TIME-DIFFERENTIAL PERTURBED-ANGULAR-CORRELATION STUDY OF HYPERFINE INTERACTIONS AT CD-111([-IN-111) IN ALPHA-FE2O3

Hyperfine interactions at Cd111(111In) atoms on Fe3+ sites in -Fe2O3 are studied in a wide temperature range from 85 to 987 K by means of time-differential perturbed-angular-correlation measurements. The observed hyperfine interaction is unique at high temperatures above 700 K but is distributed at lower temperatures owing to the aftereffects of 111111Cd electron-capture decay. The spectra modified by the aftereffects are analyzed successfully with a static distribution of the electric field gradient (EFG) in the temperature region above the Morin temperature (TM=260 K). On the other hand, the spectra below TM are analyzed with a large distribution of hyperfine frequencies, which diminishes rapidly in a short period of about 40 ns. From these analyses the following are found for the hyperfine interaction at Cd2+111 ions in the electronic ground state. (1) The hyperfine magnetic field Hhf changes with temperature above TM in proportion to the magnitude of the magnetic moments of the Fe3+ ions in this oxide and is discontinuous in its value at TM (72.5 kOe above TM and 67.6 kOe below it). (2) The EFG is axially symmetric and exhibits no detectable change at TM. (3) The angle between Hhf and the principal axis of the EFG is virtually 90°and 0°, respectively, above and below TM. The dominant part of Hhf is reasonably explained as being the supertransferred one HSTHF originating from nine adjacent Fe3+ ions on the neighboring cation layers, and its direction is found to be parallel to the magnetic moments of these Fe3+ ions. By subtracting the contribution of the dipole magnetic field from Hhf, HSTHF is estimated to be about 75 and 65 kOe, respectively, above and below TM. The smaller value below TM is tentatively explained by a modification of spin alignment of the nearby Fe3+ ions induced as an impurity effect of Cd2+. The apparently enhanced aftereffects below TM also seem to reflect this modified spin alignment. © 1990 The American Physical Society.