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Séminaire Magnétisme non conventionnel

Jeudi 8 Décembre à 14h00,
Salle Remy Lemaire K223

Orateur : Evan Constable (Institut Néel)
"Anisotropy in a spin 1/2 ferromagnetic buckled kagome compound"


Cu3Bi(SeO3)O2Cl, or francisite, features a buckled kagome lattice made from two unique Cu spin ½ sites (Cu1 and Cu2) and displaying unusual frustration mechanisms. In fact, there is a strong competition between nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic interactions. With only isotropic exchange interactions, and despite dominant ferromagnetic couplings, this would lead to a classical degenerate ground state that should survive quantum fluctuations. However, the first experimental studies of this material have shown that it actually orders in a canted magnetic structure below 25 K and that the magnetic macroscopic behavior is highly anisotropic. A theoretical work has predicted that the essential ingredient stabilizing this magnetic order is a tiny Dzyaloshinskii-Moriya interaction. The whole behavior is complicated by the fact that this material additionally undergoes a structural distortion at 115 K with a doubling of the unit cell along the c axis that we have identified through neutron diffraction. Taking this into account, we were able to refine the magnetic structure. We have also performed inelastic neutron scattering on the triple-axis instruments ThALES and IN12 in order to identify the relevant ingredients of the Hamiltonian from the spin wave spectrum. In disagreement with the previously considered model, a global gap of the excitations is observed and the source of the anisotropy producing it is investigated.

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