Origin of Magnetism in Al-Pd-Mn and Al-Mn Quasicrystals and Approximants
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Origin of Magnetism in Al-Pd-Mn and Al-Mn Quasicrystals and Approximants Françoise Hippert1, Virginie Simonet2, Marc Audier1, Yvonne Calvayrac3, Robert Bellissent4, Guy Trambly de Laissardière5 and Didier Mayou6 1 Laboratoire des Matériaux et du Génie Physique (CNRS), ENSPG, BP46, 38402 Saint Martin d'Hères, France 2 Laboratoire de Physique des Solides (CNRS), Bât. 510, Université Paris Sud, 91405 Orsay Cedex, France 3 CECM (CNRS), 15 rue G. Urbain, F-94407 Vitry Cedex, France 4 Laboratoire Léon Brillouin (CEA-CNRS), CE Saclay, 91191 Gif sur Yvette Cedex, France 5 Laboratoire de Physique Théorique et Modélisation, Université de Cergy-Pontoise, BP 222, 95302 Cergy-Pontoise Cedex, France 6 LEPES (CNRS), 25 Avenue des Martyrs, BP166X, 38042 Grenoble Cedex, France
ABSTRACT
Localized moments are observed on a small fraction of the Mn atoms in Al-Pd-Mn and Al-Mn quasicrystals while a large proportion of magnetic moments is found in liquids in equilibrium with these quasicrystalline phases. As magnetic sites cannot be identified in quasicrystals because of the very small number of Mn atoms involved, we have studied the case of approximant phases in both Al-Mn and Al-Pd-Mn systems. Most of the approximants are nonmagnetic but magnetic moments are present on a fraction of the Mn sites in the µ-Al4Mn phase and in the Mn-rich TAlPdMn phase. From both experimental and theoretical investigations, the magnetic sites are identified in these phases. The moment formation can be explained by analysing the local environment of the Mn atoms and indirect Mn-Mn interactions mediated by conduction electrons on relatively large distances. A clear picture of the origin of magnetism in quasicrysals, approximants and liquids in the Al-Mn and Al-Pd-Mn systems can therefore be obtained.
INTRODUCTION
Most Al-based quasicrystalline alloys are characterized by a negative magnetic susceptibility nearly temperature-independent. For instance, this is the case of icosahedral AlCu-Fe, icosahedral Al-Pd-Re and decagonal Al-Ni-Co phases [1]. Such a diamagnetic behavior can be explained by the weakness of the Pauli contribution in presence of a pseudo-gap at the Fermi level. The possibility of a strong Landau diamagnetism of conduction electrons resulting from the peculiar Fermi surface of quasicrystals has also been proposed [2]. In contrast, the existence of Curie terms, and hence of localized moments, has been reported both in metastable Al-Mn and stable Al-Pd-Mn quasicrystals (see [3] and refs therein). Icosahedral and decagonal K14.2.1
phases exhibit a similar magnetic behavior in these systems [4,5] with, in particular, spin glass transitions occuring at low temperature [3,6]. The relatively low values of measured Curie constants suggest that only a few percent of Mn atoms carry a magnetic moment in all these phases. Besides, from the observation of a strong increase of the paramagnetic contribution on melting, it has been concluded that a large proportion of Mn atoms carries a magnetic moment in the liquid state [7]. The origin of magnetism i
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