Synthesis and Characterization of a High-Entropy Spinel Oxide Single Crystal

High-entropy materials generally refer to compounds which involve mixing five or more elements in nearly equimolar concentrations at an equivalent atomic site. These compounds are stabilized into a single phase by the high configurational entropy caused by the varying sizes and masses of their constituent elements. Prior work has shown competing magnetic interactions enabled by high entropy generate novel magnetic phases. Given that oxides with the spinel structure contain a variety of magnetic ordering, high-entropy spinel oxides, if successfully made, would provide a platform for the further study of high-entropy tuning of magnetism. This research aimed to synthesize a novel high-entropy oxide with the spinel structure and to determine the effects of its lattice distortions on its magnetization. In this work, an (Mg, Mn, Fe, Ni, Co)Al₂O₄ single crystal was synthesized for the first time using the optical floating zone growth technique. The sample was confirmed to be a phase pure high-entropy oxide (HEO) using X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). Through magnetization measurements, we found (Mg, Mn, Fe, Ni, Co)Al₂O₄ exhibits a spin-glass state though the parent phases show either antiferromagnetic or ferrimagnetic ordering or spin glass. Furthermore, we also found that (Mg, Mn, Fe, Ni, Co)Al₂O₄ has much greater thermal expansion than its parent compounds from neutron scattering measurements.