Spin Wave and Critical Fluctuations in Magnetite

The long-wavelength spin dynamics of the ferrimagnet magnetite (Fe3O4) have been investigated at and below the critical temperature Tc. The magnetic fluctuations were observed in zero magnetic field near the (111) Bragg peak by triple axis neutron spectrometry. Well below Tc the dynamics can be described in terms of spin-wave excitations whose energy is Dq2. Over the temperature range 0.6>1 - T/Tc>0.01, D is found to vary as (Tc - T) to the power 0.265±0.03. Within about 1% of Tc the spin-wave lines broaden rapidly and become over-critically damped. At T = Tc there are no signs of spin-wave excitations and the energy width of the neutron scattering function varies as q to the power 1.65±0.20 over the range 0.05≤q≤0.16 Å−1.