The issue, composition dependence of glass-forming ability (GFA) in metallic glasses (MG), has been investigated by systematic experimental measurements coupled with theoretical calculations in Cu-Zr and Ni-Nb alloy systems. It is found that the atomic-level packing efficiency strongly relates to their GFA. The best GFA is located at the largest difference in the packing efficiency of the solute-centered clusters between the glassy and crystal alloys in both MG systems. This work provides an understanding of GFA from atomic level and will shed light on the development of new MGs with larger critical sizes.

Basic polyhedral clusters have been derived from intermetallic compounds at near-eutectic composition by considering a dense packing and random arrangement of atoms at shell sites. Using such building units, bulk metallic glasses can be formed. Since the discovery of glassy systems, based on multicomponent alloys, in the early 1990s, bulk metallic glasses ͑BMGs͒ have been extensively studied because certain mechanical properties, such as strength, can be significantly improved over their crystalline counterparts. 1-11 BMGs produced so far usually contain three or more elements. 1-3 Until now, complex compositions have been considered necessary in order to inhibit crystallization of the liquid phase during cooling of the melt. However, simpler systems should be of great interest, fundamentally as well as technologically, since they would facilitate the atomic-structure determination for a given BMG, which has been a long-standing problem. [12][13][14][15][16][17][18][19][20][21][22][23][24] Originally, the dense-random-packed model was used to describe metallic glasses. 25 This model is based on the assumption that the glass consists of a random arrangement of spherical atoms ͑hard spheres͒ of each element. However, it has also been pointed out that localized, directional chemical bonding and the formation of groups of atoms are relevant factors to theories of glass formation and stability. 26 The local structure is well defined and similar to that of the crystalline form of the material. Recently, a topological model for metallic glass formation was proposed. 27,28 According to that model, a solute occupying either substitutional or interstitial sites in the host lattice can destabilize the lattice by producing a critical internal strain. Besides the structural models mentioned above, several criteria for structural stability of a BMG have been suggested: ͑1͒ large value of the reduced glass transition temperature T g / T l , where T g is the glass transition temperature and T l is the liquidus temperaure; 29 ͑2͒ three empirical rules for a large supercooled liquid region: ͑a͒ multicomponent system, ͑b͒ significant differences in atomic size, and ͑c͒ negative heats of mixing among the main constituent atoms; 1,3 ͑3͒ high gamma value, ␥ = T x / ͑T g + T l ͒, where T x is crystallization temperature; 30 ͑4͒ interaction between the Fermi surface and the Brillouin zone in a nearly-free-electron model; 31 ͑5͒ the critical concentration of a solute element required for amorphization decreases, reaches a minimum, and then increases with increasing ratio between the size of the solute and the solvent atoms; 27,32,33 ͑6͒ there is an optimum ratio R * between the size of the solute atom and the average size of the surrounding solvent atoms for dense packing. 28,34 In spite of the criteria mentioned above, the design of alloys with a high glass forming ability ͑GFA͒ remains to a large extent unpredictable due to lack of understanding of the local atomic structure. Although polyhedral clusters have been revealed in some met...

The atomic structure and the magnetic properties of 5 and 10 at. % Co-doped ZnO samples prepared by a high-pressure and high-temperature method have been investigated by various techniques, including x-ray diffraction with Rietveld refinements, scanning electron microscopy, high-resolution transmission electron microscopy, nanometer-sized element mapping, x-ray photoelectron spectroscopy, near-edge x-ray absorption fine structure, and extended x-ray absorption fine structure and magnetization measurements. It is found that Co ions with a valence of 2+ substitute Zn ions in the wurtzite ZnO structure. No metallic or oxidic Co-rich clusters were detected in the samples. If Co 2+ ions substitute Zn 2+ ions, no intrinsic ferromagnetism is observed in ZnO at doping levels up to 10 at. % Co even down to 5 K. A paramagnetic behavior with partially antiferromagnetic interactions is observed. Thus, in bulk Co-doped ZnO system any observation of ferromagnetism is probably due to small clusters of second phase materials. However, in thin films the situation might be more complex, since other extrinsic influences, such as strain or proximity effects, may modify the electronic and magnetic properties.

The influence of structure characteristics on the ferromagnetism of Ni-doped ZnO (ZnO:Ni) thin films prepared by pulsed laser deposition has been investigated by magnetization and resistivity measurements. Structural and optical absorption analyses revealed that Ni has been incorporated into ZnO lattice as Ni2+ substituting for Zn2+ ions and the microstructure of the films becomes more inhomogeneous with increasing Ni content. The room temperature ferromagnetic (FM) behavior only occurs when Ni content is not high and the saturation magnetic moment per Ni atom decreases with increasing Ni content. No FM signal was detected for the film with 7at.% Ni content. Interestingly, the carrier density decreases with the increase of Ni doping from 1to5at.%, whereas an increase of carrier density is observed for 7at.% doped film, which indicates that carrier density alone cannot account for the FM property in ZnO:Ni films. All the results demonstrated that the microstructure of ZnO:Ni films gets more deteriorated with increasing Ni content and it is unfavorable for the long-range FM order in the ZnO:Ni films.

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