Spin excitations in cubic maghemite nanoparticles studied by time-of-flight neutron spectroscopy

We have determined the field dependence of collective magnetic excitations in iron oxide nanoparticles of cubic shape with 8.42(2) nm edge length and a narrow log normal size distribution of 8.2(2)% using time-of-flight neutron spectroscopy. The energy dependence of the uniform precession modes was investigated up to 5 T applied field and yields a Land\'e factor $g=2.05(2)$ as expected for maghemite ($\ensuremath{\gamma}$-Fe${}_{2}$O${}_{3}$) nanoparticles. A large effective anisotropy field of ${B}_{A,\mathrm{eff}}=0.45(16)$ T was determined, in excellent agreement with macroscopic measurements. This anisotropy is attributed to enhanced shape anisotropy in these monodisperse cubic nanoparticles. The combination of our results with macroscopic magnetization information provides a consistent view of the energy scales of superparamagnetic relaxation and collective magnetic excitations in m…