Magnetic iron oxide nanoparticles have been extensively studied in the biomedical field due to their diverse application such as magnetic hyperthermia, targeted drug delivery. Recently, larger magnetic particles with a vortex ground state has been recognized as useful biomedical materials, because of high saturation magnetization and wider tunability of a particle size from sub-100 nm to submicrometer scale. The vortex particles showed a high heating efficiency, which is comparable to that of superparamagnetic nanoparticles and is enhanced with the number of vortices 1,2. However, spin reorientation process under alternating fields, for submicron particles in particular, has not been fully understood, though the understanding is of crucial importance for the improvement of the biomedical performance. Here, we report results of magnetic hysteresis scaling for hollow Fe3O4 spherical submicron particles with a vortex structure 3,4. A scaling coefficient is deduced from the power law scaling and examined to elucidate the relation with particle morphology. We examined hollow Fe3O4 spherical particles with varying 400-700 nm diameter 4. A set of symmetrical minor loops were measured with increasing field amplitude step-by-step up to 2 kOe. Fig. 1 shows a typical example of a hysteresis scaling at T = 300 K for various particle sizes. The relation between hysteresis loss and remanence of minor loops follows a power law with an exponent of ~1.5 in a limited field range (< ~ 500 Oe), where a vortex state is stabilized. Such exponent was universally obtained for other particle size and in a wide temperature range of 10-300 K. As shown in the inset, the scaling coefficient monotonically decreases with increasing inner/outer diameter ratio and temperature, indicating a decrease of domain wall pinning strength 5 and an increasing vortex mobility under alternating fields. The results demonstrate that the scaling coefficient is an useful physical parameter for evaluating a vortex mobility in a ferromagnetic particle.

References:

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