Characterization of Inhomogeneities in Amorphous Superconductors
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CHARACTERIZATION OF INHOMOGENEITIES
IN AMORPHOUS SUPERCONDUCTORS
Esen E.Alp, S.K.Malik(I), Y.Lepetre, P.A.,Montano (2), and G.K.Shenoy Materials Science and Technology Division, Argonne National Laboratory, Argonne, Ill. 60439 ABSTRACT The structure and superconducting properties of rapidly quenched Zr-Ni and Zr-V alloys have been studied by XRD,EXAFS spectroscopy,TEM,electron diffraction,ED, electrical resistivity and ac susceptibility measurements. Amorphous Zr 6 6 Ni 3 4 ribbons quenched with different cooling rates were also examined by flux pinning measurements. Characteristic differences are observed in the flux pinning mechanisms of microcrystalline and amorphous samples. The temperature dependence of upper critical fields have been measured down to 0.4 K and the results are analyzed in terms of WHH theory. INTRODUCTION The superconducting properties of amorphous transtion metal alloys are recieving increased attention [1,2]. Superconductivity is becoming an effective tool to study the effect of atomistic scale disorder on the electronic behaviour of amorphous transition metal alloys [1]. However, it should be mentioned that measurements made at different laboratories on similar alloys do not necessarily agree with each other, and there is a reproducibility problem. [3]. This is mainly because of variations in the cooling rate and the subsequent thermal relaxation following the production of these alloys. Such variations will result in microstructures that are microcrystalline with a grain size of few hundred angstroms to amorphous phase These inhomogeneities may with crystalline inhomogeneities embedded in it. vary in size and might affect the superconducting behaviour of the amorphous matrix. Since the amorphous phase itself may have compositional as well as density variations at scales comparable to the coherence length of the superconducting electrons [41, it is necessary to investigate the effect of these different type of inhomogeneities on the superconducting behavior of amorphous metals. EXPERIMENTAL The amorphous ribbons were produced by melt-spinning technique using alloy buttons prepared in an arc-furnace. Different cooling rates could be obtained by changing the wheel speed onto which the molten metal is ejected. Three different Zr-Ni and two different Zr-V alloys were prepared with wheel speeds between 40-60 m/sec, with nominal compositions of Zr 6 6 Ni 3 4 , and Zr 3 4 V6 6 . These samples failed to give crystalline peaks as examined by X-ray diffraction. The superconducting transition temperatures are determined using ac susceptibility and dc resistivity. Transition widths are found to be higher when detected by ac susceptibility. This became even more pronounced when magnetic field was applied. Moreover, for Zr-V samples more than one transi- tion were observed, indicating the inhomogenous character of the samples. X-ray diffraction measurements were carried out using Cu Ka radiation on a Rigaku scanning diffractometer, equipped with a diffracted beam monochromator. Crystallization experime
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