Exchange Narrowing: Systematic Discrepancies between Theory and Experiment

Publisher Website
Google Scholar
Abstract
The strongly exchange‐narrowed Mn++ EPR in single‐crystal MnF2, KMnF3, and RbMnF3 has been studied at X band with T≫TN. The half‐widths at half‐maximum ΔHexp of the nearly Lorentzian line profiles are 260±10, 59±3, and 58±3 G, respectively. From the accurately known values of exchange and lattice parameters we have calculated the second and fourth moments using the full dipolar Hamiltonian and obtained the half‐width at half‐maximum ΔHth of an assumed truncated Lorentizan line profile. For MnF2, KMnF3, and RbMnF3 we find ΔHth=112.5, 20.8, and 20.8 G with ΔHexp/ΔHth=2.3±0.1, 2.8±0.2, and 2.8±0.2, respectively. Similarly, a comparison between theory and experiment for the exchange‐narrowed hyperfine‐broadened 19F NMR in these same materials yields ΔHexp/ΔHth=1.48±0.04, 2.20±0.11, and 2.10±0.10 for MnF2, KMnF3, and RbMnF3, respectively. The systematic nature of the discrepancies suggests that the decay of the spin autocorrelation functions are inadequately characterized by a single decay constant and that the stochastic processes are more complex than have hitherto been assumed.

This publication has 14 references indexed in Scilit: