  The ferrimagnetic inverse spinel NiCo2O4 has attracted extensive research
interests 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.
