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Seminaire Magnétisme frustré

Jeudi 15 Mai à 14h00,
Salle Remy Lemaire, K223

orateur : Vladimir MARYASIN (SPSMS, INAC CEA Grenoble)
"Order by structural disorder in frustrated Heisenberg triangular antiferromagnet"

Abstract

Heisenberg triangular antiferromagnet is a paradigmatic spin model, exhibiting magnetic frustration. In external field its ground state is degenerate with different spin configurations having the same classical energy. The famous thermal “order by disorder” mechanism lifts this accidental degeneracy, resulting in a rich and complex magnetic phase diagram. Quantum fluctuations, once taken in consideration, tend to favor the same spin states, thus enhancing the selection of the most collinear configurations.
I discuss the effect of impurities on the phase diagram of classical Heisenberg triangular antiferromagnet. I present analytical arguments that structural disorder in the form of weak impurities or weak bond disorder favor non-coplanar conical or umbrella spin states. This is manifested by an effective positive bi-quadratic exchange term in the energy, which should be compared with a similar negative bi-quadratic exchange arising from thermal fluctuations. Thus ground state spin configuration is determined by the competition between structural and conventional thermal order by disorder. This scenario is confirmed by the results of Monte-Carlo simulations of Heisenberg triangular antiferromagnet with random vacancies.

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