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Séminaire Magnétisme Frustré

Jeudi 6 Novembre, 14h00,
Salle Remy Lemaire, K223

Orateur : Romain Sibille (LDM, PSI, Suisse)
"The ‘forgotten’ pyrochlore magnets Tb2Hf2O7 and Ce2Sn2O7"


Insulating pyrochlore oxides, denoted by the general formula A2B2O7 where A is a rare-earth magnetic ion and B is a transition metal ion or a group IV ion, have attracted much attention because both A and B sites form a network of corner-sharing tetrahedra, a great playground for studying geometrical frustration in three dimensions. The case of rare-earth ions having a strong uniaxial anisotropy constrained to be pointing along the <111> direction of the cubic unit cell (Pr3+, Tb3+, Dy3+, Ho3+) reverses, in theory, the roles of ferromagnetic and antiferromagnetic couplings with regard to frustration : the ferromagnetic case becomes highly frustrated, and can lead to the spin ice state, while the antiferromagnetic one is not. However, pyrochlore Ising antiferromagnets have proved able to markedly defy the expectations of long ranged ordering. A famous example is the enigmatic terbium titanate, Tb2Ti2O7, which forms a so-called spin liquid state with power-law correlations and magnetoelastic excitations. I will first present ongoing investigations on the Hafnium analogue of this compound, which displays large similarities with its historical counterpart. Second, I will present recent results on a Ce3+ pyrochlore, Ce2Sn2O7. In particular, I will show that it is also an Ising antiferromagnet, and that it develops a short range correlated state instead of entering a long range ordered phase. Its small effective magnetic moments at low temperature, together with antiferromagnetic exchange interactions one order of magnitude larger than the expected ferromagnetic dipolar couplings, unveil a valuable system for exchange-based quantum spin liquids.

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