Magnetism at surfaces and defects in icosahedral Al-Pd-Mn quasicrystals

In our recent work [Krajčí and Hafner, Phys. Rev. B 78, 224207 (2008)] we have demonstrated that the ground state of bulk i-Al-Pd-Mn quasicrystals is nonmagnetic. Mn atoms located at specific sites can acquire a large magnetic moment if they have at least two Pd neighbors in the first coordination shell. Such configurations can be created by substitutional Al/Pd defects which can be formed at low energetic cost because at these sites the overlap between the pseudo-Mackay and Bergman clusters building the quasicrystalline structure leads to conflicting assignments of the chemical decoration. Besides the large magnetic moments formed on these special Mn sites, we have found a broad diffuse background of smaller magnetic moments on many different Mn atoms induced by the large Mn moments. In the present work we extend these investigations to magnetism at the five-fold surface of the quasicrystal and at isolated point defects with particular attention to the formation of induced moments. For the stable five-fold surface we find that Mn atoms located in the surface layer carry indeed large magnetic moments of up to 3 μB and that smaller magnetic moments (aligned both parallel and antiparallel to the surface moments) are induced at distances of up to 10 Å below the surface. We have considered three types of isolated point defects (and substitutional defects) around a Mn atom and investigated the formation of magnetic moments on the Mn atom and of induced moments on the surrounding sites. For both the magnetization induced below a magnetic surface and the magnetization induced around an Al/Pd substitutional defect, the induced moments show an irregular dependence on the distance from the inducing ”source” moment, but a marked dependence on the location of the Mn atom in the occupation domain in six-dimensional hyperspace. Mn atoms with a large coordinate in perpendicular space show large induced moments. Based on the analysis of the local paramagnetic density of states we demonstrate that the formation of a large induced moment is caused by a large polarizability of these Mn atoms, which is related in turn to a rather loosely packed local environment. Per Mn atom, the sum of the source moment (created by a special substitutional defect in the bulk quasicrystal or by the reduced coordination of a Mn atom at the surface) and of the induced moments reaches values of 6 to 8 μB, i.e., much higher than the limit set by Hund’s rule for the spin moment of a free Mn atom.