Zero-Field Manganese Nuclear Magnetic Resonance in Antiferromagnetic Manganese Fluoride

The zero-field NMR of ${\mathrm{Mn}}^{55}$ has been observed directly in the antiferromagnetic state of Mn${\mathrm{F}}_2$. A single resonance, with linewidth $\Delta {\nu }^{55}\simeq 1.3$ Mc/sec, was observed in the frequency range of 650-675 Mc/sec and the temperature range of 1.3-20.5°K. The extrapolated ${\mathrm{Mn}}^{55}$ NMR frequency at 0°K is found to be ${{\nu }_0}^{55}=671.4\mathrm{}\pm 0.2$ Mc/sec. Combining the 0°K ${\mathrm{Mn}}^{55}$ NMR frequency together with the dipolar field $H_{\mathrm{dip}}=+5.770\mathrm{}$ kOe and the hyperfine coupling constant $A^{55}=-(90.78\mathrm{}\pm 0.3)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, measured for ${\mathrm{Mn}}^{2+}$ in Zn${\mathrm{F}}_2$, gives a value for the zero-point spin deviation of $1-\frac{〈S〉}{S}=(0.43\mathrm{}\pm 0.34)\%$. This value is to be compared with the value predicted by spin-wave theory of 2.37%. The observed temperature dependence of the ${\mathrm{Mn}}^{55}$ NMR frequency agrees, within experimental error, with the temperture dependence of the ${\mathrm{F}}^{19}$ zero-field NMR in antiferromagnetic Mn${\mathrm{F}}_2$. Upper and lower limits of 1300 kc/sec and 600 kc/sec are placed on the contribution to the ${\mathrm{Mn}}^{55}$ NMR linewidth in antiferromagnetic Mn${\mathrm{F}}_2$ by the Suhl-Nakamura interaction.