Lattice distortion effects on the frustrated spin-1 triangular-antiferromagnet A3NiNb2O9 ( A=

Abstract

In the geometrically frustrated materials with the low-dimensional and small spin moment, the quantum fluctuation could interfere with the complicated interplay of the spin, electron, lattice and orbital interactions, and host exotic ground states such as nematic spin-state and chiral liquid phase. While the quantum phases of the one-dimensional chain and S -½ twodimensional triangular-lattice antiferromagnet (TLAF) had been more thoroughly investigated by both theorists and experimentalists, the work on S = 1 TLAF has been limited. We induced the lattice distortion into the TLAFs, A3NiNb2O9 (A = Ba, Sr, and Ca) with S (Ni 2+ ) = 1, and applied the thermodynamic, magnetic and neutron scattering measurements. Although A3NiNb2O9 kept the non-collinear 120° antiferromagnetic phase as the ground state, the Ni 2+ -lattice changed from the equilateral triangle (A = Ba) into isosceles triangle (A = Sr and Ca). The inelastic neutron scattering data were simulated by the linear spin-wave theory, and the competition between the single-ion anisotropy and the exchange anisotropy from the distorted lattice was discussed. I. INTRODUCTIONIn the geometrically frustrated system, the complicated interaction(s) between electron, phonon, spin and orbital could lead to degenerate ground states, which introduce exotic properties and attracted a lot attention over the past decades [1][2][3]. Meanwhile, those degenerate states would easily be held by the symmetrical inconsistency and be destroyed by significant quantum fluctuations, not only induced by the complicated interactions among low dimensionality, geometrical frustration, small spin and the applied magnetic field, but also modified the classical Heisenberg model [4][5]. The triangular-lattice antiferromagnet (TLAF), one of the simplest frustrated two-dimensional (2D) material, had been suggested to be strongly influenced by the strong quantum spin fluctuations with small effective spin (S = 1/2 or 1) and exhibited a rich variety of interesting physics [6][7][8].A striking example of these quantum phenomena is the transition from a non-collinear 120° spin configuration at 0 T into fractional-magnet lattice under a finite range of applied magnetic field, such as a collinear up-up-down (uud) phase corresponding to a magnetization plateau with one-third of its saturation value [9][10][11][12][13].Recently, the theoretical research indicated the uud state as a commensurate analogue of the incommensurate spin-density-wave which was predicted and observed for frustrated one-dimensional spin-1 chains and S = ½ TLAF [14, 15], and suggested the possibility for exotic magnetic excitations. While there was an enormous theoretical activity in the domain, a full consensus on the origin of the uud state, (even the ground magnetic state, non-collinear 120 o at zero field) was limited. This is true as well for the state-of-the art experimental investigation of its spin excitations due to the lack of a triangular-lattice materials. Although the lattice distortion has been believed to influence t...

Non-magnetic ion site disorder effects on the quantum magnetism of a spin-1/2 equilateral triangular lattice antiferromagnet

“…This spatial anisotropy, on top of the easy-plane anisotropy, can lead to a two-step transition with canted spins. For example, Ba 3 NiNb 2 O 9 has an equilateral triangular lattice for spin-1 Ni 2+ ions and orders at 4.9 K with a 120 degree spin structure in the ab plane [60]. By replacing Ba 2+ ion with Ca 2+ ion with smaller ionic size, a structural distortion is introduced into the system and therefore Ca 3 NiNb 2 O 9 now has a isosceles triangular lattice for Ni 2+ ions.…”

“…By replacing Ba 2+ ion with Ca 2+ ion with smaller ionic size, a structural distortion is introduced into the system and therefore Ca 3 NiNb 2 O 9 now has a isosceles triangular lattice for Ni 2+ ions. One consequence is that Ca 3 NiNb 2 O 9 orders with a two-step transition at 4.2 K and 4.8K [60]. Future studies on the local structure of Sr-BCSO is desirable to exam this possibility and better understand its magnetic ground state.…”

“…Another fact is that the triple perovskite structure for BCSO is very flexible on its chemical composition. For example, one can replace Ba 2+ ions with Sr 2+ or Ca 2+ ions [60,61], or replace Sb 5+ ions with Nb 5+ or Ta 5+ ions [62][63][64] while keeping the Co-triangular lattice. In fact, Ba 3 CoNb 2 O 9 [62] exhibits a two-step ordering process at 1.36 K and 1.10 K with easy axis anisotropy and also exhibits the QSSTs.…”

Spin-liquid behavior of the three-dimensional magnetic system Ba3NiIr2O9 with S=1

“…However, the magnetic behavior of these S = 1 systems at low temperature differs widely from each other, even in the compounds with similar structural geometry. For example, unlike the wellknown 120 • spin structure [18], as observed in A 3 NiNb 2 O 9 [16], Ba 3 NiSb 2 O 9 shows characteristic spin-liquid behavior [7,19,20], whereas NiGa 2 S 4 hosts a spin-nematic phase [21]. The interaction of such Ni-based S = 1 triangular lattice with another magnetically active sublattice might result in an exotic magnetic phase in 3D materials.…”

“…Since experimental reports of QSL with either spin (S) 1 [7,8] or 3D arrangement of spins [9][10][11][12][13] are very few, it can be easily anticipated that chances of having a QSL with both higher spin and 3D geometry is extremely low [14]. Many of the attempts to obtain spin-1 QSL have been focused on stabilizing Ni within various structural networks [7,8,[14][15][16][17]. However, the magnetic behavior of these S = 1 systems at low temperature differs widely from each other, even in the compounds with similar structural geometry.…”