Nuclear Magnetic Resonance and Magnetic Ordering in NiF2

Measurements of the nuclear magnetic resonance of the ${\mathrm{F}}^{19}$ nucleus in single crystals of Ni${\mathrm{F}}_2$ have been made at temperatures between 4.2°K and 298°K. In the paramagnetic state at 298°K, for arbitrary directions of $H_0$ two resonances were observed, while for $H_0$ in a (100) plane only one resonance was observed as expected from the crystal symmetry. The shift of the resonances from the normal resonance field $\frac{\omega }{{\gamma }_N}$ allows one to calculate three independent parameters of the hyperfine interaction between the ${\mathrm{F}}^{19}$ nuclei and the magnetic electrons. In conjunction with measurements made in the antiferromagnetic state, it was possible to determine the individual components of the hyperfine interaction in units of ${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ as ${A_s}^{\mathrm{I}}=36.4\mathrm{}\pm 1.8$, ${A_s}^{\mathrm{II}}=42.1\mathrm{}\pm 2.1$, ${A_{\sigma }}^{\mathrm{I}}=9.1\mathrm{}\pm 0.9$, and ${A_{\sigma }}^{\mathrm{II}}=9.9\mathrm{}\pm 0.9$. Superscripts refer to the two different bonds with ${\mathrm{Ni}}^{++}$ and subscripts $s$ and $\sigma$ refer to isotropic and anisotropic contributions. From these we calculate that the fractions of unpaired spin in the corresponding fluoride ion $s$ and $p_{\sigma }$ orbitals are ${f_s}^{\mathrm{I}}=(0.46\mathrm{}\pm 0.02)\%$, ${f_s}^{\mathrm{II}}=(0.54\mathrm{}\pm 0.03)\%$, ${f_{\sigma }}^{\mathrm{I}}=(4.1\mathrm{}\pm 0.4)\%$, and ${f_{\sigma }}^{\mathrm{II}}=(4.5\mathrm{}\pm 0.4)\%$. Since geometrical considerations almost completely forbid magnetic $p_{\pi }$ interactions in Ni${\mathrm{F}}_2$, there is a negligible cancellation of $p_{\sigma }$ interactions by the