Magnetic Structures and Magnetic Phase Diagram of the Mixed-Valence Iron Phosphate Fe ₇ (PO ₄ ) 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 <i>T</i>_N1 = 47 K and <i>T</i>_N2 = 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 <i>T</i>_N1, magnetic propagation vector is <b><i>k</i></b>_<b>1</b> = (1/2, 0, 1/2), and magnetic moments on Fe³⁺ sites are nearly fully ordered; while moments on Fe²⁺ sites are significantly reduced. Below <i>T</i>_N2, the second propagation vector appears <b><i>k</i></b>_<b>2</b> = (0, 1/2, 0) and coexists with <b><i>k</i></b>_<b>1</b>; moments on Fe²⁺ sites are fully ordered (at 2 K) reaching 4.5 μ_B 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 <i>T</i>_N1 is nearly magnetic-field independent up to 90 kOe, while a more complex behavior was observed near <i>T</i>_N2 at magnetic fields around 50 kOe.