The effect of thermal treatment on the magnetic state and cluster-related disorder of icosahedral Al-Pd-Mn quasicrystals

The evolution of the magnetic state and the cluster-related disorder upon thermal treatment of the icosahedral i-Al-Pd-Mn quasicrystals were studied on single-crystalline samples subjected to different thermal annealing sequences involving short and long annealing times and slow or fast cooling modes. In this way, thermal equilibration of the samples was systematically changed by varying the amount of vacancies and thermal strains. The results show that different thermal treatments can reduce or increase reversibly the paramagnetic magnetization of the i-Al-Pd-Mn samples of nominally the same composition by a factor up to 26. Magnetic susceptibility and magnetization measurements have demonstrated comparable importance of the vacancies and thermal strains on the size of the mean effective magnetic moment per Mn atom. While the reduced vacancy concentration reduces the mean effective moment, thermal strains induced by rapid cooling increase it. Theoretical ab-initio calculations of the effect of vacancies and thermal strains on the magnetic properties of a periodic approximant model to the i-Al-Pd-Mn phase predict that vacancies at specific Al sites introduce magnetic moments at the Mn sites, which are otherwise nonmagnetic in the perfect structure. Removal of thermal strains by relaxation of the structure containing vacancies acts in the opposite way by decreasing the moments. X-ray diffraction experiments with full reciprocal space sampling revealed that the cluster-related disorder cannot be improved noticeably by annealing at either 800 degrees C or 600 degrees C even for annealing times as long as three months, suggesting that this disorder may be intrinsic to the i-Al-Pd-Mn phase, needed for the electronic or entropic stabilization of the structure. (C) 2009 Elsevier Ltd. All rights reserved