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Tsai-type 1/1 RE-Au-Al approximant crystal (AC) phases RE₁₄Au_xAl_86-x (RE = Gd, Tb) display a large homogeneity range (50 < x < 75) across which distinct changes of magnetic properties – from spin glass to ferromagnetic (FM) to antiferromagnetic (AFM) – and intricate changes in structural properties occur. Here we analyzed selected 1/1 Ho- and Sm-Au-Al AC phases in the range 53 < x < 71 with single crystal X-ray diffraction and magnetometry. We find that the Al/Au ordering behavior and associated structural changes with x are similar in both systems, and match that observed for RE = Gd, suggesting that the structural evolution with x is rather independent from the type of RE. On the other hand, the magnetic properties, and notably the type of long-range order obtained at large x, are strongly depending on the RE. Ho₁₄Au_xAl_86-x samples with x = 61–71 consistently displayed a ferrimagnetic behavior whereas Sm₁₄Au_xAl_86-x samples with x = 67 and 71 revealed AFM behavior.

The effect of Eu doping in the Tsai quasicrystal (QC) GdCd_7.88 and its periodic 1/1 approximant crystal (AC) GdCd₆ are investigated. This represents the first synthesis of Eu-containing stable QC samples, where three samples with the final composition Gd_1-xEu_xCd_7.6±α at Eu doping concentrations x = 0.06, 0.13, and 0.19 are obtained (α ∼ 0.2). They are compared to two 1/1 ACs with compositions Gd_1-xEu_xCd₆ (x = 0.12, 0.16). In addition, a new type of 1/1 AC, differing only by the inclusion of extra Cd sites unique to the Eu₄Cd₂₅ 1/1 AC, has been discovered and synthesized for the concentrations Gd_1-xEu_xCd_6+δ (x = 0.25, 0.33, 0.45, 0.69, 0.73, and 0 < δ ≤ 0.085). Due to the preferred cube morphology of its single grains, we refer to them as c-type 1/1 ACs and to the conventional standard ones as s-type. In both QCs and s-type ACs, the Eu content appears to saturate at a concentration of similar to 20%. On the other hand, any Gd| Eu ratio is allowed in the c-type ACs, varying continuously between GdCd₆ and Eu₄Cd₂₅. We describe and contrast the changes in composition, atomic structure, specific heat, and magnetic properties induced by Eu doping in the quasicrystalline phase and the s-type and c-type 1/1 ACs. By comparing our results to the literature data, we propose that the occupancy of the extra Cd sites can be used to predict the stability of Tsai-type quasicrystalline phases.

This work will begin with an introduction to quasicrystals (QCs) and their structure, defining the unit cell-equivalent for icosahedral quasiperiodicity: a 3D tiling based on the golden ratio and aperiodic in all three dimensions. We briefly present the cut-and-project method used to generate QCs from a higher dimension hypercrystal, and how to build the 3D Penrose tilings, composed of the two golden rhombohedra. We introduce the atomic elementary units used to understand Tsai-type icosahedral QCs and their related approximant crystals (ACs). Approximants are conventional periodic crystals with local atomic environments very similar to their QC counterparts. Two methods for synthesis of Tsai-type QCs and approximants are introduced, the self-flux method as well as a rapid quench method. We describe briefly the differences between the two methods and aspect about phase stability for QCs and ACs obtained in binary and ternary systems. Various types of structural modulations can be induced in ACs which are absent from QCs. We present a new criterion discovered during the doctoral studies, which links the structure of existing ACs to the existence of stable QC counterparts. Basic concepts of magnetism relevant to Tsai systems are then introduced, with a description of the 4f shell magnetism in lanthanide elements, how it differs from transition metal magnetism, with localized (and relatively large) magnetic moments. In intermetallic systems such as Tsai-type QCs and ACs, the main type of magnetic interaction is of RKKY type. We link their structure to the magnetic behavior and phase transition that can occur in frustrated systems: spin glass, reentrant spin glass, spin ice, etc. and how they can be related to various Tsai systems.

The dynamical magnetic properties of the icosahedral i-Tb-Cd quasicrystal (QC) were investigated by means of squid magnetometry. At low temperatures, below T ∼ 4.5 K, we observe conventional spin glass features including aging, memory, and rejuvenation. Interestingly, from T ∼ 4.5 to 10 K, the spin configuration is also found to evolve with time (i.e., age as in spin glass), yet without memory. This suggests a reentrantlike behavior for the i-Tb-Cd QC, where upon cooling from high temperatures, the system first displays a magnetically frustrated phase before turning into a spin glasslike one at the lowest temperatures. We discuss the nature and possible origin of this magnetic behavior, in the light of the results obtained in the i-Gd-Cd QC, and ternary Tb-based quasicrystal approximants.

The magnetic properties of binary Gd-Cd and ternary Gd-Au-Ge crystals obtained from the newly introduced low-melt peritectic formation (LMPF) synthesis method were investigated. This method consists of a rapid quenching of the metallic melt followed by an annealing treatment at a relevant temperature. In the first system, both quasicrystal (QC) and approximant crystal (AC) phases can be stabilized, whereas only the AC phase is obtainable in the pseudo-binary Gd-(Au-Ge) system. The magnetic properties of the crystals obtained from LMPF for each system are compared to those of larger single grain crystals obtained from the self-flux (SF) method.

A single-crystal neutron diffraction study was conducted on a Ho13.6Au61.1Al25.3 Tsai-type quasicrystal approximant synthesised by the self-flux method. The magnetisation measurements reveal a ferrimagnetic behaviour with a transition below 6 K. In agreement with the magnetometry data, a non-coplanar whirling spin order in the icosahedral clusters around the crystallographic [1 1 1] direction with a ferrimagnetic arrangement was observed from single-crystal neutron diffraction measurements below 5.5 K. The magnetic structure of Ho13.6Au61.1Al25.3 is compared to the previously published magnetic structures of related RE-Au-SM (RE = Tb and Ho; SM = Al, Si and Ga) systems.

Gd14AuxAl86-x Tsai-type 1/1 quasicrystal approximants (ACs) exhibit three magnetic orders that can be finely tuned by the valence electron concentration (e/a ratio). This parameter has been considered to be crucial for controlling the long-range magnetic order in quasicrystals (QCs) and ACs. However, the nonlinear trend of the lattice parameter as a function of Au concentration suggests that Gd14AuxAl86-x 1/1 ACs are not following a conventional solid solution behavior. We investigated Gd14AuxAl86-x samples with x values of 52, 53, 56, 61, 66, and 73 by single-crystal X-ray diffraction. Our analysis reveals that increasing Au/Al ordering with increasing x leads to distortions in the icosahedral shell built of the Gd atoms and that trends observed in the interatomic Gd-Gd distances closely correlate with the magnetic property changes across different x values. Our results demonstrate that the e/a ratio alone may be an oversimplified concept for investigating the long-range magnetic order in 1/1 ACs and QCs and that the mixing behavior of the nonmagnetic elements Au and Al plays a significant role in influencing the magnetic behavior of the Gd1(4)Au(x)Al(86-x) 1/1 AC system. These findings will contribute to improved understanding towards tailoring magnetic properties in emerging materials.

Quasicrystals constitute a special category of crystals exhibiting long-range order without periodicity. This strange feature allows them to exhibit rotational symmetries prohibited in regular crystals, such as 5-, 8-, 10- or 12-fold symmetry. Amongst them, icosahedral quasicrystals are the only type that is aperiodic along all 3 dimensions. The present work will focus on Tsai-Type quasicrystals and their approximants, which are the most studied type of icosahedral quasicrystals. We will introduce how they can be generated froma hyperspace perspective, their exotic structure and how it relates to their physical properties. Pressure is a key parameter which can be used to alter the condensed matter properties of a material. By reducing the size of samples and using diamond anvil cells, it is possible to obtain the highest experimentally achievable pressures and collect informations such as magnetization, electrical resistivity or Raman signal. This work will introduce the specialized diamond anvil cells we used to investigate electron transport and magnetization under pressure, explain in details how to circumvent the most common difficulties arising with high pressure setups. and describe how high pressure affects Tsai quasicrystals and approximants in terms of structure and magnetic properties.

We observe nonequilibrium dynamical magnetic behavior in the magnetically ordered phase of a Tsai-type Tb-Au-Si quasicrystal approximant system. The magnetic texture in the ordered phase is found to exhibit scalar spin chirality (SSC) order, inferring that SSC is the order parameter of the present magnetic system. We further find that the introduction of “pseudo-Tsai” clusters, associated with additional Tb atoms in the structure, induces spin-glass dynamics. We discuss the observed dynamical magnetic behavior in the Tb-Au-Si systems, considering the effect of the pseudo-Tsai clusters on the magnetic configuration and local spin chirality.

We report the effect of magnetic dilution on the physical properties of (Gd_1−xY_x)Cd₆ approximant crystals (ACs), close siblings of their corresponding quasicrystal (QC). Compared to the pure system GdCd₆, we observe remarkable changes in the thermodynamic and magnetic bulk properties near and below the static-ordering temperatures from diluting the magnetic Gd atoms with nonmagnetic Y atoms by only 1–3% (x=0.01–0.03). On the other hand, the corresponding QC system exhibits a monotonic change in its spin-glass behavior upon the magnetic dilution. We discuss the origin of the magnetic-dilution behavior in the present AC system in terms of possible magnetic frustration and short-range magnetic correlation that can be linked to its peculiar structure.