We investigate the magnetic phase diagram of a solid solution BaFe2(S1-xSex)3, which is categorized into a new family of Fe-based superconductors hosting a quasi-one-dimensional ladder lattice. We show that the antiferromagnetic order is largely suppressed due to bicritical fluctuations in the vicinity of the magnetic transition at x=0.23 from the stripe-type to block-type ordering. Moreover, we observed large magnon contributions in the thermal conductance in a wide x range. These results indicate the quasi-one-dimensional nature of spin systems and give a solid foundation for modeling the material toward unraveling the microscopic mechanism of superconductivity.
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