Magnetic phase diagram of $A_{2}$[FeCl$_{5}$(H$_{2}$O)] ($A$ = K, Rb, NH$_{4}$)

TL;DR

This study maps the magnetic phase diagram of erythrosiderites A2FeX5H2O with A = K, Rb, NH4, revealing how superexchange interactions and anisotropies determine their magnetic ground states, including cycloidal and collinear structures.

Contribution

It provides a comprehensive theoretical analysis of the magnetic phases in erythrosiderites using Monte Carlo simulations, explaining experimental observations through superexchange ratios and anisotropy effects.

Findings

Cycloidal spins stabilized for J4/J2 > 0.95.

Collinear spins stabilized for J4/J2 < 0.95.

Single-ion anisotropy differences are crucial for intermediate states.

Abstract

Erythrosiderites with the formula A2FeX5H2O, where A = Rb, K, and (NH4) 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. 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 J4/J2. For J4/J2 > 0.95 a cycloidal spins structure is stabilized as observed in (NH4)2FeCl5H2O and otherwise, a collinear spin structure is stabilized as observed in (K,Rb)2FeCl5H2O. 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)2FeCl5H2O.