Field and temperature dependence of critical magnetic relaxation in the anisotropic ferromagnet CrBr3

We present measurements of the dynamic longitudinal susceptibility near the Curie temperature of Cr${\mathrm{Br}}_3$ in magnetic fields up to 30 mT applied parallel to the easy magnetic axis. The frequency dependence of $\chi$ is monodispersive except very close to $T_c$, where a second unknown relaxation process comprising a small part of the total $\chi$ appears at low frequencies. Above $T_c$ the critical slowing down of the main (fast) process is explained with the help of recent mode-coupling results as arising from spin-spin relaxation in an uniaxial ferromagnet. With rising magnetic field the relaxation rate increases steeply below the dipolar crossover ($\epsilon <0.03\mathrm{}$), while above it a gradual decrease is found. Both observations seem to be inconsistent with existing ideas about the field effect on critical relaxation. Below $T_c$ the fast (para-) process is observed to speed up again.