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Sökning: WFRF:(Denoel Fernand)

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1.
  • Denoel, Fernand (författare)
  • Electronic and magnetic properties of Tsai quasicrystals and their approximants under pressure
  • 2022
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • 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.
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2.
  • Denoel, Fernand, et al. (författare)
  • Eu Doping in the GdCd7.88 Quasicrystal and Its Approximant Crystal GdCd6
  • 2024
  • Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 63:11, s. 5040-5051
  • Tidskriftsartikel (refereegranskat)abstract
    • The effect of Eu doping in the Tsai quasicrystal (QC) GdCd7.88 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 Gd1-xEuxCd7.6±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 Gd1-xEuxCd6 (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 Gd1-xEuxCd6+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.
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3.
  • Denoel, Fernand (författare)
  • Frustrated magnetism in icosahedral structures : The influence of order-disorder in quasicrystals and approximant crystals
  • 2024
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • 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.
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4.
  • Denoel, Fernand, et al. (författare)
  • Magnetic frustration and glassiness in an icosahedral i-Tb-Cd quasicrystal
  • 2024
  • Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 109:18
  • 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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5.
  • Gebresenbut, Girma Hailu, 1982-, et al. (författare)
  • Atomic-Scale Tuning of Tsai-Type Clusters in RE-Au-Si Systems (RE = Gd, Tb, Ho)
  • 2020
  • Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 59:13, s. 9152-9162
  • 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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6.
  • Huang, Yu-Chin, et al. (författare)
  • Structural Analysis of the Gd-Au-Al 1/1 Quasicrystal Approximant Phase across Its Composition-Driven Magnetic Property Changes
  • 2023
  • Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 62:36, s. 14668-14677
  • 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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7.
  • Phuyal, Dibya, et al. (författare)
  • Ferroelectric properties of BaTiO3 thin films co-doped with Mn and Nb
  • 2019
  • Ingår i: AIP Advances. - : American Institute of Physics. - 2158-3226. ; 9:9
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on properties of BaTiO3 thin films where the bandgap is tuned via aliovalent doping of Mn and Nb ions co-doped at the Ti site. The doped films show single-phase tetragonal structure, growing epitaxially with a smooth interface to the substrate. Using piezoforce microscopy, we find that both doped and undoped films exhibit good ferroelectric response. The piezoelectric domain switching in the films was confirmed by measuring local hysteresis of the polarization at several different areas across the thin films, demonstrating a switchable ferroelectric state. The doping of the BaTiO3 also reduces the bandgap of the material from 3.2 eV for BaTiO3 to nearly 2.7 eV for the 7.5% doped sample, suggesting the viability of the films for effective light harvesting in the visible spectrum. The results demonstrate co-doping as an effective strategy for bandgap engineering and a guide for the realization of visible-light applications using its ferroelectric properties.
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8.
  • Serrano, Ismael G., et al. (författare)
  • Two-Dimensional Flexible High Diffusive Spin Circuits
  • 2019
  • Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 19:2, s. 666-673
  • Tidskriftsartikel (refereegranskat)abstract
    • Owing to their unprecedented electronic properties, graphene and two-dimensional (2D) crystals have brought fresh opportunities for advances in planar spintronic devices. Graphene is an ideal medium for spin transport while being an exceptionally resilient material for flexible nanoelectronics. However, these extraordinary traits have never been combined to create flexible graphene spin circuits. Realizing such circuits could lead to bendable strain-spin sensors, as well as a unique platform to explore pure spin current based operations and low-power 2D flexible nanoelectronics. Here, we demonstrate graphene spin circuits on flexible substrates for the first time. Despite the rough topography of the flexible substrates, these circuits prepared with chemical vapor deposited monolayer graphene reveal an efficient room temperature spin transport with distinctively large spin diffusion coefficients ∼0.2 m2 s–1. Compared to earlier graphene devices on Si/SiO2 substrates, such values are up to 20 times larger, leading to one order higher spin signals and an enhanced spin diffusion length ∼10 μm in graphene-based nonlocal spin valves fabricated using industry standard systems. This high performance arising out of a characteristic substrate terrain shows promise of a scalable and flexible platform towards flexible 2D spintronics. Our innovation is a key step for the exploration of strain-dependent 2D spin phenomena and paves the way for flexible graphene spin memory–logic units and planar spin sensors.
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9.
  • Shiino, Takayuki, et al. (författare)
  • Nonequilibrium dynamical behavior in noncoplanar magnets with chiral spin texture
  • 2022
  • Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 105:18
  • 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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10.
  • Shiino, Takayuki, et al. (författare)
  • Singular magnetic dilution behavior in a quasicrystal approximant
  • 2021
  • Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:22
  • 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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11.
  • Shiino, Takayuki, et al. (författare)
  • Singular magnetic dilution behavior in a quasicrystal approximant
  • 2021
  • Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 104:22
  • 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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12.
  • Shiino, Takayuki, et al. (författare)
  • Superconductivity at 1 K in Y-Au-Si quasicrystal approximants
  • 2021
  • Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 103:5
  • 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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