Quantum Magnetization Plateau in Spin-1 Triangular-Lattice Antiferromagnet Ba₃NiSb₂O₉

Abstract

We report the results of magnetization and specific heat measurements on Ba 3 NiSb 2 O 9 , which is a quasi-twodimensional spin-1 triangular-lattice antiferromagnet. We observed a nonclassical magnetization plateau at one-third of the saturation magnetization that is driven by spin frustration and quantum fluctuation. Exact diagonalization for a 21-site rhombic cluster was performed to analyze the magnetization process. Experimental and calculated results agree well.KEYWORDS: Ba 3 NiSb 2 O 9 , triangular-lattice antiferromagnet, magnetization plateau Triangular-lattice antiferromagnet (TLAF), which is one of the simplest frustrated systems, exhibits a rich variety of physics.1-3) A symbolic quantum effect predicted for smallspin Heisenberg TLAF is that magnetization is quantized at one-third of the saturation magnetization M s in magnetic fields. [4][5][6][7][8] For classical spin, the equilibrium condition in a magnetic field H is given by S 1 þ S 2 þ S 3 ¼ g B H=ð3JÞ with the sublattice spins S i . Because the number of the equations is smaller than the number of parameters to determine the spin configuration, the stable state is not uniquely determined; thus, the ground state is infinitely degenerate. This classical degeneracy can be lifted by the quantum fluctuation that is remarkable for small spin, and a specific spin state is selected as the ground state. Consequently, the up-up-down state is stabilized in a finite field range, which can be observed as a magnetization plateau at 1 3 M s . 9)In experiments, the quantum magnetization plateau at 1 3 M s was actually observed in Cs 2 CuBr 4 , [11][12][13][14][15][16] 12,13) Furthermore, precise thermodynamic measurements revealed the cascade of field-induced quantum phase transitions.17) These unusual observations should be attributed to the spatially anisotropic triangular lattice and the Dzyaloshinsky-Moriya interaction, [18][19][20][21][22] but the overall explanation is still an open question.The motivation of this study comes from a question on whether the ground states for Heisenberg TLAFs with integer and half-integer spins are qualitatively the same. It is known that in the case of kagome-lattice Heisenberg antiferromagnet that is closely related to Heisenberg TLAF, the ground states for spin-1 2 and spin-1 are qualitatively different.23) For spin-1 Heisenberg TLAF, the ground states in zero and finite magnetic fields have not been well understood. NiGa 2 S 4 is known as spin-1 Heisenberg TLAF with incommensurate spin correlation. [24][25][26] In this nickel compound, no clear magnetization plateau has been observed, although the magnetization shows a tiny undulate anomaly at around 1 3 M s .27) To investigate the magnetization process of spin-1 Heisenberg TLAF, we have performed high-field magnetization measurements on Ba 3 NiSb 2 O 9 . As shown below, a nonclassical magnetization plateau was observed at 1 3 M s . Ba 3 NiSb 2 O 9 crystallizes in a high symmetric hexagonal structure, P6 3 =mmc, 28,29) which is the same as the hexagonal BaTiO 3 struct...

The Magnetization Process of the Spin-One Triangular-Lattice Heisenberg Antiferromagnet

“…1 B /(Ni 2þ T), which has not been done in Ref. 16. Due to the finite-temperature effect, small single-ion anisotropy and small anisotropy of the g factor the experimental curve is smeared out around the endpoints of the 1/3-plateau, H c1 and H c2 .…”

“…One spectacular feature of this model system for a strongly frustrated magnet is the unconventional magnetization process of the triangular-lattice Heisenberg antiferromagnet. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] While for the classical isotropic Heisenberg model at zero temperature the magnetization M increases linearly with the applied magnetic field H, thermal or quantum fluctuations induce a plateau at 1/3 of the saturation magnetization M sat (''order from disorder'' phenomenon). For the extreme quantum case, i.e., spin quantum number s ¼ 1=2, this plateau at T ¼ 0 has been widely discussed.…”

“…16,18,19) In Ba 3 NiSb 2 O 9 , the uniform triangular lattice of magnetic ions is realized as in Ba 3 CoSb 2 O 9 . Indeed, recently it has been found that the magnetization curve of this compound also exhibits a clear 1/3 plateau.…”

“…Indeed, recently it has been found that the magnetization curve of this compound also exhibits a clear 1/3 plateau. 16) By contrast to the wellinvestigated spin-half case there are much less reliable theoretical studies of the s ¼ 1 model relevant for Ba 3 NiSb 2 O 9 . Motivated by the excellent agreement of theoretical predictions based on exact diagonalization (ED) and coupled-cluster method (CCM) and experimental results for the spin-half compound Ba 3 CoSb 2 O 9 reported in Ref.…”

Umbrella-coplanar transition in the triangular XXZ model with arbitrary spin

“…20,22 However, we can not exclude that a refinement of the theoretical model should be pursued to explain further experimental results. 17,19 This line of research is being expanded to other compounds with identical or very similar structure, such as Ba 3 27 While in general all these materials feature magnetic ions occupying the sites of triangular lattice layers which are arranged in a simple AA type of stacking, it becomes interesting to understand how various details of the single material, such as the value of S , the dimensionality (strength of the interlayer coupling), certain spin anisotropies, etc., influence the actual observations.…”