Response of magnetic nanoparticles to microwaves

Two important processes in ferromagnetic resonance are the first-order absorption of a photon and creation of a single magnon, and a second-order process in which the absorption of a photon results in the creation of two magnons of equal and opposite wave vector [M. Sparks, Ferromagnetic Relaxation (McGraw–Hill, New York, 1964)]. We have found that under resonance conditions for the second-order process, samples containing ∼0.1% magnetite absorb energy from the microwave field at the same rate as a solid magnetite sample. The resultant very high-energy density in the magnetic nanoparticles, coupled with a significant thermal energy barrier with the matrix, leads to a large temperature difference between the grains and their surroundings that makes it possible to magnetize and demagnetize the sample with a relatively small increase in sample temperature.