We have developed silica stabilized ferrimagnetic magnetite nanocubes (MNCs) as effective heat mediators for magnetic hyperthermia. To do that, large hydrophobic magnetite nanoparticles, with a mean size of 79 and 124 nm were synthesized and transferred to water by silica surface modification. The heating ability of these water suspensions was studied by measuring the Specific Power Absorption (SPA) applying an AC magnetic field of amplitude H₀ = 45 kA/m and frequency f = 360 kHz. High SPA values were obtained, ranging from 560 to 1160 W/g. The normalization of the SPA values by the inverse of the frequency and squared field amplitude was performed in order to compare our results with these described in literature. The SPA value for our samples, normalized in this way, yielded values of 0.7-1.5 nH m²/kg.