Hexagonal RMnO3: a model system for two-dimensional triangular lattice antiferromagnets

The hexagonal RMnO3(h-RMnO3) are multiferroic materials, which exhibit the coexistence of a magnetic order and ferroelectricity. Their distinction is in their geometry that both results in an unusual mechanism to break inversion symmetry and also produces a two-dimensional triangular lattice of Mn spins, which is subject to geometrical magnetic frustration due to the antiferromagnetic interactions between nearest-neighbor Mn ions. This unique combination makes the h-RMnO3 a model system to test ideas of spin-lattice coupling, particularly when both the improper ferroelectricity and the Mn trimerization that appears to determine the symmetry of the magnetic structure arise from the same structure distortion. In this review we demonstrate how the use of both neutron and X-ray diffraction and inelastic neutron scattering techniques have been essential to paint this comprehensive and coherent picture of h-RMnO3. (c) 2016 International Union of Crystallography110111scopu

Renormalization of spin excitations in hexagonal HoMnO3 by magnon-phonon coupling

Hexagonal HoMnO3, a two-dimensional Heisenberg antiferromagnet, has been studied via inelastic neutron scattering. A simple Heisenberg model with a single-ion anisotropy describes most features of the spin-wave dispersion curves. However, there is shown to be a renormalization of the magnon energies located at around 11 meV. Since both the magnon-magnon interaction and magnon-phonon coupling can affect the renormalization in a noncollinear magnet, we have accounted for both of these couplings by using a Heisenberg XXZ model with 1=S expansions [1] and the Einstein site phonon model [13], respectively. This quantitative analysis leads to the conclusion that the renormalization effect primarily originates from the magnon-phonon coupling, while the spontaneous magnon decay due to the magnon-magnon interaction is suppressed by strong two-ion anisotropy.Comment: 5 pages, 4 figure

Properties of spin 1/2 triangular lattice antiferromagnets: CuRE2Ge2O8 (RE=Y, La)

We found new two-dimensional (2D) quantum (S=1/2) antiferromagnetic systems: CuRE2Ge2O8 (RE=Y and La). According to our analysis of high-resolution X-ray and neutron diffraction experiments, the Cu-network of CuRE2Ge2O8 (RE=Y and La) exhibits a 2D triangular lattice linked via weak bonds along the perpendicular b-axis. Our bulk characterizations from 0.08 to 400 K show that they undergo a long-range order at 0.51(1) and 1.09(4) K for the Y and La systems, respectively. Interestingly, they also exhibit field induced phase transitions. For theoretical understanding, we carried out the density functional theory (DFT) band calculations to find that they are typical charge-transfer-type insulators with a gap of Eg = 2 eV. Taken together, our observations make CuRE2Ge2O8 (RE=Y and La) additional examples of low-dimensional quantum spin triangular antiferromagnets with the low-temperature magnetic ordering.Comment: 15 pages, 6 figures, and 1 tabl

Spin texture induced by non-magnetic doping and spin dynamics in 2D triangular lattice antiferromagnet h-Y(Mn,Al)O3

Novel effects induced by nonmagnetic impurities in frustrated magnets and quantum spin liquid represent a highly nontrivial and interesting problem. A theoretical proposal of extended modulated spin structures induced by doping of such magnets, distinct from the well-known skyrmions has attracted significant interest. Here, we demonstrate that nonmagnetic impurities can produce such extended spin structures in h-YMnO3, a triangular antiferromagnet with noncollinear magnetic order. Using inelastic neutron scattering (INS), we measured the full dynamical structure factor in Al-doped h-YMnO3 and confirmed the presence of magnon damping with a clear momentum dependence. Our theoretical calculations can reproduce the key features of the INS data, supporting the formation of the proposed spin textures. As such, our study provides the first experimental confirmation of the impurity-induced spin textures. It offers new insights and understanding of the impurity effects in a broad class of noncollinear magnetic systems.Comment: 18 pages, 4 figures and supplementary Information. Accepted for publication in Nature Communication

Robust singlet dimers with fragile ordering in two-dimensional honeycomb lattice of Li2_2RuO3_3

When an electronic system has strong correlations and a large spin-orbit interaction, it often exhibits a plethora of mutually competing quantum phases. How a particular quantum ground state is selected out of several possibilities is a very interesting question. However, equally fascinating is how such a quantum entangled state breaks up due to perturbation. This important question has relevance in very diverse fields of science from strongly correlated electron physics to quantum information. Here we report that a quantum entangled dimerized state or valence bond crystal (VBC) phase of Li2RuO3 shows nontrivial doping dependence as we perturb the Ru honeycomb lattice by replacing Ru with Li. Through extensive experimental studies, we demonstrate that the VBC phase melts into a valence bond liquid phase of the RVB (resonating valence bond) type. This system offers an interesting playground where one can test and refine our current understanding of the quantum competing phases in a single compound.Comment: Scientific Reports (in press

Doping effects on the ferroelectric transition of multiferroic Y(Mn,Al/Ga) O3

Multiferroic hexagonal manganites RMnO3 have a very high ferroelectric transition temperature around and above 1200 K, depending on the rare-earth elements, and a reasonably large electric polarization of about 5.5μC/cm2 at room temperature. It is generally believed that the ferroelectric transition is driven by the combination of R-OP displacement and MnO5 tilting, and hence called improper ferroelectric. In order to better understand the improper ferroelectric transition, we studied doping effects, using two elements with the same valence but a different ionic size: Al and Ga on the Mn site of YMnO3. Through detailed structural studies and nanoscale measurements of piezoresponse force microscopy (PFM) we conclude that there is a drastic doping effect for Al, whose ionic size is much smaller than Mn. It is in stark contrast with our observation that Ga, having a slightly smaller ionic size with Mn, does not change the ferroelectric transition up to 50% doping. This drastic difference in the doping effect is due to local strain induced by the difference in the ionic size of Al and Mn as compared with that of Mn, and sheds light on the intriguing nature of the improper ferroelectric transition. © 2018 American Physical Societ

Frustrated antiferromagnetic honeycomb-tunnel-like lattice CuR2Ge2O8 (R=Pr, Nd, Sm, and Eu)

New frustrated antiferromagnetic compounds CuR2Ge2O8 (R=Pr, Nd, Sm, Eu) have been investigated using high-resolution x-ray diffraction, magnetic, and heat capacity measurements. These systems show different magnetic lattices depending on rare-earth element. The nonmagnetic Eu compound is a S=1/2 two-dimensional triangular antiferromagnetic lattice oriented in the ac plane with geometrical frustration. On the other hand, the Pr, Nd, and Sm compounds show a three-dimensional honeycomb-tunnel-like lattice made of R3+ running along the a axis with the characteristic behavior of frustrated antiferromagnets. © 2017 American Physical Society101sciescopu