Magnetic and Mössbauer studies of quasicrystalline Al 70-x Pd 15 T 15 M x (T = Mn, Fe; M = Si, Ge; 0 ≤ x ≤8) D. Bahadur a,* , C.M. Srivastava a , M. Foldeaki b , A. Giguere b , R.A. Dunlap c a Department of Metallurgical Engineering and Material Science, Indian Institute of Technology, Powai, Bombay 400076, India b Institut de Recherche Sur 1’ Hydrogène, Universitè du Quèbec ã, Trois-Rivières, Trois-Rivières, Que., Canada c Department of Physics, Dalhousie University, Halifax, Canada N.S. Abstract Our magnetisation and Mössbauer effect investigations of magnetic icosahedral quasicrystals (Al, Ge) 70 Pd 15 Mn 15 and (Al, Si) 70 Pd 15 Fe 15 have shown that the magnetic atoms Mn and Fe occupy two types of sites, one of which is magnetic and one which is non-magnetic. In certain cases the magnetic atoms couple ferromagnetically. Keywords: Quasicrystals; Magnetisation; Mössbauer effect; Icosahedral 1. Introduction The moment formation in Al-based quasicrystals when 3D (Fe/Mn/Cr/Co) and 4D (Pd) magnetic atoms are simul- taneously present has recently been studied [1–4] Evidence of magnetic ordering in such alloys have also been reported when metalloid such as Boron or Germenium is used as a fourth component [4–6]. However, a clear understanding of the magnetic behaviour of these alloys is not yet available. Yokoyama et al. [4] and Bahadur et al. [5] have studied the magnetic properties of Al-Pd-Mn-B icosahedral quasicrys- tals and have concluded that the Mn atoms occupy two types of sites, on one of which it has no moment while on the other it has a small moment that can enter into ferromag- netic interactions with its near neighbours through an indi- rect coupling of 3D moments. The moment formation has been explained on the basis of Anderson’s model of localised magnetic states in metals [5]. On replacing B by Si or Ge the changes in lattice constant are expected. The magnetic moment formation when a magnetic atom T (Mn or Fe) is introduced in a non-magnetic matrix Al-M (M = metalloid) is not understood at present and efforts have been made to study this problem for a fixed value of T and Pd. In the present study we have synthesised Al 70-x Pd 15 T 15 M x (M = Si, Ge; 0 ≤ x ≤ 10; T = Mn, Fe) by melt spining. These alloys have been characterised by X-ray diffraction, room temperature Mössbauer spectroscopy and magnetic suscep- tibility measurements in the temperature range 4 to 300 K. 2. Experimental methods Alloys of composition Al 70-x T 15 Pd 15 M x with 0 ≤ x ≤ 10, M = Si; Ge, T = Mn, Fe were prepared by rapid quenching from the melt. X-ray diffraction measurements were made on a Siemens D-500 scanning diffractometer us- ing Cu K radiation. Magnetic susceptibility measurements were carried out between 4 and 300 K in applied fields of 1T using a SQUID magnetometer. Room temperature 57 Fe Mössbauer measurements were made using a Pd 57 Co source and a Wissel System II spectrometer with an intrinsic 57 Fe line width of 0.23 mm s -1 (FWHM). 3. Results and discussion X-ray diffraction results show that all samples are single-phase icosohedral alloys. Fig. 1 shows the plot of quasilattice constant versus metalloid concentration in Al 70-x Pd 15 T 15 M x (T = Mn, Fe; M = Ge, Si; x values be- tween 0 and 10). This plot shows that on substituting Si for Al, the lattice parameter decreases linearly with increasing Si content and follows Vegard’s law. On the other hand, on substituting Ge for Al, the lattice constant remains almost unchanged as has been reported elsewhere [4]. This effect may be readily understood on the basis of the similarity of the ionic radii of Al 3+ and Ge 4+ (0.51 and 0.5 Å), respec- tively, and the much smaller ionic radius of Si 4+ (0.42 Å). The change from Mn to Fe lowers the quasilattice constant by nearly 0.06 Å. The room temperature Mössbauer spectra of Al 70-x Fe 15 Pd 15 M x (M = Si, Ge, 2 ≤ x ≤ 10) exhibit