Magnetic phase diagram of A2[FeCl5(H2O)] (A = K, Rb, NH4)

Erythrosiderites with the formula A 2 Fe X 5 ⋅ H ₂O, where A = Rb, K, and (NH ₄) and X = Cl and Br are intriguing systems that possess various magnetic and electric phases, as well as multiferroic phases in which magnetism and ferroelectricity are coupled. In this report, we study the magnetic phase diagram of erythrosiderites as a function of superexchange interactions and magnetic anisotropies. To this end, we perform classical Monte Carlo simulations on magnetic Hamiltonians that contain five different superexchange interactions with single-ion anisotropies. Our phase diagram contains all magnetic ground states that have been experimentally observed in these materials. We argue that the ground states can be explained by varying the ratio of J 4 J 2 . For J 4 J 2 > 0 . 95 a cycloidal spins structure is stabilized as observed in (NH ₄) ₂FeCl 5 ⋅ H ₂O and otherwise a collinear spin structure is stabilized as observed in (K,Rb) ₂FeCl 5 ⋅ H ₂O. We also show that the difference in the single-ion anisotropy along a - and c - axes is essential to stabilize the intermediate state observed in (NH) ₂FeCl 5 ⋅ H ₂O.