Internal Field Studies of Ferromagnetic CrI3by Means of the Mössbauer Effect inI129

The Mössbauer effect in the 27.8-keV transition of ${\mathrm{I}}^{129}$ was used to measure the coupling constants of the internal fields of the insulating ferromagnet Cr${\mathrm{I}}_3$ above and below the Curie temperature ($T_C$). Above $T_C$, at 78°K, the following parameters were obtained: $e^{2}qQ\left({\mathrm{I}}^{127}\right)=+662\mathrm{}\pm 8$ Mc/sec; $\eta =0.35\mathrm{}\pm 0.05$; isomer shift =+0.23±0.05 mm/sec with respect to a ${\mathrm{Zn}}^{66}$ ${\mathrm{Te}}^{129}$ ($T_{\frac{1}{2}}=70\mathrm{}$ m) source at 80°K. Below $T_C$, at 21°K, we deduce the following: $H_i=25\mathrm{}\pm 5$ kOe; $cos\beta =0.45\mathrm{}\pm 0.05$. The $e^2\mathrm{qQ}$ value at 21°K was 727 Mc/sec, 10% greater than that at 78°K.