Nuclear Magnetic Resonance in Antiferromagnetic MnF2under Hydrostatic Pressure

Abstract

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\ifmmode^\circ\else\textdegree\fi{}K, 20.4\ifmmode^\circ\else\textdegree\fi{}K, and 35.7\ifmmode^\circ\else\textdegree\fi{}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 N\'eel temperature. This deduction gives $(\frac{1}{A})(\frac{\mathrm{dA}}{\mathrm{dP}})=+(1.9\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}/(\mathrm{kg}/{\mathrm{cm}}^{2})$ and $(\frac{1}{{T}_{N}})(\frac{d{T}_{N}}{\mathrm{dP}})=+(4.4\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}/(\mathrm{kg}/{\mathrm{cm}}^{2})$. 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 ${\mathrm{F}}^{\ensuremath{-}}$ 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.