Tailoring Magnetic Behavior in the Tb-Au-Si Quasicrystal Approximant System

Abstract

A novel synthesis method, "arc-melting-self-flux", has been developed and a series of five Tsai-type 1/1 approximant crystals in the Tb-Au-Si system have been synthesized. The synthesis method, by employing a temperature program which oscillates near the melting and nucleation points of the approximants, has provided high-quality and large single crystals in comparison to those obtained from the standard arc-melting-annealing and self-flux methods. The atomic structures of the approximants have been determined from single-crystal X-ray diffraction data and described using concentric atomic clusters with icosahedral symmetry. The compounds are nearly isostructural with subtle variations; two types of atomic clusters which mainly vary at their cluster centers are observed. One type contains a Tb site at the center, and the other contains a disordered tetrahedron decorated with Au/Si mixed sites. Both cluster types can be found coexisting in the approximants. The compounds have different average weighted ratios of central Tb to disordered tetrahedron in the bulk material. Furthermore, a strategy for chemically tuning magnetic behavior is presented. Magnetic property measurements on the approximants revealed that the magnetic transition temperature (Tc) decreases as the occupancy of the central Tb site increases. Tc decreased from 11.5 K for 0% occupancy of the central Tb to 8 K for 100% occupancy. Enhanced magneto crystalline anisotropy is observed for the approximants with higher central Tb occupancy in comparison to their low central Tb occupancy counterparts. Hence, the previously reported "ferrimagnetic-like" magnetic structure model remains valid.

Long-range magnetic order in real icosahedral quasicrystals

“…On the other hand, for i Au65Ga20Tb15, the M magnitude is suppressed to ~6B/Tb 3+ at 7 T, about two-thirds of the full moment of a Tb 3+ ion (9B/Tb 3+ ). This behaviour is closely consistent with those of Tb-containing ACs [25][26][27] , which was attributed to the existence of the strong uniaxial anisotropy of the Tb 3+ spins. The M suppression is ascribed to the formation of non-coplanar spin configuration due to this strong uniaxial anisotropy of Tb 3+ spins in Au-Si-Tb 1/1 AC 28 .…”

“…3a and 3b, we note that χ increases sharply at 23.4 K and 16.0 K for the Au65Ga20R15 (R = Gd, Tb) i QCs, respectively, suggesting the occurrence of a ferromagnetic transition for both i QCs. For i Au65Ga20Tb15, a deviation of χ between the FC and ZFC susceptibilities is observed below 16.0 K; this behaviour is similar to that observed for Tb-containing ACs 25,27 . The M-H curves for Au65Ga20R15 (R = Gd, Tb)…”

Atomic-Scale Tuning of Tsai-Type Clusters in RE–Au–Si Systems (RE = Gd, Tb, Ho)

“… 21 Further, Gebresenbut et al showed that a successive replacement of the central tetrahedron unit in the Tsai cluster by a single Tb atom leads to a lowering of the magnetic ordering temperature. 15 However, millimeter-sized single crystals of TAS(CC) synthesized from excess flux as in the present case were not available at the time.…”

“…Our previous study of the Tb–Au–Si system indicates a solid solution behavior, at least for RE = Tb. 15 …”

“… 13 , 14 Recently, we studied the Tsai-type 1/1 AC in the Tb–Au–Si system and could show that a successive replacement of the tetrahedron by a single Tb atom may be achieved by diligent control of the starting compositions and elaborate heat treatments of samples. 15 Yet, several synthesis steps were necessary for achieving a complete substitution of the tetrahedron, and although samples large enough for single-crystal X-ray diffraction could be obtained, they were not large enough or had insufficient quality to perform more elaborate physical property measurements, surface studies, or single-crystal neutron diffraction measurements. An important finding of this study, however, was that tuning the chemistry of the cluster center leads to altered magnetic properties for the Tb–Au–Si AC.…”