Relating Magnetic Properties and High Hyperthermia Performance of Iron Oxide Nanoflowers

We investigated, in depth, the interrelations among
structure, magnetic properties, relaxation dynamics and magnetic
hyperthermia performance of magnetic nanoflowers. The nanoflowers
are about 39 nm in size, and consist of densely packed iron oxide
cores. They display a remanent magnetization, which we explain by
the exchange coupling between the cores, but we observe indications
for internal spin disorder. By polarized small-angle neutron
scattering, we unambiguously confirm that, on average, the nanoflowers
are preferentially magnetized along one direction. The extracted
discrete relaxation time distribution of the colloidally dispersed particles
indicates the presence of three distinct relaxation contributions.
We can explain the two slower processes by Brownian and classical Néel relaxation, respectively. The additionally observed
very fast relaxation contributions are attributed by us to the relaxation of disordered spins within the nanoflowers. Finally,
we show that the intrinsic loss power (ILP, magnetic hyperthermia performance) of the nanoflowers measured in colloidal dispersion
at high frequency is comparatively large and independent of the viscosity of the surrounding medium. This concurs with
our assumption that the observed relaxation in the high frequency range is primarily a result of internal spin relaxation, and possibly
connected to the disordered spins within the individual nanoflowers.