Magnetic and power absorption measurements on iron oxide nanoparticles synthesized by thermal decomposition of Fe(acac)3

•7–12 nm in size IONPs were synthesized by thermal decomposition of Fe(acac)3.•Heating rate to reflux can be the most important factor which determine size.•The magnetic anisotropy constant is in the range, Keff = (17–41) kJ/m3.•In TOPO-coated 12 nm sized NPs, SAR ∼ H02f2 due to lower magnetic anisotropy.•Our systems dissipate power exclusively through the Néel relaxation processes.