Ferromagnetic transition in Au- based Approximants

The structure of Tsai-type magnetic quasicrystals and its related compounds (called approximants) are characterized by the space-filling of an icosahedral cluster which has a rare-earth icosahedron [1]. From an experimental point of view, such compounds have been known to show the spin glass like behavior without exception [2]. However, the discovery of the antiferromagnetic phase transition in the Cd-Tb approximant [3] gives a counterexample to this trend. Moreover, ferromagnetic transitions were observed in the Au-based approximant recently. In this paper, magnetic phase transitions in Au-Si-R (R= Gd ,Tb, Dy and Ho) approximants are discussed. In all the systems, the temperature dependence of magnetization show ferromagnetic transition at Tc. On the other hand, the magnetization curves below Tc are different between Gd-compound and non-Gd compounds. The difference in the magnetization may be attributed to the existence of the CEF effect in the non-Gd compounds which have non-zero orbital angular momentum.

Synthesis and magnetic property of a Tsai-type 2/1 approximant

Various Tsai-type quasicrystals and approximants have been synthesized by replacing the constituent elements of the Cd-RE systems, based on the condition, (e/a)~2.0,. However, systems containing transition metals (TMs) have been rarely reported and, therefore, in this work we aimed to synthesize TM-bearing Tsai-type quasicrystals and approximants. Starting from the composition of the Ga-Pd-Gd 1/1 approximant [1], a search of TM-bearing approximants was performed by replacing Pd (1.376 Å) by Pt (1.378 Å). In this research, alloys of various compositions around the reported composition of 1/1 Ga-Pd-Gd were prepared by arc melting and they were then annealed at various conditions. The phase constitution was studied by X-ray diffraction and the thermal stability of the phases was examined by differential thermal analysis (DTA) for samples before and after annealing. As a result, we have observed formation of both 1/1 and 2/1 approximants with a=14.37 Å and 23.23 Å, respectively. The DTA curve exhibits no exothermic peak in the heating run up to the melting point (Tm=1180 K) for the 2/1 approximant, which suggests that the obtained 2/1 approximant is thermally stable up to Tm. The magnetic properties of the 1/1 and 2/1 approximants will be reported in the presentation.

Synthesis and thermal stability of Au-based quasicrystals and its approximants

Various Tsai-type quasicrystals and crystalline approximants have been found in a variety of systems. These compounds are made of Tsai-type icosahedral clusters. Recently, the existence of long-range magnetic orders was reported in Cd6R and Au-SM-R approximants[1,2]. Therefore, it is of interest to investigate the magnetic properties of new alloys systems which are also composed of Tsai-type clusters. In the present work, in order to search new Tsai-type quasicrystals as well as approximants, we have investigated the phase constitution and phase stability in the Au-In-R(R=rare-earth) systems. In this research, alloys of various compositions were prepared by arc melting and were subsequently annealed under various conditions. The phase constitution and phase stability were investigated by X-ray diffraction and differential thermal analysis. As a result, we have observed the formation of 1/1 approximants in the Au-In-R systems and found that they are stable up to high temperatures of ~1000K. The formation condition and stability of icosahedral quasicrytals will be also reported in the presentation.

Phase transition and atomic structure of Cd₆Tb 1/1 approximant to quasicrystal

We present an in-situ temperature study of the atomic structure of the 1/1 Cd6Tb approximant to an icosahedral quasicrystal. It belongs to the `Tsai' type family of quasicrystal and approximants whose archetype is the well-studied Cd-Yb system[1]. Its high temperature structure can be described as a bcc packing of a large Tsai atomic cluster, whose inner shell is a disordered tetrahedron at room temperature. As for most of the Cd6RE (RE=rare earth) approximant, the Cd6Tb phase undergoes a phase transition at 190 K to a phase of lower symmetry, resulting from an ordering of the inner tetrahedron[2]. Moreover, it has been shown that this phase undergoes a magnetic phase transition below 20 K, with an antiferromagnetic ordering of the Tb moment bearing atoms. It is thus particularly important to have a detailed structural study of this phase. We have carried out a systematic in situ measurement on a single grain from room temperature down to 40K on the crystal beam line located at the Soleil synchrotron. The structural phase transition is observed at about 190K. Using different attenuation, we have collected integrated intensity in a large dynamical range, leading to more than 60000 unique reflections in the C2/c monoclinic low temperature phase. The final wR2 values for room temperature and 40K are equal to 0.0726 and 0.0905 respectively. The resulting atomic structure will be compared to the well-studied approximant Zn6Sc, which is isostructural to Cd6Tb[3]. The ordering of the innermost tetrahedron leads to the distortion of the successive shells. The evolution of the high temperature phase, in particular just above Tc where pretransitional diffuse scattering is observed will be presented.