Strain and Defect-Tailored Magnetotransport in NiCo ₂ O ₄ Thin Films and Freestanding Membranes

Magnetic spinel oxides are high-performance spintronic materials due to their high Curie temperature, high spin polarization, fast spin dynamics, and strain-tunable magnetic anisotropy. Epitaxial strain and disorder can significantly modify the electronic and magnetic energy landscapes, while their interplay remains elusive. Here, we use epitaxial NiCo₂O₄ thin films and freestanding NiCo₂O₄ membranes as model systems to reveal the complex roles of strain and defects in determining the metallicity and magnetotransport properties of the ferrimagnetic spinel. Unlike the perpendicular magnetic anisotropy and 2-fold sinusoidal anisotropic magnetoresistance (AMR) observed in metallic NiCo₂O₄ films on spinel substrates, NiCo₂O₄ on perovskite substrates and NiCo₂O₄ membranes exhibit insulating behaviors and spin canting, with an additional 4-fold AMR component emerging due to disorder-induced spin scattering and strain-induced tetragonal magnetocrystalline anisotropy. The amplitude ratio between the 4-fold and 2-fold AMR components provides critical information on the disorder types that contribute to the AMR. Electron microscopy studies reveal structural and chemical phase separation in the membranes similar to those in disordered films, which explains the highly consistent magnetotransport properties for NiCo₂O₄/Sr₃Al₂O₆ films and NiCo₂O₄ membranes. Our study provides effective material strategies for engineering spin transport and magnetic anisotropy in NiCo₂O₄ and presents a promising venue for designing flexible magnetic memory, sensor, and spintronic applications.