Magnetization Steps Promoted by Structural Modulation in BaCoX₂O₇ (X = As, P)

Abstract

BaCo 2 X 2 O 7 (X = P, As) compounds are formed of chains of Co 2+ O 8 dimers. In a first approximation, the structural chains appear strongly isolated, which gives a pronounced one-dimensiona (1D) magnetic character to the systems. GGA+U calculations show ferromagnetic (FM) exchanges inside the dimers and predominant antiferromagnetic coupling between them along the chains, as confirmed by the refined magnetic structure. This 1D antiferromagnetic topology between S = 3 FM dimers does not support the existence of two broad overlapped magnetization plateaus observed around H c1 = 5 T (ferrimagnetic state → ∼Ms/3) and H c2 = 7 T (ferromagnetic state → Ms). In the crystal, an incommensurate structural modulation promotes important atomic displacement waves that transform the 1D chains into two-dimensional layers. In the layers, strongly frustrated topologies between Co 2+ triangles of S = 3 dimers are distributed accordingly to the modulation wave. The exchanges calculated for the maximal frustrated geometry offered by the modulation show that they are mainly responsible for the stepped magnetization.

Synthesis and Characterization of Ba[CoSO]: Magnetic Complexity in the Presence of Chalcogen Ordering

“…Similar values of the effective U yield reasonable spin‐polarized electronic ground‐states in DFT+U calculations for other low‐dimensional Co oxides 6. 31 By also testing values of J=0 and 1.3 eV and different values of U, we checked that within the described bounds, the calculation stays consistent. However, it should be noted that the DFT+U method depends on implementation.…”

Magnetic Structure of Ground and Field Induced Ordered States of Low-Dimensional γ-CoV2O6

“…The intrachain ordering (b axis) along edge-sharing CoO 6 octahedra is expected ferromagnetic (see ref 26) as usually observed for plethora of systems with similar topologies. 2,[16][17][18]23,29 The second propagation vector modulates the first one, tuning the orientation of the spin in a 8-fold unit cell. This hypothesis is supported by the fact that the magnetic saturation is not totally reached at the F plateau and could explain the slope of the magnetization curve above 1 T (Figure 2a).…”

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

“…The crystal structure of the original BaFeP 2 O 7 member is modulated similarly to BaCoX 2 O 7 (X = P, As, V). [ 21,24 ] Its triclinic (S.G. P‐1 , cell parameters a = 5.3855(2) Å, b = 7.5783(3) Å, c = 7.1496(4) Å, α = 102.302(3)°, β = 93.811(3)°, γ = 91.315(3)°) average structure, consists of infinite chains of edge sharing Fe 2 O 8 square–pyramid dimers running along the a ‐axis (see Figure 1b), connected by pyroarsenates As 2 O 7 bridges but disconnected in the ac plane. These layers are stacked along the b ‐axis and separated by Ba 2+ cations.…”