dielectric properties of the NiFe₂O₄(111) thin film and cause partially reversible dielectric phase transition of the thin film surface region with temperature. Low energy electron diffraction (LEED) images of the NiFe₂O₄(111) thin film showed that the surface of the thin film was highly crystalline throughout the reversible dielectric (non-metallic) to metallic phase transition of the thin film, thus the creation of a massive number of oxygen vacancies does not normally occur under the experimental conditions. Temperature dependent intensities of LEED gave us effective surface Debye temperature of 249.7 ± 11.1 K. The effective bulk Debye temperature of 629.10 ± 58.22 K and 787.03 ± 52.81 K were estimated using temperature dependent intensities of Fe 2p_3/2 and Ni 2p_3/2 XPS spectra respectively indicating different sites for Fe and Ni. As expected, lower effective Debye temperature was found for the surface region compared to the bulk region of the NiFe₂O₄(111) thin film. This work fundamentally provides insights into the material properties during an electronic phase transition of some dielectric oxide thin films and opens a door to non-volatile memory devices utilizing thin film oxides like NiFe₂O₄.<br/>[1] A. Subedi <i>et al.,</i> 2024, <i>J. Phys. D: Appl. Phys., in press,</i> https://doi.org/10.1088/1361-6463/ad5aa8