Z. Islam

Systematic study of anisotropic transport and magnetic properties of RAgSb2 (R=Y, La–Nd, Sm, Gd–Tm)

Abstract Anisotropic measurements of the magnetic and transport properties have been performed on high-quality single crystals of the RAgSb 2 series of compounds with R=Y, La–Nd, Sm, Gd–Tm. For most of these compounds, strong magnetic anisotropies, created by crystalline electric field (CEF) splitting of the Hunds rule ground state, confine the moments to the basal plane. Additional anisotropy within the basal plane is also observed in DyAgSb 2 , leading to a series of metamagnetic transitions. All of the compounds order antiferromagnetically at low temperatures except for the nonmagnetic compounds LaAgSb 2 and YAgSb 2 and the ferromagnetic compound CeAgSb 2 . For R=Y, La–Nd, and Sm clear de Haas–van Alphen oscillations in fields as low as 30 kOe and at temperatures as high as 25xa0K can be measured. Throughout the series, the low-temperature transverse magnetoresistance is large [Δ ρ ( H )/ ρ (0)≈60 in SmAgSb 2 at 55xa0kOe] and deviates strongly from quadratic behavior with field the dependence of Δ ρ ( H )/ ρ (0) ranging between H 0.8 and H 1.5 .

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...

Growth of large single-grain quasicrystals from high-temperature metallic solutions

Abstract Flux-growth techniques have been successfully applied to the preparation of several families of quasicrystals and related approximant phases. Large (up to 1xa0cm 3 ) single-grain samples have been obtained for icosahedral R–Mg–Zn (R=Y, Gd, Tb, Dy, Ho and Er), decagonal Al–Ni–Co, icosahedral Al–Pd–Mn, icosahedral Al–Ga–Pd–Mn, and the ξ′ Al–Pd–Mn approximant phase. The samples are obtained by slowly cooling ternary (or quaternary) melts of a composition that intersects the primary solidification surface of the desired phase in the equilibrium ternary alloy phase diagram (i.e. this is a self-flux technique). The technique clearly reveals the growth habit, and results in remarkably strain-free quasicrystals.

Magnetic and transport properties of single-crystal R2Cu2In (R = Gd-Tm, Lu)

Abstract Magnetic and transport properties of the heavy-rare-earth-containing series of compounds R 2 Cu 2 In (R=Gd–Lu, excepting Yb) are presented for the first time. Measurements were made using single-crystal samples grown via a self-flux technique. The magnetic rare-earth compounds all order ferromagnetically. Ordering temperatures vary between 26.7 (R=Ho) and 85.5xa0K (R=Gd), but strong crystal electric field anisotropies result in T C values that do not scale with the de Gennes factor of the rare earths. The compound Dy 2 Cu 2 In shows clear evidence of a second phase transition at a lower temperature than T C =45.5xa0K to an antiferromagnetic ground state at T N =22.0xa0K.

Magnetic properties of icosahedral R–Mg–Zn quasicrystals (R=Y, Tb, Dy, Ho and Er)

Abstract Large (up to 0.5 cm3) single grains of icosahedral R–Mg–Zn quasicrystals (R=Y, Tb, Dy, Ho and Er) have been grown via a self-flux technique. The samples have a composition of approximately R9Mg34Zn57, and are extremely well-ordered, with little or no evidence of phason strain. These samples have allowed a detailed investigation of the magnetic behaviour of local 4f moments in a quasiperiodic environment. Neutron scattering experiments using powdered single-grains show no evidence for long range magnetic order in the R–Mg–Zn quasicrystals. However, magnetization and magnetic susceptibility measurements indicate a spin-glass state, with freezing temperatures Tf=5.8, 3.6, 2.0 and 1.3 K for R=Tb, Dy, Ho and Er respectively. Deviations from a Curie-Weiss temperature dependence of the susceptibility are small, and only occur for temperatures up to 5 K above Tf, indicating the absence of any significant cluster glass behaviour. In addition, the rare earth dilution series (Y1−xGdx)–Mg–Zn have lower freezing temperatures than (Y1−xTbx)–Mg–Zn, despite the larger de Gennes factor of Gd3+ over Tb3+, indicating that crystal electric fields play a significant role in the freezing phenomenon. This difference between Heisenberg and non-Heisenberg moments is further explored via several (Gd1−xRx)–Mg–Zn pseudo-ternary quasicrystal series.

Susceptibility Anisotropy in an Iron Arsenide Superconductor Revealed by X-Ray Diffraction in Pulsed Magnetic Fields

In addition to unconventional high-T(c) superconductivity, the iron arsenides exhibit strong magnetoelastic coupling and a notable electronic anisotropy within the a-b plane. We relate these properties by studying underdoped Ba(Fe(1-x)Co(x))2As2 by x-ray diffraction in pulsed magnetic fields up to 27.5 T. We exploit magnetic detwinning effects to demonstrate anisotropy in the in-plane susceptibility, which develops at the structural phase transition despite the absence of magnetic order. The degree of detwinning increases smoothly with decreasing temperature, and a single-domain condition is realized over a range of field and temperature. At low temperatures we observe an activated behavior, with a large hysteretic remnant effect. Detwinning was not observed within the superconducting phase for accessible magnetic fields.

Robust but disordered collapsed-volume phase in a cerium alloy under the application of pulsed magnetic fields

We report synchrotron x-ray powder diffraction measurements of Ce0.8La0.1Th0.1 subject to pulsed magnetic fields as high as 28 Tesla. This alloy is known to exhibit a continuous volume collapse on cooling at ambient pressure, which is a modification of the gamma -> alpha transition in elemental cerium. Recently, it has been suggested on the basis of field-cooled resistivity and pulsed field magnetization measurements that the volume collapse in this alloy can be suppressed by the application of magnetic fields. Conversely, our direct diffraction measurements show a robust collapsed phase, which persists in magnetic fields as high as 28 Tesla. We also observe nanoscale disorder in the collapsed phase, which increasingly contaminates the high temperature phase on thermal cycling.

THE MAGNETIC STRUCTURES OF DyNi2Ge2

Abstract In this paper we report the results of neutron diffraction measurements of the magnetic structures of a single crystal of DyNi2Ge2 in zero applied magnetic field. There are two distinct magnetic transitions in this material; one is from the paramagnetic phase to an amplitude modulated (AM) antiferromagnetic phase at the Neel temperature, TN = 8.3±0.1 K, and the other is at a lower temperature, Tt = 3.1±0.2 K, from this phase to an equal moment (EM) antiferrromagnetic structure. The EM structure is described by a set of three wave vectors, namely, τ 1 = (0 0 3 4 ), τ 2 = ( 1 2 1 2 0) and τ 3 = ( 1 2 1 2 1 2 ). A weak third harmonic, τ 1 ′ = (0 0 1 4 ), related to τ 1 , also appears indicating that the magnetic structure is not purely sinusoidal. In this phase Dy moments have their full saturation value of μs = 10 μB. The AM phase is described by a single propagation vector τ 1 . In both the phases there is an ordered component of the Dy moments in the basal plane. The angle between the ordered Dy moments and the the c axis is estimated to be 17°±6° at 1.5 K. These results are consistent with the dc susceptibility as a function of temperature data which shows weak anisotropy and two transitions.