Random Fields and the Partially Paramagnetic State of ${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{o}}_{0.83}{\mathrm{M}\mathrm{g}}_{0.17}{\mathrm{B}\mathrm{r}}_3$: Critical Scattering Study

We have performed measurements of the critical neutron scattering on ${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{o}}_{0.83}{\mathrm{M}\mathrm{g}}_{0.17}{\mathrm{B}\mathrm{r}}_3$, a dilute stacked triangular lattice (STL) Ising antiferromagnet (AF). A two component line shape associated with the critical fluctuations appears at a temperature coincident with $T_{N1}$ observed in pure ${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{o}\mathrm{B}\mathrm{r}}_3$. Such scattering is indicative of fluctuations in prototypical random field Ising model (RFIM) systems. The random field domain state arises in this case due to geometrical frustration within the STL Ising AF, which gives rise to a three sublattice Néel state, in which one sublattice is disordered. Magnetic vacancies nucleate AF domains in which the vacancies reside on the disordered sublattice thereby generating a RFIM state in the absence of an applied magnetic field.