Orbital Selective Magnetism in the Spin-Ladder Iron Selenides Ba$_{1-x}$K$_{x}$Fe$_2$Se$_3$

TL;DR

This study reveals how orbital-dependent magnetism in Ba$_{1-x}$K$_{x}$Fe$_2$Se$_3$ evolves from block to stripe antiferromagnetic order, influenced by electron count, atomic displacements, and orbital bandwidths, with implications for magnetic behavior in iron selenides.

Contribution

It demonstrates orbital-selective magnetism and structural changes across the Ba-K substitution series in iron selenide spin ladders, highlighting the role of orbital physics in magnetic order.

Findings

Transition from block to stripe antiferromagnetic order.

Reduction in magnetic moment larger than expected from electron count change.

Intermediate compositions show insulating behavior and possible spin-glass formation.

Abstract

Here we show that the 2.80(8) {\mu}B/Fe block antiferromagnetic order of BaFe2Se3 transforms into stripe antiferromagnetic order in KFe2Se3 with a decrease in moment to 2.1(1) {\mu}B/Fe. This reduction is larger than expected from the change in electron count from Ba$^{2+}$ to K$^{+}$, and occurs with the loss of the displacements of Fe atoms from ideal positions in the ladders, as found by neutron pair distribution function analysis. Intermediate compositions remain insulating, and magnetic susceptibility measurements show a suppression of magnetic order and probable formation of a spin-glass. Together, these results imply an orbital-dependent selection of magnetic versus bonded behavior, driven by relative bandwidths and fillings.