MAGNETISM IN LUDLAMITE, FE3(PO4)2.4H2O

The magnetic structure of the weak ferromagnet ludlamite, Fe 3(PO4)2.4H2O, has been re-examined by both polarized and unpolarized neutron diffraction from single crystal samples of the mineral. The crystal structure has been refined from three dimensional data collected at 20 K, the unit cell being monoclinic, a=10.542(8), b=4.650(2), c=9.278(7) AA, beta =100.75(7) degrees at this temperature. At 4.2 K the contributions to the weak b-axis ferromagnetic moment produced by an external field of 1.5 T parallel to (010), determined from (h0l) polarized beam flipping ratios, are 1.73(3) and 0.62(2) mu B for the Fe2+ ions at the centre of symmetry and in a general position, respectively. Limited three-dimensional unpolarized data collected at 4.0 K were used to refine the complete magnetic structure. The moments on the closely linked triads of ions are predominantly ferromagnetically coupled, with the inter-moment angle of 26(2) degrees . The total moment at 4 K and in zero field is best fitted by the values: Fe1, 4.51(6) mu B at angles Theta =30(2) and phi =48(2) degrees ; Fe2, 4.00(5) mu B at Theta =6(2), phi =90(7) degrees , where Theta is measured from the c axis and phi from the a-c plane. The orientations and magnitudes of the ionic moments are related to the competition between exchange and spin-orbit coupling.