We have investigated two original hydrated cobalt arsenates based on Co 2+ octahedral edgesharing chains. Their different magnetocrystalline anisotropies induce different types of metamagnetic transition, spin-flop versus spin-flip. In both compounds, a strong local anisotropy (Ising spins) is favored by the spin-orbit coupling present in the CoO6 octahedra while ferromagnetic (FM) exchanges predominate in the chains. Co2(As2O7), 2H2O (1) orders antiferromagnetically below TN = 6.7 K. The magnetic structure is a non-collinear antiferromagnetic spin arrangement along the zigzag chains with DFT calculations imply frustrated chains and weakened anisotropy. A metamagnetic transition suggests a spin-flop process above 0H = 3.2 T. On the contrary, BaCo2As2O8.2H2O (2) linear chains are arranged in disconnected layers, with only inter-chain ferromagnetic exchanges and therefore increasing its magnetocrystalline anisotropy. The magnetic structure is collinear with a magnetic easy axis that allows a spin-flop to a sharp spin-flip transition below TN= 15.1 K and above 0H = 6.2 T.

A new paradigm in multiferroics is observed in BaCoX2O7 (X = As, P) compounds. They consist of one dimensional (1D) antiferromagnetic chains undulated by incommensurate structural modulations with unusually large atomic displacive waves, giving a mixed 1D/2D “real” magnetic topology. The magnetic ground state is antiferromagnetic (AFM) with k = [½ 0 0], leading to a nonmodulated collinear spin lattice despite the aperiodic atomic framework, and allows developing spin‐induced multiferroicity below TN. Severe arguments against the identified mechanisms for type‐II multiferroics, i.e., by inverse Dzyaloshinskii–Moriya, exchange striction and spin‐dependent p–d hybridizations, suggest an original scenario in which the atomic waves, the collinear magnetic structure, and magnetic dipole–dipole interactions may interact as crucial ingredients of the spin‐induced ferroelectric phase. Here, the specific role of the Co2+ spin–orbit coupling in the magnetoelectric (ME) phase diagram is demonstrated by comparison with the novel Heisenberg BaFeP2O7 isomorph, similarly structurally modulated. This compound shows a noncollinear modulated AFM ordering, while no ME coupling is detected in its case. Accordingly, both BaCoX2O7 and BaFeP2O7 also undergo metamagnetic transitions above 5–6 T promoted by the modulated distribution of spin exchanges, but the spin‐flop progressive alignment of the spins in the noncollinear spin structure (Fe2+ case) turns into an abrupt flip‐like transition in the uniaxial spin structure (Co2+ case).

The standard redox potentials of the Fe3+/Fe2+ (+0.77 V) and H3AsO4/H3AsO3 (+0.56 V) stabilize the easy crystallization of ferric arsenates. However, activating solvothermal reactions with a reducing agent such as...