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Abstract Improper ferroelectricity in hexagonal rare-earth manganites (h-RMnO₃, R = Ho–Lu, Y, Sc) arises from a geometric distortion as the primary order parameter, resilient to depolarizing fields and promising for ultrathin ferroelectric devices. However, the substrate-induced interface clamping effect, which suppresses the geometric distortion in the sub-nanometer regime, has thus far hindered the realization of two-dimensional improper ferroelectrics. This study demonstrates that doping with calcium can enhance ferroelectric structural distortion in h-LuMnO₃thin films. Compressively strained h-Lu_1−xCa_xMnO₃(x= 0.1, 0.2, 0.3, 0.4, 0.5) epitaxial thin films were stabilized on sapphire substrates using an h-ScFeO₃buffer layer. We have found that the interface clamping effect is entirely overcome when the doping concentration reachesx⩾ 0.2, establishing a potential quasi-2D ferroelectric system with a remarkably high estimated structural transition temperature of larger than 1200 K inferred indirectly from temperature-resolved reflection high-energy electron diffraction. This finding suggests a general strain engineering strategy to enhance improper ferroelectricity in hexagonal manganites. 

We have revisited the Kittel model that describes antiferroelectricity (AFE) in terms of two sublattices of spontaneous polarization with antiparallel couplings. By constructing a comprehensive phase diagram including the antiferroelectric, ferroelectric, and paraelectric phases in the parameter space, we have identified an AFE phase with stable antipolar states and metastable polar states (SAMP) when the coupling between the two sublattices is weak. We find that the metastability of the polar state in the SAMP AFE phase can lead to apparent ferroelectric behavior, depending on the measurement timescale—for example, the frequency of the applied electric field. This explains the observed ferroelectric behavior of orthorhombic hafnia, which is predicted to be antipolar from density functional theory calculations. 

Abstract Multiferroic hexagonal rare-earth ferrites (h-RFeO₃, R= Sc, Y, and rare earth), in which the improper ferroelectricity and canted antiferromagnetism coexist, have been advocated as promising candidates to pursue the room-temperature multiferroics, because of strong spin-spin interaction. The strong interactions between the ferroic orders and the structural distortions are appealing for high-density, energy-efficient electronic devices. Over the past decade, remarkable advances in atomic-scale synthesis, characterization, and material modeling enable the significant progresses in the understanding and manipulation of ferroic orders and their couplings in h-RFeO₃thin films. These results reveal a physical picture of rich ferroelectric and magnetic phenomena interconnected by a set of structural distortions and spin-lattice couplings, which provides guidance for the control of ferroic orders down to the nano scale and the discovery of novel physical phenomena. This review focus on state-of-the-art studies in complex phenomena related to the ferroelectricity and magnetism as well as the magnetoelectric couplings in multiferroic h-RFeO₃, based on mostly the recent experimental efforts, aiming to stimulate fresh ideas in this field. 

The atomic structures at epitaxial film–substrate interfaces determine the scalability of thin films and can result in new phenomena. However, it is challenging to control the structure of the interface. In this work, we report the strong tunability of the epitaxial interface of improper ferroelectric hexagonal ferrites deposited on spinel ferrites, achieving the artificial selection of two types of interfaces that are related by a 90° rotation of in-plane epitaxial relations and feature either disordered or hybrid reconstruction. The hybrid-type interface exhibits characteristic structures of both hexagonal ferrites and spinel ferrites, which remove the critical thickness for improper ferroelectricity. This tunable interfacial structure provides critical insight into controlling interfacial clamping to maintain robust improper ferroelectricity at the two-dimensional limit. 

The topological Hall effect (THE), a quantum phenomenon arising from the emergent magnetic field generated by a topological spin texture, is a key method for detecting non-coplanar spin structures like skyrmions in magnetic materials. Here, we investigate a bilayer structure of Pt and the conducting ferrimagnet NiCo2O4 (NCO) of perpendicular magnetic anisotropy and demonstrate a giant THE across a temperature range of 2–350 K. The absence of THE in a single-layer Pt and NCO, as well as in Pt/Cu/NCO, suggests its interfacial origin. The maximum THE occurring just before the NCO coercive field indicates its connection to magnetic nucleation centers, which are topologically equivalent to skyrmions. The large normalized THE, based on the emergent-field model, points to a high population density of small magnetic nucleation centers. This aligns with the seemingly unresolvable domain structures by the employed techniques during magnetization reversal, even though clear domain structures are detected after zero-field cooling. These results establish heavy metal/NCO as a promising system for exploring topological spin structures. 

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 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. 

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 The switching characteristics of ferroelectrics and multiferroics are influenced by the interaction of topological defects with domain walls. We report on the pinning of polarization due to antiphase boundaries in thin films of the multiferroic hexagonal YbFeO₃. We have directly resolved the atomic structure of a sharp antiphase boundary (APB) in YbFeO₃thin films using a combination of aberration-corrected scanning transmission electron microscopy (STEM) and total energy calculations based on density-functional theory (DFT). We find the presence of a layer of FeO₆octahedra at the APB that bridges the adjacent domains. STEM imaging shows a reversal in the direction of polarization on moving across the APB, which DFT calculations confirm is structural in nature as the polarization reversal reduces the distortion of the FeO₆octahedral layer at the APB. Such APBs in hexagonal perovskites are expected to serve as domain-wall pinning sites and hinder ferroelectric switching of the domains.