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

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Physical Review B
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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=13. The neutron powder diffraction data claims that the magnetic structure of LuyY1−yMnO3 is described as a mixture of Γ3 (P6′3cm′) and Γ4 (P6′3c′m) at the xMn position for y=0.15, 0.30, and 0.45. The ratio of Γ3 and Γ4 depends on temperature and composition and the fraction of Γ3 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 Γ 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 Lu0.3Y0.7MnO3 could be an ideal case to investigate the trimerization, frustrated magnetism, and magnetoelastic coupling often observed in two-dimensional triangular lattice Heisenberg antiferromagnet systems.