Search for: All records

Abstract X-ray photoelectron spectroscopy (XPS) shows that dramatic changes in the core level binding energies can provide strong indications of transitions between more dielectric and more metallic CoFe₂O₄and NiCo₂O₄thin films. These significant variations in the XPS core level binding energies are possible with a combination of annealing and oxygen exposure; however, the behaviors of the CoFe₂O₄and NiCo₂O₄thin films are very different. The XPS Co and Fe 2p_3/2core levels for the CoFe₂O₄thin film at room temperature show large photovoltaic surface charging, leading to binding energy shifts, characteristic of a highly dielectric (or insulating) surface at room temperature. The photovoltaic charging, observed in the XPS binding energies of the Co and Fe 2p_3/2core levels, decreases with increasing temperature. The XPS core level binding energies of CoFe₂O₄thin film saturated at lower apparent binding energies above 455 K. This result shows that the prepared CoFe₂O₄thin film can be dielectric at room temperature but become more metallic at elevated temperatures. The dielectric nature of the CoFe₂O₄thin film was restored only when the film was annealed in sufficient oxygen, indicating that oxygen vacancies play an important role in the transition of the film from dielectric (or insulating) to metallic. In contrast, the XPS studies of initially metallic NiCo₂O₄thin film demonstrated that annealing NiCo₂O₄thin film led to a more dielectric or insulating film. The original more metallic character of the NiCo₂O₄film was restored when the NiCo₂O₄was annealed in sufficient oxygen. Effective activation energies are estimated for the carriers from a modified Arrhenius-type model applied to the core level binding energy changes of the CoFe₂O₄and NiCo₂O₄thin films, as a function of temperature. The origin of the carriers, however, is not uniquely identified. This work illustrates routes to regulate the surface metal-to-insulator transition of dielectric oxides, especially in the case of insulating NiCo₂O₄thin film that can undergo reversible metal-to-insulator transition with temperature. 

Abstract Intrinsic exchange bias is known as the unidirectional exchange anisotropy that emerges in a nominally single-component ferro-(ferri-)magnetic system. In this work, with magnetic and structural characterizations, we demonstrate that intrinsic exchange bias is a general phenomenon in (Ni, Co, Fe)-based spinel oxide films deposited on $\alpha$-Al₂O₃(0001) substrates, due to the emergence of a rock-salt interfacial layer consisting of antiferromagnetic CoO from interfacial reconstruction. We show that in Ni_xCo_yFe_3−x−yO₄(111)/ $\alpha$-Al₂O₃(0001) films, intrinsic exchange bias and interfacial reconstruction have consistent dependences on Co concentrationy, while the Ni and Fe concentration appears to be less important. This work establishes a family of intrinsic exchange bias materials with great tunability by stoichiometry and highlights the strategy of interface engineering in controlling material functionalities. 

Abstract In an effort to reconcile the various interpretations for the cation components of the 2p_3/2observed in x-ray photoelectron spectroscopy (XPS) of several spinel oxide materials, the XPS spectra of both spinel alloy nanoparticles and crystalline thin films are compared. We observed that different components of the 2p_3/2core level XPS spectra, of these inverse spinel thin films, are distinctly surface and bulk weighted, indicating surface-to-bulk core level shifts in the binding energies. Surface-to-bulk core level shifts in binding energies of Ni and Fe 2p_3/2core levels of NiFe₂O₄thin film are observed in angle-resolved XPS. The ratio between surface-weighted components and bulk-weighted components of the Ni and Fe core levels shows appreciable dependency on photoemission angle, with respect to surface normal. XPS showed that the ferrite nanoparticles Ni_xCo_1−xFe₂O₄(x= 0.2, 0.5, 0.8, 1) resemble the surface of the NiFe₂O₄thin film. Surface-to-bulk core level shifts are also observed in CoFe₂O₄and NiCo₂O₄thin films but not as significantly as in NiFe₂O₄thin film. Estimates of surface stoichiometry of some spinel oxide nanoparticles and thin films suggested that the apportionment between cationic species present could be farther from expectations for thin films as compared to what is seen with nanoparticles. 

Abstract Understanding intrinsic exchange bias in nominally single‐component ferromagnetic or ferrimagnetic materials is crucial for simplifying related device architectures. However, the mechanisms behind this phenomenon and its tunability remain elusive, which hinders the efforts to achieve unidirectional magnetization for widespread applications. Inspired by the high tunability of ferrimagnetic inverse spinel NiCo₂O₄, the origin of intrinsic exchange bias in NiCo₂O₄(111) films deposited on Al₂O₃(0001) substrates are investigated. The comprehensive characterizations, including electron diffraction, X‐ray reflectometry and spectroscopy, and polarized neutron reflectometry, reveal that intrinsic exchange bias in NiCo₂O₄(111)/Al₂O₃(0001) arises from a reconstructed antiferromagnetic rock‐salt Ni_xCo_1‐xO layer at the interface between the film and the substrate due to a significant structural mismatch. Remarkably, by engineering the interfacial structure under optimal growth conditions, it can achieve exchange bias larger than coercivity, leading to unidirectional magnetization. Such giant intrinsic exchange bias can be utilized for realistic device applications. This work establishes a new material platform based on NiCo₂O₄, an emergent spintronics material, to study tunable interfacial magnetic and spintronic properties.