Magnetic properties of the icosahedral quasicrystals silver indium gadolinium and zinc iron scandium, and the 11 approximant silver indium gadolinium

This thesis is a study of magnetic properties of new icosahedral quasicrystals Ag50In36Gd14, Zn77Fe7Sc 16 and a 1/1 crysta lline approximant Ag50In36Gd 14. The Ag50In36Gd14 and Zn77 Fe7Sc16 quasicrystals are shown to have a primitive six-dimensional Bravais lattice with a six-dimensional hypercubic lattice constant of, respectively, 7.805(2) and 7.087(1) A. The crystal structure of the approximant Ag50In36Gd14 is found to be of the YbCd6 type (space group lm3¯) with a lattice constant of 15.202(1) A. No evidence is found for a transition to a ground state with long-range magnetic order in the temperature range 2--300 K for all three alloys. These three alloys are shown to be spin glasses. The spin freezing temperature of the Ag50In36Gd14 and Zn77Fe 7Sc16 quasicrystals is, respectively, 4.25(5) and 7.75(2) K. The unusual spin freezing in the approximant is found to occur in two stages: at ∼3.6 K spins develop short-range correlations but they co ntinue to fluctuate at low frequencies, and then long-range freezing is achieved upon further cooling to below ∼2.4 K. The Ag50In36Gd 14 quasicrystal and its 1/1 approximant are shown to belong to a category of strongly geometrically frustrated magnets. The frequency dependence of the freezing temperature in all three alloys is shown to be equally well accounted for by the Vogel-Fulcher law and the power law. Analysis of the ageing effects observed in the Zn77Fe 7Sc16 quasicrystal leads to a major discovery that the nature of the spin-glass state in this quasicrystal is fundamentally different from that of a canonical spin glass. The effective magnetic moment at Gd atoms in the Ag50In 36Gd14 quasicrystal and its 1/1 approximant is found to be close to that of a Gd+3 ion. The effective magnetic moment at Fe atoms in the Zn77Fe7Sc16 quasicrystal is the largest ever found in an Fe-containing quasicrystal. The hyperfine magnetic fields at 155Gd and 57Fe nuclei in the Ag50In36Gd14 and Zn77 Fe7Sc16 quasicrystals are shown to set in at temperatures larger than the corresponding freezing temperatures. The presence of the distribution of the electric quadrupole splitting in the 155Gd Mossbauer spectra of the Ag50In36Gd14 quasicrystal indicate s the existence of the multiplicity of Gd sites. The binomial distribution of the electric quadrupole splitting and the hyperfine magnetic field derived from the 57Fe Mossbauer spectra Zn77Fe 7Sc16 quasicrystal indicates the existence of two classes of Fe sites. The Debye temperatures of the quasicrystals Ag50In 36Gd14, Zn77Fe7Sc16, and the approximant Ag50In36Gd14 determined from the Mossbauer spectra are, respectively, 199(2),443(8), and 199(1) K