Exchange Narrowing: Systematic Discrepancies between Theory and Experiment

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.