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

BaCo2X2O7 (X = P, As) compounds are formed of chains of Co2+O8 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 ID 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 -> similar to 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 ID chains into two-dimensional layers. In the layers, strongly frustrated topologies between Co2+ 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.