The magnetic phase diagram of an extended J1-J2 model on a modulated square lattice and its implications for the antiferromagnetic phase of KyFexSe2

TL;DR

This study maps the magnetic phases of an extended J1-J2 model on a modulated square lattice, revealing the stability of block-spin antiferromagnetic order relevant to KFeSe compounds and analyzing their spin dynamics.

Contribution

It provides a comprehensive magnetic phase diagram for a J1-J2 model on a vacancy-ordered lattice, highlighting the stability of experimentally observed block-spin states and their spin-wave properties.

Findings

Block-spin antiferromagnetic phase occupies a significant phase space.

Spin-wave degeneracy reveals exchange coupling information.

Different vacancy orderings influence magnetic phase stability.

Abstract

Motivated by the experimentally observed $\sqrt{5} \times \sqrt{5}$ iron vacancy order and a block spin antiferromagnetic phase with large magnetic moment in $\mathrm{K}_{0.8}\mathrm{Fe}_{1.6}\mathrm{Se}_2$, we study the magnetic phase diagram of an extended $J_1-J_2$ model on a 1/5-depleted square lattice with $\sqrt{5} \times \sqrt{5}$ vacancy order, using a classical Monte Carlo analysis. The magnetic phase diagram involves various antiferromagnetically ordered phases, and most of them have higher order commensuration. We find that the experimentally relevant block-spin state occupies a significant portion of the phase diagram, and we discuss the spin dynamics of this phase using a linear spin-wave analysis. Based on our spin wave calculations in different parameter regimes corresponding to the block spin antiferromagnetic phase, we show how spin-wave degeneracy along the high symmetry directions of the magnetic Brillouin zone can provide information regarding the underlying exchange couplings. We have also analyzed the magnetic phase diagram of a $J_1-J_2$ model on two different modulated square lattices relevant for $\mathrm{K}_y\mathrm{Fe}_{1.5}\mathrm{Se}_2$, which respectively exhibit 1/4-depleted $2\times2$ and $4\times2$ vacancy ordering.