Er-driven incommensurate to commensurate magnetic phase transition of Fe in the spin-chain compound <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>BaErFeO</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:math>

Magnetic phase transitions and structures of the spin chain compound BaErFeO4 were investigated by measurements of magnetic properties and neutron diffraction. The lattice geometry of the orthorhombic crystal structure (space group Pnma) of the BaRFeO4 compounds (R = Dy–Yb, Y) supports frustrations which lead to multiple magnetic phase transitions with complex magnetic structures. BaErFeO4 undergoes three successive magnetic phase transitions at TN1 = 49 K, TN2 = 33.4 K and TN3 = 3.4 K. In contrast to the previously investigated BaRFeO4 (R = Y, Tm, Yb) compounds, all with incommensurate magnetic propagation vectors k1 = (0, 0, kz), BaErFeO4 is the first member in this series that shows a phase transition from an incommensurate (k1 below TN1) to a commensurate magnetic structure with k2 = (1/2, 0, 1/2) below TN2. In the crystal structure all magnetic ions are part of chains propagating along the b-axis. Below TN1, strong antiferromagnetic Fe-Fe spin-exchange coupling between square pyramidal (Fe1) and octahedral (Fe2) centres generates a collinear antiferromagnetic structure with a constant size of the ordered Fe moments and a constant magnetic phase inside each chain of Fe3+ cations. Exchange coupling between the Fe chains is much weaker. At TN2, 3d–4f exchange interactions induce an ordered moment at the Er3+ ions, which results in a change of the direction of the ordered Fe moments from the b-direction (above TN2) to inside the ac-plane (below TN2) and a change from an incommensurate (k1 above TN2) to a commensurate (k2 below TN2) antiferromagnetic structure. Towards lower temperature, 4f–4f exchange interactions become stronger and create at TN3 a constant magnetic phase inside each chain of Er3+ cations. At TN2 and at TN3, the magnetic susceptibility shows sharp decreases that coincide with large increases of the correlation length of the magnetic structure. The unique magnetic structures of BaErFeO4 are compared with those of other BaRFeO4 compounds by considering experimental and theoretical aspects.