Structural distortion and incommensurate noncollinear magnetism in ${\mathrm{EuAg}}_{4}{\mathrm{As}}_{2}$

Structural distortion and incommensurate noncollinear magnetism in ${EuAg}_{4} {As}_{2}$

Bing Shen, Chaowei Hu, Huibo Cao, Xin Gui, Eve Emmanouilidou, Weiwei Xie, and Ni Ni

Phys. Rev. Materials 4, 064419 – Published 25 June 2020

Abstract

Layered pnictide materials have provided a fruitful platform to study various emergent phenomena, including superconductivity, magnetism, charge density waves, etc. Here we report the observation of structural distortion and noncollinear magnetism in layered pnictide ${EuAg}_{4} {As}_{2}$ via transport, magnetization, single crystal x-ray, and neutron diffraction data. ${EuAg}_{4} {As}_{2}$ single crystal shows a structural distortion at 120 K, where two sets of superlattice peaks with the propagation vectors of $q_{1} = \pm$ (0, 0.25, 0.5) and $q_{2} = \pm$ (0.25, 0, 1) emerge. Between 9–15 K, the hexagonal ${Eu}^{2 +}$ sublattice enters an unpinned incommensurate magnetic state, with magnetic Bragg reflections pictured as circular sectors. Below 9 K, it orders in an incommensurate noncollinear antiferromagnetic state with a well-defined propagation wavevector of (0, 0.1, 0.12) and a very rare magnetic structure, which is helical along the $c$ axis and cycloidal along the $b$ axis with a moment of 6.4 $μ_{B} / {Eu}^{2 +}$ . Furthermore, rich magnetic phases under magnetic fields, large magnetoresistance, and strong coupling between charge carriers and magnetism in ${EuAg}_{4} {As}_{2}$ are revealed.

Received 19 September 2018
Revised 18 May 2020
Accepted 2 June 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.064419

Physics Subject Headings (PhySH)

Antiferromagnetism Phase transitions

Single crystal materials

Methods in transport Neutron diffraction X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Bing Shen^1,*, Chaowei Hu^1,*, Huibo Cao ², Xin Gui³, Eve Emmanouilidou¹, Weiwei Xie³, and Ni Ni^1,†

¹Department of Physics and Astronomy and California NanoSystems Institute, University of California, Los Angeles, California 90095, USA
²Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
³Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, USA

^*These authors contributed equally to this work.
^†Corresponding author: nini@physics.ucla.edu

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Vol. 4, Iss. 6 — June 2020

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Figure 1
Single-crystal x-ray diffraction precession image of ${EuAg}_{4} {As}_{2}$ : (a), (d), and (g): the $(0 k l)$ , $(h 0 l)$ and $(h k 0)$ planes in the reciprocal lattice at 170 K. (b), (e), and (h): the $(0 k l)$ , $(h 0 l),$ and $(h k 0)$ planes in the reciprocal lattice at 100 K. (c), (f), and (i): the zoom-in plot of the selected area of (b), (e), and (h). The blue and olive arrows indicate the prorogation vectors. Some main Bragg peaks are indexed.
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Figure 2
Bulk physical properties of ${EuAg}_{4} {As}_{2}$ : (a) The temperature-dependent resistivity from 2–300 K. Top left: A single crystal of ${EuAg}_{4} {As}_{2}$ against 1-mm scale. Left Inset: The zoom-in resistivity from 100–120 K upon warming and cooling. Right inset: The zoom-in resistivity and the first derivative of the resistivity from 2–20 K. (b) The temperature-dependent $M / H$ from 2–300 K with $H / / c$ and $H / / a b$ and $H = 1$ T. Left inset: Temperature-dependent $H / M$ with the Curie-Weiss fit for $H / / c$ and $H / / a b$ . Right inset: $M / H$ at $H = 0.1$ T from 2–20 K with $H / / c$ and $H / / a b$ . (c) The temperature-dependent specific heat of ${EuAg}_{4} {As}_{2}$ from 2–150 K. The specific heat of ${SrAg}_{4} {As}_{2}$ is used as a nonmagnetic reference. Left inset: The entropy release from 2–40 K. Right inset: The zoom-in specific heat from 2–20 K.
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Figure 3
An isotropic isothermal magnetization and magnetotransport of ${EuAg}_{4} {As}_{2}$ . (a) Transverse resistivity $ρ_{x x}$ , Hall resistivity $ρ_{y x}$ , and magnetization measured under with the magnetic field parallel to $c$ at 2 K. (b) Transverse resistivity $ρ_{x x}$ and magnetization measured under the magnetic field parallel to $a b$ at 2 K. The dashed lines in (a) and (b) mark the fields of the transitions.
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Figure 4
Magnetic order parameter plot of ${EuAg}_{4} {As}_{2}$ . Black symbols: The intensity of the magnetic Bragg peaks (1, 0, 1) $+ k_{m}$ and (1, 0, 4) $+ k_{m}$ , where $k_{m}$ is the magnetic prorogation vector (0, 0.1, 0.12). Pink curve: The temperature-dependent $d ρ_{x x}$ / $d T$ . The dashed vertical lines mark the transition temperatures.
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Figure 5
The noncollinear magnetic structure of ${EuAg}_{4} {As}_{2}$ . (a) the structure in one unit cell at 4 K. Blue sphere: Eu. Gray sphere: As. Orange sphere: Fully occupied Ag. Light orange sphere: Partially occupied Ag. (b)–(c) Periodicity beyond one unit cell along the $c$ and and $b$ direction, respectively. Only ${Eu}^{2 +}$ ions are shown.
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Figure 6
Single-crystal neutron diffraction of ${EuAg}_{4} {As}_{2}$ near the (2, 0, 1) nuclear peak: (a)–(b) Slices at $Δ k_{z} = \pm 0.026 {nm}^{- 1}$ taken at 4 K. Coordinates are given relative to the (2, 0, 1) nuclear peak. (c)–(d) Slices at $Δ k_{z} = \pm 0.026 {nm}^{- 1}$ taken at 10.3 K. (e)–(f) Cylindrical mapping of the magnetic reflection, with integrated magnetic diffraction intensity along each azimuthal angle $θ$ and polar angle $ϕ$ at 4 K and 10.3 K, respectively.
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Physical Review Materials

Structural distortion and incommensurate noncollinear magnetism in EuAg4As2

Bing Shen, Chaowei Hu, Huibo Cao, Xin Gui, Eve Emmanouilidou, Weiwei Xie, and Ni Ni

Phys. Rev. Materials 4, 064419 – Published 25 June 2020

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Structural distortion and incommensurate noncollinear magnetism in ${EuAg}_{4} {As}_{2}$