Eur. Phys. J. B 22, 429-442 (2001)
https://doi.org/10.1007/s100510170093

Crystal and magnetic structures of the oxyphosphates MFePO₅ (M = Fe, Co, Ni, Cu). Analysis of the magnetic ground state in terms of superexchange interactions

¹ Faculté des Sciences, Département de Physique, Av. Ibn Batouta, BP 1014 Rabat, Maroc
² Laboratoire Léon Brillouin (CEA-CNRS), CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France

Corresponding author: ^a juan@llb.saclay.cea.fr

Received: 15 December 2000
Revised: 25 June 2001
Published online: 15 August 2001

Abstract

We have studied in detail the crystal and magnetic structures of the oxyphosphates MFePO₅ (M: divalent transition metal) using neutron powder diffraction as a function of temperature. All of them are isomorphic to the mixed valence compound α-Fe₂PO₅ with space-group Pnma. No disorder exists between the two metallic sites. The M²⁺O₆ octahedra share edges between them and faces with Fe³⁺O₆ octahedra building zigzag chains running parallel to the b-axis that are connected by PO₄ tetrahedra. The topology of this structure gives rise to a complex pattern of super-exchange interactions responsible of the observed antiferromagnetic order. The magnetic structures are all collinear with the spin directed along the b-axis except for M = Co. The experimental magnetic moments of Cu⁺² and Ni²⁺ correspond to the expected ionic value, on the contrary the magnetic moment of Fe³⁺ is reduced, probably due to covalence effects, and that of Co²⁺ is greater than the spin-only value indicating a non negligible orbital contribution. Using numerical calculations we have established a magnetic phase diagram adapted for this type of crystal structure and determined the constraints to be satisfied by the values of the exchange interactions in order to obtain the observed magnetic structure as the ground state.