Phase evolution and magnetic transitions in non-equiatomic Ni0.7R0.1Mn0.1Fe1.1O3+x (R = Cr, Ru, and La) high-entropy oxide nanostructures

The structural, magnetic, and magnetocaloric properties of Ni0.7R0.1Mn0.1Fe1.1O3+x (R = Cr, Ru, and La) oxide nanostructures were systematically investigated and compared with pristine Fe2O3. All compounds were synthesized via an entropy-assisted sol–gel route and labeled as S1, S2, S3, and S0, respectively. The nanoparticles exhibited irregular morphologies, a tendency toward agglomeration, and a broad size distribution ranging from 10 to 120 nm. EDS analysis confirmed the expected stoichiometry, with configurational entropy values near 1R. Rietveld refinement of XRD data revealed that S1(Cr) and S3(La) crystallize in a single-phase cubic spinel structure (Fd3¯m space group), whereas S2 (Ru) exhibits a mixed spinel-tetragonal structure, consistent with Raman analysis. HRTEM and SAED images displayed well-defined lattice fringes and dislocations, and XPS analysis identified multiple oxidation states associated with configurational disorder. Magnetic measurements indicated second-order magnetic transitions near 230 K for S1 through S3, as verified by Arrott plots and n-parameter analysis, whereas the pristine Fe2O3 sample exhibited a first-order spin-canted transition around 237 K. The maximum magnetic entropy change (|ΔSM|) reached 1.55 J kg−1 K−1 for S2, followed by S1 (1.11 J kg−1 K−1) and S3 (0.75 J kg−1 K−1), with RCPs of 24.78, 15.75, and 12.01 J kg−1 at 5 T, respectively. Estimated TEC and NRC values for the oxide nanostructures are in good agreement with the corresponding RCP and the |ΔSM|, confirming the consistency of the magnetocaloric assessment. The findings demonstrate that cationic disorder and entropy-driven lattice distortion significantly enhance the magnetocaloric performance of these non-equiatomic high-entropy oxides.