Spin waves and magnetic exchange interactions in the spin ladder   compound RbFe$_2$Se$_3$
  We report an inelastic neutron scattering study of the spin waves of the one-dimensional antiferromagnetic spin ladder compound RbFe$_2$Se$_3$. The results reveal that the products, $SJ$'s, of the spin $S$ and the magnetic exchange interactions $J$'s along the antiferromagnetic (leg) direction and the ferromagnetic (rung) direction are comparable with those for the stripe ordered phase of the parent compounds of the iron-based superconductors. The universality of the $SJ$'s implies nearly universal spin wave dynamics and the irrelevance of the fermiology for the existence of the stripe antiferromagnetic order among various Fe-based materials. 
Meng Wang, Ming Yi, Shangjian Jin, Hongchen Jiang, Yu Song, Huiqian   Luo, A. D. Christianson, C. De la Cruz, E. Bourret-Cou...
cond-mat.str-el cond-mat.supr-con