Far-Infrared Magnetic Resonance in NiF2

Measurements of the far-infrared transmission of the canted-spin antiferromagnet Ni${\mathrm{F}}_2$ using the technique of Fourier-transform spectroscopy are described. Resonant absorptions were found at ${\nu }_1=3.33\mathrm{}\pm 0.05$ ${\mathrm{cm}}^{-1\mathrm{}}$ and ${\nu }_2=31.14\mathrm{}\pm 0.1$ ${\mathrm{cm}}^{-1\mathrm{}}$ with applied field $H=0\mathrm{}$ and temperature $T\approx 0$. Using the resonance relations derived by Joenk and Bozorth from Moriya's model, these give anisotropy and exchange parameters $E=1.66\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and 8 $JD=482.2\mathrm{}$ ${\mathrm{cm}}^{-2\mathrm{}}$. A good fit is obtained to the measured magnetic-field dependence of both modes for $g_{\perp }=2.35\mathrm{}$ which, along with the measured perpendicular susceptibility, gives $8J=125$ ${\mathrm{cm}}^{-1\mathrm{}}$ and $D=3.86\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The temperature dependence of ${\nu }_2$ was measured between 1.2 and 65°K ($T_N=73.2\mathrm{}$°K) and is found to be in agreement with the Zener theory when the temperature dependence of the sublattice magnetization is estimated from neutron-diffraction intensity measurements. The linewidth and strength of mode 2 were also measured as a function of $T$, and are discussed qualitatively.