Direct Mn55 NMR Absorption in Antiferromagnetic KMnF3

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Abstract
The direct observation of Mn55 nuclear magnetic resonance absorption in the nearly cubic antiferromagnet KMnF3 is reported. The striking features of the resonance are its unusual intensity and its strong temperature and field dependence. Both effects are associated with the strong coupling between nuclear and electronic modes in magnetic materials with low anisotropy and large nuclear magnetization. The nuclear linewidth at 4.2°K is 740 kc/sec between points of half‐maximum absorption. This width is in agreement with that expected from the Suhl‐Nakamura indirect coupling between nuclear spins. At lower temperatures the linewidth increases greatly, approaching nearly 3 Mc/sec at 2.1°K. This unexpected increase in linewidth is attributed to a low‐temperature inhomogeneity in the antiferromagnetic excitation spectrum.