Magnetic Frustration and Iron-Vacancy Ordering in Iron-Chalcogenide

TL;DR

This paper models magnetic and vacancy orders in iron-chalcogenides using a $J_1-J_2-J_3$ framework, explaining how vacancy patterns influence magnetic frustration and transition temperatures, supported by spin wave calculations.

Contribution

It introduces a unified model explaining magnetic and vacancy orders in iron-chalcogenides, linking vacancy patterns to magnetic frustration reduction and transition temperature enhancement.

Findings

Vacancy order reduces magnetic frustration in 122 systems.

Iron-vacancy pattern correlates with maximum frustration reduction.

Calculated spin wave excitations exhibit novel features supporting the model.

Abstract

We show that the magnetic and vacancy orders in the 122 $(A_{1-y}Fe_{2-x}Se_2)$ iron-chalcogenides can be naturally derived from the $J_1-J_2-J_3$ model with $J_1$ being the ferromagnetic (FM) nearest neighbor exchange coupling and $J_{2}, J_3$ being the antiferromagnetic (AFM) next and third nearest neighbor ones respectively, previously proposed to describe the magnetism in the 11(FeTe/Se) systems. In the 11 systems, the magnetic exchange couplings are extremely frustrated in the ordered bi-collinear antiferromagnetic state so that the magnetic transition temperature is low. In the 122 systems, the formation of iron vacancy order reduces the magnetic frustration and significantly increases the magnetic transition temperature and the ordered magnetic moment. The pattern of the 245 iron-vacancy order ($\sqrt{5}\times \sqrt{5}$) observed in experiments is correlated to the maximum reduction of magnetic frustration. The nature of the iron-vacancy ordering may hence be electronically driven. We explore other possible vacancy patterns and magnetic orders associated with them. We also calculate the spin wave excitations and their novel features to test our model.