A NEUTRON-DIFFRACTION STUDY OF THE MAGNETIC-STRUCTURE AND PHASE-TRANSITION IN NI-3(11)B7O13CL BORACITE

The nuclear and magnetic structures and the magnetic phase transition of (Ni3B7O13Cl)-B-11 [Ni-Cl] boracite have been investigated by means of high resolution and high flux neutron powder diffraction between 1. 5 and 30 K. The temperature dependence of the magnetic intensities sho ws an unsharp onset at T-N=9 K, indicating the existence of a second-o rder magnetic phase transition. Refinements of the magnetic structure give rise to the magnetic space group Pc'a2(1)' for the magnetic phase below T-N, with the magnetic modes C-x, F-y and A(z) for the three Ni sublattices. The three Ni sites display two kinds of magnetic moment with different values and orientations: Ni(1) has a moment parallel to the main antiferromagnetic axis along [100](or), while Ni(2) and Ni(3 ) have their antiparallel moments along [010](or)-directions. The tota l corresponds to a coplanar two-dimensionally canted antiferromagnetic arrangement, which is characteristic for the magnetic structure of Ni -Cl. The refined magnetic moment values at 1.5 K are: mu(1x)=1.65(2) m u(B) for Ni(1), and mu(2y)=-mu(3y)=0.79(4) mu(B) for Ni(2) and Ni(3), respectively, showing a weak magnetic superexchange interaction compar ed with the spin only moment 2 mu(B)/Ni2+ observable by neutrons. The magnetic ordering process and the possible frustration effects due to the interwoven configurations between the three Ni sites and the forma tion of isolated Ni3Cl groups, are discussed based on the proposed mag netic structure.