Consistent model of magnetism in ferropnictides

TL;DR

This paper introduces a simple effective spin model with biquadratic interactions that explains the magnetic properties and phase transitions in ferropnictides, reconciling experimental observations with theoretical understanding.

Contribution

It presents a novel spin model with biquadratic interactions that captures key magnetic features of ferropnictides beyond traditional Heisenberg models.

Findings

Explains the spin wave spectrum peculiarities

Accounts for thin domain walls and phase transition crossover

Provides insights into nematic phase emergence

Abstract

The discovery of superconductivity in LaFeAsO introduced the ferropnictides as a major new class of superconducting compounds with critical temperatures second only to cuprates. The presence of magnetic iron makes ferropnictides radically different from cuprates. Antiferromagnetism of the parent compounds strongly suggests that superconductivity and magnetism are closely related. However, the character of magnetic interactions and spin fluctuations in ferropnictides, in spite of vigorous efforts, has until now resisted understanding within any conventional model of magnetism. Here we show that the most puzzling features can be naturally reconciled within a rather simple effective spin model with biquadratic interactions, which is consistent with electronic structure calculations. By going beyond the Heisenberg model, this description explains numerous experimentally observed properties, including the peculiarities of the spin wave spectrum, thin domain walls, crossover from first to second order phase transition under doping in some compounds, and offers new insight in the occurrence of the nematic phase above the antiferromagnetic phase transition.