Magnetic Properties of Quasicrystals

The aperiodic T-A180Mn20 phase is magnetic with an effective moment 1.3 pg / Mn. Below 4.1 K a spin glass state is observed. The T-phase and the quasi-crystalline A173Mn21Si6 phase have similar magnetic properties. The crystalline phase, obtained by annealing the T-phase, is less magnetic than the T-phase. Quasi-crystalline (qc) phases, first discovered in Al-Mn based alloys, are aperiodic crystals which present a five-fold symmetry diffraction pattern (see [I] for a survey): One can wonder whether the electronic properties of qc differ-from those of crystalline phases with the same composition. To this purpose the AlsoMnm T-phase is an interesting case as it is known to present a periodic order in one direction and a qc structure in the perpendicular plane [2]. In magnetism studies differences do appear between crystalline and 3 dim qc phases. Indeed the magnetisation of qc-Al7sMnz1Si~ increases strongly as the temperature decreases while the crystalline phase obtained by annealing the qc one has a temperature independent susceptibility [3, 41. However NMR studies of this qcphase [5] have shown that a majority of Mn are not magnetic. It has been suggested that these non magnetic Mn, forming virtual bound states, would be responsible for the enhancement of the overall (s-p-d) density of states at the Fermi level detected by specific heat measurements [6]. In a previous paper [4], by comparing the initial susceptibility and the saturation magnetisation, we could estimate the fraction of magnetic Mn to be about 15 at. % . We have also shown that the magnetic Mn freeze in a spin-glass state below Tg = 5.2 K. We discuss here the magnetic properties of the T-AlsoMn20 phase and of the crystalline phase obtained by annealing. The T-phase sample was provided by C. Janot and J. M. Dubois. Elaboration details can be found in [I . Structural studies show that this sample is single phase [7]. Part of it was annealed at 700 "C for 10 hours under vacuum. We checked by neutron diffraction in situ that all the characterisic diffraction lines of the T-phase have disappeared. However the data precision does not allow to determine the respective proportion of the X and pAlsoMn20 crystalline phases in the annealed sample. The magnetisation of the T-A180Mnzo alloy is found temperature dependent (Fig. 1) but does not follow a Curie-Weiss type law, M / H = xo + C / (T + 8) , in the whole temperature range. Good fits can only be obtained in restricted temperature ranges: see table I. Moreover as the three parameters (8, C, x ~ ) are not independent the values given in table I should only be considered as indicative. The ac susceptibility exFig. 1. Temperature dependence of the magnetisation for the aperiodic T-Al~oMn2~ (A) and for the crystalline 1;AlsoMnzo (0). Fig. 2. T-A18~Mnz0: field-cooled (FC) and zero field cooled (ZFC) magnetisations. Insert: ac susceptibility cusp (Tg = 4.1 K). hibits a maximum at T, = 4.1 K (Fig. 2). Below Tg the magnetisation is found to depend on the cooling process. The magnetisation measured when field cooling is temperature independent while the magnetisation measured when warming up the sample, after cooling in zero field, shows a broad maximum around T, (Fig. 2). Thus the T-AlsoMn20 phase, like the qcA173Mn21si6 studied previously [4], exhibits qualitatively the magnetisation irreversibilities characteristic of spin glasses. Both samples are actually very similar, having close Tg and similar Curie-Weiss parameters (Tab. I). Magnetic measurements have been reported for 3 dim qc A~~oIVinzo [8] but some care should be taken in the data analysis since the samples of reference [8] contained fcc A1 and likely a non negligible amount of T phase. However when comparing the present results and those of reference [8] it appears that the magnetic properties of the 3 dim qc AlsoMnzo phase and of the T-AlsoMn~o phase are similar. The Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888104 C8 236 JOURNAL DE PHYSIQUE Table I. Magnetic properties of quasi-crystalline and crystalline alloys. Susceptibility data were obtained either by ac susceptibility technique (in zero static field) or by magnetisation measurements in fields up to 10 kG. At high temperature M increases linearly with H up to 10 kG so M (H = 10) provides a good determination of the initial susceptibility xi. This is no longer true below 15 K where M has to be measured in 2 kG to get xi. The parameters deduced from CurieWeiss fits, x = xo + C/ (T + 8) , depend on the fitted temperature range. The effective moment pee was deduced from C by assuming that all the Mn have the same moment, which is certainly wrong, but allows comparisons between difierent samples. The spin value S was then deduced from petr assuming g = 2. annealed ~ A l ~ ~ M n ~ o phase is different. It is still found partly magnetic (Fig. 1) but at 4.2 K its magnetisation is 4 times smaller than that of the T ~hase . The susceptibility increases continuously by a factor 3 between 4.2 K and 1.5 K. These results are somewhat surprisSample qc A 1 7 3 M n ~ ~ S i ~ P A173Mn21Si6 T A1s0Mn20