Multiferroic BaCoX₂O₇ (X = P, As) Compounds with Incommensurate Structural Waves but Collinear Spin Ingredients

Abstract

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).

Ferromagnetic Mn3+ Sawtooth Chains in A2(Mn2O)(SeO3)3 (A = K, Rb) Quaternary Selenites

“…53 Multiferroic possibilities and magnetoelectric couplings are out of the scope of this paper, but it is expected that dealing with spin-clusters or macrospins may enhance these phenomena, which may order together in a f rustrated noncolinear manner or develop incommensurate magnetic periods. 54 Finally, it is evident that the rapid magnetization of high moments under an applied magnetic field offered by the FM ST chains is a prerequisite for important entropy changes, as recently demonstrated in original low-dimensional magnetocalorics. 55 Thus, future efforts should concentrate on characterizing and chemically increasing the magnetocrystalline anisotropy of the FM ST chains, which generally promote both mutiferroism and magnetocaloric potentialities.…”

Fragile magnetic ordering between robust 2D-ferrimagnets in the AFe3(SeO3)2F6 (A = K, Rb, Cs) series

“…25 MDD interactions are scaled by 1/r 3 (where r is the interlayer distance given by the unit cell parameter c) and were estimated between the external Fe(1) and Fe(1), Fe(2) atoms of the next block in a sphere delimited by the cutoff distance Fe-Fe r 12 Å, which correspond to the sum of 440 individual contributions. We use the spin orientations from the refined magnetic structures and the following formula recently applied to BaCoAs 2 O 7 multiferroics 26 for individual contributions:…”

Compressibility of structural modulation waves in the chain compounds BaCoX 2O7 (X = As, P): a powder study

“…The details of polycrystalline sample synthesis procedures by solid-state reaction at high temperature were reported elsewhere (David et al, 2013;Leclercq et al, 2021).…”

“…The average crystal structures of BaCoX 2 O 7 (X = As, P, hereafter BCAO and BCPO) and also BaFeP 2 O 7 were determined previously from single-crystal diffraction by Riou et al (1988), David et al (2013) and Leclercq et al (2021). All compounds crystallize in the triclinic symmetry (space group P " 1 1, Z = 2) as shown in Fig.…”

“…In BaCoX 2 O 7 , the modulated distribution of Ising-spin exchanges induces a magnetic complex phase diagram with competition between ISSN 2052-5206 collinear AFM, charge density wave (CDW) and ferromagnetic (FM) structures increasing the applied field. An original mechanism at the origin of the multiferroicity in these compounds was proposed recently (Leclercq et al, 2021). The spin-induced ferroelectric phase results from a unique combination of the atomic waves, the collinear magnetic structure and magnetic dipole-dipole interactions.…”