Takanobu Hiroto

Ferromagnetism and re-entrant spin-glass transition in quasicrystal approximants Au-SM-Gd (SM = Si, Ge)

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.

Sign of canted ferromagnetism in the quasicrystal approximants Au-SM-R (SM = Si, Ge and Sn / R = Tb, Dy and Ho)

Magnetic susceptibility and magnetization of the quasicrystal approximants Au-SM-R (SM = Si, Ge or Sn / R = Gd, Tb, Dy or Ho) are investigated. Ferromagnetic transitions are observed in all of these compounds, in contrast to the spin-glass behavior reported in similar compounds, Ag-In-R (R = Eu, Gd, Tb or Dy). Au-SM-Gd (SM = Si, Ge or Sn) exhibit a simple ferromagnetic transition at 22.5, 13 and 9 K, respectively, whereas Au-Si-(Tb, Dy or Ho) show indications of a canted ferromagnetic transition at 8.3, 5.9 and 3.8 K, respectively. The latter are attributed to a crystal electric field effect that is absent in the Gd-bearing compounds. The ferromagnetic behavior in Au-SM-R may be understood to be a consequence of the short R-R distances compared to those for Cd-R and Ag-In-R.

Antiferromagnetic order and the structural order-disorder transition in the Cd6Ho quasicrystal approximant

It has generally been accepted that the orientational ordering of the Cd4 tetrahedron within the Cd6 R quasicrystal approximants is kinetically inhibited for R = Ho, Er, Tm and Lu by steric constraints. Our high-resolution X-ray scattering measurements of the Cd6Ho quasicrystal approximant, however, reveal an abrupt (first-order) transition to a monoclinic structure below T S = 178 K for samples that have ‘aged’ at room temperature for approximately one year, reopening this question. Using X-ray resonant magnetic scattering at the Ho L 3-edge we have elucidated the nature of the antiferromagnetic ordering below T N = 8.5 K in Cd6Ho. The magnetic Bragg peaks are found at the charge forbidden H + K + L = 2n + 1 positions, referenced to the high-temperature body-centred cubic structure. In general terms, this corresponds to antiferromagnetic arrangements of the Ho moments on adjacent clusters in the unit cell as previously found for Cd6Tb.

Magnetic structure and excitations in the Au–Si–Tb quasicrystalline approximant

The icosahedral quasicrystal is a solid with a quasiperiodic arrangement of Matryoshka-like icosahedral atomic clusters, giving rise to sharp Bragg reflections with forbidden rotational symmetry, such as the five-fold symmetry. Approximant is a conventional periodic crystalline phase made of the same atomic clusters found in the icosahedral quasicrystals. Study on the magnetic behavior of the approximant phases is of fundamental interest not only to understand related quasicrystalline magnetism, but also for its own sake, i.e., to reveal collective behavior of magnetic moments in high symmetry magnetic clusters. The recently found Au-Si-Tb approximant is of particular interest, since it shows long-range magnetic order at low temperatures below Tc = 8.3 K [1]. From the extensive macroscopic measurements, it is suggested that the long-range order is either ferroor ferrimagnetic. The crystal structure of Au-Si-Tb approximant has been solved using conventional x-ray diffraction [2], and the local atomic structure around the Tb magnetic ion is well characterized, having unique pseudo five-fold axis in addition to the mirror plane. In this work, we have performed both neutron diffraction and inelastic scattering experiments on the Au-Si-Tb approximant to reveal its magnetic structure and crystalline electric field (CEF) effect. The neutron diffraction was performed using HB-3A diffractometer at ORNL, whereas the inelastic scattering was carried out using HRC spectrometer at J-PARC. From neutron diffraction data, we confirmed the ferrimagnetic structure below Tc. The neutron inelastic scattering measurement shows a single very broad inelastic peak centered at 4 meV (Fig. 1) in a wide temperature range, indicating that the dominant CEF term is the B20 term. Nonetheless, the finite five-fold term is necessary to reproduce the significantly broadened feature of the inelastic peak, as indicated by the calculated solid lines in the figure. The five-fold term (B65) in the CEF Hamiltonian is prominent unique feature in this Au-Si-Tb approximant, which has been suggested in theory decades ago [3], and has been long sought after for observation in real magnetic materials.

Phenomenological Magnetic Model in Tsai-Type Approximants

Motivated by the recent discovery of canted ferromagnetism in the Tsai-type approximants Au–Si–REs (RE = Tb, Dy, and Ho), we propose a phenomenological magnetic model reproducing their magnetic structure and thermodynamic quantities. In the model, the cubic symmetry (\(m\bar{3}\)) of the approximately regular icosahedra plays a key role in the peculiar magnetic structure determined by a neutron diffraction experiment. Our magnetic model does not only explain magnetic behaviors in the quasicrystal approximants, but also provides a good starting point for the possibility of the coexistence between the magnetic long-range order and the aperiodicity in quasicrystals.

Ferromagnetic transition in Au- based Approximants

The structure of Tsai-type magnetic quasicrystals and its related compounds (called approximants) are characterized by the spacefilling 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 of Pd-based Tsai-type 1/1 approximants

Recently, a new Tsai-type 1/1 approximant (AP) was discovered in the Pd-Ge-Yb system by Kashimoto et al[1]. In this alloy, the Yb valence was found to be in an intermediate state between Yb2+ and Yb3+. On the other hand, various magnetic transitions were found in other rare-earth (RE) bearing Tsai-type APs. In this work, the formation condition and magnetic property of Pd-M-RE appximants were investigated. Mother ingots of various conditions based on Pd-M-Yb[1], were prepared by arc melting followed by various heattreatment conditions to obtain homogeneous equilibrium phases. The Phase constitutions were studied by powder X-ray diffraction (XRD). The obtained XRD patterns can be indexed as 1/1 AP. The details of the formation conditions as well as magnetic properties will be presented in the poster.