  There has been renewed interest in the use of magnetic nanoparticles to
convert high frequency electromagnetic energy into heat. During the last
decade, numerous examples in the field of catalysis, lightweight thermoplastic
composites for aeronautical and automotive engineering, and biomedical
applications as well, have flourished. In a recent Letter "Ultrathin Interface
Regime of Core-Shell Magnetic Nanoparticles for Effective Magnetism Tailoring",
Moon et al. DOI: 10.1021/acs.nanolett.6b04016 claim to have observed a new
interface spin canting phenomenon in core-shell ferrrites that pushes heating
efficiencies above 10,000 W/g. If this were true, it represents new
opportunities for further tailoring nanoscale agents to be used in drug
delivery and selective destruction of tumours by hyperthermia. But here I argue
that these claims are inconsistent with the facts of physics.
