The first study of antiferromagnetic eosphorite-childrenite series (Mn_1−xFe_x)AlP(OH)₂H₂O (x=0.5)

This study presents for the first time the antiferromagnetic structure of the eosphorite-childrenite series (Mn_1-xFe_x)AlPO₄(OH)₂H₂O (x=0.5), based on neutron single crystal diffraction at 3 K in combination with group theoretical representation analysis. The new magnetic structure is described in the magnetic space group P_Cmnb, maintaining the atomistic unit cell size (a×b×c) with a ~6.9 Å, b ~10.4 Å, c ~13.4 Å. Mn-rich and Fe-rich zones within solid solution crystals are expanded up to several hundred micrometers, as seen in electron microprobe and polarisation microscopy. Magnetic susceptibility and specific heat measurements on two different eosphorite-childrenite crystals show the magnetic transition temperature between 6.5 K and 6.8 K as the Mn²⁺/Fe²⁺ ratio varies over single compositional zones. Below the Néel temperature, a magnetic field between 1.5 T and 2 T parallel to the a-axis causes a 180° spin-flip, reaching the saturation (5.25 μ_B pfu) toward high magnetic fields.