Two-orbital quantum spin model of magnetism in the iron pnictides

TL;DR

This paper introduces a two-orbital spin model for iron pnictides, revealing how ferromagnetic second-neighbor interactions lead to variable magnetic moments and complex magnetic phases, including hidden stripe order and canted states.

Contribution

The study develops a variational cluster method to analyze a two-orbital spin model, uncovering the role of ferromagnetic second-neighbor interactions in magnetic moment variability and phase diversity.

Findings

Ferromagnetic second-neighbor exchange explains small ordered moments.

Identification of a partially hidden stripe order with tunable moments.

Confirmation of a canted magnetic state and discovery of additional phases.

Abstract

We study a two-orbital spin model to describe (pi,0) stripe antiferromagnetism in the iron pnictides. The "double-spin" model has an on-site Hunds's coupling and inter-site interactions extending to second neighbors (inter- and intra-orbital) on the square lattice. Using a variational method based on a cluster decomposition, we optimize wave functions with up to 8 cluster sites (up to 2^16 variational parameters). We focus on the anomalously small ordered moments in the stripe state of the pnictides. To account for it, and large variations among different compounds, we show that the second-neighbor cross-orbital exchange constant should be ferromagnetic, which leads to "partially hidden" stripe order, with a moment that can be varied over a large range by small changes in the coupling constants. In a different parameter region, we confirm the existence of a canted state previously found in spin-wave theory. We also identify several other phases of the model.