Experimental Observation of Long-Range Magnetic Order in Icosahedral Quasicrystals

Quasicrystals (QCs), first discovered in 1984, generally do not exhibit long-range magnetic order. Here, we report on long-range magnetic order in the real icosahedral quasicrystals (i QCs) Au-Ga-Gd and Au-Ga-Tb. The Au₆₅Ga₂₀Gd₁₅ i QC exhibits a ferromagnetic transition at T_C = 23 K, manifested as a sharp anomaly in both magnetic susceptibility and specific heat measurements, along with an appearance of magnetic Bragg peak below T_C. This is the first observation of long-range magnetic order in a real quasicrystal, in contrast to the spin-glass-like behaviors observed for the other magnetic quasicrystals found to date. Moreover, when Gd is replaced by Tb, i.e., for the Au₆₅Ga₂₀Tb₁₅ i QC, a ferromagnetic behavior is still retained with T_C = 16 K. Although the sharp anomaly in the specific heat observed for the Au₆₅Ga₂₀Gd₁₅ i QC becomes broadened upon Tb substitution, neutron diffraction experiments clearly show marked development of magnetic Bragg peaks just below T_C, indicating long-range magnetic order for the Au₆₅Ga₂₀Tb₁₅ i QC also. Our findings can contribute to the further investigation of exotic magnetic orders formed on real quasiperiodic lattices with unprecedented highest global symmetry, i.e., icosahedral symmetry.