The ferrimagnetic inverse spinel NiCo₂O₄ has attracted extensive research interest for its versatile electrochemical properties, robust magnetic order, high conductivity, and fast spin dynamics, as well as its highly tunable nature due to the closely coupled charge, spin, orbital, lattice, and defect effects. Single-crystalline epitaxial thin films of NiCo₂O₄ present a model system for elucidating the intrinsic physical properties and strong tunability, which are not viable in bulk single crystals. In this Perspective, we discuss the recent advances in epitaxial NiCo₂O₄ thin films, focusing on understanding its unusual magnetic and transport properties in light of crystal structure and electronic structure. The perpendicular magnetic anisotropy in compressively strained NiCo₂O₄ films is explained by considering the strong spin–lattice coupling, particularly on Co ions. The prominent effect of growth conditions reveals the complex interplay between the crystal structure, cation stoichiometry, valence state, and site occupancy. NiCo₂O₄ thin films also exhibit various magnetotransport anomalies, including linear magnetoresistance and sign change in anomalous Hall effect, which illustrate the competing effects of band-intrinsic Berry phase and impurity scattering. The fundamental understanding of these phenomena will facilitate the functional design of NiCo₂O₄ thin films for nanoscale spintronic applications.

The layered van der Waals CuInP₂S₆ (CIPS) features interesting functional behaviors, including reversible ferroelectric polarization, Cu ion migration, negative capacitance effect, etc. Here, the CIPS flakes were exfoliated from the CVT-grown high-quality single crystals, which were fabricated into metal/CIPS/metal heterostructures by conventional photolithography. It was found that the CIPS flakes persisted in the dominant out-of-plane polarization and the minor in-plane polarization. Clear hysteresis current–voltage (I–V) loops, as well as the rectifying character, were revealed in metal/CIPS/metal heterostructures, indicating the potential application as a memory device. Additionally, the different metal electrode could significantly modulate the Schottky-like barrier at metal/CIPS interfaces, resulting in symmetric or asymmetric I–V loops. The complicated I–V curves may have originated from the voltage-induced Cu ion migration, reversible ferroelectric polarization, and carrier (ion) trapping/detrapping. This work may facilitate the metal electrode selection for the ferroelectric CIPS-based device application.

The use of lead-free ceramic film materials with positive temperature coefficient of resistivity (PTCR) is widespread in temperature heaters and sensors in micro-electromechanical systems. In this research, the out of plane transport properties of the BiFeO₃ (BFO) films have been studied. Surprisingly, PTCR was found in the BFO ceramic films due to the strongly correlated interaction between the multiferroic material BFO and the superconductor YBCO perovskite oxides. To our knowledge, this is the first report on the PTCR effect of BFO films. The BFO/YBCO interface and the bulk conductivity of BFO are important for the PTCR effect, as they make it possible to compare the transport properties of Au/BFO/YBCO- and YBCO/BFO/YBCO-type structures. PTCR was observed in Au/BFO/YBCO at a bias voltage of more than 2 V, but not in the YBCO/BFO/YBCO, even with a 40 V bias voltage. PTCR was found after BFO breakdown of a YBCO/BFO/YBCO capacitor. This indicated that the conductivity of BFO is critical for PTCR. The dependence of PTCR on the superconducting transition temperature illustrates that a cooper-pair can be injected into BFO. Our work presents a method by which to produce a lead-free ceramic film material with PTCR.