Cation-Cation Interaction Contributions to the Hyperfine Interaction. The "Supertransferred Hyperfine Interaction"

The change in hyperfine field at a central cation site when the nearest-neighbor cation sites are occupied by magnetic ions is calculated. Two mechanisms dominate this effect: one an orthogonalization of the neighboring magnetic ion's $\sigma$ orbitals to the inner $s$ shells of the central cation; the other a charge transfer process from the neighboring magnetic ion's $\sigma$ orbitals to the unoccupied $s$ orbitals on the central cation. Specific application is made to ${\mathrm{Mn}}^{2+}$ in two hosts, KMg${\mathrm{F}}_3$ and MgO. It is shown that the hyperfine field at th ${\mathrm{Mn}}^{2+}$ site increases by 10.8 kG and (more approximately) 21.1 kG, respectively, for KMn${\mathrm{F}}_3$ and MnO, as compared with the values in the dilute salts. This increase, together with the observed nuclear resonance hyperfine-field frequency in the antiferromagnetic state, allows for a reduction in sublattice magnetization, due to zero-point motion, of 3.2% for KMn${\mathrm{F}}_3$ and 2.5% in MnO, in essential agreement with the predictions of spin-wave theory. A qualitative discussion is given of similar changes in ${\mathrm{Fe}}^{3+}$ salts; and of ${\mathrm{Cr}}^{3+}$ salts where the effect is shown to be of opposite sign.