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.

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