Field Dependence of the Magnetization and Spin-Wave Correlations in Ferromagnetic CrBr3

Using the ${\mathrm{Cr}}^{53}$ NMR, the field dependence of the magnetization $M(T_0,H)$ at elevated temperatures of single-crystal ferromagnetic Cr${\mathrm{Br}}_3$ has been determined. While an accurate fit to the $M(T_0,H)-\mathrm{v}\mathrm{s}-H$ data can be obtained using a two-exchange-parameter first-order renormalized spin-wave theory, the parameters required differ appreciably from those required for an accurate fit to the previously obtained $M(T,0\mathrm{})\mathrm{}-\mathrm{v}\mathrm{s}-T$ data with the same theory. A $t$-matrix two-parameter-model theory, correct to lowest order in the magnon density, but to all orders in the magnon-magnon interaction, was constructed. Although both the first-order and $t$-matrix corrections to the spin-wave energies are sizable, the full $t$-matrix results only change the large first-order corrections to $M(T,H)\mathrm{}$ by 15%. However, even if one employs the full $t$-matrix renormalization there are no pairs of values of the two exchange parameters which simultaneously fit the $M(T,0\mathrm{})\mathrm{}-\mathrm{v}\mathrm{s}-T$ and $M(T_0,H)-\mathrm{v}\mathrm{s}-H$ data. We attribute the inability of the more sophisticated theory to provide agreement with measurements of more than one thermodynamic function to be an inadequacy of the two-parameter model rather than an intrinsic failure of the theoretical approach.