Oxygen stoichiometry-driven charge compensation and Ruddlesden–Popper defects in ferromagnetic high-entropy manganite thin films

High-entropy oxides (HEOs) originate from an innovative materials design strategy that stabilizes single-phase solid solutions despite the inclusion of multiple principal elements into a single cation sublattice. While prior efforts have largely focused on cation disorder, the impact of anion defects on the structure and properties of HEOs remains unexplored. Here, we examine the influence of oxygen non-stoichiometry on the nanostructure and magnetic properties of single-crystal high-entropy manganite (HE-Mn) films, (Gd$_{0.2}$La$_{0.2}$Nd$_{0.2}$Sm$_{0.2}$Sr$_{0.2}$)MnO$_3$. The films were deposited on single-crystal (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ (001) substrates under varying oxygen partial pressures p(O$_2$). Phase-pure cube-on-cube epitaxy is maintained across all growth conditions. However, distinct nano-columnar Ruddlesden-Popper (RP) faults formed in oxygen deficient HE-Mn films. Unlike in conventional manganites, low-pressure-deposited films show no change in cation oxidation state, indicating the concurrent oxygen and manganese deficiency. This coupled cation-anion deficiency preserves the Mn$^{3+}$/Mn$^{4+}$ ratio and drives RP fault formation. ... mehrConsequently, ferromagnetic ordering persists even in the low p(O$_2$) HE-Mn films, demonstrating their resilience to oxygen non-stoichiometry. Additionally, an in-plane to out-of-plane magnetic anisotropy crossover was observed, likely arising from spatial variation in the c-axis lattice constant. These findings establish oxygen non-stoichiometry as an effective control parameter for defect nanostructuring and magnetic property tuning in HEO epitaxial films.