T. A. Wiener

Anisotropy and metamagnetism in the RNi2Ge2 (R=Y, La–Nd, Sm–Lu) series

Abstract Single crystals of a complete series of the RNi 2 Ge 2 (R=Y, La–Nd, Sm–Lu) ternary intermetallics were grown and characterized by measurements of the anisotropic magnetization and in-plane resistivity in zero and applied magnetic field. The majority of moment-bearing members of the series order antiferromagnetically and have a series of transitions in zero applied magnetic field and there is a rough de Gennes scaling of Θ D and T N for the heavy rare-earth members of the series. Signatures of the metamagnetic transitions in the magnetically ordered state and magnetic transitions in zero field are observed for a number of the members of the series. TbNi 2 Ge 2 was studied in more details. Angular-dependent magnetization measurements support the hypothesis that the spins in the ordered states are confined to the c -axis. Complex H–T (H‖c) phase diagram for this material was derived from the magnetization measurements at different temperatures ( M ( H )) and applied fields ( M ( T )). It is suggested that TbNi 2 Ge 2 is a promising candidate for the `devils staircase’ material. In addition, YbNi 2 Ge 2 is suggested to have strong 4f-conduction electron hybridization and is characterized as a 4f-hole analog of the previously studied CeNi 2 Ge 2 moderate heavy fermion compound.

On the growth of decagonal Al-Ni-Co quasicrystals from the ternary melt

Abstract High-quality, single-Frain samples of decagonal Al-Ni-Co quasicrystals with volumes of up to 0.8 cm3 and an approximate composition of Al72Ni11 Co17 have been grown from the ternary melt. The precise growth methods and conditions are discussed in detail. X-ray diffraction patterns of crushed single grains and of single-grain samples are exceptionally sharp indicating a high degree of structural order with no evidence of second phases. Transmission electron micrographs also reveal sharp diffraction patterns in the even n layers but diffuse scattering in the odd n layers, indicative of a structure intermediate between the Al73Co27 and Al69Ni31 end members. The electrical resistivity measured directly from cut bars for currents flowing perpendicular to the crystalline direction (i.e. perpendicular to the c axis) is non-metallic, with a broad maximum around 180 K, and a value of approximately 310 μω cm at 300 K. The resistivity for currents flowing parallel to the c axis is metallic and is almost an ...

On the growth of icosahedral Al-Pd-Mn quasicrystals from the ternary melt

Abstract Icosahedral Al71Pd21,Mn8 quasicrystals have been grown from the ternary melt via a ‘self-flux’ technique. The nucleation is exceptionally well controlled, and individual grains can have masses of up to almost 30 g (limited only by the dimensions of the growth crucible). The quasicrystals grown by this technique are readily separated from the melt by decanting and clearly manifest a dodecahedral morphology with pentagonal facets. X-ray and electron-diffraction data indicate a remarkably well-ordered structure. with a correlation length of over 1000 A. The magnetic susceptibility has a temperature-independent diamagnetic component and a temperature-dependent paramagnetic term. For temperatures greater than approximately 60 K, the paramagnetic term follows a Curie-Weiss temperature dependence characterized by an average effective moment of 0.62 ± 0.02μB/Mn and a Weiss temperature of −12.8 ± 0.1 K. Deviations from the Curie-Weiss temperature dependence for temperatures less than approximately 60 K ma...

Optical and magneto-optical investigation on electronic structure of ordered ferromagnetic Fe3Pt

The optical and magneto-optical properties of ordered Fe3Pt have been investigated by spectroscopic ellipsometry and magneto-optical Kerr spectroscopy. The diagonal component of the optical conductivity tensor of the compound exhibits a broad absorption peak at about 2 eV, which is shifted by about 0.5 eV to lower energies from the corresponding one in pure bcc Fe. The Kerr angle spectrum of the compound disperses quite similarly in both spectral trend and magnitude to that of pure Fe below 3.5 eV but differently above it. The lower-energy shift of the 2-eV-absorption structure of the compound is interpreted as due to the shift of the minority-spin Fe-d states toward EF through the hybridization with Pt-d states. The Kerr effect of the compound is attributable to a large spin-orbit coupling in Pt as well as the well-hybridized spin-polarized bands.

Formation and morphological development of porosity in icosahedral Al–Pd–Mn alloys

Abstract Directional solidification and quenching experiments have been conducted to examine the formation and origin of the faceted porosity commonly observed in icosahedral Al–Pd–Mn alloys. Faceted porosity has been previously observed in both Czochralski grown and Bridgman grown single crystals during single-phase solidification of the icosahedral phase. Detailed observations in Bridgman grown single crystals indicate that the distribution and quantity of pores are non-uniformly distributed along the length of the as-grown ingot. This non-uniform distribution suggests that their formation is influenced by both chemical segregation during solidification as well as convection in the liquid. Controlled solidification of this alloy followed by quenching of the solid–liquid interface provides for a time–temperature history of the porosity formation and morphological development as a function of processing conditions.

The magnetic characteristics of the Tb(Ni1−xCox)2Ge2 system

Abstract The compounds TbNi 2 Ge 2 and TbCo 2 Ge 2 both crystallize with the ThCr 2 Si 2 structure. TbNi 2 Ge 2 has a complex H – T phase diagram while TbCo 2 Ge 2 has a much simpler, though disputed phase diagram. In order to investigate the effects on the magnetism of TbNi 2 Ge 2 due to cobalt substitution, single crystals of Tb(Ni 1− x Co x ) 2 Ge 2 with x ranging from 0 to 1 have been grown from a psuedoternary melt. We have traced the Neel and Weiss temperatures as a function of cobalt concentration, x . Close to x =0.5, the Weiss temperature is positive, indicating predominately ferromagnetic interactions (in contrast to x x >0.6 for which the Weiss temperatures are negative), χ ( T ≈ T N ) is a maximum and the critical field for metamagnetism is a minimum. Nevertheless, the compound orders antiferromagnetically, similar to all other members of the dilution series.