Narrowing of ESR lines by an exchange field in a superconductor

An analysis is made of narrowing of inhomogeneously and dipole-broadened ESR lines by a long-range exchange in a superconductor. The mechanism of this narrowing differs qualitatively from the well-known Anderson-Weiss mechanism when narrowing is due to a rapid exchange of states between spins which are in different local fields. In the present case a dynamic coupling between spins is provided by an exchange field, which alters greatly the narrowing behavior. A simple formula is derived for the width of an exchange-narrowed ESR line: it generalizes the familiar Anderson-Weiss formula and combines the field and fluctuation narrowing effects. Estimates are obtained of the magnitude of the narrowing and these are used to account for the strong narrowing of an ESR line as a result of the superconducting transition in the LaEr system. A numerical analysis is made of the resonance line profile and it is shown that the line asymmetry as a result of the field narrowing increases the asymmetry of the detected ESR signal, as found experimentally