Noncollinear spin-density-wave antiferromagnetism in FeAs

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Noncollinear spin-density-wave antiferromagnetism in FeAs

E. E. Rodriguez, C. Stock, K. L. Krycka, C. F. Majkrzak, P. Zajdel, K. Kirshenbaum, N. P. Butch, S. R. Saha, J. Paglione, and M. A. Green

Phys. Rev. B 83, 134438 – Published 28 April 2011

Abstract

The nature of the magnetism in the simplest iron arsenide is of fundamental importance in understanding the interplay between localized and itinerant magnetism and superconductivity. We present the magnetic structure of the itinerant monoarsenide FeAs with the B31 structure. Powder neutron diffraction confirms incommensurate modulated magnetism with wave vector $q = (0.395 \pm 0.001) c^{*}$ at 4 K, but can not distinguish between a simple spiral and a collinear spin-density-wave structure. Polarized single-crystal diffraction confirms that the structure is best described as a noncollinear spin-density wave arising from a combination of itinerant and localized behavior with spin amplitude along the $b$ -axis direction being ( $15 \pm 5$ ) $%$ larger than in the $a$ direction. Furthermore, the propagation vector is temperature dependent, and the magnetization near the critical point indicates a two-dimensional Heisenberg system. The magnetic structures of closely related systems are discussed and compared to that of FeAs.

Received 17 December 2010

DOI:https://doi.org/10.1103/PhysRevB.83.134438

Authors & Affiliations

E. E. Rodriguez¹, C. Stock^1,2, K. L. Krycka¹, C. F. Majkrzak¹, P. Zajdel³, K. Kirshenbaum⁴, N. P. Butch⁴, S. R. Saha⁴, J. Paglione⁴, and M. A. Green^1,5

¹NIST Center for Neutron Research, NIST, 100 Bureau Dr., Gaithersburg, Maryland 20878, USA
²Indiana University, 2401 Milo B. Sampson Lane, Bloomington, Indiana, 47408, USA
³Division of Physics of Crystals, Institute of Physics, University of Silesia, Katowice, PL-40-007, Poland
⁴Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, Maryland 20742, USA
⁵Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA

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Vol. 83, Iss. 13 — 1 April 2011

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Images

Figure 1
Two views of the orthorhombic crystal structure of FeAs with the unit cell drawn. For the view down the [010] direction, the four Fe atom sites are labeled. For the view down the [100] direction, the structure of FeAs is superimposed on the NiAs-type structure (un-rendered) and its hexagonal cell (thin dashed line). The Fe cations (orange rendered) are octahedrally coordinated to the arsenic anions (purple rendered).Reuse & Permissions
Figure 2
The geometry of the spin-polarized neutron experiment with the polarization direction $P_{0}$ perpendicular to the scattering vector $Q$ . Since the magnetic structure factor $F_{M} \propto S_{⊥}$ , in this geometry, the spin contribution to the $ζ$ and $η$ axes can be measured. Illustration of the different modulated magnetic structures in FeAs arising from either a simple spiral or a collinear spin-density wave (As atoms left out for clarity). The simple spiral can be modeled by a combination of representations as $Γ_{3} + Γ_{4}$ , whereas the SDW can be modeled by a single representation such as $Γ_{4}$ . Atoms Fe1 and Fe2 belong to one orbit (red arrows), while Fe3 and Fe4 belong to the other (blue arrows). The spin projections onto the $ab$ plane for a simple spiral and noncollinear SDW. The former traces out a circle, and the other an ellipse, with the long axis along $b$ for FeAs.Reuse & Permissions
Figure 3
(a) Contour plot of the (0 0 $2 + δ$ ) magnetic reflection versus temperature. (b) Integrated intensity of the magnetic reflection near the Nèel point and an order-parameter fit to the data. The critical exponent $β$ was found to be $0.16 \pm 0.02$ , which is close to a two-dimensional Heisenberg system such as $K_{2}$ MnO $_{4}$ ( $β = 0.15 \pm 0.01$ ) (Ref. 26). (c) Plot of the center of the magnetic reflection and, therefore, the value of the propagation vector $q$ versus temperature. A quadratic function fit to the data with the coefficients $c_{0} = 0.389$ , $c_{1} = - 5.360 \times 10^{- 5}$ , and $c_{2} = 3.991 \times 10^{- 6}$ . For comparison, the (002) nuclear peak’s center is plotted to show the thermal expansion of the lattice, which makes a minimal contribution to the increase of $q$ with temperature. (d) Plot of the FWHM of the (0 0 $2 + δ$ ) magnetic reflection up to the Nèel point. All error bars represent an uncertainty of $\pm σ$ .Reuse & Permissions
Figure 4
Non-spin flip (NSF) and spin flip (SF) intensities for nuclear and magnetic reflections in FeAs. In (a), the (2 0 0) nuclear reflection was measured to obtain the flipping ratio, which was found to be $\approx 15$ . In (b), where $P_{0} ∥ b$ axis, the NSF and SF intensities for the (0 $0 + δ$ ) magnetic peak indicate that $S_{⊥ ζ}$ is larger than $S_{⊥ η}$ . In (c), where $P_{0} ∥ z$ axis, the intensities of the NSF and SF channels are reversed. In (d), the result for the (1 0 $1 + δ$ ) magnetic peak also indicates that FeAs has ( $15 \pm 5$ ) $%$ more spin polarization in the $b$ direction than the $a$ direction, a result consistent with those for the (0 $0 + δ$ ) magnetic peak. In (e), the flipping ratio (NSF/SF) of the (0 $0 + δ$ ) reflection is shown to be temperature independent up to the Néel point. The data presented in (a)–(c) were taken at 15 K.Reuse & Permissions

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