Kondo “proximity” effect

PflYSICA

Physica B 194-196 (1994) 983-984 North-Holland

Kondo "Proximity" Effect M. A. Blachly and N. Giordano Department of Physics, Purdue University, West Lafayette, IN 47907, USA We have studied the Kondo effect in bilayer structures in which the bottom film is composed of the Kondo alloy Au(Fe), while the top fihn is the nonmagnetic host Au. We find that the Kondo contribution to the resistivity of the Au(Fe) is enhanced by the presence of the Au.

1. I N T R O D U C T I O N The Kondo effect concerns the behavior of a magnetic impurity in a non-magnetic host. This is a "classic" many-body problem, that has been of interest for several decades [1]. One aspect of the Kondo problem which has not attracted much attention is the behavior in systems of reduced dimensionality. Recent experiments have shown that the Kondo contribution to the resistivity, ApK, becomes smaller when the sample size is reduced [2-4]. In several different Kondo alloys the critical length scale at which this decrease occurs has been found to be in the range 0.1-1 pm. A key question which is not yet resolved is the origin of this length scale. It was originally proposed [2] that the so-called Kondo length Rk = 5VF/27rkBTg, which sets the scale for the spin correlations [5], is the relevant length scale, but it now appears that some (currently unknown) physics distinct from the simplest Kondo effect plays a dominant role [6]. In this paper we report a new study of bilayer structures consisting of a Kondo film in contact with a pure nonmagnetic film. Our goal was to study how the presence of the nonmagnetic film affects the behavior of the adjacent Kondo film. 2. E X P E R I M E N T S The sample geometry is shown in the inset to Fig. 1. The b o t t o m film was Au(Fe) deposited by evaporation, with a concentration of ~ 50 ppm Fe. The top film was pure Au, also deposited by evaporation. Each sample batch consisted a series of samples in which the b o t t o m films, i.e., the Au(Fe), were all deposited at the same time. This guarantees that the Au(Fe) layers in each sample had the same thickness and Fe concentration.

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Figure 1. Results for a series of Au(Fe)/Au bilayers, which had dhu(Fe) = 180 /~. The thickness of the Au layer in each sample is given in the figure.

Then, without breaking vacuum, the Au layers were deposited; a system of shutters allowed us to independently vary the thickness of the Au layer from sample to sample. A test film of just Au was also deposited, to confirm that the Au layers did indeed have a negligible Kondo effect. Figure 1 shows results for ApK(bilayer) as a function of temperature, for samples with different top layer thicknesses, dhu. If the Kondo contribution to the Au(Fe) resistivity was independent of dhu, then Apg(bilayer ) should decrease as dAu is increased. This is because in that case the only effect of the Au layer would be to reduce the average Fe concentration. However, we see fl'om Fig. 1 that such behavior is definitely not observed. Instead, ApK(bilayer ) increases when the Au layer is added. Since the resistivity of the

0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E1072-T

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layers of different thicknesses. It is seen that for fixed dAu, the enhancement becomes larger when the Au(Fe) thickness is reduced. This seems intuitively reasonable since the suppression of the Kondo effect in a single film is greater when dAu(Fe) is reduced [2]. We also note that preliminary results for Cu(Fe)/Cu bilayers are very similar to those reported here for Au(Fe)/Au, implying again that neither the Kondo length RK, nor any of its derivatives [2], is the controlling length scale for this process.

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Figure 2. Kondo enhancement factor as a function of the thickness of the All layer, dAu. The two data sets correspond to two different values of dhu(Fe) as indicated in the figure. The solid curves are guides to the eye.

Au layer is temperature independent, this result means that the Kondo effect in the Au(Fe) fihn is enhanced by the presence of the Au layer; i.e., there is a sort of "proximity" effect. Careful analysis of the results in Fig. 1 shows that the temperature dependence of Apg(bilayer) does not depend on dAu, only the magnitude of Apg(bilayer) is affected. We can therefore characterize the effect of the Au layer on the Kondo effect in the bottom layer by a single factor, which we will refer to as the "enhancement factor." From results like those in Fig. 1, together with the measured film thicknesses, etc., one can extract the resistivity of just the Au(Fe) layer (i.e., the b o t t o m layer) as a function of T and dAu; we will refer to this function as Apb(dAu ). We then define an enhancement factor

Apb(dAu) E -

Apb(dA u = 0) -- 1 ;

(1)

the case E = 0 thus corresponds to no enhancement. As we have already discussed, the data in Fig. 1 imply.a nonzero enhancement, E > 0. Results for E as a function of the thickness of the Au layer are shown in Fig. 2. Here we plot results for two different batches, which had Au(Fe)

In summary, we find that the presence of a nonmagnetic layer can significantly enhance the Kondo effect in an adjacent Kondo layer. So far as we know there is no theoretical explanation of this effect. We thank P. F. Muzikar and A. Zawadowski for helpfld discussions. This work was supported by the NSF through grant DMR-9220455. REFERENCES

1. See, for example, J. Kondo, Prog. Theor. Phys. 32, 37 (1964); K. Fischer in Springer Tracts in Modern Physics , Vol. 54, edited by G. HShler (Springer-Verlag, Berlin, 1970), p. 1; M. Daybell in Magnetism, Vol. 5, edited by G. T. Rado and H. Suhl (Academic Press, New York, 1973); K. G. Wilson, Rev. Mod. Phys. 47, 773 (1975). 2. G. Chen and N. Giordano, Phys. Rev. Lett. 6 6 , 2 0 9 (1991). 3. J.F. DiTusa, K. Lin. M. Park, M. S. Isaacson, and J. M. Parpia, Phys. Rev. Lett. 68, 1156 (1992). 4. M . A . Blachly and N. Giordano, Phys. Rev. B. 46, 2951 (1992). 5. E. M/iller-Hartmann, Z. Phys. 223, 277 (1969); H. Ishii, Prog. Theor. Phys. 55, 1373 (1976). 6. M . A . Blachly and N. Giordano, in the proceedings of the Second International Sympo-

sium on New Phenomena in Mesoscopic Systems, Hawaii, 1992.