Tailoring B-Site Preferential Occupations for Tunable Magnetism in Spinel High-Entropy Oxides via Nonmagnetic Al3+ Probe

The rich diversity of transition-metal pairs bridged by oxygen contributes to the fascinating magnetic behaviors in high-entropy oxides (HEOs), while the complex interplay between multisite lattice configurations, spin states, and magnetic exchange interactions remains yet to be explored. Here, we report the Al3+-driven octahedral site preferential occupations and tunable magnetic frustration in spinel-type [5M1/(5+x)Alx/(5+x)]3O4 (x = 0–5; M = Cr/Fe/Mn/Zn/Co) HEOs. Nonmagnetic Al3+ preferentially occupies octahedral sites (PB), which significantly distorts the lattice with expansion in tetrahedral sites while shrinkage in PB–O. This structural evolution enables tuning of magnetic properties, e.g., competition between ferromagnetic and antiferromagnetic states, spin glass, and spin-flop frustration states. Alternating-current susceptibility presents apparent spin frustration that follows a critical slowing-down model, which is suppressed when the Al3+ level is beyond x = 2 because of the weakly frozen and disordered magnetic moments. These findings bring insights into modulating magnetic exchange interactions within multiple structural complexities for designing HEOs with tunable magnetic properties.