Effect of iron vacancies on magnetic order and spin dynamics of the spin ladder BaFe2-delta S1.5Se1.5

Quasi-one-dimensional iron chalcogenides possess various magnetic states depending on the lattice distortion, electronic correlations, and presence of defects. We present neutron diffraction and inelastic neutron scattering experiments on the spin ladder compound BaFe2-delta S1.5Se1.5 with similar to 6% iron vacancies. The data reveal that long-range magnetic order is absent, while the characteristic magnetic excitations that correspond to both the stripe-and block-type antiferromagnetic correlations are observed. First-principles calculations support the existence of both stripe- and block-type antiferromagnetic short-range orders in the experimental sample. The disappearance of long-range magnetic order may be due to the competition between these two magnetic orders, which is greatly enhanced for a certain concentration of iron vacancies, which we calculate to be about 6%, consistent with the measured iron vacancy concentration. Our results highlight how iron vacancies in the iron-based spin ladder system strongly influence the magnetic ground state.