Random Fields and the Partially-Paramagnetic State of $CsCo_{0.83}Mg_{0.17}Br_3$: A Critical Scattering Study

Critical neutron scattering measurements have been performed on CsCo$_{0.83}$Mg$_{0.17}$Br$_3$, a dilute stacked triangular lattice (STL) Ising antiferromagnet (AF). A two component lineshape associated with the critical fluctuations appears at a temperature coincident with T$_{N1}$ observed in pure CsCoBr$_3$. 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 3 sublattice Neel 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.