> The second field realigned the nanoparticles, effectively tapping the brakes on heat production.
The heating slowed fastest in areas with more nanoparticles
Probably very stupid question, but can this be used also for cooling? Let the nanoparticles absorb some heat, then use a magnetic field to align them, then repeat?
There is no mechanism for cooling the magnetite particles below the ambient temperature.
A ferromagnetic material that is placed in an alternating magnetic field is heated by the so-called hysteresis losses, which happen when the material is magnetized, demagnetized and remagnetized. This can be seen e.g. in an electric transformer with a ferrite core, where the transformer core will become hot during operation (the iron cores used in low-frequency transformers are also heated by eddy currents, like an induction stove, but ferrites are insulators, so they are heated only by hysteresis losses).
Because the magnetite nanorods are anisotropic, the heat produced will depend on their orientation vs. the alternating magnetic field.
This is used to modify the heating rate, but there is no way to do cooling.
There exist magnetic refrigerators, which are based on the magnetocaloric effect, but they use very different magnetic materials, e.g. based on gadolinium or other rare-earth metals. Those are efficient only at extremely low temperatures, close to zero kelvin, so they would be useless for freezing animal tissues.
Probably very stupid question, but can this be used also for cooling? Let the nanoparticles absorb some heat, then use a magnetic field to align them, then repeat?