Control of the third dimension in copper-based square-lattice antiferromagnets

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Control of the third dimension in copper-based square-lattice antiferromagnets

Paul A. Goddard, John Singleton, Isabel Franke, Johannes S. Möller, Tom Lancaster, Andrew J. Steele, Craig V. Topping, Stephen J. Blundell, Francis L. Pratt, C. Baines, Jesper Bendix, Ross D. McDonald, Jamie Brambleby, Martin R. Lees, Saul H. Lapidus, Peter W. Stephens, Brendan W. Twamley, Marianne M. Conner, Kylee Funk, Jordan F. Corbey, Hope E. Tran, J. A. Schlueter, and Jamie L. Manson

Phys. Rev. B 93, 094430 – Published 25 March 2016

Abstract

Using a mixed-ligand synthetic scheme, we create a family of quasi-two-dimensional antiferromagnets, namely, $[Cu ({HF}_{2}) {(pyz)}_{2}] {ClO}_{4}$ [pyz = pyrazine], $[Cu L_{2} {(pyz)}_{2}] {({ClO}_{4})}_{2} [L$ = pyO = pyridine-N-oxide and 4-phpy-O = 4-phenylpyridine-N-oxide. These materials are shown to possess equivalent two-dimensional ${[Cu {(pyz)}_{2}]}^{2 +}$ nearly square layers, but exhibit interlayer spacings that vary from 6.5713 to 16.777 Å, as dictated by the axial ligands. We present the structural and magnetic properties of this family as determined via x-ray diffraction, electron-spin resonance, pulsed- and quasistatic-field magnetometry and muon-spin rotation, and compare them to those of the prototypical two-dimensional magnetic polymer $Cu {(pyz)}_{2} {({ClO}_{4})}_{2}$ . We find that, within the limits of the experimental error, the two-dimensional, intralayer exchange coupling in our family of materials remains largely unaffected by the axial ligand substitution, while the observed magnetic ordering temperature (1.91 K for the material with the ${HF}_{2}$ axial ligand, 1.70 K for the pyO and 1.63 K for the 4-phpy-O) decreases slowly with increasing layer separation. Despite the structural motifs common to this family and $Cu {(pyz)}_{2} {({ClO}_{4})}_{2}$ , the latter has significantly stronger two-dimensional exchange interactions and hence a higher ordering temperature. We discuss these results, as well as the mechanisms that might drive the long-range order in these materials, in terms of departures from the ideal $S = 1 / 2$ two-dimensional square-lattice Heisenberg antiferromagnet. In particular, we find that both spin-exchange anisotropy in the intralayer interaction and interlayer couplings (exchange, dipolar, or both) are needed to account for the observed ordering temperatures, with the intralayer anisotropy becoming more important as the layers are pulled further apart.

Received 30 January 2016
Revised 1 March 2016

DOI:https://doi.org/10.1103/PhysRevB.93.094430

Physics Subject Headings (PhySH)

Antiferromagnetism Molecular magnetism

Antiferromagnets Polymers

Electron spin resonance Magnetization measurements Muon spin resonance X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Paul A. Goddard^1,*, John Singleton^2,3, Isabel Franke^3,†, Johannes S. Möller^3,‡, Tom Lancaster⁴, Andrew J. Steele³, Craig V. Topping³, Stephen J. Blundell³, Francis L. Pratt⁵, C. Baines⁶, Jesper Bendix⁷, Ross D. McDonald², Jamie Brambleby¹, Martin R. Lees¹, Saul H. Lapidus^8,9, Peter W. Stephens⁸, Brendan W. Twamley¹⁰, Marianne M. Conner¹¹, Kylee Funk¹², Jordan F. Corbey¹¹, Hope E. Tran¹¹, J. A. Schlueter¹², and Jamie L. Manson^11,§

¹Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, United Kingdom
²National High Magnetic Field Laboratory, Los Alamos National Laboratory, MS-E536, Los Alamos, New Mexico 87545, USA
³Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom
⁴Department of Physics, Durham University, South Road, Durham, DH1 3LE, United Kingdom
⁵ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom
⁶Paul Scherrer Institut, Laboratory for Muon-Spin Spectroscopy, CH-5232 Villigen PSI, Switzerland
⁷Department of Chemistry, University of Copenhagen, Copenhagen DK-2100, Denmark
⁸Department of Physics and Astronomy, State University of New York, Stony Brook, New York 11794, USA
⁹X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL, 60439, USA
¹⁰University Research Office, University of Idaho, Moscow, Idaho 83844, USA
¹¹Department of Chemistry and Biochemistry, Eastern Washington University, Cheney, Washington 99004, USA
¹²Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

^*Author to whom correspondence should be addressed: p.goddard@warwick.ac.uk
^†Present address: Helios, 29 Hercules Way, Aerospace Boulevard, Aero Park, Farnborough GU14 6UU, United Kingdom.
^‡Present address: Neutron Scattering and Magnetism, Laboratory for Solid State Physics, ETH Zurich, CH-8093 Zurich, Switzerland.
^§Author to whom correspondence should be addressed: jmanson@ewu.edu

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Vol. 93, Iss. 9 — 1 March 2016

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Images

Figure 1
Experimentally determined crystal structures viewed along the copper-pyrazine planes. All structures shown were determined via single-crystal x-ray diffraction at 100 K, except for No. 1 for which both 300 and 50 K structures are presented. Here, pyz = pyrazine, pyO = pyridine-N-oxide, 4-phpyO = 4-phenyl-pyridine-N-oxide; as shown in the inset key, Cu = brown, Cl = green, C = gray, N = blue, H = cyan, O = red, and F = light green. All hydrogens from organic molecules are omitted for clarity. Broken lines denote a single unit cell. All four systems are based on two-dimensional arrays of Cu-pyz in which pyz orbitals mediate the dominant exchange interactions. In No. 1, the triclinic structure is supported by strong $F \dots H \dots F$ hydrogen bonds that form bridging ligands separating the layers [11]. The three other systems have nonbridging ligands along the interlayer direction and a monoclinic structure. The nonbridging ligands in Nos. 2 and 3 are pyO and 4-phpyO, respectively. While the ${ClO}_{4}$ molecules are noncoordinating anions in Nos. 1, 2, and 3, they coordinate to the Cu ions in No. 4.
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Figure 2
(a) Powder ESR spectra of $Cu {(pyz)}_{2} {({ClO}_{4})}_{2}$ (No. 4) measured at a frequency of 71 GHz. (b) Similar powder spectra for $[Cu {(4-phpy-O)}_{2} {(pyz)}_{2}] {({ClO}_{4})}_{2}$ (No. 3) measured at a frequency of 70.3 GHz. Above 10 K the $g$ -factor anisotropy in both cases is $g_{z} = 2.25$ and $g_{x y} = 2.04$ . (c) The $sin (2 θ)$ angle dependance of the $g$ -factor of $[Cu {(pyO)}_{2} {(pyz)}_{2}] {({ClO}_{4})}_{2}$ (No. 2) as field is rotated between perpendicular to $(0^{\circ})$ and parallel to $(90^{\circ})$ the Cu–pyz planes, measured at a temperature of 20 K. Again the $g$ -factor anisotropy is $g_{z} = 2.25$ and $g_{x y} = 2.04$ . (d) The frequency-magnetic field scaling of the ESR line measured on single crystal $[Cu ({HF}_{2}) {(pyz)}_{2}] {ClO}_{4}$ (No. 1) with the field oriented perpendicular to the planes.
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Figure 3
Pulsed-field magnetization vs field. Data shown are recorded in increasing fields and at a temperature of 0.60 K. (a) and (b) show single-crystal data for $[Cu ({HF}_{2}) {(pyz)}_{2}] {ClO}_{4}$ (No. 1) and $[Cu {(pyO)}_{2} {(pyz)}_{2}] {({ClO}_{4})}_{2}$ (No. 2), respectively, with the field applied parallel and perpendicular to the copper-pyrazine planes. (c) Polycrystalline data for $[Cu {(4-phpy-O)}_{2} {(pyz)}_{2}] {({ClO}_{4})}_{2}$ (No. 3) and $Cu {(pyz)}_{2} {({ClO}_{4})}_{2}$ (No. 4). (d) All of the data vs $H / H_{c}$ , where $H_{c}$ is the saturation field.
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Figure 4
(a) Muon asymmetry observed at 0.41 K (No. 1), 0.1 K (No. 2), 0.26 K (No. 3), and 0.34 K (No. 4); points are data, while lines represent the fits described in the text. The data for No. 1 and No. 4 are taken from Refs. [14] and [12]. (b) Temperature evolution of the oscillation frequencies $ν$ . The solid lines are fits to the functional form $ν_{i} (T) = ν_{i} (0) {[1 - {(T / T_{c})}^{α}]}^{β}$ as described in the text.
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Figure 5
(a)–(d) Low-field magnetization data (points) for polycrystalline samples for Nos. 1, 2, and 3 taken at 500 mK and a single crystal of compound No. 4. For the latter data, taken from Ref. [70], the magnetic field was applied both parallel (solid circles) and perpendicular (open circles) to the copper-pyrazine planes. The anisotropy fields, $H_{A}$ , are determined by the peaks in $d^{2} M / d H^{2}$ (solid line). (e) Experimentally determined values of $T_{c} / J$ and the $X Y$ -type spin-exchange anisotropy, $Δ = H_{A} / H_{c}$ , for the compounds mentioned in the text (points). Also shown is the theoretical relation (line) determined via quantum Monte Carlo simulations [8].
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