Static and Dynamical Properties of the Spin-1/2Equilateral Triangular-Lattice AntiferromagnetBa3CoSb

Abstract

We present single-crystal neutron scattering measurements of the spin-1/2 equilateral triangular-lattice antiferromagnet Ba3CoSb2O9. Besides confirming that the Co 2+ magnetic moments lie in the ab plane for zero magnetic field and then determining all the exchange parameters of the minimal quasi-2D spin Hamiltonian, we provide conclusive experimental evidence of magnon decay through observation of intrinsic line-broadening. Through detailed comparisons with the linear and nonlinear spin-wave theories, we also point out that the large-S approximation, which is conventionally employed to predict magnon decay in noncollinear magnets, is inadequate to explain our experimental observation. Thus, our results call for a new theoretical framework for describing excitation spectra in low-dimensional frustrated magnets under strong quantum effects. The equilateral triangular-lattice quantum antiferromagnet Ba 3 CoSb 2 O 9 was synthesized recently [24][25][26][27][28][29]. The Co 2+ ion has a Kramers doublet ground-state due to the spin-orbit coupling, and this doublet can be described as an effective spin-1/2 moment. In addition, the high symmetry of the hexagonal crystal structure, P6 3 / mmc [24-28], forbids DM interaction for pairs up to third nearest-neighbor (NN) in the same abplane and between any pair of spins along the c-axis [25].Powder neutron diffraction measurements presented the noncollinear 120• structure with the magnetic wavevector Q = (1/3, 1/3, 1) [24]. The Néel temperature was found to be ≈ 3.8 K and a rich temperature-magnetic field phase diagram was reported up to 32 T [25][26][27][28]. Electronic spin resonance (ESR) [27] and nuclear magnetic resonance (NMR) [28] measurements suggested a spin model with small easy-plane exchange anisotropy and an exchange interaction along the caxis much weaker than the NN intralayer exchange. This observation is consistent with the alternation of magnetic (Co 2+ ) and nonmagnetic (Sb 2 O 9 bioctahedra) layers along the cdirection. While more precise determination of the model parameters requires inelastic neutron scattering (INS) measurements, such detailed information is indeed physically relevant.

Hierarchy of Exchange Interactions in the Triangular-Lattice Spin Liquid YbMgGaO4

“…However, precise numerical calculations for the spin-1=2 TLHAF, which take into account nearest-neighbor interactions only [24][25][26], indicate that quantum fluctuations are not enough to suppress magnetic ordering and the actual ground state is a noncollinear long-range ordered spin structure. Experiments on various spin-S triangular-lattice antiferromagnets have overwhelmingly confirmed this picture [27][28][29][30], with a few noteworthy exceptions [31][32][33]. Several perturbations from the pure TLHAF have been proposed to enhance quantum fluctuations: next-nearest neighbor (NNN) interactions [34][35][36][37][38], ring-exchange terms [39,40], and anisotropic exchange [41][42][43][44], although it remains theoretically unclear if the latter mechanism alone can stabilize a QSL [45][46][47][48].…”

Exact diagonalization and cluster mean-field study of triangular-lattice XXZ antiferromagnets near saturation

“…Recent advances in experiments have allowed for preparing a variety of quasi-two-dimensional (quasi-2D) TLAF materials and accessing their magnetic properties in strong magnetic fields up to the saturation field H s [11][12][13][14][15][16][17][18][19]. Since the seminal work by Kawamura and Miyashita [20], it has been known that the simplest model of TLAFs with isotropic Heisenberg interactions possesses an accidental continuous degeneracy of the classical ground state for finite magnetic fields.…”

“…For instance, the layered TLAF materials such as Ba 3 CoSb 2 O 9 [11][12][13][16][17][18] and Ba 3 CoNb 2 O 9 [14,15] possess XXZ-type anisotropy in the spin-exchange interactions on each layer. The interactions between the spins on the single layer are modeled by the following XXZ Hamiltonian:…”

Magnetic properties of the triangular lattice magnets A4B′B2O12 ( A=Ba<

“…There are several examples: (i) Ba 3 CoSb 2 O 9 7-10 , with equilateral Co 2+ (effective spin-1/2) triangular lattice, exhibits a 120 degree ordered state at zero field and an up up down phase with applied field. Although its spin wave spectrum can be reasonably described by the XXZ model with a small easy plane anisotropy, the quantum spin fluctuations still lead to abnormal magnon decay and line broadening that cannot be accounted for by linear and nonlinear spin wave theories 11 ; (ii) Ba 21 is a rare TLM that shows ordering with a collinear alternating stripe pattern which could be related to its strong easy axis anisotropy; (iv) AAg 2 M(VO 4 ) 2 (A = Ba, Sr; M = Co, Ni) 22,23 are TLMs possessing ferromagnetic (FM) orderings due to superexchange via the bridging vanadates on a triangular lattice; (v) NaVO 2 24,25 is a rare TLM exhibiting ordering of the V 3+ orbitals. Consequently, this orbital ordering relieves the geometrical frustration and leads to long range magnetic ordering; (vi) shown recently, the exotic quantum spin liquid (QSL) state is realized in YbMgGaO 4 26-29 with an effective spin-1/2 Yb 3+ triangular lattice in which the spin anisotropy and next-nearest neighbor interactions play important roles.…”