We studied the magnetic heating properties of nearly monodispersed iron oxide nanoparticles (NPs) suspended in liquid media. Iron oxide NPs with a narrow size distribution (sigma similar to 0.1) with various median diameters were synthesized by the seed-mediated growth method. The effect of size on specific absorption rate (SAR) was investigated for median diameters ranging from 9 to 15 nm at intervals of 1 nm. The maximum value of SAR was found at about 13 nm for the samples dispersed in a viscous liquid media. Calculated SAR using linear response theory explains the results qualitatively but showed quantitative disagreement. This discrepancy was explained by assuming the existence of a magnetically frustrated layer on the seed NPs. (C) 2014 AIP Publishing LLC.