Superparamagnetic iron oxide-based nanoparticles (SPIONS) have attracted an enormous amount of attention for their potential use in biomedical applications, due to their good biocompatibility and low toxicity. The current study considers citric acid-conjugated manganese ferrite and its synergy to be used in MRI and in hyperthermia treatment, thus showing theragnostic applications. High colloidal stability was obtained with this functionalization. SPIONS with superparamagnetic behavior of crystal sizes of approximately 20 nm were obtained via an electrochemical synthesis method. One of the highest specific absorption rate (SAR) values was achieved in this work (1661 W g(-1)), under a magnetic field of 30 mT at 717 kHz frequency, compared with other magnetic ferrites in the literature. These nanoparticles dissipate heat through Neel relaxation and, together with the high SAR value obtained, indicate an excellent material for hyperthermia treatment of cancer. In addition, these nanoparticles exhibit transverse relaxivity behavior, with an r(2) value of 394 mM(-1) s(-1), i.e., at least two times higher than the value of a commercial magnetic contrast agent based on iron oxides. Finally, no toxicity effects of these nanoparticles are evidenced; as a result, these nanoparticles are appropriate for in vivo application.