Sinusoidal magnetic structure in a three-dimensional antiferromagnetic Co2(OH)AsO4: Incommensurate-commensurate magnetic phase transition

Abstract

Co2 OH AsO4 has been prepared by hydrothermal synthesis and characterized from x-ray and neutron powder diffraction. The structure consists of a three-dimensional framework in which Co 1 O5-trigonal bipyramid dimers and Co 2 O6-octahedra chains are simultaneously present. The magnetic structure has been determined by neutron D2B and D1B powder-diffraction data. Below 22 K, the Co2 OH AsO4 phase shows an incommensurate antiferromagnetic structure along the b direction. The propagation vector 0, ,0 is temperature dependent with a value of =0.430 at the lowest temperature 1.8 K . Magnetization measurements of Co2 OH AsO4 show a complex magnetic behavior with the presence of three different signals. Between 6 and 21 K, a strong dependence of the magnetic field is observed with a shift of the inflexion point associated to the three-dimensional antiferromagnetic ordered from 18 K at 1 kOe to 20.1 K at 90 kOe. The small splitting observed in the zero-field-cooled-field-cooled curves at low temperatures is characteristic of ferromagnetic interactions but saturation is not reached even up to 90 kOe. Heat-capacity measurements show an unusual dependence on the magnetic field for antiferromagnetic transitions with a jump at the Neél temperature quite small 2 J/Kmol . The magnetic contribution exhibits a -type anomaly associated to the three-dimensional antiferromagnetic ordering. Surprisingly, the anomaly grows with the magnetic field and becomes better defined. Neutron powder diffraction in different fields shows a magnetic phase transition. The incommensurate magnetic structure evolves at low temperatures toward a collinear AF phase for fields higher than 35 kOe.