Sinusoidal magnetic structure in a three-dimensional antiferromagneticCo2(OH)AsO4: Incommensurate-commensurate magnetic phase transition

Abstract

Co 2 ͑OH͒AsO 4 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 Co͑1͒O 5 -trigonal bipyramid dimers and Co͑2͒O 6 -octahedra chains are simultaneously present. The magnetic structure has been determined by neutron ͑D2B and D1B͒ powder-diffraction data. Below 22 K, the Co 2 ͑OH͒AsO 4 phase shows an incommensurate antiferromagnetic structure along the b direction. The propagation vector ͑0,␦ ,0͒ is temperature dependent with a value of ␦ = 0.430 at the lowest temperature ͑1.8 K͒. Magnetization measurements of Co 2 ͑OH͒AsO 4 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/Kmol͒. 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.

Synthesis, Spectroscopic and Magnetic Properties of the Co2(OH)(PO4)1–x(AsO4)x [0 ≤ x ≤ 1] Solid Solution

“…In the case of Co 2 (OH)(PO 4 ) 0.1 (AsO 4 ) 0.9 , a similar behavior to that of the arsenate compound, Co 2 (OH)AsO 4 , [19] is observed (see Figure 6). The χ m increases as the temperature falls below room temperature and reaches a rounded maximum at approximately 30 K that could be attributed to different origins: i) bidimensional magnetic ordering, ii) short-range magnetic interactions or iii) a crystalline electrical field.…”

“…The χ m increases as the temperature falls below room temperature and reaches a rounded maximum at approximately 30 K that could be attributed to different origins: i) bidimensional magnetic ordering, ii) short-range magnetic interactions or iii) a crystalline electrical field. [19] Below 22 K, a strong increase in the magnetic signal is observed and this can be associated to the threedimensional antiferromagnetic ordering as in the case of the arsenate compound. [19] However, the inflexion point located close to 6 K in the Co 2 (OH)AsO 4 phase disappears with the [19] substitution of 10 % of AsO 4 3-by PO 4 3-ions.…”

“…[19] Below 22 K, a strong increase in the magnetic signal is observed and this can be associated to the threedimensional antiferromagnetic ordering as in the case of the arsenate compound. [19] However, the inflexion point located close to 6 K in the Co 2 (OH)AsO 4 phase disappears with the [19] substitution of 10 % of AsO 4 3-by PO 4 3-ions. These changes show the important effect of the anion on the magnetic behavior of these phases.…”

“…[16,19] The ZFC-FC curves at 0.1 T are given in Figure 7. For the Co 2 (OH)(PO 4 ) 1-x (AsO 4 ) x [0.1 Յ x Յ 0.75] samples, a paramagnetic behavior without any difference between ZFC and FC magnetization is observed at temperatures above T N .…”

“…[17,18] The substitution of the PO 4 3-by AsO 4 3-anions in the Co 2 (OH)PO 4 phase substantially modifies the magnetic exchange Co(1)-O(3)-Co(2) angle from 107°to 116.9°for the phosphate and arsenate, respectively relaxing the magnetic frustration system of Co 2 (OH)PO 4 and leading to an incommensurate phase with antiferromagnetic interactions (sinusoidal amplitude-modulated) in Co 2 (OH)AsO 4 . [19] In all of them the magnetic interactions propagated via |OH| and |XO 4 | tetrahedra (X = P, As) give rise to a three-dimensional antiferromagnetic coupling.…”