Low-Temperature Behavior of the Hyperfine Splitting and the Absolute Sign of the Hyperfine Constant

An anomalous temperature dependence of the splitting between hyperfine lines for ${\mathrm{Mn}}^{2+}$ in Sr${\mathrm{F}}_2$ has been observed at low temperatures. This behavior is explained in terms of high electron spin polarization at low temperatures and the second-order hyperfine contribution. It is shown that for Sr${\mathrm{F}}_2$:${\mathrm{Mn}}^{2+}$ the actual hyperfine coupling constant $A$ is the same within experimental error at $T=77,4.\mathrm{}2, \mathrm{and} 1.3$ K. In general this type of anomalous behavior of the hyperfine splitting can be used to determine the absolute sign of $A$ for a paramagnetic ion with electronic spin $S\ge 1$ when the fine-structure splitting is vanishing or very small. For ${\mathrm{Mn}}^{2+}$ in Sr${\mathrm{F}}_2$ the sign of $A$ is determined to be negative.