Structural, magnetic, and electronic evolution of the spin-ladder system BaFeS Se with isoelectronic substitution.

We report experimental studies of a series of BaFeS Se (0 ⩽ ⩽ 3) single crystals and powder specimens using x-ray diffraction, neutron-diffraction, muon-spin-relaxation, and electrical transport measurements. A structural transformation from (BaFeS) to (BaFeSe) was identified around = 0.7 - 1. Neutron-diffraction measurements on the samples with = 0.2, 0.4, and 0.7 reveal that the Néel temperature of the stripe antiferromagnetic order is gradually suppressed from ~120 to 85 K, while the magnitude of the ordered Fe moments shows very little variation. Similarly, the block antiferromagnetic order in BaFeSe remains robust for 1.5 ⩽ ⩽ 3 with negligible variation in the ordered moment and a slight decrease of the Néel temperature from 250 K ( = 3) to 225 K ( = 1.5). The sample with = 1 near the and border shows coexisting, two-dimensional, short-range stripe- and block-type antiferromagnetic correlations. The system remains insulating for all , but the thermal activation gap shows an abrupt increase when traversing the boundary from the stripe phase to the block phase. The results demonstrate that the crystal structure, magnetic order, and electronic properties are strongly coupled in the BaFeS Se system.