The magnetic scattering of Fe atoms in Au is measured as a function of temperature in the range between 0.08 and 4 K by means of weak localization. We find a strong temperature dependence with maximum scattering at the characteristic temperature TK ~ 1 K of this well-known Kondo system. This is a new and direct experimental approach to observation of the theoretically predicted Kondo maximum and is analogous to pair-breaking experiments in superconductivity.PACS numbers: 75.20.Hr, 71.55Jv During the last few years weak localization has been developed into a powerful new method in solid-state physics (see, for example, Bergmann 1 and Lee and Ramakrishnan 2 ). Magnetoresistance measurements on disordered thin films correspond to a time-of-flight experiment with conduction electrons, yielding their inelastic lifetime, and spin-orbit and magnetic scattering times. In particular, the opportunity of directly observing magnetic scattering rates promises new insights into the Kondo problem which is recently again receiving considerable interest. 3 " 5The scattering of conduction electrons by Kondo impurities can be separated into two contributions, the spin-nonflip and the spin-flip parts. Only the latter causes dephasing in superconductivity and in weak localization. This has been intensively studied in superconducting alloys where it is possible to obtain this rate from the T c depression at different concentrations 6 and the dependence of H c i on temperature. 7 These measurements show an increase of the spin-flip rate with decreasing temperature 8 in the temperature range above the Kondo temperature T&. The predicted maximum of the rate at T& and its decrease for T < T& could, however, not be verified by this method. Therefore, the theoretical prediction of a maximum in the spin-flip scattering waited almost 20 yr for an experimental proof. Using the method of weak localization we find for the first time a maximum of the spin-flip rate in the Kondo system AuFe and a clear decrease of this rate below T&.Weak localization as a method to measure the spin-flip rate has a number of advantages over the superconducting experiments. It does not require a superconducting host, nor does it need different samples for every point in the T dependence. It can be applied to convenient values of 7"K and small concentrations (to stay within the single-impurity limit) and measures the temperature dependence of the spin-flip scattering rate in a wide temperature range.When weak localization is employed it is favorable to use disordered thin films. l To prepare very homogeneous and highly disordered films, they are quench condensed inside a dilution refrigerator while the substrate is maintained at 4 K. This has the very desirable feature that one can produce multilayers step by step and analyze them after each evaporation step. Therefore, it is possible to separate the additional scattering caused by a small fraction of an atomic layer of Fe atoms from the total phase-breaking scattering. It is not necessary to obtain the informatio...

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The magnetic scattering rate of Kondo alloys with a finite concentration of magnetic impurities is measured by means of weak localization. The three investigated systems-Fe in Mg and Co on the surface of Mg and Cu-show magnetic interactions at liquid-helium temperatures. At much lower temperatures the magnetic scattering rate decreases drastically with a T 112 temperature dependence in the experimental range. This suggests a cooperative effect between the different impurities which appears to suppress the magnetic moments, although the condition for a Landau Fermi liquid is not fulfilled. PACS numbers: 75.20.Hr, 71.55.Jv, 72.15.QmMagnetic impurities in metals interact strongly with the conduction electrons. l One of the interesting questions is their low-temperature behavior. Do the magnetic moments remain active at low temperature or do they lose their magnetic character? The latter case is denoted as the "Fermi-liquid" behavior of the system. Two extreme examples are well known: the single Kondo impurity and the periodic Anderson problem, also called the heavy-fermion system. In our present investigation we are interested in dilute Kondo systems. The single Kondo impurity forms at low temperature a singlet state with the conduction electrons. At zero temperature the magnetic moment is suppressed and neither yields spinflip scattering nor does it act as a random field. At finite temperature T<£TK, the Fermi-liquid theory 2 yields a T 2 law for the magnetic scattering rate. Recent experiments 3 on thin disordered Mg/Fe/Mg sandwiches with about 300 ppm Fe showed in the temperature range between 4.5 and 20 K qualitatively a T 2 behavior on top of a residual scattering. At He temperatures (which is -JV of the Kondo temperature of 4 about 45 K) the magnetic scattering appeared to saturate at a finite rate. It could be shown that this saturation is due to interaction between the impurities. 5 In this Letter we try to answer the question, What is the ground state of interacting impurities? Do they form a spin-glass or do they approach a Fermi-liquid behavior? We extend the investigation to temperatures of about 80 mK, far below the Kondo temperature.A rather dramatic indication for the suppression of the magnetic behavior is the occurrence of superconductivity in some heavy-fermion systems, which means that the pair breaking due to the magnetic moments is suppressed. The magnetic moments no longer dephase the Cooper pairs. However, this detection of the nonmagnetic character is restricted to potential superconductors.We use a rather similar method which detects the dephasing of the electrons by the magnetic impurities -weak localization. By this method, which has been described in several review articles, 6 " 9 one can quantitatively measure the dephasing rate due to magnetic impurities.When one is using weak localization, it is very favorable to use disordered thin films. Our films are quench condensed in situ in an especially built dilution refrigerator where the substrate is maintained at 4.2 K during the evaporatio...