Antiferromagnetic Resonance Linewidth in MnF2 near the Transition Temperature

Abstract

The antiferromagnetic resonance (AFMR) linewidth of MnF₂ has been studied near the Néel temperature T_N. In the presence of an applied magnetic field, the linewidth diverges as ${\mathrm{|\hspace{0.17em}T-T}}_N{\mathrm{\hspace{0.17em}|}}^{-\frac{5}{6}}$ . In the absence of an applied field, the linewidth is larger and diverges approximately as ${\mathrm{|\hspace{0.17em}T-T}}_N{\mathrm{\hspace{0.17em}|}}^{-\frac{4}{3}}$ . The linewidth is independent of magnetic field direction and is the same for both branches of the AFMR spectrum. Near T_N the AFMR frequency varies as |T−T_N|^0.483±0.025 whereas the simplest molecular field theory predicts a |T−T_N|^½ variation. From this temperature dependence, together with existing data on the fluorine nuclear magnetic resonance in MnF₂, we find that the anisotropy energy E_A is proportional to M^2.89±0.16 compared to Zener's M³ and Oguchi's M^2.9 theoretical results. Above T_N, the electron paramagnetic resonance in MnF₂ has been observed. For H₀ ∥ c the linewidth varies as (T−T_N)^−3/8 near T_N. For H₀⊥c the linewidth increases more slowly as T_N is approached.