Regensburg 2016 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 42: Correlated Electrons: Frustrated Magnets - Cu-based Systems & FeCr

TT 42.2: Talk

Wednesday, March 9, 2016, 09:45–10:00, H18

Peculiarities of structural distortions in kagome francisites — •Alexander A. Tsirlin¹, Danil Prishchenko², Vladimir Tsurkan³, and Vladimir G. Mazurenko² — ¹EP VI, Electronic Correlations and Magnetism, University of Augsburg, Germany — ²Ural Federal University, Ekaterinburg, Russia — ³EP V, Electronic Correlations and Magnetism, University of Augsburg, Germany

Francisites Cu₃Bi(SeO₃)₂O₂X (X = Cl, Br) are spin-1/2 kagome materials with predominantly ferromagnetic exchange couplings. Despite this tendency toward ferromagnetism, they entail strong magnetic frustration related to antiferromagnetic couplings between next-nearest neighbors. The infinite classical degeneracy of the ground state is lifted by anisotropic Dzyaloshinsky-Moriya couplings that stabilize canted magnetic order within the kagome planes. The canting angle θ reveals strong dependence on the halogen atom, chlorine (θ≃ 70^∘) or bromine (θ=52^∘), which is difficult to rationalize assuming that these ions are not part of the kagome planes and have no influence on relevant superexchange couplings. By combining high-resolution synchrotron x-ray diffraction with ab initio calculations of lattice dynamics, we show that the Cl francisite undergoes a structural phase transition triggered by the weakly bonded Cl ions located between the kagome planes. This transition has strong impact on the in-plane magnetism and alters magnetic interactions considerably. In the Br francisite, no structural phase transition is observed, and a more regular version of the kagome lattice is formed.