Antiferromagnetic-Resonance Linewidths in Mn<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">F</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

The fundamental relaxation mechanisms by which the uniform and magnetostatic modes decay in an antiferromagnet are identified by antiferromagnetic-resonance linewidth observations in Mn${\mathrm{F}}_{2}$ at 4.2\ifmmode^\circ\else\textdegree\fi{}K in fields of 85 kOe. The dependence of the linewidth---0.2 Oe in the narrowest instance---on sample geometry, surface preparation, and impurity concentrations makes quantitative comparison with the Loudon-Pincus theory possible. The predominant relaxation process is surface pit scattering into the degenerate spin-wave manifold in all but the most impure crystals.

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