Noncollinear Polar Magnet Fe₂(SeO₃)₃(H₂O)₃ with Inequivalent Fe Sites

The emergence of novel magnetic states becomes more likely when the inversion symmetry of the crystal field, relative to the center between two spins, is broken. We propose that placing magnetic spins in inequivalent sites in a polar lattice can promote a realization of nontrivial magnetic states and associated magnetic properties. To test our hypothesis, we study Fe₂(SeO₃)(H₂O)₃ as a model system that displays two distinct Fe(1) and Fe(2) magnetic sites in a polar structure (R3c space group). At low fields µ₀H 0.06 T, the material undergoes an antiferromagnetic ordering with T_N1 = 77 K and a second transition at T_N2 4 K. At µ₀H 0.06 T and 74 K T 76 K, a positive entropy change of approximately 0.12 mJ mol^-1 K^-1 can be associated with a metamagnetic transition to possibly nontrivial spin states. At zero field, Fe(1) is nearly fully ordered at T 25 K while Fe(2) features magnetic frustration down to T = 4 K. The magnetic ground state, a result corroborated by single-crystal neutron diffraction and ⁵⁷Fe Mössbauer spectroscopy, is a noncollinear antiparallel arrangement of ferrimagnetic Fe(1)-Fe(2) dimers along the c-axis. The results demonstrate that placing distinct magnetic sites in a polar crystal lattice can enable a new pathway to modifying spin, orbital, and lattice degrees of freedom for unconventional magnetism.