| 1. |
- Denoel, Fernand, et al.
(författare)
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Magnetic frustration and glassiness in an icosahedral i-Tb-Cd quasicrystal
- 2024
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 109:18
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Gebresenbut, Girma Hailu, 1982-, et al.
(författare)
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Atomic-Scale Tuning of Tsai-Type Clusters in RE-Au-Si Systems (RE = Gd, Tb, Ho)
- 2020
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 59:13, s. 9152-9162
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Tidskriftsartikel (refereegranskat)abstract
- Tsai-type quasicrystals and approximants are distinguished by a cluster unit made up of four concentric polyhedral shells that surround a tetrahedron at the center. Here we show that for Tsai-type 1/1 approximants in the RE-Au-Si systems (RE = Gd, Tb, Ho) the central tetrahedron of the Tsai clusters can be systematically replaced by a single RE atom. The modified cluster is herein termed a pseudo-Tsai cluster and represents, in contrast to the conventional Tsai cluster, a structural motif without internal symmetry breaking. For each system, single-phase samples of both pseudo-Tsai and Tsai-type 1/1 approximants were independently prepared as millimeter-sized, faceted, single crystals using the self-flux synthesis method. The full replacement of tetrahedral moieties by RE atoms in the pseudo-Tsai 1/1 approximants was ascertained by a combination of single-crystal and powder diffraction studies, as well as energy dispersive X-ray spectroscopy (EDX) analyses with a scanning electron microscope (SEM). Differential scanning calorimetry (DSC) studies revealed distinctly higher decomposition temperatures, by 5-35 K, for the pseudo-Tsai phases. Furthermore, the magnetic properties of pseudo-Tsai phases are profoundly and consistently different from the Tsai counterparts. The onset temperatures of magnetic ordering (T-mag) are lowered in the pseudo-Tsai phases by similar to 30% from 24 to 17 K, 11.5 to 8 K, and 5 to 3.5 K in the Gd-Au-Si, Tb-Au-Si, and Ho-Au-Si systems, respectively. In addition, the Tb-Au-Si and Ho-Au-Si systems exhibit some qualitative changes in their magnetic ordering, indicating decisive changes in the magnetic state/structure by a moment-bearing atom at the cluster center.
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| 3. |
- Gebresenbut, Girma Hailu, 1982-, et al.
(författare)
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Chemical speciation in Gd-Cd-M (M=Zn, Au) quasicrystal approximants
- 2021
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Ingår i: Zeitschrift für Anorganische und Allgemeines Chemie. - : John Wiley & Sons. - 0044-2313 .- 1521-3749. ; 647:2-3, s. 86-90
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the effect of partial replacement of Cd by M=Au and Zn in the crystal structure of the 1/1 Tsai-type quasicrystal approximant (AC) GdCd6. Compositionally homogeneous single crystal samples Gd(Cd0.87Zn0.13(1))(6) and Gd(Cd0.80Au0.20(1))(6) were grown from melts Gd-5(Cd0.8Zn0.2)(100) and Gd-1(Cd0.9Au0.1)(100), respectively, and isolated by centrifugation. The M for Cd substitution in GdCd6 is accompanied with a sizeable reduction of the cubic unit cell parameter, from 15.514(2) angstrom to 15.329(1) angstrom (Zn) and 15.314(1) angstrom (Au). Site preferences were established from single crystal X-ray diffraction data. A clear preference of atomic sites for Au and Zn is noted which is compared to earlier reported Yb(Cd0.75Mg0.25)(6). Three and two out of in total seven crystallographic sites defining the Cd partial structure accept preferably metals more and less electronegative than Cd, respectively, and are classified as negatively and positively polarized sites in the binary 1/1 AC.
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| 4. |
- Gebresenbut, Girma Hailu, 1982-, et al.
(författare)
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Effect of pseudo-Tsai cluster incorporation on the magnetic structures of R-Au-Si (R = Tb, Ho) quasicrystal approximants
- 2022
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 106:18
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Tidskriftsartikel (refereegranskat)abstract
- In cluster-based quasicrystals, tetrahedra located in conventional Tsai clusters may be replaced by single rare-earth (R) ions at the cluster centers (pseudo-Tsai clusters). In this study, we investigate the effect of the pseudo-Tsai cluster incorporation on the magnetic structures of two approximants, the Tsai-type Tb-Au-Si [denoted TAS(0)] and Ho-Au-Si [denoted HAS(52)] with partial replacement of conventional Tsai clusters by pseudo-Tsai clusters, up to 52%. The mixture of Tsai and pseudo-Tsai clusters can be considered a different source of randomness/disorder other than the conventional chemical mix sites (Au/Si). The effect of the latter has been previously discussed regarding the origin/cause of spin-glass-like ordering and Anderson localization of electronic states in quasicrystals and approximant crystals. Single crystal neutron diffraction experiments at 2 K were performed and bulk physical properties (magnetization and specific heat) were investigated. In addition, earlier collected powder neutron diffraction data of TAS(14) with 14% replacement was reanalyzed in light of the results on TAS(0) and HAS(52). We find that the arrangement of ordered magnetic spins in the icosahedral shells of these phases is similar, while the cluster-center R magnetic states are different. In the case of TAS(14), the cluster-center Tb magnetic moments seem to affect the arrangement of surrounding icosahedral magnetic moments, and the magnetic structure of the icosahedral shell deviates from that of TAS(0). In the case of HAS(52), however, the icosahedral R magnetic moments are less affected by the cluster-center R, while the averaged cluster-center R magnetic moments are significantly diminished. We discuss these results considering the magnetic ordering effect on the bulk physical properties.
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| 5. |
- Gebresenbut, Girma Hailu, 1982-
(författare)
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Quasicrystal Approximants in the RE-Au-SM systems (RE = Gd, Tb, Ho, Yb; SM = Si, Ge) : Syntheses, structures and properties
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this study, new Tsai-type 1/1 quasicrystal approximants (ACs) in the RE-Au-SM systems (RE = Gd, Tb, Ho, Yb; SM = Si, Ge) were synthesized using high temperature synthesis techniques such as self-flux, arc-melting-annealing and novel arc-melting-self-flux methods. The syntheses not only provided appropriate samples for the intended structural and physical property measurements but could also be adapted to other systems, especially where crystal growth is a challenge. The newly developed arc-melting-self-flux method uses a temperature program that oscillates near the nucleation and melting points of the intended phase in order to obtain large single crystals. Self-flux methods employed to synthesize Ho-Au-Si and Tb-Au-Si ACs using a precursor alloy ≈Au79Si21 resulted in 100 mm3 and 8 mm3 single crystals, respectively.The crystal structures of the compounds are determined by either one or combinations of the following diffraction techniques; single crystal x-ray, powder x-ray, powder neutron and single crystal neutron diffraction methods. The crystal structure refinements indicated that the compounds are essentially iso-structural with the prototype Tsai-type 1/1 approximant crystal, YbCd6. In the present compounds there are some structural variations at the cluster center and in the so-called cubic interstices located at (¼, ¼, ¼).For the current ACs; either thermoelectric, magnetic or both properties were investigated. The measured properties were understood further by correlating the properties with the atomic structures of the ACs. Significant differences are observed in the thermoelectric properties, particularly on the lattice thermal conductivities (Kphonon) of Gd-Au-Si, Gd-Au-Ge and Yb-Au-Ge ACs. The difference is attributed to the presence of chemical and positional disorder.Magnetic susceptibility and specific heat measurements revealed ferromagnetic transitions at low temperatures, Tc ≈ 22.5 K for Gd-Au-Si and Tc ≈ 13.1 K for Gd-Au-Ge. For a Tb-Au-Si AC with 14 % central-Tb occupancy, a ferrimagnetic-like transition was observed at Tc ≈ 9 K. Later, it was noted that the Tc and other magnetic properties depend on the occupancy of the central-RE site. Consistent decrease of Tc with increasing central-Tb occupancy is observed. The dependency of magnetic behavior with central-RE occupancy was clarified by solving the magnetic structure of the Tb-Au-Si AC.
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| 6. |
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| 7. |
- Gebresenbut, Girma Hailu, 1982-
(författare)
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Syntheses, structure determination, magnetic and thermoelectric properties of quasicrystal approximants in RE-Au-SM systems (RE = Gd, Tb and Yb and SM = Si, Ge)
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this study, new compositions of Tsai-type 1/1 quasicrystal approximants Gd(14)Au(70)Si(16), Gd(14)Au(67)Ge(19), Tb(14)Au(70)Si(16) and Yb(16)Au(65)Ge(19)are synthesized using both self-flux and arc-melting-annealing techniques. Both syntheses routes resulted single phase samples. The crystal structures of the compounds are determined by collecting single crystal X-ray and/or powder X-ray and powder neutron diffraction intensities. The atomic structure refinements indicated that the compounds are essentially iso-structural with the prototype Tsai-type 1/1 approximant crystal, YbCd6. However, there are subtle structural variations at their cluster centers and in the so-called cubic interstices which affects some of their physical properties.Thermoelectric and magnetic properties of the compounds are investigated. Significant differences are observed in the thermoelectric properties of Gd(14)Au(70)Si(16), Gd(14)Au(67)Ge(19) and Yb(16)Au(65)Ge(19) compounds which are explained on the bases of their crystal structures and chemical compositions. Magnetic susceptibility and specific heat measurements revealed ferromagnetic transitions at low temperatures, Tc ≈ 22.5 K for Gd(14)Au(70)Si(16) and Tc ≈ 13.1 K for Gd(14)Au(67)Ge(19), whereas, for the Tb(14)Au(70)Si(16) compound a ferrimagnetic-like transition is observed at Tc ≈ 9 K. Moreover, a re-entrant spin-glass transition is observed at TRSG ≈ 3.3 K for Gd(14)Au(67)Ge(19) compound. Finally, the magnetic structure of the Tb(14)Au(70)Si(16) compound was determined from powder neutron diffraction data which is to our knowledge the first magnetic structure refinement report in the family of quasicrystals and approximants.
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| 8. |
- Hiroto, Takanobu, et al.
(författare)
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Ferromagnetism and re-entrant spin-glass transition in quasicrystal approximants Au-SM-Gd (SM = Si, Ge)
- 2013
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Ingår i: Journal of Physics. - IOP Publishing in Bristol, UK : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:42, s. 426004-
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic susceptibility and specific heat measurements on quasicrystalline approximants Au–Si–Gd and Au–Ge–Gd reveal that a ferromagnetic (FM) transition occurs at Tc = 22.5(5) °K for Au–Si–Gd and at Tc = 13(1) °K for Au–Ge–Gd, which are the first examples of ferromagnetism in crystalline approximants. In addition, a re-entrant spin-glass (RSG) transition is observed at TRSG = 3.3 °K for Au–Ge–Gd in contrast to Au–Si–Gd. The different behaviors are understood based on the recent structural models reported by Gebresenbut et al (2013 J. Phys.: Condens. Matter 25 135402). The RSG transition in Au–Ge–Gd is attributed to a random occupation of the center of the Gd12 icosahedron by Gd atoms; a central Gd spin hinders the long-range FM order.
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| 9. |
- Huang, Yu-Chin, et al.
(författare)
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Structural Analysis of the Gd-Au-Al 1/1 Quasicrystal Approximant Phase across Its Composition-Driven Magnetic Property Changes
- 2023
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 62:36, s. 14668-14677
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Tidskriftsartikel (refereegranskat)abstract
- 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 Gd14AuxAl86–x 1/1 AC system. These findings will contribute to improved understanding towards tailoring magnetic properties in emerging materials.
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| 10. |
- Joshi, Deep C., et al.
(författare)
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2D crystal structure and anisotropic magnetism of GdAu6.75−xAl0.5+x (x ≈ 0.54)
- 2022
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 12
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Tidskriftsartikel (refereegranskat)abstract
- Exploration of the gold-rich part of the ternary Gd–Au–Al system afforded the intermetallic compound GdAu6.75−xAl0.5+x (x ≈ 0.54) which was structurally characterized by single crystal X-ray diffraction (Pnma, a = 18.7847(4) Å, b = 23.8208(5) Å, c = 5.3010(1) Å). GdAu6.75−xAl0.5+x crystallizes in a previously unknown structure type featuring layers of Gd2(Au, Al)29 and Gd2(Au, Al)28 clusters which are arranged as in a close-packing parallel to the ac plane. The Gd substructure corresponds to slightly corrugated 36 nets (dGd–Gd = 5.30–5.41 Å) which are stacked on top of each other along the b direction with alternating short (5.4, 5.6 Å, within layers) and long distances (6.4 Å, between layers). The title compound has been discussed with respect to a quasicrystal approximant (1/1 AC) GdAu5.3Al in the same system. The magnetic properties of GdAu6.75−xAl0.5+x were found to be reminiscent to those of some ternary ACs, with sharp peaks in the temperature dependent magnetization, and metamagnetic-like transitions. The material becomes antiferromagnetic below 25 K; magnetometry results suggest that the antiferromagnetic state is composed of ferromagnetic ac planes, coupled antiferromagnetically along the b direction.
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| 11. |
- Joshi, Deep C., et al.
(författare)
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Memory and rejuvenation in a quasicrystal
- 2020
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 132:2
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Tidskriftsartikel (refereegranskat)abstract
- The glassy features of a single crystal of the icosahedral quasicrystal i-GdCd7.5 were investigated by means of squid magnetometry. The temperature-dependent zero-field-cooled magnetization was recorded on re-heating from low temperatures after halts in the cooling. The results evidence dynamical features akin to those of archetypal spin glasses, such as aging, memory, and rejuvenation. The results are compared to those of model spin glasses with different spin dimensionality, suggesting a qualitative similarity to the behaviour of metallic RKKY "Heisenberg" spin glasses. Copyright (C) 2020 The author(s)
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| 12. |
- Shiino, Takayuki, et al.
(författare)
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Examination of the critical behavior and magnetocaloric effect of the ferromagnetic Gd-Au-Si quasicrystal approximants
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 106:17
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the critical behavior and magnetocaloric effects of the Gd-Au-Si (GAS) ferromagnetic quasicrystal approximants, Gd13.7 Au72.7 Si13.6 [referred to as GAS(0)] and Gd15.4 Au68.6 Si16.0 [GAS(100)]. The former is a conventional Tsai-type 1/1 approximant crystal, while the latter has a slightly different atomic decoration from the Tsai type (thus referred to as “pseudo-Tsai” type). Their critical exponents at the ferromagnetic transitions are close to those of the mean-field theory. Both GAS systems exhibit an interesting magnetic-field dependence of the specific heat, which is reflected in the behavior of their magnetocaloric effect (MCE). The MCE is characterized by an adiabatic cooling (heating) effect over a relatively broad temperature range below ∼30 K, which stems from a broad feature in the specific heat.
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| 13. |
- Shiino, Takayuki, et al.
(författare)
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Nonequilibrium dynamical behavior in noncoplanar magnets with chiral spin texture
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 105:18
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 14. |
- Shiino, Takayuki, et al.
(författare)
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Singular magnetic dilution behavior in a quasicrystal approximant
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:22
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Tidskriftsartikel (refereegranskat)abstract
- We report the effect of magnetic dilution on the physical properties of (Gd1−xYx)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.
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| 15. |
- Shiino, Takayuki, et al.
(författare)
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Superconductivity at 1 K in Y-Au-Si quasicrystal approximants
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 103:5
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Tidskriftsartikel (refereegranskat)abstract
- We report the structural and physical properties of two Y-Au-Si (YAS) compounds, Y(14.1)AU(69.2)Si(16.7) and Y15.4Au68.6Si16.1, which are 1/1 approximant crystals of a Tsai-type quasicrystal without intrinsic magnetic moments. The compounds differ by the presence of either a tetrahedron (Au,Si)(4) or a single Y atom at the center of their characteristic structural building unit consisting of concentric polyhedral shells. Both compounds exhibit bulk superconductivity, which seems to be of a conventional type-II BCS type. The compound with Y atoms at the cluster center has a slightly higher transition temperature with a sharper step in the specific heat than the compound with tetrahedral units. We discuss the occurrence of this superconducting state in the light of the specific structural and physical properties of these quasicrystal approximants.
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