Disorder-to-order transition in the magnetic and electronic properties of URh_2Ge_2

We present a study of annealing effects on the physical properties of tetragonal single--crystalline URh_2Ge_2. This system, which in as-grown form was recently established as the first metallic 3D random-bond heavy-fermion spin glass, is transformed by an annealing treatment into a long-range antiferromagnetically (AFM) ordered heavy-fermion compound. The transport properties, which in the as-grown material were dominated by the structural disorder, exhibit in the annealed material signs of typical metallic behavior along the crystallographic a axis. From our study URh_2Ge_2 emerges as exemplary material highlighting the role and relevance of structural disorder for the properties of strongly correlated electron systems. We discuss the link between the magnetic and electronic behavior and how they are affected by the structural disorder.Comment: Phys. Rev. B, in print (scheduled 1 Mar 2000

AMORPHOUS COBALT-PHOSPHORUS ALLOYS : ATOMIC ARRANGEMENTS AND MAGNETIC PROPERTIES

Des mesures de diffraction des rayons X, de densité physique, et de magnétisation ont été effectuées sur une série d'alliages Co-P amorphes électrodéposés de teneur en phosphore comprise entre 19 et 23,6 %. Bien que la fonction de distribution radiale des alliages à 22 % en P présente les caractéristiques du modèle de Bernal (empilement dense au hasard de sphères incompressibles de mêmes dimensions), une étude plus détaillée indique qu'un empilement au hasard binaire avec des sphères plus petites représentant les atomes P, serait un modèle structural plus approprié. Pour cet alliage, la distance entre plus proches voisins (dominée par les paires Co-Co) est 2,58 ± 0,5 Å et le premier nombre de coordination moyen est 13,0 ± 0,5. Des mesures entre 5 et 700 K à faible et intense champ de magnétisation indiquent que les alliages sont ferromagnétiques, avec une température de magnétisation qui suit, à basse température, une loi en T3/2, ceci presque jusqu'à Tc/4. Ces mesures montrent aussi que les points de Curie Tc sont entre 550 et 750 K et que le moment magnétique par atome de Co à 0 K varie de 1,15 à 1,36 magnétons de Bohr. Tc et nB diminuent tous deux en fonction de l'augmentation de la teneur en phosphore.X-ray diffraction, physical density, and magnetization measurements have been performed on a series of electrodeposited amorphous Co-P alloys between 19.0 and 23.6 at. % P. Although the radial distribution function of the 22.0 at. % P alloy exhibits characteristic features of a Bernal dense random packing of equal size hard spheres, more detailed comparisons indicate that binary random packing, with smaller spheres representing P atoms, would be a more appropriate structural model. For this alloy the nearest neighbour distance (dominated by Co-Co pairs) is 2.58 ± 0.5 Å and the average first coordination number is 13.0 ± 0.5. Low and high field magnetization measurements from 5 to 700 K indicate that the alloys are ferromagnetic with low temperature demagnetization following a T3/2 temperature dependence almost to Tc/4, that the Curie temperatures Tc are between 550 and 750 K, and that nB, the magnetic moment per Co atom at 0 K, varies from 1.15 to 1.36 Bohr magnetons. Both Tc and nB decrease with increasing P content

Total-electron-yield current measurements for near-surface extended x-ray-absorption fine structure

A total-electron-yield technique is described in which near-surface extended x-ray-absorption fine-structure (EXAFS) data are obtained from direct measurements of specimen current. Experiments with several model systems—amorphous germanium and crystalline germanium, nickel, and cobalt; and arsenic ion implanted into silicon—demonstrate that this technique can reproduce EXAFS χ(k) functions obtained from transmission and fluorescence measurements. Experiments also reveal that EXAFS amplitudes from total-electron-yield data can be 5–10 % smaller than those from transmission measurements for samples where the very-near-surface structure, at depths of tens to hundreds of angstroms, differs from the bulk structure. Measurements with buried layers confirm that the sampling depth for this total-electron-yield technique is determined primarily by the penetration ranges of Auger electrons emitted from the absorbing atoms. For the model systems listed above, LMM Auger electrons have ranges of hundreds of angstroms and KLL Auger electrons have ranges of thousands of angstroms. Expressions are derived for the sampling depth for total-electron-yield EXAFS experiments. The total-electron-yield technique described here is particularly useful for studying impurities within a few thousand angstroms of the surface of single crystals, where Bragg diffraction complicates the use of fluorescence measurements