Multi-element ferrite nanoparticles (NPs) were synthesized as heat agents for use in magnetic hyperthermia treatments, specifically, Co0.8Zn0.2Fe₂O₄ NPs coating with polyethylene glycol. The crystal structures of these particles were examined by X-ray diffraction. Particle diameters were controlled to be approximately 10 nm by controlling the annealing temperature and time. The modification of polyethylene glycol (PEG) on the particles was confirmed by mass spectrometry and Fourier-transform infrared spectrometry. The heat dissipation characteristics of the particles were investigated by measuring AC magnetic susceptibility and temperature increase in AC magnetic fields. A peak in the imaginary part of AC magnetic susceptibility χ″ appeared, depending on the frequency. The value of χ″ was found to contribute to the effective heat dissipation according to the Neel relaxation system. The temperature increase of the particles was measured in AC magnetic fields of 64-146 Oe, with an observed temperature increase of ~10 K. Finally, to test the applications of these particles in theranostics, in vitro experiments using human breast cancer cells were conducted.