The superconducting isotope effect of iron in U6Fe

Solid State Communications,

Vol. 13, pp. 1355—1356, 1973.

Pergamon Press.

Printed in Great Britain

THE SUPERCONDUCTING ISOTOPE EFFECT OF IRON IN U6Fe* J.J. Engelhardt Institute for Pure and Applied Physical Sciences. University of California, San Diego. La Jolla, California 92307, U.S.A. (Received 13 July 1973 by H. SuhI)

The iron isotope effect on the superconducting transition temperature of the compound U6Fe was found to be very small or zero.

IN THE seriesof compounds U6X(X = Mn, Fe, Co. Ni), Hill and Matthias’ found that there is a surprisingly close relationship between the superconducting transition temperature ~ of U6X and the saturation magnetization of the the constituent pure X elements. To in help under. stand the role of elements these compounds, measurements of the isotope effect on T~ have been made. The result for the isotope effect on uranium in U 2 to be more or less has been found effect on iron in normal, T~, 6Fe M°~37. The isotope this compound is reported here.

The results are shown in the figure where the bridge imbalance is plotted against the temperature. The reproducibility is indicated by the difference between the transitions of thefrom U6Fedifferent natural sources and 56 samples which come U6 Feprobably have different impurity contents. The and transition midpoints occur randomly with 10 millidegrees of each other, and the spread seems to

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The samples were made by melting the elements together in an argon atmosphere arc furnace 9 times. turning the ingot over each time. Weight losses were less than 0.02 per cent. The actual molar ratio U/Fe was 6.1 here instead of 6 since the T~,of U 6Fe is concentration independent2on theingots uranium side The wererich wrapped of the stoichiometric ratio. in a quartz tube for 26 in tantalum foil and annealed hat 770°C.The Fe isotopes were obtained from Oak Ridge in the form Fe 2 03 and were reduced to the metal in hydrogen at 1000°Cfor 2 h. The Fe natural was ultra-pure grade from Batelle Memorial Institute, and the uranium was depleted and 99.9 per cent pure. The samples were measured in a self-inductance bridge operating at 95 Hz immersed in liquid helium. *

~.

F.54

968

54.07

Fe”

--

5585

~ F,

,..,

~ch-

9993 9073

7

56 00

~

5691

lJ,F,

F~

/

‘

380

390

400

TEMPERATURE ‘I~ .

FIG. 1. Su~erconductingtransitions 56,and of the U com7pounds 5 U6Fe ,U6Fe natural, U6Fe 6Fe approach zero toward the onset of the transitions. If

Research sponsored by the Air Force Office of

all the transitons had the same width, then the spread F-44620-72.C-0017.

m 1 s would appear to be a few millidegrees at most. 1355

1356

SUPERCONDUCTING ISOTOPE EFFECT OF IRON IN U6Fe

in terms of the iron mass alone, the fractional spread in T~is vanishingly small compared to the fractional change in mass, and thus the isotope effect is zero. Alternatively, if we assume that the average mass density is important for superconductivity, then the U and Fe isotope effects would have the same ratio as31 the molar ratio Theofuranium T~‘-M°~ corresponds to aU/Fe. dT~/dM about 6result millidegrees per atomic mass unit. An Fe isotope effect assumed to be

1/6 of this would produce a 7, spread of 3 millidegrees for the present experiment. The two results are therefore compatible. This indicates that the iron mass either has no effect on the superconductivity of U6Fe or at most merely contributes to the average mass of a dominant uranium lattice. Acknowledgements I would like to thank Hunter Hill for his help. —

REFERENCES 1. 2.

HILL H.H. and MATTHIASB.T.,Phys. Rev. 168,464(1968). WHITE R.W., LINDSAY J.D.G. and FOWLER R.D.,Solid State Commun. 13, 531 (1973).

Der Isotopieeffekt des Elsens in der Supraleiten Verbindung U 6Fe wurde als sehr klein oder Null gernessen.

Vol. 13, No.9