Magnetism dependent phonon anomaly in LaFeAsO observed via inelastic x-ray scattering

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Ames Laboratory

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Magnetism dependent phonon anomaly in

LaFeAsO observed via inelastic x-ray scattering

Steven E. Hahn

Iowa State University, quantumsteve@gmail.com

Gregory S. Tucker

Iowa State University, tucker@iastate.edu

J.-Q. Yan

Iowa State University

A. H. Said

Argonne National Laboratory

B. M. Leu

Argonne National Laboratory

See next page for additional authors

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Recommended Citation

Hahn, Steven E.; Tucker, Gregory S.; Yan, J.-Q.; Said, A. H.; Leu, B. M.; McCallum, R. William; Alp, E. E.; Lograsso, Thomas A.; McQueeney, Robert J.; and Harmon, Bruce N., "Magnetism dependent phonon anomaly in LaFeAsO observed via inelastic x-ray scattering" (2013).Ames Laboratory Conference Papers, Posters, and Presentations. 4.

Magnetism dependent phonon anomaly in LaFeAsO observed via

inelastic x-ray scattering

Abstract

The phonon dispersion was measured at room temperature (above the Néel temperature TN) along (0,0,L) in the tetragonal phase of LaFeAsO using inelastic x-ray scattering. Magnetostructural effects are well

documented in the AFe2As2-based (A = Ca, Sr, Ba, Eu) systems. Only recently have single crystals of

LaFeAsO become available. The experimentally observed splitting between twoA1gphonon modes at 22 and 26 meV is only reproduced in spin-polarized calculations. Magnetostructural effects similar to those observed in the AFe2As2materials are confirmed to be present in LaFeAsO. This is discussed in terms of the strong

antiferromagnetic correlations that are known to persist above TNand into the tetragonal phase.

Keywords

Physics and Astronomy, antiferromagnetic materials, iron compounds, lanthanum compounds, magnetic structure, phonon dispersion relations, superconducting materials, X-ray scattering

Disciplines

Condensed Matter Physics | Metallurgy

Comments

Copyright 2013 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.

The following article appeared inJournal of Applied Physics113 (2012): 17E153 and may be found at

http://dx.doi.org//10.1063/1.4800657.

Authors

Steven E. Hahn, Gregory S. Tucker, J.-Q. Yan, A. H. Said, B. M. Leu, R. William McCallum, E. E. Alp, Thomas A. Lograsso, Robert J. McQueeney, and Bruce N. Harmon

Magnetism dependent phonon anomaly in LaFeAsO observed via inelastic

x-ray scattering

S. E. Hahn, G. S. Tucker, J.-Q. Yan, A. H. Said, B. M. Leu et al.

Citation: J. Appl. Phys. 113, 17E153 (2013); doi: 10.1063/1.4800657

View online: http://dx.doi.org/10.1063/1.4800657

View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v113/i17

Published by the AIP Publishing LLC.

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Magnetism dependent phonon anomaly in LaFeAsO observed via inelastic

x-ray scattering

S. E. Hahn,1,2G. S. Tucker,1,2J.-Q. Yan,2,b)A. H. Said,3B. M. Leu,3R. W. McCallum,2 E. E. Alp,3T. A. Lograsso,2R. J. McQueeney,1,2,a)and B. N. Harmon1,2

1

Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA 2

Division of Materials Sciences and Engineering, Ames Laboratory US-DOE, Iowa State University, Ames, Iowa 50011, USA

3

Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA

(Presented 16 January 2013; received 4 November 2012; accepted 22 January 2013; published online 16 April 2013)

The phonon dispersion was measured at room temperature (above the Neel temperature TN) along

(0,0,L) in the tetragonal phase of LaFeAsO using inelastic x-ray scattering. Magnetostructural effects are well documented in the AFe2As2-based (A¼Ca, Sr, Ba, Eu) systems. Only recently have single crystals of LaFeAsO become available. The experimentally observed splitting between two A1g phonon modes at 22 and 26 meV is only reproduced in spin-polarized calculations.

Magnetostructural effects similar to those observed in the AFe2As2materials are confirmed to be present in LaFeAsO. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist above TNand into the tetragonal phase.VC 2013 AIP Publishing LLC

[http://dx.doi.org/10.1063/1.4800657]

Despite rather convincing arguments that superconduc-tivity in the AFe2As2 (A¼Ca, Sr, Ba, Eu) and RFeAsO (R¼La, Ce, Pr, Nd, Sm, Gd)-based compounds does not originate from conventional electron-phonon coupling,1 these systems do display significant sensitivity to the lattice geometry.2 While these magnetostructural effects are well documented in the AFe2As2-based systems, the difficulty of synthesizing RFeAsO in single-crystalline form allowed only for a limited quantitative confirmation of similar mag-netostructural coupling across the AFe2As2 and RFeAsO systems.

LaFeAsO single crystals were synthesized in NaAs flux at ambient pressure as described elsewhere.3Inelastic x-ray scattering measurements were performed on the HERIX instrument at sector 30-ID-C of the Advanced Photon Source at Argonne National Laboratory.4,5Scattering is described in terms of the tetragonal P4/nmm unit cell. In order to under-stand the features of the phonon dispersion, the experimental measurements were compared toab initiocalculations of the phonons using Density Functional Theory (DFT) and Density Functional Perturbation Theory (DFPT).6The exper-imental lattice parameters at room temperature in the tetrag-onal phase (a¼4.03533 A˚ andc¼8.74090 A˚ ) were used for all calculations.7,8 The calculated relaxed positions, where all forces were zero, were used for the internalz-parameters determining the position of lanthanum and arsenic atoms. Structural parameters used for the nonmagnetic and spin-polarized calculations as well as experimental results are given in Table I. The pseudopotentials chosen used the Perdew-Burke-Ernzerhof (PBE) exchange correlation

functional.9,10 Additional experimental and computational details are given elsewhere.11

Figure1shows a scan consisting of several phonon excita-tions atQ¼ ð0;0;8:4Þat room temperature. Experimental data are given by green dots and pseudo-Voigt fit by a solid black line. The peak positions for this and other scans were used to construct the dispersion of phonon branches along different directions, as shown in Fig.2. The intensity of the phonon modes multiplied by the energy is also represented in Fig. 2 by the diameter of the circles. At (0,0,8.4), the acoustic mode and a nearby optical mode are present at 7 and 11meV, along with three other modes at 22, 26, and 34 meV, respectively. Over the entire range measured, the Lorentz fraction gvaries between 0.48 and 1.0 and the full width at half maximumCvaries between 1.85 and 3.96.

Fig.3shows several calculations of the dynamical struc-ture factor atQ¼ ð0;0;8:4Þ, which can be directly compared to Fig. 1. The red dotted line is a nonmagnetic calculation. Frequencies for the acoustic and lowest optical modes are in reasonable agreement with the experiment, but the calculated intensity of the optical mode is too high. The phonon excita-tion near 24 meV consists of two modes separated by less than 1 meV. These two A1g modes consist of As and La

motion polarized along thec-axis. This result from the non-magnetic calculation is inconsistent with the measurements, where these two modes are clearly split by 4 meV at (0,0,8.4). At this value of Q, the 32 meV feature consists of both Fe and As motion, but the intensity is extremely weak.

In the spin-polarized calculation corresponding to the observed spin density wave (SDW) structure, the effect of the Fe magnetization is to strongly split the two 24 meV branches at (0,0,8.4) with the 21 meV excitation, contain-ing As motion, lowercontain-ing its energy by approximately 7.8%. The ratio of intensities between the acoustic and the nearby optical mode moves in the same direction as

a)_{Electronic mail: mcqueeney@ameslab.gov.} b)

This work was originally done at Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA and Department of Materials and Engineering, The University of Tennessee, Knoxville, Tennessee 37996, USA.

0021-8979/2013/113(17)/17E153/3/$30.00 113, 17E153-1 VC2013 AIP Publishing LLC

in the experiment. In order to better understand the impor-tance of the specific magnetic order and the size of the Fe moment on the lattice dynamics, two additional calcula-tions were performed for hypothetical magnetic structures. First is the “checkerboard” magnetic structure. It is a tet-ragonal space group, where Fe neighbors have opposite spins. Second is the CeFeAsO structure,12 also referred to as “striped.” It is an orthorhombic space group with ferro-magnetic coupling of Fe moments along the c-axis. The dynamical structure factor for this material is shown with black dashes in Fig.3.

Fig.2compiles all of the experimental data and calcula-tions of the nonmagnetic and SDW magnetic structure by showing several contour plots of the dynamical structure fac-tor along ð0;0;LÞ. Calculations with different magnetic ordering are similar. Values of calculated intensities range from no intensity (blue) to high intensity (red) and have been multiplied by the energy to improve visibility of the optical modes. The white dots show the experimentally determined frequencies with the size of each dot being proportional to the intensity. Including calculations with a magnetic moment on the Fe shows splitting between the twoA1g modes near

24 meV. Overall changes in the phonon frequencies and intensities indicate the complex and subtle effects that mag-netic ordering has on the lattice dynamics.

Despite the changes introduced by the magnetic order, all the spectra are qualitatively similar for different magnetic

alignments, and in better agreement with the experiment com-pared to the nonmagnetic calculations. This behavior can be interpreted as a consequence of Fe moments still being pres-ent above TN, albeit without long-range order.13–15Compared to nonmagnetic calculations, imposing an antiferromagnetic (AFM) ordering better describes phonons in LaFeAsO. Consequently, it is likely that the presence of Fe moments, or-dered or not, affects the force constants. On-site Fe and As force constants show significant softening compared to

FIG. 1. Energy scan at constant-Q atQ¼ ð0;0;8:4Þmeasured at room tem-perature on LaFeAsO. Experimental data are given by solid green dots. The black line is a fit using a pseudo-Voigt function.

FIG. 2. Contour plots of the calculated dynamical structure factor along (0,0,L). Values range from no intensity (blue) to high intensity (red), and have been multiplied by the energy to improve visibility of the optical modes. The white dots show the experimentally determined energies, as described in the text, with the intensity (also multiplied by the energy) shown by the size of the dot. (a) Nonmagnetic calculation and (b) calcula-tion with SDW magnetic order.

FIG. 3. Dynamical structure factor calculation of constant-Q line scan at

Q¼ ð0;0;8:4Þ. The dotted red line corresponds to nonmagnetic calculations of the dynamical structure factor. The solid green line corresponds to spin-polarized calculations imposing the SDW AFM ordering observed at lower temperatures. The black dashed line and blue dashed-dotted lines correspond to spin-polarized calculations with a striped (ferromagnetic along c) and checkerboard ordering, respectively. The experimentally observed frequen-cies in Fig.1are shown with vertical grey lines. The inset zooms in on the anomalous modes near 24 meV.

TABLE I. Theoretically relaxed and experimentally observed z-position for La and As atoms at room temperature, and the associated magnetic moment per Fe atom observed below TN and total energy. In each case, the room temperature experimental lattice parameters of (a¼4.03533 A˚ and

c¼8.74090 A˚ ) were used.7,8

NM SDW Striped Checkerboard Exp.7,8

zLa 0.13993 0.13875 0.13883 0.13887 0.14154

zAs 0.63829 0.64820 0.64770 0.64401 0.6512

lFe 0.0 2.32 2.30 1.91 0.36–0.78

E (Ry) 0.0 0.032 0.033 0.009

17E153-2 Hahnet al. J. Appl. Phys.113, 17E153 (2013)

nonmagnetic calculations; however, an investigation of the real-space force constants associates the magnetoelastic cou-pling with a complex renormalization instead of softening of a specific pairwise force.

In summary, we have measured the phonon dispersion along (0,0,L) in the tetragonal phase of LaFeAsO at room temperature, well above the magnetic ordering temperature of 138 K. Nonmagnetic calculations fail to reproduce the observed splitting between twoA1g phonon modes at 22 and

26 meV. Spin-polarized first-principles calculations impos-ing a number of hypothetical antiferromagnetic orders are qualitatively similar and in better agreement with the experi-mental results than non-spin-polarized calculations. The presence of Fe-spins is necessary to predict the observed spectrum above TN; however, the renormalization of the force constants is quite complex and cannot be reduced to a single pair-wise force constant.

This research supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Contract No. DE-AC02-07CH11358. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. The construction of HERIX was partially supported by the NSF under Grant No. DMR-0115852.

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