Electrical resistivity of quench-condensed cerium films

Solid State Communications,

Vol. 13, pp. 615-619, 1973.

Pergamon Press.

Printed in Great Britain

ELECTRICAL RESISTIVITY OF QUENCH-CONDENSED CERIUM FILMS E. L6ffler and J.A. Mydosh Institut flir Festk6rperforschung, Kernforschungsanlage Jtilich, 517 Jtilich, Wo Germany

(Received 30May 1973 by G. Leibfried)

Quench-condensed (500-1500 A) films of cerium were prepared at liquid helium temperature, and the electrical resistivity was measured from 1.5 to 300 K. A strongly disordered form of a-Ce was stabilized. An initial 7 e term was found in the resistivity. The overall behavior of the resistivity suggests the existence of nearly magnetic regions, due to the large inherent lattice disorder, and the applicability of localized spin fluctuation theory.

The characteristics of the Ce films depended very sensitively on the sample preparation. Reproducible film behavior was obtained by using the following method. The bulk starting material, 99.9% pure Ce from Rare Earth Products Ltd., was first carefully scraped with a Mo spatula and melted in high vacuum within a tungsten evaporation coil, this was then etched in HF acid, and finally, any remaining fluorides or other impurities were outgassed. The Ce material, ready for evaporation, was bright and shiny. A special in situ cryostat, similar to the one described by Bergmann, 1° was used for the quenched condensation with a vacuum better than 3 X 10-7 Torr and with the substrate always less than 10 K during deposition. A standard four-terminal technique was employed to measure the voltage across the film via a 5 1/2 position digital voltmeter and regulated constant current supply. A resistance stability of about one part in lO s was obtained with this arrangement. The temperature was measured and controlled using a calibrated Ge thermometer at low temperatures and a Pt resistor for temperatures above 100 K. The film geometry was well-defined by masking the deposition, and the film thickness was determined by Tolansky interferometry after the film had been removed from the cryostat.

THERE has recently been great activity in measurements of the electrical1 - 5 and magnetic 2,a,6 properties of the low temperature intermediate, collapsed f.c.c., a phase of cerium. This phase 7 exists between the magnetic (2.5pB/Ce atom) 7 and/3 phases 03-Ce orders antiferromagnetically at TN = 12.5 K), and the superconducting a' phase, a The exact determination of the properties of pure a-Ce is handicapped by the persistence and trapping in the a-Ce of the magnetic/3 and 7 phases. A minimum hydrostatic pressure of about 4 kbar 4,a and a slow cooling process4,5 are needed to obtain the pure, non-strained, form of a-Ce. The present picture of a-Ce is that of an exchange enhanced Pauli paramagnet similar to Pd, but with interesting magnetic enhancement effects caused by slight pressure inhomogeneities 1 and lattice strains or defects5 which are related to a 'more-magnetic' (larger enhancement factor) form of Ce dispersed throughout the non-magnetic a-Ce matrix. We have approached this problem of a-Ce in a different way by using a vacuum deposition technique at liquid helium temperatures to quench-condense Ce films, and we have measured the electrical resistivity, p, for a series of such ~_ 1000 A films. In this letter we wish to present our preliminary results for the reproducible behavior of five Ce films, and to attempt to interpret these results consistently with the other experiments and to offer an overall picture for a-Ce.

Experiencen with many types of elemental metallic films has shown that for an f.c.c, or 'closed packed' structure, a crystalline film is formed which would duplicate the structure existing in bulk 615

616

ELECTRICAL RESISTIVITY OF QUENCH-CONDENSED CERIUM FILMS

Vol. 13, No. 5

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FIG. 1. Electrical resistivity versus temperature for bulk pressure-prepared ot-Ce, mostly/~-Ce, and a quench-condensed Ce fdm.

material at the given temperature and pressure. Therefore, based upon this experience, it is expected that metallic a-Ce will grow during the quenchcondensation as the stable form. In Fig. 1 we show the overall temperature dependence of the resistivity for one such quenchcondensed film compared with the results of Nicolas-Francillon and Jerome 4 for pressure-prepared a-Ce, and a mostly/~-Ce bulk sample. TM There is an obvious similarity between the film resistivity and that ofa-Ce, with the quench-condensed film having a much higher residual resistivity due to the inherent lattice disorder of films prepared in such a manner. These closely similar resistivity behaviors allow us to argue that the f'tim is also a-Ce. It should be noted that, in spite of our measurement intervals of 0.2 K, there are no traces in the f'tim resistivity of the antiferromagnetic 'kink' at 12.5 K, observed for slight amounts offl-Ce in a-Ce,z,3 and further that a welldefined, irreversible, a ~ 7 phase transition exists at the temperature expected for bulk a-Ce.

So based upon these arguments, we believe we have obtained a-Ce films with high crystalline disorder. However, as we will show in what follows from a close examination of the resistivity behavior, neither of these magnetic phases can be present as such. For samples with greater than 2 per cent/3-Ce in a-Ce an effect in the resistivity at 12.5 K would be noticeable,2 so that large clusters of 13-Ceare unlikely. Also, since ~ and 7-Ce have well-localized magnetic moments, trace impurities of these phases in the exchange enhanced a-Ce host, would be expected to result in a different resistivity behavior, than we measured, characteristic of giant moments or the Kondo effect. Figure 2 shows a series of resistivity-temperature cycles for one typical film. The small vertical lines attached to the resistivity curves indicate the maximum annealing temperatures to which the film was cycled, and the arrows along the resistivity curves represent completely reversible behavior up to this temperature. The initial temperature dependence of

Vol. 13, No. 5

ELECTRICAL RESISTIVITY OF QUENCH-CONDENSED CERIUM FILMS 100

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FIG. 2. The temperature dependence of the resistivity for a typical quench-condensed Ce film with various annealing cycles. The vertical lines attached to the resistivity curves represent the maximum annealing temperature. Reversible behavior is indicated by the opposing arrows. The 300 K-resistivity value was measured after a one week anneal. Insert: The T2-coefficient A and the linear coefficient B as a function of the maximum annealing temperature Ta. the resistivity for the various cycles exhibits a strong low temperature T 2 law. A log (p - ~ ) vs log T plot of this initial temperature dependence is given in Fig. 3 for three different cycles of p(T) from Fig. 2. Here the T 2 behavior is dearly seen, and this behavior is observed for all our fdms. The small deviations (the upward bending in Fig. 3) from this T ~ behavior, at temperatures about 10 K, are probably due to the phonon TS-term which is beginning to be significant. Returning to the insert of Fig. 2, the T 2coefficient A is plotted as a function of the annealing temperature. This coefficient is constant for different temperature cyclings below 80 K, then it begins to increase. If the film is annealed at temperatures above 220°K the low temperature resistivity also obeys a T2-1aw,but the temperature coefficient A is 5 - 1 0 times larger than the 175 K value; furthermore, a knee at about 13 K can be seen which clearly indicates the existence of the/3-phase. Although the residual resistivity Po continues to decrease as expected with annealing temperature (see Fig. 2

where the onset of each p(T) cycle begins at successively lower resistivity values), A is either constant or increasing. This leads us to conclude that the T 2 term in the resistivity is related to magnetic effects and has nothing to do with non-magnetic latticedisorder scattering. At about 35 K the resistivity behavior changes over into a linear dependence. The linear temperature coefficient, B, is also plotted as function of annealing temperature in the insert of Fig. 2. Surprisingly B increases with annealing temperature, in contrast to the behavior 9f non-magnetic quenchcondensed Films.la This would indicate that the increase of B is also a magnetic effect and is related to the increase erA. We feel that our results permit a consistent interpretation with respect to the previous resistivity experiments on t~-Ce.1 - 5 Basically o~-Ceis an exchange enhanced metal similar to Pd. The enhancement factor varies little with pressure, 7 again analogous to Pd. 14 There should exist a small T 2 term in

ELECTRICAL RESISTIVITY OF QUENCH-CONDENSED CERIUM FILMS

618

Vol. 13, No. 5

Table 1. lnitial temperature T2-coefficient, A, from recent experiments on Ce and Pd T 2-coefficient T2"Law A (10-a/a~2 cmK-2) Pressure Prepared Bulk a-Ce Katzman and Mydosh I

Remarks Measured at pressures between 3 and 9 kbar.

yes

0.1to4.5

yes

0.6

Cooled hydrostatically at 3 kbar, measured at zero pressure.

?

41

Slow cooled at a hydrostatis pressure of 8.5 kbar, measured between 0 and 8.5 kbar.

yes ?

2 <0.1

Quenched and measured at 10 kbar. Slow cooled at 18 kbar and measured at 0.10 and 18 kbar.

Quench-condensed films Present work

yes

t.3to3.0

Pure Pd Schindler and Rice z5

yes

0.03

Pd + 0.5 to 1.7 at. % Ni Schindler and Rice 15

yes

0.1 to 0.5

Grimberg, Schinkel and Zandee 2,8 Nicolas-Francillion and Jerome 4

Brodsky and Friddle 5

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Five films, thickness 450-1350 A deposited at substrate temperatures between 7 and 9 K during evaporation.

the resistivity of a-Ce at low temperatures. However, this term is difficult to detect (as for Pd zs) and is usually considered to be caused by uniform spin fluctuations. 16 In order to increase the spin fluctuations, one alloys Pd with small amounts of Ni TM and there is good agreement between the behavior of the excess resistivity and the local spin fluctuation theory of Kaiser and Doniach. 16 One cas also vary the magnetic resistivity contribution in a-Ce, as was done by Katzman and Mydosh 1 in their variable pressure experiments with slight pressure inhomogeneities. That is, a 'more-magnetic' (larger localized enhancement factor) form of Ce is created at lattice defects, grain boundaries, or anywhere in the sample where a slight displacement of Ce atoms is possible. Now, with the quench-condensed films of Ce, similar effects are found - a large T 2 term, comparing well with Katzman and Mydosh's magnitude. Table 1 contains a summary of the T 2 coefficients. It should be noted that the magnitude of A is directly related to the strength of the spin fluctuations. At higher temperature, there is a linear dependence of p. Thus, the applicability of the localized spin fluctuation theory is also seems possible

Vol. 13, No. 5

~LELIRICAL RESISTIVITY OF QUENCH-CONDENSED CERIUM FILMS

here. If the magnetic contribution to the resistivity is connected with lattice defects, pressure inhomogeneities etc., one must expect a rather large Acoefficient for the quench-condensed f'dms. This we have consistently found (see Table 1) in our Ce fdm measurements.

619

urgical nature would be suggested, e.g. electron diffraction and electron microscopy.

Acknowledgements - We wish to acknowledge a series of helpful discussions with our colleagues at J~ilich: W. Buckel, G. Bergmann, J. Wittig and H. Wfihl.

Additional experiments on t~-Ce along a more metall-

REFERENCES 1.

KATZMAN H. and MYDOSH J.A., Phys. Rev. Lett. 20,998 (1972).

2.

GRIMBERG A.J.T., SCHINKEL C.J. and ZANDEE A.P.L.M., Solid State Commun. 11, 1579 (1972).

3.

GRIMBERG A.J.T., Thesis, University of Amsterdam (1973).

4.

NICOLAS-FRANCILLON M. and JEROME D., SolidState Commun. 12, 523 (1973).

5.

BRODSKY M.B. and FRIDDLE R.J., Phys. Rev. B7, 3255 (1973).

6.

MACPHERSON M.R., EVERETT G.E., WOHLLEBEN D. and MAPLE M.B., Phys. Rev. Lett. 26, 20 (1971).

7.

For a recent review of the experimental and theoretical situation for cerium, see COQBLIN B., J. Phys. 32, C1-599 (1971).

8.

WlTTIG J.,Phys. Rev. Lett. 21, 1250 (1968).

9.

GSCHEIDNER K.A., ELLIOTT R.O. and MCDONALD R.R., Jr. Phys. Chem. Solids 23,555 (1962).

10.

BERGMANN G., Phys. Rev. B (to be published).

11.

NOWlCK A.S., Comments on Solid State Physics 2, 155 (1970).

12.

ITSKEVICHE.S.,Zh. Eksp. Theor. Fiz. 42,1173(1962),SovietPhys. JETP15,811(1962). Seealso MEADEN G.T., ElectricaIResistance o f Metals, p. 40, Heywood Books, London (1966).

13.

MONCH W., Z. Phys. 170, 93 (1962).

14.

BEYERLEIN R.A. and LAZARUS D., Phys. Rev. B7, 511 (1973).

15.

SCHINDLER A.I. and RICE M.J.,Phys. Rev. 164,759 (1967).

16.

KAISER A.B. and DONIACH S., lnt. J. Magn. 1, 11 (1970).

Abschreckend kondensierte Cer-Fflme (500-1500 A) wurden bei HeTemAperatur hergestellt, und der elektrische Widerstand wurde yon 1.5 bis 300 ~ K gemessen. Eine stark gest6rte Form yon a-Cer entstand. Bei tiefen Temperaturen folgte der Widerstand einem T2-Gesetz. Das Gesamtverhalten des Widerstandes 1/it~tdie Existenz fast magnetischer Bereiche vermuten, die mit der starken Gitterst6rung in Zusammenhang stehen, und die Theorie lokalisierter Spinfluktuationen scheint anwendbar zu sein.