Frustrated magnetism in two new Cu2+-based kagome minerals

Orateur : Goran NILSEN

G. J. Nilsen1, Y. Okamoto2, H. Ishikawa2, L. C. Chapon1, H. Mutka1, T. Hansen1, Z. Hiroi2 1Institut Laue Langevin, 6, rue Jules Horowitz, 38042 Grenoble, France 2ISSP, University of Tokyo, 5-1-5 Kashiwanoha, 277-8581 Kashiwa, Chiba, Japan The discovery of a clean experimental realization of the quantum kagome Heisenberg anti-ferromagnet model remains one of the holy grails of frustrated magnetism. So far, the search has focused on Cu2+-containing minerals, where the kagome lattices are made up of edge-sharing Cu octahedra. Due to the Jahn-Teller instability associated with the d9 electronic con-guration, orbital ordering in a variety of patterns is observed for all such compounds at room temperature. In most cases, orbital order brings with it loss of the 3-fold axis perpendicular to the kagome plane, and therefore also the spatial isotropy of the kagome lattice. Nevertheless, much interesting physics is found to arise from the rich interplay between spin and orbital degrees of freedom in distorted kagome systems ; in volborthite (C2=m), for example, only short range magnetic correlations are observed down to T = 50 mK [1]. We have recently prepared and characterized two new Cu2+-containing kagome minerals, KCu3As2O7(OH)3 (C2=m) [2] and PbCu3(AsO4)2(OH)2 (bayldonite, C2=c), both of which are distorted. The magnetic susceptibility, -, of the former reveals a ferromagnetic - = 15 K at high T, followed by long range antiferromagnetic order at TN = 7:05 K. Neutron scattering at T < TN shows the ordered structure to be an incommensurate coplanar helix, with k = (0:77 0 0:11). The observed wavevector, magnetic structure, and some features of the excitation spectrum may be reproduced using a simple semi-classical model including two further neighbour couplings. Unlike KCu3As2O7(OH)3, bayldonite exhibits no signs of long range order down to 1:8 K, despite a large ferromagnetic - - 60 K. Instead, neutron di-raction at T = 1:8 K reveals only short range 2D correlations with an estimated correlation length of - = 100(30) -A. The net ferromagnetic interactions in both materials are thought to arise from the geometry of the -3-OH bridge between nearest neighbour Cu2+ ions. [1] Z. Hiroi et. al, J. Phys. Soc. Japan 70, 3377 (2001) ; M. Yoshida et. al, Phys. Rev. Lett. 103, 077207 (2009) ; G. J. Nilsen et. al, Phys. Rev. B 84, 172401 (2011). [2] Y. Okamoto, H. Ishikawa, G. J. Nilsen, and Z. Hiroi, J. Phys. Soc. Japan 81, 033707 (2012)

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