[Paper Review] Magnetic interactions in FeSe studied by first principle calculations

Shuai Wang, Fa Wang|arXiv (Cornell University)|Oct 19, 2015

Iron-based superconductors research3 citations

TL;DR

This study uses first-principles calculations to investigate magnetic interactions in monolayer and bulk FeSe, revealing that increasing lattice constant drives a transition from anti-ferromagnetic pair-checkboard (PAFM) to stripe collinear (CAF) order in monolayer FeSe, while bulk FeSe only adopts PAFM order under structural reconstruction. The J₁–J₂–K₁ model fitting shows strong, frustrated magnetic interactions that increase slightly with lattice expansion, suggesting magnetism may underlie the enhanced superconductivity in FeSe/SrTiO₃ heterostructures.

ABSTRACT

Based on first principle calculations we have investigated the evolution of magnetism in free-standing monolayer FeSe with respect to lattice constant and magnetism in bulk FeSe. The computational results show that the magnetic order in free-standing monolayer FeSe will change from anti-ferromagnetic pair-checkboard order to stripe collinear order along with enlarging lattice constant. The magnetic order in bulk FeSe will change from stripe collinear order to anti-ferromagnetic pair-checkboard order only if structure reconstruction is allowed. We use J$_1$-J$_2$-K$_1$ model to fit the calculated total energies of different magnetic orders to study magnetic interaction strengths in FeSe. The fitting results of J$_1$-J$_2$-K$_1$ indicate that magnetic interactions in FeSe are quite strong and highly frustrated, and increase slowly with enlarging lattice parameter.

Motivation & Objective

• To re-examine the influence of tensile strain (via lattice constant variation) on magnetism in monolayer FeSe.
• To resolve inconsistencies in prior reports on magnetic interaction strengths in FeSe.
• To determine whether magnetic order in bulk FeSe can transition from stripe collinear to pair-checkboard order under structural reconstruction.
• To quantify magnetic exchange interactions using the J₁–J₂–K₁ model and assess their role in superconducting Tc enhancement.

Proposed method

• Non-collinear magnetic calculations using VASP with PAW pseudopotentials and GGA-PBE exchange-correlation functional.
• Use of dense k-meshes (24×24×1 for monolayer, 18×18×18 for bulk) and 500 eV plane-wave cutoff for convergence.
• Inclusion of 15Å vacuum to decouple interlayer interactions in monolayer FeSe.
• Systematic calculation of total energies for multiple magnetic states: Néel, collinear, spiral, tetrahedral, bi-collinear, and pair-checkboard orders.
• Fitting of J₁–J₂–K₁ spin Hamiltonian parameters to total energies with variable magnetic moment sizes.
• Linear spin wave theory applied to compute magnon dispersions using fitted J₁, J₂, K₁ parameters for comparison with neutron scattering.

Experimental results

Research questions

• RQ1 How does increasing lattice constant affect the magnetic ground state of free-standing monolayer FeSe?
• RQ2 What magnetic order emerges in bulk FeSe when structural reconstruction is permitted?
• RQ3 How do the magnetic exchange interactions (J₁, J₂, K₁) in FeSe depend on lattice expansion?
• RQ4 To what extent is the magnetic interaction in FeSe frustrated, and how does this relate to the absence of long-range magnetic order?
• RQ5 Can the spin wave dispersion of bulk FeSe be predicted from the J₁–J₂–K₁ model, and how does it compare to known systems like BaFe₂As₂?

Key findings

• Monolayer FeSe transitions from anti-ferromagnetic pair-checkboard (PAFM) order to stripe collinear (CAF) order as lattice constant increases from 3.765 Å to 4.045 Å.
• Bulk FeSe exhibits PAFM order only when structural reconstruction is allowed; otherwise, CAF order remains the ground state.
• The J₁–J₂–K₁ model fitting yields a J₂/J₁ ratio of approximately 0.5 in both monolayer and bulk FeSe, indicating strong magnetic frustration.
• Magnetic interaction strengths increase slowly with increasing lattice constant, suggesting a weak dependence of exchange on strain.
• Spin wave dispersions in bulk FeSe under CAFM order show a slightly larger bandwidth than in BaFe₂As₂, a prediction testable via inelastic neutron scattering.
• The study resolves discrepancies with prior works by using denser k-meshes, including non-collinear states, and accounting for moment-size-dependent energy fitting.