Electron magnetic resonance in magnetic nanoparticles: dependence on the particle size and applicability of the modified giant spin model

Superparamagnetic nanoparticles containing hundreds and thousands of coupled electron spins are on the boundary between classical and quantum behavior, and demonstrate features which are typical for paramagnetic spins and absent in macroscopic ferromagnetic systems. In order to better understand the evolution of magnetization dynamics from quantum to classical behavior with the increase in the system size, we study the electron magnetic resonance signal in suspensions of iron oxide nanoparticles as the function of the particle size. The experimental data are compared with numerical simulations based on the giant spin approach.