Large magnetocaloric effect in the frustrated antiferromagnet EuIr$_2$P$_2$.

We present a theoretical analysis of the magnetocaloric effect in the frustrated antiferromagnet EuIr$_2$P$_2$. Based on a magnetic Hamiltonian with parameters estimated using total energy density functional theory calculations, we perform classical Monte Carlo simulations to obtain the magnetocaloric effect as a function of the temperature and the external magnetic field. We find a large magnetic entropy change $\Delta S_M^{\text{max}} \sim 5.14$ J kg$^{-1}$K$^{-1}$ at $T=2.45$K for a moderate change in the external magnetic field $\Delta H=1.2$ Tesla. Magnetic frustration in EuIr$_2$P$_2$ leads to a rich phase diagram with a first order (spin-flop) transition between two antiferromagnetic phases and a continuous transition between a non-collinear antiferromagnetic phase and a high entropy collinear antiferromagnet. A paramagnetic phase for high fields or temperatures completes the phase diagram. A phenomenological Landau functional analysis suggests that the four phases meet at a multicritical point $(T_m,H_m)$, and provides a simplified description of the phase diagram and of the magnetocaloric effect. The highest value of $\Delta S_M$ is obtained for a temperature $T\sim T_m$ but it persists with similar values for a broad range of temperatures $T\lesssim T_m$.