  Magnetic particle hyperthermia, in which colloidal nanostructures are exposed
to an alternating magnetic field, is a promising approach to cancer therapy.
Unfortunately, the clinical efficacy of hyperthermia has not yet been
optimized. Consequently, routes to improve magnetic particle hyperthermia such
as designing hybrid structures comprised from different phase materials are
actively pursued. Here we demonstrate enhanced hyperthermia efficiency in
relative large spherical Fe/Fe-oxide core/shell nanoparticles through the
manipulation of interactions between the core and shell phases. Experimental
results on exemplary samples with diameters in the range 30-80 nm indicated a
direct correlation of hysteresis losses to the observed temperature elevation
rate with a maximum efficiency of around 0.9 kW/g. The absolute particle size,
the core/shell ratio, and the interposition of a thin w\"ustite interlayer, are
shown to have powerful effects on the specific absorption rate. By comparing
our measurements to micromagnetic calculations we have unveiled topologically
non-trivial magnetisation reversal modes under which interparticle interactions
become negligible, aggregates formation is minimized, and the energy that is
converted into heat is increased. This information has been overlooked till
date and is in stark contrast to the existing knowledge on homogeneous
particles.
