Magnetoelastic coupling, negative thermal expansion, and two-dimensional magnetic excitations in FeAs

J. L. Niedziela, L. D. Sanjeewa, A. A. Podlesnyak, L. DeBeer-Schmitt, S. J. Kuhn, C. de la Cruz, D. S. Parker, K. Page, and A. S. Sefat

Phys. Rev. B 103, 094431 – Published 19 March 2021

Abstract

We present a temperature-dependent investigation of the local structure and magnetic dynamics of the FeAs binary. The magnetic susceptibility $χ (T)$ result shows an anomalous broad feature up to 550 K, with χ continuing to increase above the Néel antiferromagnetic ordering temperature ( $T_{N} = 70 K$ ), peaking at ∼250 K, then decreasing gently above. It is remarkable that this peak susceptibility temperature corresponds to the onset of anisotropic negative thermal expansion in both the $a$ and $c$ axes, suggesting that magnetic interactions are affecting the structure even well above the Néel point. A systematic investigation into local bonding correlations, from time-of-flight neutron pair distribution function analyses, shows the octahedral volume around each Fe site growing monotonically while adjacent octahedra tilt toward one another before relaxing away past this peak in the anomalous magnetic susceptibility. We use inelastic-neutron scattering to map spin-wave excitations in FeAs at temperatures above and below the $T_{N}$ . We find magnetic excitations near $T_{N}$ to be very different from the excitations in the ground state at 1.5 K. Spin waves measured at 1.5 K are three dimensional (3D), however, in the vicinity of the magnetic transition, the magnetic fluctuations clearly indicate two-dimensional (2D) character in this intrinsically 3D crystal structure. Unlike the undoped 2D parents of iron-arsenide superconductors, where the magnetic correlations are considerably weaker along the $c$ axis than in the $ab$ plane, inelastic neutron scattering here shows that the spin fluctuations in the 3D FeAs binary are nearly 2D in the $bc$ plane at 90 K. These results demonstrate the importance of short-range correlations in understanding the magnetic properties of transition-metal binaries, and suggest how 2D excitations, even in a 3D structure, can potentially become a breeding ground for unconventional superconductivity.

Received 26 May 2020
Revised 28 January 2021
Accepted 26 February 2021

DOI:https://doi.org/10.1103/PhysRevB.103.094431

Physics Subject Headings (PhySH)

Iron-based superconductors

Inelastic neutron scattering Neutron pair-distribution function analysis

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

J. L. Niedziela ^1,*,†, L. D. Sanjeewa ^2,*, A. A. Podlesnyak ³, L. DeBeer-Schmitt ³, S. J. Kuhn², C. de la Cruz ³, D. S. Parker ², K. Page ^3,4, and A. S. Sefat²

¹National Security Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
²Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
³Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁴Materials Science and Engineering Department, University of Tennessee, Knoxville, Tennessee 37996, USA

^*These authors contributed equally to this work.
^†Corresponding author: niedzielajl@ornl.gov

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 103, Iss. 9 — 1 March 2021

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Two views of the FeAs structure: (a) The distorted $Fe {As}_{6}$ octahedra, where Fe ions are sixfold coordinated by As. Bonds belonging to the buckled equatorial plane of each octahedra are of equal length, while the axial bonds are separated into long axial $(L_{a})$ and short axial $(S_{a})$ bond distances. (b) The bonding network between the octahedra, where the bonding connections alternate between short and long axial connections along the $a$ axis. (c) Magnetic susceptibility of FeAs, showing $T_{N}$ at 70 K with a broad anomalous hump centered around 250 K.
Reuse & Permissions
Figure 2
Representative PDF data and P1 model fits for 2-, 300-, and 500-K data as a function of fitting range $r$ . Experimental data are shown as blue circles, modeled PDF as red lines, and green lines the model-data residual. $R_{w}$ values reflect the model goodness of fit.
Reuse & Permissions
Figure 3
(a) Change of unit cell parameters with temperature. (b) Temperature dependence of Fe-As short axial $(S_{a})$ , long axial $(L_{a})$ , and equatorial ( $e q_{1}, e q_{2}$ ) bonds. (c) Temperature dependence of As-As bond distances (using the labeling in Fig. 1). All the parameters were derived from the PDF models resulting from 2- to 5-Å range fits to data.
Reuse & Permissions
Figure 4
Elastic neutron scattering map of FeAs at temperature (a) $T = 1.5 K$ and (b) $T = 75 K$ in the ( $H 0 L$ ) scattering plane. (c) Low-energy part of magnetic excitations along the ( $10 L$ ) direction taken at CNCS at $T = 1.5 K$ , (d) $T = 75 K$ . The intensity is averaged over an interval of $\pm 0.05$ reciprocal lattice units (r.l.u.) in the $H$ and $K$ directions. (e) The energy cuts at the antiferromagnetic wave vector at $T = 75 K$ , indicating the opening of the spin-wave energy gap at low temperature. (f) The INS spectra taken at SEQUOIA in the ( $10 L$ ) direction at $T = 5.0 K$ , and (g) $T = 85 K$ , i.e., in the antiferromagnetic and paramagnetic state. The intensity is averaged over an interval of $\pm 0.1 r . l . u .$ in the $H$ and $K$ directions. (h) Energy dependence of the inelastic signal integrated within one Brillouin zone for $T = 5$ and 85 K, demonstrating that the spin-wave bandwidth of FeAs is $\sim 35 meV$ . For better energy coverage and statistics we symmetrized the data according to the crystal symmetry before integration.
Reuse & Permissions
Figure 5
Constant energy transfer $ℏ ω = 12 meV$ slices in the ( $H 0 L$ ) plane taken at SEQUOIA at temperatures (a) $T = 5.0 K$ , and (b) $T = 90 K$ . Inelastic features at the corners around $q = (\pm 2 0 \pm 2)$ are phonons. (c), (d) $Q$ dependence of the spin wave excitations below (filled black circles) and above (open red diamonds) $T_{N}$ obtained through constant energy cut as shown by solid lines in the upper panels. Lines are guide to the eyes for the observed scattering.
Reuse & Permissions

Physical Review B

covering condensed matter and materials physics