Weak trimerization in the frustrated two-dimensional triangular Heisenberg antiferromagnet LuyY1-yMnO3

To understand the 2D triangular Heisenberg antiferromagnetic system, we investigated the magnetic structures and the dynamics of LuyY1-yMnO3 in detail. The substitutions are adjusted to the Mn atomic position close to xMn = 3 . The neutron powder diffraction data claims that the magnetic structure of LuyY1-yMnO3 is described 1 as a mixture of Gamma 3 (P6'3cm') and Gamma 4 (P6'3c'm) at the xMn position for y = 0.15, 0.30, and 0.45. The ratio of Gamma 3 and Gamma 4 depends on temperature and composition and the fraction of Gamma 3 increases upon cooling, while no clear trimerization was observed at the xMn position. We estimated exchange parameters from the analysis of the low-energy part of the spin waves. The results showed a weak trimerization effect on cooling because the nearest -neighbor exchange interaction is slightly enhanced. The temperature dependence of the spin-wave dispersion around the Gamma point shows that the spin gap closes with increasing temperature because the exchange interactions in the nearest Mn-Mn neighbor become smaller. Gapless diffusive magnetic excitation from a Mn triangular lattice has been observed in a wide range in Q and E space of LuyY1-yMnO3. We found that Lu0.7Y0.3MnO3 could be an ideal case to investigate the trimerization, frustrated magnetism, and magnetoelastic coupling often observed in two-dimensional triangular lattice Heisenberg antiferromagnet systems.