We have investigated ac hysteresis loops of hollow Fe3O4 submicron particles with variable particle size of d = 100−696 nm by micromagnetic simulations to investigate the possible application to magnetic hyperthermia. For the hollow particle with the inner/outer diameter ratio of γ = 0.5, the hysteresis loss increases with increasing d and maximizes at d ∼ 300 nm, whereas the hysteresis loss generally increases with γ, but its behavior strongly depends on d. A specific absorption rate, calculated from the loop area at the field frequency of 500 kHz, attains 560 W/g for d = 296 nm and γ = 0.5, which is comparable to that for conventional superparamagnetic nanoparticles. This enhanced specific absorption ratio for the hollow particles can be explained by strong irreversibility between vortex states with different orientation of the vortex core, i.e. along the magnetic field and ⟨111⟩ easy axes.
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