The analysed samples were ɣ-Fe2O3 nanoparticles of 8 nm, and uncoated and APS coated 12 nm and 14 nm; all the samples were synthetized by coprecipitation method. Nanoparticles of Fe3O4 with a size of 33 nm and a hybrid system of magnetite and gold were also analysed. A more complex system composed by ZnMnFe2O4@Fe3O4 in water and in isooctane was also measured as well as 6 nm ɣ-Fe2O3 nanoparticles with two different coatings, not being able to obtain any results of the measures since they did not warm up. All the colloids were previously characterized by TEM, XRD, DLS and SQUID magnetometry. From the SQUID and calorimetric measurements it has been seen that the coercive field and the SAR increase with the nanoparticle size until around 33 nm (monodomain-multidomain limit), having this Fe3O4 sample the higher heating efficiency of all, due probably to Brown relaxation. The comparison between uncoated and APS coated 12 nm maghemite shows an increase of the heating efficiency with the coating. For 12 nm and 14 nm maghemite samples, no differences were observed until around 30 mT; at higher fields, the 14 nm NPs have a SAR 13 % higher than the former. In the case of hybrid system of magnetite and gold, the magnetite NPs have a higher heating efficiency at low field, but at higher fields is the hybrid system the one with better performance.