Effect of Zn doping on the antiferromagnetism in kagome ${\mathrm{Cu}}_{4\ensuremath{-}x}{\mathrm{Zn}}_{x}{(\mathrm{OH})}_{6}\mathrm{FBr}$

Effect of Zn doping on the antiferromagnetism in kagome ${Cu}_{4 - x} {Zn}_{x} {(OH)}_{6} FBr$

Zili Feng, Yuan Wei, Ran Liu, Dayu Yan, Yan-Cheng Wang, Jianlin Luo, Anatoliy Senyshyn, Clarina dela Cruz, Wei Yi, Jia-Wei Mei, Zi Yang Meng, Youguo Shi, and Shiliang Li

Phys. Rev. B 98, 155127 – Published 16 October 2018

Abstract

Barlowite ${Cu}_{4} {(OH)}_{6} FBr$ shows three-dimensional (3D) long-range antiferromagnetism, which is fully suppressed in ${Cu}_{3} Zn {(OH)}_{6} FBr$ with a kagome quantum spin liquid ground state. Here we report systematic studies on the evolution of magnetism in the ${Cu}_{4 - x} {Zn}_{x} {(OH)}_{6} FBr$ system as a function of $x$ to bridge the two limits of ${Cu}_{4} {(OH)}_{6} FBr (x = 0)$ and ${Cu}_{3} Zn {(OH)}_{6} FBr (x = 1)$ . Neutron-diffraction measurements reveal a hexagonal-to-orthorhombic structural change with decreasing temperature in the $x = 0$ sample. While confirming the 3D antiferromagnetic nature of low-temperature magnetism, the magnetic moments on some ${Cu}^{2 +}$ sites on the kagome planes are found to be vanishingly small, suggesting strong frustration already exists in barlowite. Substitution of interlayer ${Cu}^{2 +}$ with ${Zn}^{2 +}$ with gradually increasing $x$ completely suppresses the bulk magnetic order at around $x = 0.4$ but leaves a local secondary magnetic order up to $x \sim 0.8$ with a slight decrease in its transition temperature. The high-temperature magnetic susceptibility and specific-heat measurements further suggest that the intrinsic magnetic properties of kagome spin liquid planes may already appear from $x > 0.3$ samples. Our results reveal that the ${Cu}_{4 - x} {Zn}_{x} {(OH)}_{6} FBr$ may be the long-thought experimental playground for the systematic investigations of the quantum phase transition from a long-range antiferromagnet to a topologically ordered quantum spin liquid.

Received 20 December 2017
Revised 18 September 2018

DOI:https://doi.org/10.1103/PhysRevB.98.155127

Physics Subject Headings (PhySH)

Antiferromagnetism Quantum spin liquid

Kagome lattice

DC susceptibility measurements Neutron diffraction Specific heat measurements

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Zili Feng^1,2, Yuan Wei^1,2, Ran Liu¹, Dayu Yan^1,2, Yan-Cheng Wang^1,3, Jianlin Luo^1,2,4, Anatoliy Senyshyn⁵, Clarina dela Cruz⁶, Wei Yi⁷, Jia-Wei Mei⁸, Zi Yang Meng^1,9, Youguo Shi^1,*, and Shiliang Li^1,2,4,†

¹Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
²School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China
³School of Physical Science and Technology, China University of Mining and Technology, Xuzhou 221116, China
⁴Collaborative Innovation Center of Quantum Matter, Beijing 100190, China
⁵Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Garching D-85747, Germany
⁶Neutron Scattering Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁷Semiconductor Device Materials Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
⁸Institute for Quantum Science and Engineering, and Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China
⁹CAS Center of Excellence in Topological Quantum Computation, University of Chinese Academy of Sciences, Beijing 100190, China

^*ygshi@iphy.ac.cn
^†slli@iphy.ac.cn

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Vol. 98, Iss. 15 — 15 October 2018

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Figure 1
Neutron powder-diffraction intensities of ${Cu}_{4} {(OD)}_{6} FBr$ (red dots) at (a) 300 K, (b) 20 K, and (c) 3.3 K measured at SPODI. The calculated intensities are shown by the black lines. Short vertical green and red lines represent nuclear and magnetic Bragg peak positions, respectively. The blue line shows the difference between measured and calculated intensities. The weighted profile $R$ factor ( $R_{w p}$ ) is 4.22%, 4.46%, and 4.29% for (a)–(c), respectively. (d) Comparison between the calculated intensities of the $P n m a$ (red) and $C m c m$ (blue) structures at 20 K. The labeled peaks are in the $P n m a$ notation and cannot be indexed in the $C m c m$ structure.
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Figure 2
(a) Nuclear structure at 300 K (left) and 20 K (right) in the view vertical to the kagome plane. The diamond and rectangle are in-plane unit cells at 300 and 20 K, respectively. The subscripts of “ $H$ ” and “ $O$ ” denote hexagonal and orthorhombic structures, respectively. We note that the $c$ axis at room temperature becomes the $b$ axis at low temperatures. The three overlapping Cu atoms at 300 K represent three positions that the atom may actually occupy. (b) Detailed low-temperature structure showing only Cu (large blue circles) and O (small red circles) in the view parallel to orthorhombic $b$ (left) and orthorhombic $a$ (right), respectively. (c) Temperature dependence of the ${(1, 0, 2)}_{O}$ structural peak measured at HB-2A. The inset shows $2 θ$ scans around this peak at 20 and 300 K.
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Figure 3
(a) The three magnetic peaks obtained by subtracting the 20-K data from the 3.3-K data. The solid lines are calculated results of three types of magnetic structures. The inset shows the refined results of raw data measured at SPODI. The subscript “ $M$ ” denotes the magnetic structure. (b) Temperature dependence of ${(0, 0, 1)}_{M}$ (open red squares) and ${(1, 0, 1)}_{M}$ (filled blue circles) magnetic peaks measured at HB-2A. The solid lines are the fitted results as described in the main text. (c) Magnetic structure with the arrows indicating the sizes and directions of magnetic moments on Cu(3) ions (red and orange balls). The yellow and blue balls are Cu(1) and Cu(2) ions, respectively, whose structures cannot be determined as discussed in the main text. The dashed black lines indicate the magnetic unit cell with the origin shifted.
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Figure 4
(a) Room-temperature x-ray-diffraction data of selected ${Cu}_{4 - x} {Zn}_{x} {(OH)}_{6} FBr$ samples. The intensity axis is plotted with the logarithmic scale. (b) Evolution of lattice constants with Zn substitution level $x$ at room temperature. We note that $a_{H} = b_{H}$ . (c) Doping dependence of the splitting $Δ$ of Cu(3) from its average position as defined in the inset.
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Figure 5
(a) Low-temperature specific heat of ${Cu}_{4 - x} Zn {(OH)}_{6} FBr$ . All the samples were measured down to 2 K except for the $x = 0.92$ sample, which has already been reported previously [40]. The arrows indicate the bulk magnetic transition temperature $T_{N}$ , which is determined as the middle point of the drop of $Δ C / T$ during the transition. The low-temperature hump for the $x = 0.92$ sample is indicated by the star symbol. (b) Temperature dependence of $Δ S$ as defined in the main text. The red filled circles are results considering $C / T$ above 2 K, whereas the temperature in obtaining the data of the blue open squares is down to 50 mK. (c) Field dependence of magnetization at 2 K for selected samples at low fields. (d) Doping dependence of retentivity $M_{R}$ . The solid line is an exponential fit. (e) Low-temperature magnetic moments of ${Cu}_{4 - x} {Zn}_{x} {(OH)}_{6} FBr$ measured by the field-cooling process at 50 Oe. The solid lines are fitted results as described in the main text. The inset shows the data with the logarithmic scale. (f) Doping dependence of $T_{N}$ from specific heat ( $T_{N}^{C}$ , red squares) and susceptibility ( $T_{N}^{M}$ , blue circles) measurements. The $T_{N}$ of the $x = 0.92$ sample is manually set as zero. The dashed lines are guides to the eye.
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Figure 6
(a) Temperature dependence of $1 / χ$ for ${Cu}_{4 - x} {Zn}_{x} {(OH)}_{6} FBr$ . The dashed lines are fitted results between 150 and 300 K. (b) Doping dependence of the Curie temperature $θ_{C W}$ (left axis) and Curie constant $C_{C W}$ (right axis). (c) Field dependence of magnetization at 2 K for selected samples at high fields. The dashed lines are linear fitted results from 30 to 50 KOe. (d) Doping dependence of slope $B$ fitted from (c). (e) High-temperature specific heat of selected samples. (f) Doping dependence of $C (x) / C (0)$ at several temperatures.
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Physical Review B

covering condensed matter and materials physics

Effect of Zn doping on the antiferromagnetism in kagome ${Cu}_{4 - x} {Zn}_{x} {(OH)}_{6} FBr$

Zili Feng, Yuan Wei, Ran Liu, Dayu Yan, Yan-Cheng Wang, Jianlin Luo, Anatoliy Senyshyn, Clarina dela Cruz, Wei Yi, Jia-Wei Mei, Zi Yang Meng, Youguo Shi, and Shiliang Li

Phys. Rev. B 98, 155127 – Published 16 October 2018

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Physical Review B

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Effect of Zn doping on the antiferromagnetism in kagome Cu4−xZnx(OH)6FBr

Zili Feng, Yuan Wei, Ran Liu, Dayu Yan, Yan-Cheng Wang, Jianlin Luo, Anatoliy Senyshyn, Clarina dela Cruz, Wei Yi, Jia-Wei Mei, Zi Yang Meng, Youguo Shi, and Shiliang Li

Phys. Rev. B 98, 155127 – Published 16 October 2018

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Effect of Zn doping on the antiferromagnetism in kagome ${Cu}_{4 - x} {Zn}_{x} {(OH)}_{6} FBr$