Giant biquadratic interaction induced magnetic anisotropy in the iron-based superconductor AxFe2-ySe2

TL;DR

This paper reveals that anisotropic biquadratic interactions cause magnetic anisotropy in AxFe2-ySe2, and introduces a universal method to accurately determine magnetic interactions, aligning well with experimental data.

Contribution

It introduces a universal method for calculating magnetic interactions that accurately captures magnetic properties in AxFe2-ySe2 without high-energy configurations.

Findings

Magnetic anisotropy arises from anisotropic biquadratic interactions.

The proposed method accurately predicts spin wave spectra.

Results support the local moment picture for magnetism in AxFe2-ySe2.

Abstract

The emergence of the electron-pocket only iron-based superconductor AxFe2-ySe2 (A = alkali metal) challenges the Fermi-surface nesting picture established in iron-pnictides. It was widely believed that magnetism is correlated with the superconductivity in AxFe2-ySe2. Unfortunately, the highly anisotropic exchange parameters and the disagreement between theoretical calculations and experimental results triggered a fierce debate on the nature of magnetism in AxFe2-ySe2. Here we find that the strong magnetic anisotropy is from the anisotropic biquadratic interaction. In order to accurately obtain the magnetic interaction parameters, we propose a universal method, which does not need including other high energy configurations as did in conventional energy mapping method. We show that our model successfully captures the magnetic interactions in AxFe2-ySe2 and correctly predicts the spin wave spectrum, in quantitative agreement with the experimental observation. These results suggest that the local moment picture, including the biquadratic term, can describe accurately the magnetic properties and spin excitations in AxFe2-ySe2, which sheds new light on the future study of the high-Tc iron-based superconductors.