Magnetomicelles are amphiphilic block copolymer micelles, which contain magnetic nanoparticles in the core. Magnetomicelles have been proposed as magnetic fluid hyperthermia agents for cancer treatment. Magnetic interactions between nanoparticles within a micelle affect the amount of heat that they dissipate. Here, magnetomicelles are prepared varying the molecular weight of the core-forming polymer block. The magnetomicelles are magnetically characterized in liquid suspensions and embedded in a polymer matrix. The results show that as the molecular weight of the core-forming polymer block increases, both the interparticle magnetic interactions and specific absorption rate decrease. This is probably due to mixing between the polymer and the magnetic nanoparticles, which increases the interparticle distance.