Although spinel ferrite nanostructures with adjustable magnetic properties have long been considered for different applications such as microwave, biomedical and magnetic memory devices, detailed magnetic behavior and structural properties of manganese-cobalt ferrites are yet to be investigated and understood. Here, cobalt ferrite nanoparticles (NPs) substituted with manganese (Co1–x Mnx Fe2O4); 0 ≤ x ≤ 1) are synthesized using a co-precipitation method followed by annealing at 800 °C. X-ray diffraction patterns confirm the formation of cubic spinel structure, which is accompanied with α- \({\text{Fe}}_{2}{\text{O}}_{3}\) secondary phase when increasing the manganese content of NPs, as also observed using field emission scanning electron microscopy. Hysteresis loop measurements demonstrate an increasing trend of magnetic properties with increasing x, leading to maximum values of saturation magnetization (Ms= 65 emu/g) and coercivity (Hc= 1815 Oe) for \({\text{Co}}_{0.8} {\text{Mn}}_{0.2} {\text{Fe}}_{2} {\text{O}}_{4}\) NPs. Meanwhile, first-order reversal curve (FORC) analysis manifests distinct soft and hard magnetic phases induced by the slight enhancement in the manganese content. The magnetic hardening of single domain manganese-cobalt ferrite NPs at room temperature is revealed by an increase in FORC coercivity up to 2480 Oe, being about 90% higher than that for pure cobalt ferrite NPs. Further increasing x weakens magnetic properties, minimizing \({M}_{s}\) and \({H}_{c}\) to 10 emu/g and 35 Oe when Mn completely substitutes Co in the spinel ferrite structure. In this case, FORC analysis shows suppressed inter-particle magnetostatic interactions and predominant superparamagnetic contribution for \({\text{MnFe}}_{2}{\text{O}}_{4}\) NPs.
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