One-dimensional antiferromagnetism in the piezoelectric layered compound BaMnF4

The magnetic properties of BaMnF4 have been studied by means of magnetic susceptibility using SQUID magnetometry and neutron diffraction. In the antiferromagnetic (AF) phase (AMP) below TN ≈ 27 K, the Mn2+ spins are uniaxially aligned parallel to the b-axis. Under an applied magnetic field above a critical strength the spins are flopped and uniaxially aligned perpendicularly to the b-axis. In the intermediate magnetic phase (IMP) between TN and TA ≈ 45 K, the Mn2+ spins are easily aligned by an applied magnetic field B, 5 mT⩽B⩽ 2 T, although the spontaneous spin order is uniaxially AF. From neutron diffraction in zero applied magnetic field it is found that three-dimensional AF spin order (3-DAFS) of Mn2+ ions in the AMP occurs in a magnetic unit cell a×2b×2c, where a, b and c are the lattice constants of the room-temperature phase. Within TN ⩽ T ⩽ TA, plane-like magnetic diffuse scattering with a reduced wavenumber qM=(ξ,ξ,12)M; 0.0⩽ξ⩽1.0 suggests that BaMnF4 has an one-dimensional AF spin order (1-DAFS) of the Mn2+ ions in the IMP. It arises from superexchange interaction between the Mn2+ spins within chains of MnF6 octahedra along the c-axis, but neither from intrachain superexchange interaction between neighboring spins in the zigzag-like (–Mn–F–Mn–F–Mn–) chains along the incommensurate a-axis nor from interplane AF interaction between pseudo two-dimensional MnF6 networks (folding screens), which are alternatively stacked along the b-axis. The 3-DAFS completes the AF interaction among the MnF6 chains in the folding screen and among the folding screens. Structural and dielectric anomalies and their relationship to magnetic order is briefly discussed.