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This paper correlates the magnetic properties of iron-oxide nanoparticles in the size range 5-18 nm with the occurring heating loss mechanisms when magnetic nanoparticle colloidal suspensions are subjected to high-frequency ac magnetic fields. The narrow size distribution of the nanoparticles enabled their clear classification into: 1) the superparamagnetic region (as large as 10 nm) where heating is mainly attributed to Neel relaxation; 2) the intermediate superparamagnetic-ferromagnetic transition region (10-13 nm); and 3) the ferromagnetic region (above 13 nm) where hysteresis losses dominate. The results from specific loss power measurements suggest that for size and concentration optimization, superparamagnetic nanoparticles may release significant amounts of heat to the surroundings, while the hysteresis losses mechanism appears to be much more efficient and the heat transfer provided through may be easier tuned for magnetically driven hyperthermia applications.
Date of Publication: April 2012