Magnetic properties of σ- and hexagonal-Mn76Si18Cr6 approximant phases of a dodecagonal quasicrystal

The tetragonal sigma phase and the hexagonal (hex) phase in the Mn-Si-V(Cr) transition-metal-alloy systems are stable approximant phases of a dodecagonal (12-fold) quasicrystal that can be prepared in bulk quantities. We have synthesized samples of the sigma and hex phases of composition Mn76Si18Cr6 and determined their magnetic properties. In the sigma-Mn76Si18Cr6, a spin-freezing transition to a canonical spin-glass phase was detected below T-f approximate to 8 K, characterized by a maximum in the zero-field-cooled susceptibility, a frequency-dependent cusp in the ac susceptibility, M(H) hysteresis, and ultraslow time decay of the thermoremanent magnetization. In contrast, no spin-glass transition was observed in the hex-Mn76Si18Cr6 phase down to the lowest investigated temperature of 2 K. The analysis of the susceptibility has shown that the coupling of spins in both phases is antiferromagnetic (AFM), but the coupling strength is considerably stronger in the sigma phase. Since both phases are structurally described by the triangle-square tiling scheme related to that of the dodecagonal quasicrystal, which imposes geometric frustration of the AFM-coupled spins on triangles, the absence of spin-glass transition in the hex-Mn76Si18Cr6 could be due to shifting of this transition below the lowest temperature of our experimental setup, as a consequence of weaker spin coupling and smaller moment sizes in the hex phase. In both investigated samples, tiny Mn3O4 inclusions that undergo a transition to a ferrimagnetic phase at T-C approximate to 42 K were detected in the magnetic signal. Geometric frustration of interactions between the AFM-coupled spins placed at the vertices of the triangle-square tiling should be a general feature of dodecagonal quasicrystals and their approximants, so that spin-glass-type ordering is expected to occur quite commonly in the dodecagonal phases.