Nuclear Magnetic Resonance in Paramagnetic MnF2

Nuclear magnetic resonances of ${\mathrm{F}}^{19}$ in single crystals of paramagnetic Mn${\mathrm{F}}_2$ have been observed in the temperature region 68°K to 300°K. Shifts of the resonances from the predicted field, $\frac{\omega }{\gamma }$, were as large as eight percent. With the external magnetic field not in the $\mathrm{ab}$ plane, two separate resonances were observed corresponding to the two different fluorine positions in the unit cell. Both the shifts and the splittings were functions of magnetic field, temperature, and crystal orientation. As the temperature was increased, the line widths increased from 25 to 42 oe. These observations were explained by postulating a small degree of covalency in the ${\mathrm{F}}^{-}$ bond to the paramagnetic ${\mathrm{Mn}}^{++}$ ion. Each bond includes 0.48±0.02% fluorine $2s$ character. The contributions of the $2p_{\sigma }$ and $2p_{\pi }$ orbitals are discussed and the effects of the covalency upon the long-range order and the superexchange mechanism are considered.