Chemical pressure effect on the magnetic properties of the S = 1 2 structural screw chain antiferromagnets A Co 2 ( Te 3 O 8 ) ( SO 4 ) ( A = Hg , Cd )

Chemical substitution is an effective route to manipulate the magnetic exchange couplings and modify the magnetic properties. However, replacement between ${\mathrm{Cd}}²⁺$ and ${\mathrm{Hg}}²⁺$ cations, both belonging to IIB group in the periodic table but having quite close ionic radii, is rarely studied. In this work, the chemical-pressure effect in $A{\mathrm{Co}}₂({\mathrm{Te}}₃{\mathrm{O}}₈)({\mathrm{SO}}₄)\phantom{\rule{4pt}{0ex}}(A=\mathrm{Hg},\mathrm{Cd})$ is reported, which is an $S=1/2$ structural screw-chain antiferromagnetic series with two unique Co positions. Magnetic susceptibility, specific heat, and high-field magnetization measurements on both polycrystal samples suggest that Hg compound experiences two successive antiferromagnetic transitions at ${T}_{\mathrm{N}1}=25.5\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}=21.5\phantom{\rule{4pt}{0ex}}\mathrm{K}$ in zero field, and multiple field-induced transitions at ${B}₁=10.5\phantom{\rule{4pt}{0ex}}\mathrm{T}, {B}₂=13\phantom{\rule{4pt}{0ex}}\mathrm{T}$, and ${B}₃=19.5\phantom{\rule{4pt}{0ex}}\mathrm{T}$ at 1.6 K. By contrast, a single magnetic transition takes place at ${T}_{\mathrm{N}}=20$ K and ${B}_{\mathrm{c}}=10.7\phantom{\rule{4pt}{0ex}}\mathrm{T}$ in Cd compound. Different twists of ${\mathrm{CoO}}₆$ octahedra are discussed to be responsible for the distinct magnetic behaviors. To further understand the influence of the chemical pressure, additional anisotropic characterizations are performed on Hg single crystal and subsequently the magnetic field $\mathrm{versus}$ temperature phase diagrams are established. Large magnetic anisotropy is observed with the moments lying within the $bc$ plane. Besides, a possible field-induced order-disorder transition is uncovered under the application of a magnetic field along the $b$ axis. The opposite features of the anisotropic magnetic susceptibility in Hg compound might demonstrate separate antiferromagnetic orders of the two Co sublattices, the discrepancy of which is reduced due to the weakened twist of ${\mathrm{CoO}}₆$ octahedra in Cd compound, leading to a simultaneous occurrence of these transitions instead.