Abstract
We have investigated the low-temperature specific heat of the quasi-crystalline (Q-C) Al73.2Mn21Si5.8 related first approximant and hexagonal crystalline phases. All measurements were performed on small, 2-mg, single-phase crystals. The structure of each phase was carefully analyzed by x-ray diffraction and for the Q-C phase all pics positions were correctly indexed to a ‘‘six-dimensional crystal’’ model.1 The samples where obtained by planar flow casting and for the hexagonal phase by an adequate annealing of the Q-C phase. The low-temperature specific heat is analyzed in a standard way as the sum of three electronic, magnetic, and phonon contributions. This analysis indicates unambiguously the presence in the Q-C phase of a large magnetic term (C/T=11 mJ/mole K K) that is supposed to be caused by the thermal excitations of a spin-glass-like ordering.2 Such a large magnetic contribution is observed neither in the hexagonal phase, which has a different local symmetry, nor in the cubic approximant with the same local icosanedral symmetry. Thus it is proposed that the random freezing of the Mn spins observed in the AlMnSi Q-C is not a canonical spin-glass state but a specific property of spins on a quasi-periodic lattice.
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