A comparative study of the magnetic properties of the 1/1 approximant Ag(50)In(36)Gd(14) and the icosahedral quasicrystal Ag(50)In(36)Gd(14).

We report on measurements of the dc and ac magnetic susceptibility, (155)Gd Mössbauer spectra, and specific heat of the 1/1 approximant Ag(50)In(36)Gd(14), and of the ac magnetic susceptibility of the icosahedral quasicrystal Ag(50)In(36)Gd(14). These alloys are shown to be spin glasses. For the icosahedral quasicrystal Ag(50)In(36)Gd(14), spin freezing occurs at T(f) = 4.3 K, and the frequency dependence of T(f) is well accounted for by the Vogel-Fulcher and power laws. Spin freezing in the 1/1 approximant Ag(50)In(36)Gd(14) occurs in two stages: at T(f(1)) = 3.7 K, Gd spins develop short-range correlations but continue to fluctuate, and then long-range freezing is achieved at T(f(2)) = 2.4 K. The frequency dependences of T(f(1)) and T(f(2)) can be accounted for by means of the Vogel-Fulcher law and the critical slowing down dynamics. It is shown that the spin freezing in both alloys is a nonequilibrium phenomenon rather than a true equilibrium phase transition. The (155)Gd Mössbauer spectra of the 1/1 approximant Ag(50)In(36)Gd(14) confirm that the Gd spins are frozen at 1.5 K and are fluctuating at 4.6 K. The magnetic specific heat exhibits a maximum at a temperature that is 30% larger than T(f(1)), but the temperature derivative of the magnetic entropy peaks at T(f(1)). The Debye temperature of the 1/1 approximant Ag(50)In(36)Gd(14) is 199(1) K as determined from the Mössbauer data, and 205(2) K as determined from the specific heat data.