[Paper Review] Itinerant Spin Excitations in SrFe2As2 Measured by Inelastic Neutron Scattering

R. A. Ewings, T. G. Perring+6 more|arXiv (Cornell University)|Nov 16, 2010

Iron-based superconductors research3 citations

TL;DR

This study uses inelastic neutron scattering to measure spin excitations in SrFe2As2 across the full Brillouin zone and up to 260 meV, finding that a local-moment J1–J2 model fails to describe the data, while a 5-band itinerant mean-field model quantitatively explains the high-energy spectra and the lack of soft mode at Q=(0.5,0.5) above T_N,s. The results imply that electronic nematic or orbital order is not required to explain the magnetic response, supporting itinerant magnetism as the dominant description.

ABSTRACT

We report inelastic neutron scattering measurements of the magnetic excitations in SrFe2As2, the parent of a family of iron-based superconductors. The data extend throughout the Brillouin zone and up to energies of ~260meV. An analysis with the local-moment J_1-J2 model implies very different in-plane nearest-neighbor exchange parameters along the $a$ and $b$ directions, both in the orthorhombic and tetragonal phases. However, the spectrum calculated from the J1-J2 model deviates significantly from our data. We show that the qualitative features that cannot be described by the J1-J2 model are readily explained by calculations from a 5-band itinerant mean-field model.

Motivation & Objective

• To determine whether magnetic interactions in SrFe2As2 are anisotropic in the antiferromagnetic state, as seen in other 122-arsenides.
• To investigate whether the spin excitation spectrum changes significantly upon warming above the Néel/structural transition temperature T_N,s = 192 K.
• To assess the validity of the local-moment description (J1–J2 model) versus itinerant electron models in explaining the observed magnetic excitations.
• To clarify whether the absence of a soft mode at Q=(0.5,0.5) in the paramagnetic phase requires additional symmetry-breaking mechanisms such as electronic nematic or orbital order.

Proposed method

• Performed inelastic neutron scattering (INS) measurements on single crystals of SrFe2As2 over a wide energy range (5–260 meV) and throughout the entire two-dimensional Brillouin zone.
• Measured the magnetic excitation spectrum both below and above the Néel/structural transition temperature T_N,s = 192 K to probe the evolution of spin dynamics across the phase transition.
• Compared experimental data to predictions from a local-moment J1–J2 model including nearest-neighbor (J1a, J1b) and next-nearest-neighbor (J2) exchange interactions, along with single-ion anisotropy terms.
• Used a 5-band itinerant mean-field model to calculate spin excitation spectra, incorporating itinerant electron effects and spin-orbit coupling.
• Applied linear spin-wave theory with damped harmonic oscillator functions to model finite excitation lifetimes and extract response functions S^yy and S^zz.
• Used the neutron scattering cross-section formalism S^αβ(Q,ω) = S_eff × (A_Q ± C - D_Q) / (ħω) × {n(ω)+1} × δ(ω - ω_α(Q)) with damping to fit experimental data.

Experimental results

Research questions

• RQ1 Is the magnetic exchange anisotropy in SrFe2As2 preserved above T_N,s, as observed in other 122-arsenides?
• RQ2 Does the spin excitation spectrum exhibit a significant soft mode at Q=(0.5,0.5) in the paramagnetic tetragonal phase, as predicted by local-moment models?
• RQ3 How well does the J1–J2 local-moment model describe the full momentum- and energy-resolved INS data in SrFe2As2?
• RQ4 Can the itinerant 5-band mean-field model quantitatively explain the high-energy magnetic excitation spectrum and the absence of a soft mode above T_N,s?
• RQ5 Is the absence of a soft mode at Q=(0.5,0.5) above T_N,s consistent with itinerant magnetism, or does it require additional symmetry-breaking mechanisms like electronic nematic or orbital order?

Key findings

• The high-energy magnetic excitation spectrum in SrFe2As2 remains largely unchanged upon warming above T_N,s, with no significant softening or mode evolution observed.
• The J1–J2 local-moment model fails to quantitatively describe the data, particularly the lack of a soft mode at Q=(0.5,0.5) in the paramagnetic phase, requiring inconsistent parameter sets.
• The 5-band itinerant mean-field model successfully explains the observed spectra, including the high-energy dispersion and the absence of a soft mode, without invoking additional symmetry-breaking orders.
• The data show no significant change in the form of the magnetic excitation spectrum across the Néel transition, indicating that the spin dynamics are robust and consistent with itinerant behavior.
• The absence of a soft mode at Q=(0.5,0.5) in the paramagnetic phase is naturally explained by the itinerant model and does not require additional phenomenology such as electronic nematic or orbital order.
• The results challenge the conventional local-moment picture and support the dominance of itinerant electron effects in shaping the magnetic response of iron-based superconductors.