Effect of temperature and antiferromagnetic ordering on the dielectric constants of MnO and MnF2

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Abstract
Temperature dependence (4.2-300K) of the static dielectric constant epsilon is reported for two antiferromagnets namely MnF2 (TN approximately=67.3K) and MnO (Tn approximately=118K) for the electric field E parallel and perpendicular to the spin easy axis. Comparison with the temperature variation of epsilon in the diamagnets ZnF2 and MgO shows that the phonon contribution in all four materials follows the equation epsilon (T)= epsilon (0)+A/(exp(h nu T/kBT)-1), nu T being the transverse optical-mode frequency. The magnetic contributions, epsilon /sub ///m and epsilon perpendicular to m for E parallel and perpendicular to the easy axes respectively, have the following characteristics: (i) in both MnF2 and MnO, epsilon perpendicular to m is negative and considerably larger than epsilon /sub ///m; (ii) in MnF2, epsilon /sub ///m is positive and varies nearly as sigma 2 ( sigma =order parameter) whereas in MnO, epsilon /sub ///m is negligible; and (iii) in MnF2, where accurate data on the magnetic energy um is available, epsilon perpendicular to m(T)/ epsilon perpendicular to m(0) varies as um(T)/um(0) both above and below TN (somewhat similar variation is observed in MnO). Despite several common features in these results for MnO and MnF2, the signs and magnitudes of epsilon /sub ///m and epsilon perpendicular to m cannot yet be explained.