Tricritical behavior in a stacked triangular lattice Ising antiferromagnet CsCoBr3

CsCoBr3 is well described as a spin-1/2 Ising-like antiferromagnet (AF) on a stacked triangular lattice (STL). The combination of AF interactions and triangular coordination leads to geometrical frustration, which manifests itself in an unusual partially paramagnetic, long-range ordered state at low temperatures. We have studied both the order parameter as well as the fluctuations in the order parameter (the critical fluctuations), associated with this unusual magnetic phase transition at TN=(28.23±0.04) K, in a high quality single crystal sample of CsCoBr3 using neutron scattering techniques. Critical scattering measurements were performed through two equivalent ordering wave vectors (-13,-13,1) and (-23,-23,1). These measurements are well described by a single, anisotropic Lorentzian line shape over a wide range of about two decades in reduced temperature. The magnetic phase transition is characterized by a set of three critical exponents ν=(0.54±0.05), γ=(1.05±0.08), and β=(0.28±0.02), which are consistent with the scaling laws. These results are very close to expectations of tricritical behavior. They are inconsistent with expectations from both analytical theory and Monte Carlo simulation results relevant to the classical Ising model on a hexagonal lattice with isotropic AF interactions, which predict that this phase transition belongs to the 3D XY universality class. In light of experimental results, we have reexamined heat capacity measurements on the same sample and show that these results are also consistent with tricritical behavior.