Abstract
${\text{Co}}_{2}(\text{OH}){\text{AsO}}_{4}$ 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 $\text{Co}(1){\text{O}}_{5}$-trigonal bipyramid dimers and $\text{Co}(2){\text{O}}_{6}$-octahedra chains are simultaneously present. The magnetic structure has been determined by neutron (D2B and D1B) powder-diffraction data. Below 22 K, the ${\text{Co}}_{2}(\text{OH}){\text{AsO}}_{4}$ phase shows an incommensurate antiferromagnetic structure along the $b$ direction. The propagation vector $(0,\ensuremath{\delta},0)$ is temperature dependent with a value of $\ensuremath{\delta}=0.430$ at the lowest temperature (1.8 K). Magnetization measurements of ${\text{Co}}_{2}(\text{OH}){\text{AsO}}_{4}$ 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 Ne\'el temperature quite small (2 J/Kmol). The magnetic contribution exhibits a $\ensuremath{\lambda}$-type anomaly associated to the three-dimensional antiferromagnetic ordering. Surprisingly, the $\ensuremath{\lambda}$ 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.
Highlights
In the last years, the compounds showing a “metalinsulator” transition have attracted much attention in solidstate physics and chemistry since the discovery of additional fascinating magnetic phenomena
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 partial substitution of transition-metal ions in the structure severely affects the magnetic properties14,15 giving rise to the evolution of the 3D antiferromagnetic system in the Co2−xCuxOHPO4 solid solution15 up to a spin-gap system in the Cu2͑OHPO4 phase,16 the spin-glasslike state in theCo, Ni2͑OHPO4 detected below 10 K,14 or the higher ferromagnetic interactions at lower temperatures in Co1.7Mn0.3͑OHPO4 coexisting with a similar spin-glass state observed in the nonsubstituted cobalt hydroxy phosphate.17 biliTzeheansiimoniilcarfraabmilietwy oorfkAs saOnd43t−heangdrePaOte4r3−sitzeetroafhethderaAtsoOs4ta3−anions predict some differences in the crystal packing features that could modify the complex magnetic properties exhibited in these phases
Summary
The compounds showing a “metalinsulator” transition have attracted much attention in solidstate physics and chemistry since the discovery of additional fascinating magnetic phenomena. We have recently discovered in the hydroxy phosphate Co2͑OHPO4 ͑Ref. 11͒ the coexistence of antiferromagnetic ordering and a spin-glass behavior in an insulator compound In this phase, a magnetic frustration in the Co1͒ magnetic moments is observed as due to the presence of antiferromagnetic interactions between Co2͒ neighbor chains. The partial substitution of transition-metal ions in the structure severely affects the magnetic properties giving rise to the evolution of the 3D antiferromagnetic system in the Co2−xCuxOHPO4 solid solution up to a spin-gap system in the Cu2͑OHPO4 phase, the spin-glasslike state in theCo, Ni2͑OHPO4 detected below 10 K,14 or the higher ferromagnetic interactions at lower temperatures in Co1.7Mn0.3͑OHPO4 coexisting with a similar spin-glass state observed in the nonsubstituted cobalt hydroxy phosphate. biliTzeheansiimoniilcarfraabmilietwy oorfkAs saOnd43t−heangdrePaOte4r3−sitzeetroafhethderaAtsoOs4ta3−anions predict some differences in the crystal packing features that could modify the complex magnetic properties exhibited in these phases.
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