Copper–nickel (CuNi) alloy nanoparticles with Curie temperatures (Tc) from 40 to 60∘C were synthesized by several techniques. Varying the synthesis parameters and post-treatment, as well as separations by size and Tc, allow producing mediator nanoparticles for magnetic fluid hyperthermia with parametric feedback temperature control with desired parameters. In vitro and in vivo animal experiments have demonstrated the feasibility of the temperature-controlled heating of the tissue, laden with the particles, by an external alternating magnetic field.