Spin correlation, excitation, and relaxation of antiferromagnetic hematite α-Fe₂O₃ nanoparticles

We study the dynamics of spins in the canted antiferromagnetic hematite nanoparticles α-Fe₂O₃. The system includes an antiferromagnetic exchange (J) between two sublattices as well as the potential dominated by a uniaxial anisotropy and dissipates through the thermally triggered spin-phonon channel. It is known that two distinct kinds of spin dynamics can be observed in the system; the superparamagnetic relaxation along the easy axis and the collective excitation near the easy axis. We examine the role of the exchange J in both types of dynamics and find that the exchange J reduces the superparamagnetic relaxation and drives the incoherent relaxation of transverse spins (decreasing behaviors of the transverse relaxation with T) at low temperature especially when T≪JS². In particular, it is found that the transverse relaxation would have a minimum near T∼JS² and a slow upturn when T≫JS². This means the exchange J is essential in understanding the dynamics qualitatively as well as quantitatively. Results of recent inelastic neutron scattering experiments are semiquantitatively understood within a role of the exchange interaction J. Further, a reliable estimation for the relevant parameters of the system is done.