  We present a minimal model that provides a description of the magnetic and
thermodynamic properties of \Eu. The model contains two exchange coupling
parameters, which are calculated using Density Functional Theory, and a local
easy axis magnetic anisotropy term. The classical ground state of the system is
a generalization of the well known 120$^\circ$ structure observed in triangular
antiferromagnets. Monte Carlo simulations show two phase transitions as a
function of the temperature. With increasing temperature, the system
transitions from the ground state into a high-entropy collinear
antiferromagnet, which in turn at higher temperatures presents a second order
transition to a paramagnetic state. A high enough external magnetic field
parallel to the anisotropy axis produces a spin-flop transition at low
temperatures. The field also reduces the temperature range of stability of the
collinear antiferromagnet phase and leads to a single phase transition as a
function of the temperature. The reported behavior of the specific heat, the
magnetization, and the magnetic susceptibility is in agreement with the
available experimental data. Finally, we present the magnetic phase diagrams
for magnetic fields parallel and perpendicular to the easy axis.
