When annealed below T g , the melt-spun material exhibits a steadily decreasing nucleation rate, 4 which suggests heterogeneous nucleation. 5 The nucleation site has so far eluded structural or chemical detection, ruling out common sites like second phase interfaces or large impurity clusters. This suggests that the nucleation sites in quenched Al 92 Sm 8 may be a form of nanometer-length structure or medium-range order ͑MRO͒. Such structure is difficult to detect in amorphous materials using conventional techniques such as x-ray diffraction. 6 Here, we report fluctuation electron microscopy ͑FEM͒ ͑Refs. 7 and 8͒ measurements and simulations which find MRO associated with primary crystallization in amorphous Al 92 Sm 8 .FEM measures diffraction from nanoscale volumes using dark-field transmission electron microscopy ͑TEM͒ at a deliberately low ͑5-50 Å͒ image resolution. The magnitude of the spatial fluctuations in diffraction, measured by the normalized variance V as a function of scattering vector k, gives information about MRO at the length scale of the image resolution. 7,9 V͑k͒ depends on the three-and four-body atomic position correlation functions. 9 Peaks in V͑k͒ give information about the type of MRO from their position in k and the degree of MRO from their height. A polycrystalline sample is an extreme example of order: in a dark-field image each grain will appear brightest when it satisfies a Bragg condition in k, leading to a high peaks in V͑k͒ at the crystal reciprocal lattice k's.Samples of amorphous Al 92 Sm 8 were prepared by rapid quenching in a single wheel melt spinner at a tangential wheel speed of 55 m / s and by cold-rolling elemental foil multilayers at a 0.003 s −1 strain rate. Melt-spun ribbon samples were annealed at 130°C ͑ϽT g of 171°C͒ 3,10 under vacuum. TEM samples were prepared by electropolishing only, as ion milling can introduce spurious peaks in V͑k͒ of amorphous metals. 11 FEM was done in hollow-cone darkfield mode on a LEO 912 EFTEM at 120 kV and 16 Å resolution. Each V͑k͒ data set is the mean of measurements from at least seven areas of the sample, quoted with one standard deviation of the mean error bars.

No abstract

Cobalt ferrite ͑CoFe 2 O 4 ͒ possesses excellent chemical stability, good mechanical hardness, and a large positive first order crystalline anisotropy constant, making it a promising candidate for magneto-optical recording media. In addition to precise control of the composition and structure of CoFe 2 O 4 , its practical application will require the capability to control particle size at the nanoscale. The results of a synthesis approach in which size control is achieved by modifying the oversaturation conditions during ferrite formation in water through a modified coprecipitation approach are reported. X-ray diffraction, transmission electron microscopy ͑TEM͒ diffraction, and TEM energy-dispersive x-ray spectroscopy analyses confirmed the formation of the nanoscale cobalt ferrite. M-H measurements verified the strong influence of synthesis conditions on crystal size and hence, on the magnetic properties of ferrite nanocrystals. The room-temperature coercivity values increased from 460 up to 4626 Oe under optimum synthesis conditions determined from a 2 3 factorial design.

No abstract

Fluctuation electron microscopy (FEM) experiments to measure nanoscale structural order in amorphous materials come in two types: variable coherence and variable resolution. Either type can be implemented experimentally using either dark-field transmission electron microscope (TEM) imaging or nanodiffraction in a scanning TEM (STEM). We propose that the discrepancy in the magnitude between FEM signals measured with TEM and STEM is caused by a difference in coherence in the two methods. We also compare the nanoscale order length scales extracted from variable-resolution FEM data using the correlation length method proposed by Gibson et al and a method we recently proposed based on an explicit cluster model for nanoscale structural order in amorphous materials.