Icosahedral quasicrystals and magnetism

Abstract

For several years the study of magnetic properties of quasicrystals was limited mostly to Mössbauer spectroscopy and susceptibility measurements on paramagnetic and spin-glass alloys primarily in the Al–Mn(–Si) system. A distribution of low-symmetry Mn sites appears to exist with only the larger sites (making up approximately 60% of the total) allowing a Mn moment to form. Fe and V (neither of which bears a moment in this system) appear to substitute preferentially in the non-magnetic and magnetic Mn sites respectively. Recently, ferromagnetic icosahedral quasicrystals have been discovered in the Al–Mn–Si and Al–Mn–Ge systems, broadening the field of interest as well as the range of techniques that can be brought to bear on these unusual materials. While large average effective moments can characterize paramagnetic quasicrystals (2·0 μ_B per Mn atom in Al₆₀Mn₂₀Ge₂₀), ferromagnetic quasicrystals so far exhibit high Curie temperatures (up to 500 K) but very small magnetizations (less than 2e.m.u. g⁻¹). Coercivities of 2000 Oe have been observed in Al₄₀Cu₁₀Mn₂₅Ge₂₅.