Anticollinear order and degeneracy lifting in square lattice antiferromagnet ${\mathrm{LaSrCrO}}_{4}$

Letter

Anticollinear order and degeneracy lifting in square lattice antiferromagnet ${LaSrCrO}_{4}$

Jing Zhou, Guy Quirion, Jeffrey A. Quilliam, Huibo Cao, Feng Ye, Matthew B. Stone, Qing Huang, Haidong Zhou, Jinguang Cheng, Xiaojian Bai, Martin Mourigal, Yuan Wan, and Zhiling Dun

Phys. Rev. B 105, L180411 – Published 23 May 2022

Abstract

We report the static and dynamic magnetic properties of ${LaSrCrO}_{4}$ , a seemingly canonical spin-3/2 square-lattice antiferromagnet that exhibits frustration between magnetic layers—owing to their AB stacking—and offers a rare testbed to investigate accidental-degeneracy lifting in magnetism. Neutron diffraction experiments on single-crystal samples uncover a remarkable anticollinear magnetic order below $T_{N}$ = 170 K characterized by a Néel arrangement of the spins within each layer and an orthogonal arrangement between adjacent layers. To understand the origin of this unusual magnetic structure, we analyze the spin-wave excitation spectrum by means of inelastic neutron scattering and bulk measurements. A spectral gap of 0.5 meV, along with a spin-flop transition at 3.2 T, reflect the energy scale associated with the degeneracy-lifting. A minimal model to explain these observations requires both a positive biquadratic interlayer exchange and dipolar interactions, both of which are on the order of 10 $^{- 4}$ meV, only a few parts per million of the dominant exchange interaction $J_{1} \approx 11$ meV. These results provide direct evidence for the selection of a noncollinear magnetic structure by the combined effect of two distinct degeneracy lifting interactions.

Received 11 March 2022
Accepted 12 May 2022

DOI:https://doi.org/10.1103/PhysRevB.105.L180411

Physics Subject Headings (PhySH)

Frustrated magnetism Magnetic order

Antiferromagnets Transition metal oxides

Inelastic neutron scattering Models based on symmetries Neutron diffraction Spin wave theory Ultrasound techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jing Zhou ^1,2, Guy Quirion ³, Jeffrey A. Quilliam⁴, Huibo Cao ⁵, Feng Ye ⁵, Matthew B. Stone ⁵, Qing Huang⁶, Haidong Zhou⁶, Jinguang Cheng ¹, Xiaojian Bai ^7,*, Martin Mourigal ⁷, Yuan Wan^1,2,8,†, and Zhiling Dun ^1,7,‡

¹Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
²University of the Chinese Academy of Sciences, Beijing 100049, China
³Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John's, Canada A1B 3X7
⁴Institute Quantique, Départment de Physique, and RQMP, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1
⁵Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁶Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA
⁷School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
⁸Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China

^*Present address: Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
^†yuan.wan@iphy.ac.cn
^‡dun@iphy.ac.cn

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Vol. 105, Iss. 18 — 1 May 2022

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Figure 1
(a) The quasi-2D square lattice antiferromagnet with AB stacking comprises of two sublattices (dubbed A and B), each hosting a 3D Néel order. The two Néel vectors are decoupled at the mean-field level owing to the frustrated interlayer coupling. An easy-plane single-ion anisotropy forces the Néel vectors to be in the crystallographic $a b$ plane, which are then parametrized by their respective azimuthal angles ( $ϕ_{a}$ , $ϕ_{b}$ ). (b) Collinear spin structure with $ϕ_{a} = ϕ_{b} = \frac{π}{4}$ observed in ${La}_{2} {CuO}_{4}$ [26], ${Sr}_{2} {CuO}_{2} {Cl}_{2}$ [33], ${LaSrFeO}_{4}$ [32], and ${La}_{2} {CoO}_{4}$ in the orthorhombic phase [29]. (c) Collinear spin structure with $ϕ_{a} = ϕ_{b} = \frac{3 π}{4}$ for ${La}_{2} {NiO}_{4}$ [27] and possibly ${La}_{2} {CoO}_{4}$ in the low-temperature tetragonal phase [29, 30]. (d) Anticollinear state with $ϕ_{a} = 0$ , $ϕ_{b} = \frac{π}{2}$ , for ${LaSrCrO}_{4}$ reported in this work. (e) The other symmetry-inequivalent anticollinear order with $ϕ_{a} = 0$ , $ϕ_{b} = - \frac{π}{2}$ .
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Figure 2
(a) Nuclear and magnetic unit cells (represented by solid and dotted lines) of LSCrO. Colored spheres present different atoms and red/black arrows presents ${Cr}^{3 +}$ spins that are orthogonal between adjacent layers. Spin interactions $J_{1}, J_{2}, D, K$ in Eqs. (1) and (2) are labeled for selective Cr-Cr bonds. (b) Elastic neutron-scattering patterns in the (HHL) plane, measured on SEQUOIA (Spallation Neutron Source, Oak Ridge National Laboratory, Ref. [38]) at $T$ = 240 and 5 K, respectively. Intensities are integrated within $\pm 0.1$ reciprocal-lattice unit (r.l.u.) in the [ $K \bar{K} 0$ ] direction. (c) Rietveld refinement of the magnetic reflections collected on HB3a (High Flux Isotope Reactor, ORNL, Ref. [39]) at 4 K based on the magnetic structure shown in (a). (d) Temperature dependence of the magnetic diffuse scattering intensity at $Q = (0.5, 0.5, 0.5)$ and magnetic Bragg peak intensity at $Q = (0.5, 0.5, 1)$ . The onset temperatures and 2D and 3D magnetic ordering are indicated by the arrows.
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Figure 3
(a) Spin-wave excitations along high-symmetry directions in the 2D Brillouin zone (inset) measured on SEQUOIA using $E_{i}$ = 120 meV. Data are integrated within $H, K$ = $\pm 0.2$ and $L = \pm 8$ r.l.u. The flat modes near 10 and 20 meV are optical phonons at high $L$ values. (b) Dispersion along $H$ near the $M$ -point of the 2D Brillouin zone measured at $E_{i}$ = 20 meV. Data integration range is $H (K)$ = $\pm 0.03$ and $L = \pm 0.15$ r.l.u. Dashed lines in (a) (b) represent best fit to Eq. (1) from LSWT. (c) Dispersion along $[0.5, 0.5, L]$ measured at $E_{i}$ = 8 meV. Data integration range is $H, K$ = $\pm 0.02$ r.l.u. (d) Dynamic magnetic susceptibility at the $M$ point, obtained by integrating the data in (c). All data shown in this figure are collected at $T$ = 5 K and symmetrized according to the $D_{4 h}$ point group symmetry of the ${Cr}^{3 +}$ site.
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Figure 4
(a), (b) The azimuthal angle of the Néel vectors, $ϕ_{a}$ and $ϕ_{b}$ , as a function of the applied magnetic field $H$ along (a) [110] and (b) [100] directions. Inset: Configurations of the Néel vectors at selective fields. (c), (d) Differential magnetization at 2 and 200 K measured with $H$ applied along (c) [110] and (d) [100] directions. (e) Field dependence of the magnetic Bragg peak intensities at $Q$ = (0.5,0.5,0) and $Q$ = (0.5,0.5,1), measured at $T$ = 2 K on the CORELLI diffuse scattering spectrometer (SNS, ORNL, Ref. [48]) with magnetic field applied along the [1 $\bar{1}$ 0] direction. (f) Field dependence of the relative velocity variation of the transverse mode propagating along the $x$ axis and polarized along the $y$ axis $V_{L x P y}$ , measured at $T$ = 2 K for $H$ along the $a$ axis (black curve) and along the $c$ axis (blue curve).
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Physical Review B

covering condensed matter and materials physics

Anticollinear order and degeneracy lifting in square lattice antiferromagnet ${LaSrCrO}_{4}$

Jing Zhou, Guy Quirion, Jeffrey A. Quilliam, Huibo Cao, Feng Ye, Matthew B. Stone, Qing Huang, Haidong Zhou, Jinguang Cheng, Xiaojian Bai, Martin Mourigal, Yuan Wan, and Zhiling Dun

Phys. Rev. B 105, L180411 – Published 23 May 2022

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Physical Review B

covering condensed matter and materials physics

Anticollinear order and degeneracy lifting in square lattice antiferromagnet LaSrCrO4

Jing Zhou, Guy Quirion, Jeffrey A. Quilliam, Huibo Cao, Feng Ye, Matthew B. Stone, Qing Huang, Haidong Zhou, Jinguang Cheng, Xiaojian Bai, Martin Mourigal, Yuan Wan, and Zhiling Dun

Phys. Rev. B 105, L180411 – Published 23 May 2022

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Anticollinear order and degeneracy lifting in square lattice antiferromagnet ${LaSrCrO}_{4}$