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Séminaire Magnétisme frustré

Jeudi 13 Juin à 11h00,
Salle Louis Weil, E424

Orateur : Françoise Damay (LLB-CEA Saclay)
"Charge and spin orders in multiferroic LuFe2O4 "


The crystal structure and magnetic properties of a polycrystalline sample of multiferroic LuFe2O4 [1] have been investigated by means of synchrotron X‐ray diffraction, magnetization measurements and neutron scattering, between 1.5 and 350 K [2].
Charge ordering of the Fe2+ and Fe3+ species occurs simultaneously with a monoclinic distortion of the rhombohedral cell around 330 K, distortion which increases as temperature decreases. Magnetic Bragg peaks appear below TN = 240 K, and can be indexed using two propagation vectors, corresponding to either a ferromagnetic or an antiferromagnetic stacking of the iron bilayers. Neutron inelastic scattering data confirm the strong easy‐axis single‐ion anisotropy of the iron spin in its trigonal environment, of about 8 meV at 5 K. The magnetic arrangement in the ab plane follows a 1:2 pattern, which obeys the charge ordering one observed in transmission electron microscopy [3], [4]. The corresponding modeling of the neutron diffraction data shows an additional level of magnetic near‐degeneracy, at the level of the bilayer itself (Figure 1) : it results in coexisting magnetic phases with opposite signs of the intra‐bilayer interaction, and with two distinct ordering temperatures, in agreement with susceptibility measurements. The complexity of this magnetic ground state raises the question of the relationship between spin and charge orders.

[1] N. Ikeda et al., Nature (London) 436, 1136 (2005)
[2] J. Bourgeois et al., Phys. Rev. B 86, 024413 (2012)
[3] J. Bourgeois et al., Phys. Rev. B 85, 064102 (2012)
[4] M. Hervieu et al., in preparation

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