Antisymmetric Exchange and Exchange-Narrowed Electron-Paramagnetic-Resonance Linewidths

The Dzialoshinsky-Moriya (DM) antisymmetric exchange interaction between superexchange-coupled ions in antiferromagnetic salts is shown to contribute to the exchanged-narrowed electron-paramagnetic-resonance (EPR) linewidth in the paramagnetic state. The contribution of antisymmetric exchange to the second and fourth moments has been calculated and is analogous to that from symmetric anisotropic exchange and the dipole-dipole interaction. It is demonstrated that for Cu${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$ · 4${\mathrm{H}}_2$O, both the linear temperature dependence of the EPR linewidth and the temperature-independent exchange-narrowed linewidth (extrapolated to $T=0\mathrm{}$ °K from the paramagnetic region where $\mathrm{kT}>J$) can be explained by the DM antisymmetric exchange interaction and are mutually consistent with one another. The analysis yields a value of the antisymmetric exchange $\frac{\left|{\overrightarrow{\mathfrak{D}}}_{\mathrm{jk}}\right|}{g{\mu }_B}$ for Cu${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$ · 4${\mathrm{H}}_2$O of order 2700 Oe and a second moment $M_2$ more than two orders of magnitude larger than the contribution to $M_2$ from the dipoledipole interaction. Several other magnetic salts where the DM antisymmetric exchange interaction might contribute to the EPR linewidth are considered.