Magnetostructural Phase Diagram of Multiferroic (ND 4 ) 2 FeCl 5 .H 2 O
Spin and polarization flop transitions are fascinating, especially when controlled by external stimuli like magnetic and electric field and accompanied by large material responses involving multiple degrees of freedom. Multiferroics like MnWO 4 , TbMnO 3 , and Ni 3 TeO 6 are flagship examples and owe their remarkable properties, for instance field control of polarization and polarization flops combined with spin helix reorientation, to the anisotropy and heavy centers that bring in spin-orbit coupling. The family of A 2 FeX 5 .H 2 O erythrosiderites (A = K, Rb, NH 4 ; B = Fe, Mn, Co; X = Cl, Br, H 2 O) drew our attention due to the rich chemical tuning possibilities, complex phase diagrams, and topological similarities to oxide multiferroics.1 (NH 4 ) 2 FeCl 5 .H 2 O is the flagship example (Fig. 1(a)). It displays a high temperature order-disorder transition involving long-range hydrogen bonding of the NH 4 + group and two successive low temperature magnetic transitions below which non-collinear magnetic order and ferroelectricity are established.1 In addition to the magnetically-induced electric polarization that arises below 6.9 K (P = 3 μC/m 2 along a and a smaller component along b), applied field reveals a peculiar hysteretic spin flop transition near 4.5 T above which polarization flops from the a- to the c-axis. There are elastic components as well. Taken together, these findings raise questions about the interactions that induce this behavior and whether additional non-equilibrium phases might be accessed under even higher magnetic fields.