Session W51: Triangular Systems

Impact of the lattice on magnetic properties in the S=1 triangular antiferromagnet NiGa2S4

NiGa₂S₄ is a triangular lattice S=1 system with strong two dimensionality of the lattice, and a competition between ferromagnetic nearest neigbor exchange J₁ and antiferromagnetic third nearest neigbor exchange J₃. It was actively discussed as a candidate to host spin-nematic order brought about by strong quadrupole coupling. Using Raman scattering spectroscopy we identify a phonon of E_g symmetry which can modulate magnetic exchange J₁ and produce quadrupole coupling. Additionally, our Raman scattering results demonstrate a gradual loss of local inversion symmetry on lowering the temperature of the sample. We associate it with a lattice distortion due to sulfur vacancies. This loss of inversion symmetry will lead to disordered Dzyaloshinskii-Moriya interactions, which can prevent long-range magnetic order. Using magnetic Raman scattering response we identify 160 K as a temperature of an upturn of magnetic correlations. The temperature range below 160 K, but above 50 K where antiferromagnetic correlations start to increase, is a candidate for spin-nematic regime.   

Realization of Quantum Dipoles in Triangular Lattice Crystal Ba3Yb(BO3)3

We investigate the thermodynamic properties of the ytterbium-based triangular lattice compound Ba₃Yb(BO₃)­₃. These results demonstrate the absence of any long-range ordering down to 56 mK. The analyses of these results show that Ba₃Yb(BO₃)­₃ may realize an S = ½ quantum dipole lattice, in which the dominant interaction is the long range dipole-dipole coupling on the geometrically frustrated triangular lattice, and exchange interactions are subdominant or negligible.   

Crystal Growth and Characterization of Rare-earth Triangular Lattice Borates

Triangular antiferromagnetic materials have attracted attention because competing interactions on the lattice can give rise to exotic phenomena, such as quantum spin liquids. Motivated by evidence that Ba₃Yb(BO₃)₃ may host a spin-1/2 quantum dipole lattice, we have synthesized several members of the borate family Ba₃RE(BO₃)₃ using different rare-earth elements to examine the effect of placing different spins in the lattice. We have grown large single crystals of these materials using the optical floating zone technique and performed in-house thermodynamics characterization measurements, from which we find no evidence of ordering down to the lowest accessible temperatures. In this talk we will present the results of our experimental efforts.   

Spin-phonon hybridization and its direct effect on thermal transport in the frustrated triangular magnet CsYbSe2.

The thermal conductivity of CsYbSe₂, and in fact many other frustrated magnets, often exhibits strong, non-monotonic field-dependence, even when the state of magnetic lattice is disordered and a coherenet itinerant magnetic contribution is absent. We present a novel and highly-generic way in which phonon heat conduction can acquire unique nontrivial field-dependence through the hybridization of acoustic phonons and spin flip excitations via strain-modulation of the magnetic g-tensor. Analytical modelling of the hybridized spectra of this model under applied fields is successful in reproducing the qualitative features of transport data in CsYbSe₂, and is potentially widely applicable to many other insulating magnetic systems.   

Magnetism in the triangular lattice materials CeCl3 and CeBr3

There is currently great interest in systems with J = 1/2 ground state and Ising-like interactions that may be geometrically frustrated, as in the triangular lattice, or frustrated by competing interactions, like the honeycomb Kitaev model. This interest has spurred our investigation of the triangular lattice rare earth materials CeCl₃ and CeBr₃, where the Ce³⁺ ion is expected to have an effective J = 1/2 ground state. Some magnetic properties of these materials were studied many years ago but prior to this investigation there had been no direct measurements of the magnetic order or excitations using neutron scattering. Here we report neutron scattering investigations of crystal field excitations in CeBr₃, confirming the J=1/2 ground state, and neutron diffraction measurements of the magnetic order in CeCl₃, showing that the system orders antiferromagnetically at low temperature with an ordering wave-vector (1/3 1/3 1/2).   

Magnon Pairing, Interactions, and Decay in the Spin-orbital Magnet FeI2

One of the scientific frontiers in quantum magnetism is the discovery and understanding of quantum entangled and topologically ordered states in real bulk materials. At the focal point of the experimental investigation of these quantum spin networks is the identification of fractionalized excitations in transport and spectroscopic measurements. Inelastic neutron scattering has proved a powerful technique to reveal such signatures in a variety of systems ranging from quasi-1D magnets to kagome compounds and more. Recent and ongoing developments with neutron scattering instrumentation have allowed the characterization of magnetic excitations in entire volumes of momentum-energy space with high resolution. This has prompted revisiting long-overlooked materials in search of exotic spin dynamics despite seemingly classical magnetically ordered ground-states. In this talk, I will discuss such experiments on a long-known material, FeI2, and show how high-fidelity modeling brings new insights into its spin dynamics [1]. I will describe the mechanism that endows low energy quadrupolar fluctuations in FeI2 with large spectral weight and how these can be completely understood using a SU(3) representation of spin degrees of freedom. I will discuss the consequence of having several quasiparticles as the low-energy degrees of freedom in this system including the formation of heavy bound-states [2] and their mutual decay [3].    

Successive phase transitions and incommensurate magnetic ordering in a triangular-lattice antiferromagnet

The triangular-lattice antiferromagnet (TLAF) has attracted a lot of attention in the past several decades as a prototypical two-dimensional (2D) frustrated magnet for which quantum fluctuations may stabilize unconventional ground-states such as the novel up-up-down (UUD) ground state. We have conducted a systematic study on a new TLAF compound with perfect equilateral triangular lattice Na2BaMn(PO4)2 with DC and AC susceptibilities and neutron scattering measurements. DC susceptibility measurements show the saturation field to be around 7 T. AC susceptibility measurements on single-crystal samples have confirmed a long-range magnetic ordering below 1.1 K under zero field and two critical fields at 1.6 T and 2.9 T with the magnetic field applied along the c axis, which also indicates an easy-axis anisotropy. Neutron scattering measurements were conducted on powder sample. The results suggest a novel incommensurate ordering in this compound under zero and finite fields, which is seldom to be observed in TLAF. More work will be done to solve the exact magnetic structure in different magnetic phases. And specific heat measurements will be conducted to map out a complete phase diagram.   

Potts-nematic Quantum Phase in Frustrated Heisenberg Anti-ferromagnets

In this talk we consider a class of quantum anti-ferromagnets with frustrated Heisenberg exchange interactions and collinear magnetic ground states which spontaneously break discrete lattice rotation symmetry. In particular, we examine the nature of the quantum phase transition between the disordered paramagnetic state and the magnetically ordered state, and show that this transition can occur via an intermediate paramagnetic phase in which rotation symmetry is broken. To demonstrate this, we investigate a spin-1 J₁-J₂ model on the triangular lattice with a single-ion anisotropy and show that the paramagnet becomes unstable to the formation of nematic two-particle bound states as the strength of the single-ion anisotropy decreases. This indicates the existence of a Potts-nematic quantum paramagnetic phase which lacks magnetic order, but breaks the three-fold rotation symmetry of the lattice. We also discuss generalizations to other systems and thermal phase transitions.   

Emergent Potts Z3 order in the Heisenberg windmill model with biquadratic exchange interactions

We design and study a bilinear-biquadratic Heisenberg model with coexisting ferro- and antiferromagnetic exchange interactions on the windmill lattice, which consists of coupled triangular and honeycomb lattices. Spins on the triangular layer are coupled antiferromagnetically, while spins on the honeycomb layer interact ferromagnetically. We determine the classical ground state phase diagram, which includes a variety of phases including a non-coplanar canted umbrella state, canted coplanar phases and a fully ferromagnetic state, resembling the finite-temperature finite-magnetic field phase diagram of the classical Heisenberg antiferromagnet on a triangular lattice. We identify a region in the classical ground state phase diagram with a Z3-degenerate ground state. Using Monte-Carlo simulations and analytical techniques, we determine the finite-temperature phase diagram above this phase and identify a finite-temperature Potts phase transition of an emergent Z3 order parameter.   

Commensurate and incommensurate antiferromagnetic orderings in SrMn2As2 and CaMn2As2 revealed by NMR

Recently much attention has been paid to the Mn-based compounds with AMn₂Pn₂ (A= Sr, Ca, Ba, and  Pn = P, As, Sb, Bi)  since the systems show a rich variety of magnetic properties with different crystal structures. Different from BaMn₂As₂ which shows the body-centered tetragonal ThCr₂Si₂-type structure, (Ca,Sr)Mn₂Pn₂ (Pn = P, As, Sb, Bi) crystallize in the trigonal CaAl₂Si₂-type structure where the Mn ions form a triangular lattice bilayer which can be considered as a corrugated Mn honeycomb sublattice.  Among the Mn compounds, CaMn₂As₂ and SrMn₂As₂ have been reported to be antiferromagnetic (AFM) insulators with T_N = 62 K and 120 K, respectively. In this study, we have carried out ⁷⁵As nuclear magnetic resonance (NMR) measurements on those systems to investigate the magnetic properties from a microscopic point of view. In the case of SrMn₂As₂, no obvious distribution of the internal field at the As site was observed in the AFM state. In contrast, broad and complex NMR spectra were observed in CaMn₂As₂ in the AFM state, which clearly shows a distribution of internal field at the As site. Those results indicate that the antiferromagnetic states are commensurate and incommensurate in SrMn₂As₂ and CaMn₂As₂, respectively.     

Suppression of antiferromagnetic order and strong ferromagnetic spin fluctuations in Ca(Co1−xNix)2−yAs2 single crystals

The highly-frustrated itinerant magnetic system CaCo_2-yAs₂ exhibits A-type antiferromagnetic (AFM) order below T_N = 52 K. From the magnetization M versus temperature T, magnetic field H, and heat capacity C_p(T, H) measurements it is found that T_N decreases to 22 K with only 3% Ni substitution and is completely suppressed for x > 0.16. Magnetic susceptibility χ measurements reveal that strong ferromagnetic (FM) fluctuations develop for 0.11≤ x ≤ 0.52. The FM fluctuations for x = 0.21 and 0.31 are quasi-1D in nature, where χ_c >> χ_ab. The C_p(T) measurements further reveal that the FM spin fluctuations are quantum in nature, where a logarithmic T-dependent upturn in C_p(T )/T is observed at low T and H = 0, that is suppressed with increasing H. A reentrant spin-glass behavior arising out of competing AFM and FM interactions is also observed for x = 0.11 and 0.16 below T_N. No signature of any magnetic order or spin fluctuations is observed for x > 0.52 at T ≥ 1.8 K. Density functional theory (DFT) calculations show that the transition to nonmagnetic state occurs via a Stoner transition.   

Dynamic and frozen quantum magnetism in the ground states of triangular lattice magnets YbMgGaO4, ErMgGaO­4 and YbCoGaO4 from inelastic neutron scattering

The putative quantum spin liquid (QSL) state exhibited by YbMgGaO₄ is largely ascribed to the quasi-2D triangular lattice which the magnetic Yb³⁺ moments decorate in concert with anisotropic exchange and disorder in neighbouring disordered Mg²⁺/Ga³⁺ bilayers [1,2,3]. We present new inelastic neutron scattering (INS) measurements on YbMgGaO₄ as well as its isostructural sister compounds: ErMgGaO₄ and YbCoGaO₄. Each material was synthesized as a phase-pure powder and INS measurements were performed on IN6-Sharp with E_i = 3.1 meV at the Institut Laue-Langevin. We discuss the observed hallmarks of a QSL in YbMgGaO₄ and contrast these with signatures of frozen spin correlations in each of ErMgGaO₄ and YbCoGaO₄ in the INS measurements. The case of YbCoGaO4 is an interesting one as it displays the interplay between a QSL-like state associated with the Yb³⁺ magnetism, interleaved with a two-dimensional spin glass state originating from the disordered Co²⁺ magnetism. 

[1] Paddison, J., Daum, M., Mourigal, M. et al. Nature Phys 13, 117–122 (2017)

[2] Li, Y., Adroja, D., Zhang, Q. et al. Phys. Rev. Lett. 118, 107202 (2017)

[3] Zhu, Z., Maksimov, P. A., Chernyshev, A. L. et al. Phys. Rev. Lett. 119, 157201 (2017)   

First principles study of monolayer magnetic triangular lattice compounds MX2

In the quest to understand exotic low-dimensional magnetic states, magnetic triangular lattices offer a rich playground. With this study, we model effective magnetic interactions for the monolayer structures of a range of triangular lattice materials MX₂ (M={V,Mn,Ni}, X={Cl,Br,I}). We also took the opportunity of such a larger study to benchmark two ab-initio methods used to extract effective S={3/2,5/2,1} spin models for real materials. Additionally, by means of toy model parameters based on general properties such as filling and hybridization, we shed light on the microscopic mechanism behind the different behaviors and magnetic interactions displayed by these, apparently similar, systems. In particular, the considered relatively light magnetic metal ions have small spin-orbit coupling (SOC), which allows to effectively tune the SOC by interchanging the ligand elements. We find that the corresponding SOC matrix-elements differ strongly from the atomic limit and that SOC effects manifest only in anisotropic exchange and single-ion anisotropy for specific fillings. Noticeably, for monolayer NiI₂ we do find a sizeable Kitaev coupling, suggested in previous works.