Hyperfine interactions at ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$(${\leftarrow }^{111}$In) atoms on ${\mathrm{Fe}}^{3+}$ sites in α-${\mathrm{Fe}}_2$${\mathrm{O}}_3$ 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 ${}_{}{}^{111}{}_{}{}^{}\to _{}^{111}{}_{}{}^{}$Cd 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 (${\mathit{T}}_{\mathit{M}}$=260 K). On the other hand, the spectra below ${\mathit{T}}_{\mathit{M}}$ 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 ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}_{}^{2+}{}_{}{}^{}$ ions in the electronic ground state. (1) The hyperfine magnetic field ${\mathit{H}}_{\mathrm{hf}}$ changes with temperature above ${\mathit{T}}_{\mathit{M}}$ in proportion to the magnitude of the magnetic moments of the ${\mathrm{Fe}}^{3+}$ ions in this oxide and is discontinuous in its value at ${\mathit{T}}_{\mathit{M}}$ (72.5 kOe above ${\mathit{T}}_{\mathit{M}}$ and 67.6 kOe below it). (2) The EFG is axially symmetric and exhibits no detectable change at ${\mathit{T}}_{\mathit{M}}$. (3) The angle between ${\mathit{H}}_{\mathrm{hf}}$ and the principal axis of the EFG is virtually 90° and 0°, respectively, above and below ${\mathit{T}}_{\mathit{M}}$. The dominant part of ${\mathit{H}}_{\mathrm{hf}}$ is reasonably explained as being the supertransferred one ${\mathit{H}}_{\mathrm{STHF}}$ originating from nine adjacent ${\mathrm{Fe}}^{3+}$ ions on the neighboring cation layers, and its direction is found to be parallel to the magnetic moments of these ${\mathrm{Fe}}^{3+}$ ions. By subtracting the contribution of the dipole magnetic field from ${\mathit{H}}_{\mathrm{hf}}$, ${\mathit{H}}_{\mathrm{STHF}}$ is estimated to be about 75 and 65 kOe, respectively, above and below ${\mathit{T}}_{\mathit{M}}$. The smaller value below ${\mathit{T}}_{\mathit{M}}$ is tentatively explained by a modification of spin alignment of the nearby ${\mathrm{Fe}}^{3+}$ ions induced as an impurity effect of ${\mathrm{Cd}}^{2+}$. The apparently enhanced aftereffects below ${\mathit{T}}_{\mathit{M}}$ also seem to reflect this modified spin alignment.

• Received 5 September 1989

DOI:https://doi.org/10.1103/PhysRevB.41.6124