Magnetic Structures and Magnetic Phase Diagram of the Mixed-Valence Iron Phosphate Fe7(PO4)6

Mixed-valence iron compounds are fascinating materials exemplified by ferrimagnetic Fe3O4, multiferroic LuFe2O4, and a pigment Fe4[Fe(CN)6]3. Mixed-valent iron (oxy)phosphates also provide a large variety of different connections between magnetic ions, which are responsible for complex magnetism. Fe7(PO4)6 (Fe32+Fe43+(PO4)6) has well-defined Fe2+ and Fe3+ sites and shows two antiferromagnetic transitions at TN1 = 47 K and TN2 = 16 K. Here, we investigated magnetic structures of Fe7(PO4)6 (at zero magnetic field) using neutron powder diffraction and constructed a temperature–magnetic-field phase diagram. Below TN1, magnetic propagation vector is k1 = (1/2, 0, 1/2), and magnetic moments on Fe3+ sites are nearly fully ordered; while moments on Fe2+ sites are significantly reduced. Below TN2, the second propagation vector appears k2 = (0, 1/2, 0) and coexists with k1; moments on Fe2+ sites are fully ordered (at 2 K) reaching 4.5 μB suggesting noticeable spin–orbital contributions. Coexistence of two propagation vectors and triclinic symmetry results in variations of total magnetic moments on each site. Magnetic measurements up to 300 kOe detected one field-induced transition near 55.5 kOe. Data showed that TN1 is nearly magnetic-field independent up to 90 kOe, while a more complex behavior was observed near TN2 at magnetic fields around 50 kOe.