Momentum-Dependent Magnon Lifetime in the Metallic Noncollinear Triangular Antiferromagnet ${\mathrm{CrB}}_{2}$

Momentum-Dependent Magnon Lifetime in the Metallic Noncollinear Triangular Antiferromagnet ${CrB}_{2}$

Pyeongjae Park, Kisoo Park, Taehun Kim, Yusuke Kousaka, Ki Hoon Lee, T. G. Perring, Jaehong Jeong, Uwe Stuhr, Jun Akimitsu, Michel Kenzelmann, and Je-Geun Park

Phys. Rev. Lett. 125, 027202 – Published 7 July 2020

Abstract

Noncollinear magnetic order arises for various reasons in several magnetic systems and exhibits interesting spin dynamics. Despite its ubiquitous presence, little is known of how magnons, otherwise stable quasiparticles, decay in these systems, particularly in metallic magnets. Using inelastic neutron scattering, we examine the magnetic excitation spectra in a metallic noncollinear antiferromagnet ${CrB}_{2}$ , in which Cr atoms form a triangular lattice and display incommensurate magnetic order. Our data show intrinsic magnon damping and continuumlike excitations that cannot be explained by linear spin wave theory. The intrinsic magnon linewidth $Γ (q, E_{q})$ shows very unusual momentum dependence, which our analysis shows to originate from the combination of two-magnon decay and the Stoner continuum. By comparing the theoretical predictions with the experiments, we identify where in the momentum and energy space one of the two factors becomes more dominant. Our work constitutes a rare comprehensive study of the spin dynamics in metallic noncollinear antiferromagnets. It reveals, for the first time, definite experimental evidence of the higher-order effects in metallic antiferromagnets.

Received 8 April 2020
Accepted 19 June 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.027202

Physics Subject Headings (PhySH)

Frustrated magnetism RKKY interaction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Pyeongjae Park^1,2,3, Kisoo Park^2,3, Taehun Kim ^1,2,3, Yusuke Kousaka⁴, Ki Hoon Lee^2,3,5, T. G. Perring⁶, Jaehong Jeong^2,3, Uwe Stuhr ⁷, Jun Akimitsu⁸, Michel Kenzelmann⁷, and Je-Geun Park ^1,2,3,*

¹Center for Quantum Materials, Seoul National University, Seoul 08826, Republic of Korea
²Center for Correlated Electron Systems, Institute for Basic Science, Seoul 08826, Republic of Korea
³Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea
⁴Department of Physics and Electronics, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
⁵Center for Theoretical Physics of Complex Systems, Institute for Basic Science (IBS), Daejeon 34126, Korea
⁶ISIS Pulsed Neutron and Muon Source, STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire, OX11 0QX, United Kingdom
⁷Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, CH-5232 Villigen, Switzerland
⁸Research Institute for Interdisciplinary Science, Okayama University, Okayama, Okayama 700-8530, Japan

^*jgpark10@snu.ac.kr

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Vol. 125, Iss. 2 — 10 July 2020

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Figure 1
(a) Crystal and (b) noncollinear magnetic structure of ${CrB}_{2}$ with up to the 5th nearest-neighbor coupling $J_{m}$ and an interlayer coupling $J_{c}$ being marked. (c) Incommensurate cycloidal magnetic order of ${CrB}_{2}$ along the [110] direction. (d) Magnetization of single crystal ${CrB}_{2}$ along several directions measured under the magnetic field of 1 T. Inset shows inverse susceptibility consistent with the Curie-Weiss behavior above $T_{N}$ . (e) Three symmetrically equivalent magnetic Bragg peaks of ${CrB}_{2}$ with labels of $q$ points used in Fig. 2. White solid (dashed) lines indicate the momentum contours used in Figs. 2, 2, respectively.
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Figure 2
(a)–(b) INS data obtained at MAPS along the two paths shown in Fig. 1, and (c)–(d) simulations of the same spectra using LSWT convoluted with the instrumental resolution of MAPS. Black solid lines are the phonon dispersion lines from the DFT calculations, and red dotted lines are the fitted spin-wave dispersion lines from our magnetic Hamiltonian. White squares highlight the region in the $q − E$ space where there exist continuumlike excitations, which cannot be explained by LSWT.
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Figure 3
(a),(b) Comparison between INS data and LSWT simulation with (blue solid lines) and without (red dotted lines) the magnon damping effect considered. (a) and (b) A stack of constant $q$ cuts at different $q$ position along the [-2H H 0] and [-K K 0] directions, respectively. Each one corresponds to the $E_{L = 0} - F$ and $D - G$ lines in Fig. 2. For a better presentation, scale factors were applied to the undamped calculation. The two constant $q$ cuts at the bottom of (a) clearly show additional continuumlike excitations at the $q$ positions denoted by the white squares in Fig. 2. Error bars indicate the standard deviations of the data points.
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Figure 4
(a) Intrinsic HWHM plot of magnon modes [ $Γ (q, E_{q})$ ] along the [-2H H 0] direction with calculated two-magnon (red solid line) and Stoner (blue dashed line) continuum DOS at $(q, E_{q})$ . $Γ (q, E_{q})$ was extracted from the fitting (Fig. 3) with the instrumental resolution excluded. Data points in a shaded region may not be reliable due to the overlap with the continuumlike signal. Error bars indicate the standard deviations of the fitted HWHM. (b) INS data, (c) calculated two-magnon continuum DOS, and (d) calculated Stoner continuum DOS along the same direction as in (a). Red dotted lines indicate the magnon dispersion $E_{q}$ from the LSWT calculations. White rectangles indicate the region where the continuumlike excitations appear. (e)–(h) are shown along the [-K K 0] direction. For a better presentation, a logarithmic scale was used in (c)–(d) and (g)–(h).
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Physical Review Letters

Momentum-Dependent Magnon Lifetime in the Metallic Noncollinear Triangular Antiferromagnet CrB2

Pyeongjae Park, Kisoo Park, Taehun Kim, Yusuke Kousaka, Ki Hoon Lee, T. G. Perring, Jaehong Jeong, Uwe Stuhr, Jun Akimitsu, Michel Kenzelmann, and Je-Geun Park

Phys. Rev. Lett. 125, 027202 – Published 7 July 2020

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Momentum-Dependent Magnon Lifetime in the Metallic Noncollinear Triangular Antiferromagnet ${CrB}_{2}$