Magnetic inhomogeneity in the copper pseudochalcogenide CuNCN

A. Zorko, P. Jeglič, M. Pregelj, D. Arčon, H. Luetkens, A. L. Tchougréeff, and R. Dronskowski

Phys. Rev. B 97, 214432 – Published 27 June 2018

Abstract

Copper carbodiimide, CuNCN, is a geometrically frustrated nitrogen-based analog of cupric oxide, whose magnetism remains ambiguous. Here, we employ a combination of local-probe techniques, including ${}^{63, 65}{Cu}$ nuclear quadrupole resonance, ${}^{13}C$ nuclear magnetic resonance, and muon spin rotation to show that the magnetic ground state of the ${Cu}^{2 +}$ ( $S = 1 / 2$ ) spins is frozen and disordered. Moreover, these complementary experiments unequivocally establish the onset of an intrinsically inhomogeneous magnetic state at $T_{h} = 80$ K. Below $T_{h}$ , the low-temperature frozen component coexists with the remnant high-temperature dynamical component down to $T_{l} = 20$ K, where the latter finally ceases to exist. Based on a scaling of internal magnetic fields of both components, we conclude that the two components coexist on a microscopic level.

Received 18 April 2018

DOI:https://doi.org/10.1103/PhysRevB.97.214432

Physics Subject Headings (PhySH)

Frustrated magnetism Magnetic phase transitions

Muon spin relaxation & rotation Nuclear magnetic resonance Nuclear quadrupole resonance

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

A. Zorko^1,*, P. Jeglič¹, M. Pregelj¹, D. Arčon^1,2, H. Luetkens³, A. L. Tchougréeff^4,5,6, and R. Dronskowski⁴

¹Jožef Stefan Institute, Jamova c. 39, SI-1000 Ljubljana, Slovenia
²Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia
³Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
⁴Institute of Inorganic Chemistry, RWTH Aachen University, D-52056 Aachen, Germany
⁵Moscow Center for Continuous Mathematical Education, Bol. Vlasevskyi 11, 119002 Moscow, Russia
⁶A. N. Frumkin Institute of Physical Chemistry and Electrochemistry of Russian Academy of Science, Moscow, Russia

^*andrej.zorko@ijs.si

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Vol. 97, Iss. 21 — 1 June 2018

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Images

Figure 1
The unit cell of CuNCN. The on-site hyperfine interaction of the ${}^{63, 65}{Cu}$ nuclei with the ${Cu}^{2 +}$ electronic moment is denoted by $A_{hf}^{Cu}$ , while the transferred hyperfine interactions of various nuclei $n$ with the ${Cu}^{2 +}$ electronic moment are indicated by arrows and labeled as ${\tilde{A}}_{hf}^{n}$ .
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Figure 2
(a) The ${}^{63, 65}{Cu}$ NQR spectra of CuNCN at a few selected temperatures. The spectra are shifted vertically for clarity, and their intensity is normalized by the Boltzmann population factor $1 / T$ and corrected for the spin-spin relaxation time $T_{2}$ [inset in (d)]. (b) The temperature dependence of the corrected ${}^{63}{Cu}$ NQR line intensity compared to the unfrozen part of the $μ SR$ signal (reproduced from Ref. [27]). The temperature dependence of the ${}^{63, 65}{Cu}$ NQR (c) line position and (d) linewidth. The inset displays the temperature dependence of the ${}^{63}{Cu}$ NQR spin-spin relaxation rate. The arrows indicate special temperatures $T_{h}$ , $T_{l}$ , and $T^{*}$ .
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Figure 3
(a) The ${}^{13}C$ NMR spectra of CuNCN, with the two components ( ${}^{13}C_{1}$ and ${}^{13}C_{2}$ ) appearing below $T_{h} = 80$ K. The spectra are shifted vertically for clarity, and their intensity is normalized by the Boltzmann factor $1 / T$ . (b) The powder simulation (dotted line) of the 300 K spectrum (circles) with axially symmetric anisotropy. (c) The fit (solid line) of the 30 K spectrum (cycles) with two Gaussian components ${}^{13}C_{1}$ (dotted line) and ${}^{13}C_{2}$ (dashed line). (d) The temperature dependence of the spin-lattice relaxation rate, measured at the positions $ν_{A}$ and $ν_{B}$ , marked by arrows in panel (a). The solid lines display a linear temperature dependence. The arrows indicate the magnetically inhomogeneous region between $T_{h}$ and $T_{l}$ . Inset: the temperature dependence of the stretching exponent at the two frequencies.
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Figure 4
(a) Oscillation of muon asymmetry in the transverse magnetic field of 520 mT and (b) the corresponding Fourier transforms. The curves are shifted vertically for clarity. (c) The $μ SR$ signal at 50 K, demonstrating the presence of two oscillating component with significantly different damping rates. The solid lines in (a) and (c) correspond to fits with two damped cosine contributions [Eq. (1)], while in (b) the line represents a Gaussian fit.
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Figure 5
(a) The temperature dependence of the normalized intensity of the narrow $μ_{1}$ and the broad $μ_{2} μ SR$ component [defined as $A_{i} (T) / A_{0}$ , where $A_{0}$ corresponds to full asymmetry] compared to the relative intensities $I_{i} / (I_{1} + I_{2})$ of the narrow ${}^{13}C_{1}$ and broad ${}^{13}C_{2}$ NMR component and the ${}^{63}C$ NQR intensity. (b) Comparison of the $μ SR$ relaxation rates, the ${}^{13}C$ NMR linewidth of the narrow (1) and broad (2) components, and the ${}^{63}C$ NQR linewidth. The arrows indicate specific temperatures $T_{h}$ and $T_{l}$ .
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