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We prepared pure-phase Ni x Mn 1-x Co 2 O 4 (x = 0, 0.25, 0.5, 0.75 and 1) nanoparticles using a low-temperature solid-state reaction method. Magnetization measurement results showed that with Ni doping, the Curie temperature and coercivity of Ni x Mn 1-x Co 2 O 4 increased. Multiple magnetic phases that transition from paramagnetic to ferrimagnetic to ferrimagnetic and antiferromagnetic were observed to coexist in the Ni 0.5 Mn 0.5 Co 2 O 4 sample. At low temperatures, the ferromagnetic and antiferromagnetic phases coexist in Ni x Mn 1-x Co 2 O 4 (x = 0 and 0.25), and as the concentration of Ni increases, Ni x Mn 1-x Co 2 O 4 (x = 0.75 and 1) show a spin glass state. The structure of Ni x Mn 1-x Co 2 O 4 (x < 0.5) is mainly affected by cation defects, and by cation substitution when x is greater than 0.5. The results of first-principles calculations show that covalent bonds exist in Ni x Mn 1-x Co 2 O 4 and that the strength of the Ni-O bond is greater than that of the Mn-O bond. [ABSTRACT FROM AUTHOR]