Epitaxial growth and structural and electronic properties of oriented thin films of GeNi₂O₄ and GeCu₂O₄ spinels

Frustrated magnets can host exotic many-body quantum and topological phenomena. GeNi₂O₄ is a three-dimensional S = 1 frustrated magnet with an unusual two-stage transition to the two-dimensional antiferromagnetic ground state, while GeCu₂O₄ is a high-pressure phase with a strongly tetragonally distorted spinel structure and magnetic lattice formed by S = 1/2 CuO₂ linear chains with frustrated interchain exchange interactions and exotic magnetic behavior. Here, we report on the first thin-film epitaxial stabilization of these two compounds in (100) and (111) directions. The developed growth mode, surface morphology, crystal structure, and valence state were characterized by in situ reflection high-energy electron diffraction, atomic force microscopy, x-ray reflectivity, x-ray diffraction, x-ray photoelectron spectroscopy, and resonant x-ray absorption spectroscopy. Our results pave an alternative route to the investigation of the puzzling magnetic properties of these compounds and the exploration of emergent features driven by strain. Furthermore, the availability of large-area high-quality GeCu₂O₄ thin films opens a road for future experimentation to reveal the controversial nature of its ground state magnetism and elucidate the origin of multiferroicity in this compound.