Nuclear Magnetic Resonance in Antiferromagnetic MnF2under Hydrostatic Pressure

The nuclear magnetic resonance frequency of the ${\mathrm{F}}^{19}$ nucleus in antiferromagnetic Mn${\mathrm{F}}_2$, in zero external field, has been measured as a function of pressure at 4.2°K, 20.4°K, and 35.7°K using a new type very high frequency variable frequency spectrometer. From these measurements we have deduced the pressure dependence of the hyperfine coupling constant ($A$) between the manganese electrons and the fluorine nucleus, and the pressure dependence of the Néel temperature. This deduction gives $\left(\frac{1}{A}\right)\left(\frac{\mathrm{dA}}{\mathrm{dP}}\right)=+(1.9\mathrm{}\pm 0.1)\times {10}^{-6\mathrm{}}/(\mathrm{kg}/{\mathrm{cm}}^2)$ and $\left(\frac{1}{T_N}\right)\left(\frac{d{T}_N}{\mathrm{dP}}\right)=+(4.4\mathrm{}\pm 0.3)\times {10}^{-6\mathrm{}}/(\mathrm{kg}/{\mathrm{cm}}^2)$. We have also measured the compressibility of Mn${\mathrm{F}}_2$. The magnitude and pressure dependence of $A$ is explained using the theories of Mukherji and Das, and Marshall and Stuart, which permit a calculation of the dependence of $A$ on the interatomic distances, starting from the Hartree-Fock self-consistent field wave functions for ${\mathrm{Mn}}^{2+}$ and ${\mathrm{F}}^{-}$ with the ${\mathrm{Mn}}^{2+}$ wave functions properly adjusted to bring it into agreement with neutron scattering form factor measurements. The theory is in very good agreement with the experimental results.