Magnetic structures and phase transitions of Nickel-Bromine boracite Ni3B7O13Br. a neutron powder diffraction study

Abstract

The nuclear crystal structure and the magnetic ordering of the two ferroelectric/weakly ferromagnetic phases of Nickel-Bromine boracite with the Shubnikov groups of Pc'a21' (30-21K) and P1 (<21K)-postulated previously from magnetoelectric and optical data-were studied by neutron powder diffraction. The neutron spectra confirm the onset of magnetic ordering at 30 K but do not resolve the transition to the triclinic symmetry. The observed neutron intensities of the orthorhombic and triclinic phase, collected at 24 K and 8 K, respectively, were interpreted in terms of the magnetic space group Pc'a21' by an essentially antiferromagnetic ordering of the nickel ion moments. The magnetic structure refinements allowed to distinguish two different types of the magnetically ordered nickel ions among the three inequivalent nickel sites. Two possible magnetic structure models and a potential frustration phenomenon during spin ordering are discussed.