Formation Mechanism of High-Entropy Spinel Thin Film and its Mechanical and Magnetic Properties: Linking High-Entropy Alloy to High-Entropy Ceramic

Abstract High-entropy spinel thin film (HESTF) of (CrMnFeCoNi)3O4 was formed for the first time by heat-treating magnetron-sputtered thin film in Ar+O2 atmosphere. One of the most interesting observations is the formation of dual spinel structure, innermost Cr-rich spinel and outer Cr-lean spinel as the oxidation proceeds. For phase transformation from high entropy alloy (HEA) to dual high entropy spinel, the free energy minimization criterion was taken into consideration as well as the diffusivity of metal cations. The formation of innermost Cr-rich spinel is favored because of greater reduction of crystal field stable energy and octahedral site preference energy for the spinel with Cr3+ ions in the tetrahedral and octahedral sites. We propose that Cr-rich spinel form first in the initial HEA film with the oxygen supply through the short circuit diffusion paths and remnant atoms/ions diffused outward to form the outer Cr-lean spinel layer. The hardness and elastic modulus of the thin film increased from 8.4 to 13.2 GPa and 190.8 to 255 GPa, respectively. HESTF was found to be a ferrimagnetic nanomaterial with its TC being greater than 873 K. HESTF of this study was observed to maintain its blocked spins up to higher temperatures with TB value of 225K.