Sub-Kelvin Resolution X-ray Diffraction Thermometry Defies the Existence of Nanoscale Hotspots in Induction Heated Magnetic Nanoparticles

A number of recent experiments have indicated that magnetic nanoparticles can become locally hotter than their nonmagnetic surroundings during induction heating. While such nanoscale hotspot-effect is particularly attractive for applications within biomedicine and catalysis, its existence is a topic of scientific controversy. To address this, we here present simultaneous measurements of the internal temperatures of magnetic nanoparticles and their solid support material during induction heating. The supports are dry, nonmagnetic, and nonconductive porous powders serving to separate the magnetic nanoparticles. The temperatures are measured by <i>in situ</i> synchrotron X-ray diffraction, utilizing that thermal expansion of the materials cause a shift in their X-ray diffraction peak positions. With a subkelvin temperature resolution and a time resolution of 0.1 s, we find no measurable temperature difference between magnetic nanoparticles and support, <i>i.e.</i>, no significant hotspots, in agreement with existing theory. We obtain the same result for three different combinations of magnetic nanoparticles and supports. We encourage further use of X-ray diffraction thermometry in combination with other localized thermometry techniques to clarify whether potentially nonthermal effects could have been incorrectly ascribed to a local temperature increase in previous experimental studies.