MAGNETIC-ORDERING OF MN3B7O13X (X=CL, BR, I) BORACITES DETERMINED BY MAGNETIC MEASUREMENTS AND NEUTRON-DIFFRACTION

A new method for the preparation of (Mn3B7O13I)-B-11 powder sample, us ed for neutron diffraction, is presented. Magnetization measurements o n powder samples of Mn3B7O13X (X=Cl, Br, I) have shown that their Ni e l temperatures are 11+/-0.5 K for Mn3B7O13Cl, 14+/-0.5 K for Mn3B7P13B r and 26+/-0.5 K for Mn3B7O13I. The crystal and the magnetic structure s of Mn3B7O13I have been investigated by neutron diffraction. The neut ron diffraction pattern in the paramagnetic phase at 35 K was refined in space group Pca2(1)l'. The neutron diffraction data confirm the ons et of magnetic ordering at 26 K. The magnetic structure has been deter mined at 1.5 K in the magnetic space group Pc'a2(1)'. The magnetic mom ents of the three manganese sites order with a two-dimensional canted spin arrangement. The moments of two sites point along c with mu(2)(Mn -2)=mu(2)(Mn3)=3.8(2) mu(B)/atom while the Mn(1) site has a magnetic m oment of 5.4(2) mu(B)/atom located in the(101) orthorhombic plane, for ming an angle of 12.0(3) degrees with the a direction. The effect of t he antiferromagnetic/ferromagnetic compromise on the Mn-I-Mn pathways is discussed.