Spin waves in antiferromagnetic FeF2

Abstract

Spin-wave dispersion in antiferromagnetic FeF₂ has been investigated by inelastic neutron scattering using a chopper time-of-flight spectrometer. The single mode observed has a relatively flat dispersion curve rising from 53 cm^-1 at the zone centre to 79 cm^-1 at the zone boundary. A spin Hamiltonian which includes a single-ion anisotropy constant D (H_sⁱ=-DS_iz²), three exchange parameters J_l(H_ex¹²=J_lS₁.S₂) and dipolar interactions has been fitted to the measured energies. The authors find D=6.46 (+0.29, -0.10) cm^-1, J₁ (coupling neighbouring ions along the c axis)=-0.048 (+or-0.060) cm^-1, j₂ (coupling neighbouring ions at the corner and body centre of the cell)=+3.64(+or-0.10) cm^-1, and J₃ (coupling neighbouring ions along the a axis)=+0.194 (+0.060) cm^-1.