Spin waves and magnetic exchange interactions in insulating Rb0.89Fe1.58Se2

The parent compounds of iron pnictide superconductors are bad metals with a collinear antiferromagnetic structure and Neel temperatures below 220 K. Although alkaline iron selenide A(y)Fe(1.6+x)Se(2) (A = K, Rb, Cs) superconductors are isostructural with iron pnictides, in the vicinity of the undoped limit they are insulators, forming a block antiferromagnetic order and having Neel temperatures of roughly 500 K. Here we show that the spin waves of the insulating antiferromagnet Rb0.89Fe1.58Se2 can be accurately described by a local moment Heisenberg Hamiltonian. A fitting analysis of the spin wave spectra reveals that the next-nearest neighbour couplings in Rb0.89Fe1.58Se2, (Ba,Ca,Sr)Fe2As2, and Fe1.05Te are of similar magnitude. Our results suggest a common origin for the magnetism of all the Fe-based superconductors, despite having different ground states and antiferromagnetic orderings.