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We have combined electron heating experiments and noise thermometry to perform quantitative measurements of the thermopower in mesoscopic samples. This new measuring technique allows us to detect finite size effects in the thermopower of narrow AuFe wires with an Fe concentration ranging from 50 to 3000 ppm. The size effects emerge when reducing the width of the wires below $\ensuremath{\simeq}300\mathrm{nm}$ and may be related to a spin-orbit induced magnetic anisotropy close to the wire surface.

The Hamiltonian, which describes the surface-induced anisotropy for impurity spin in mesoscopic wires of dilute magnetic alloys, is derived for the wires with a rectangular cross-section and rough surfaces. We show that due to the combined effect of the four surfaces of the wire, the impurity-spin energy spectra are substantially different from those in a semi-infinite host. Moreover, we find that a moderate roughness of the surface enhances the splitting of the impurity-spin energy level.

Recently we reported on the differential resistance R(I) = dV/dI of narrow AuFe spin-glass wires (Phys. Rev. Let., 77 (1996) 2276). Surprisingly, R(I) was found to be asymmetric in I for the smallest samples. In this letter we present differential resistance measurements performed on a AuFe wire with a multiprobe geometry. The new experiments reveal that the asymmetric behavior of R(I) can be explained in terms of a mesoscopic thermoelectric effect. Our experiments also indicate that the thermopower S of a AuFe spin-glass alloy becomes size dependent for length scales below a few hundred nanometers.

The placement of a transition metal impurity in a noble metal host induces a collective response in the electron gas which attempts to screen the magnetic moment of the impurity. At low impurity concentration, enhanced scattering of electrons from the screened impurity leads to a characteristic logarithmic increase of the resistance with decreasing temperature, the well-known Kondo effect [1]. If the concentration of impurities is high, the interaction of the screening electrons around one impurity with those around other impurities leads to an effective impurity-impurity interaction, the RKKY (Ruderman-Kittel-Kasuya-Yosida) interaction [2]. At high temperatures, the thermal energy of the impurity spins is sufficient to overcome the RKKY interaction, and each individual spin is free to rotate independently. As the temperature is reduced, however, the impurity spins are increasingly fixed in random orientations by the RKKY interaction. The onset of this spin-glass order is signaled by a drop in the resistance of the host metal due to the reduced magnetic scattering of the conduction electrons. In combination with the Kondo effect at higher temperatures, this gives rise to a maximum in the resistance as a function of temperature which is characteristic of spin glasses [2]. Earlier work on magnetic impurities in metals concentrated on the properties of bulk materials; recently, with the opportunities presented by nanolithography, interest has focused on the properties of samples whose dimensions are comparable to relevant microscopic length scales. A number of such microscopic length scales have been proposed for both the Kondo effect [3] and spin glasses [4]. The hope is that measurements on mesoscopic samples would allow one to verify directly the existence of these microscopic length scales. However, the experimental evidence in both Kondo systems and spin glasses has so far been inconclusive. For example, measurements by some groups [5,6] of the Kondo effect in thin films, wires, and small point contacts defined by break junctions show a definite size dependence, but on vastly different length scales, while measurements by other groups [7] on AuFe wires found no size dependence on the Kondo effect. The situation is similar for samples in the spin-glass regime [8 ‐ 11]. Thus the issue of the existence of fundamental length scales in both the spin-glass and the Kondo regimes remains open. In this Letter, we report on measurements of the low temperature differential resistance RsId › dV ydI of AuFe wires as a function of dc current bias I .A s reported previously by other groups in measurements on CuCr wires [9] and point contact break-junction devices [10], we find that the shape of RsId reflects the behavior of the temperature dependent resistance RsT d, in that it has a maximum at a particular current Im. In addition, however, we find that RsId is asymmetric in I, even in zero magnetic field. The asymmetry is small at high temperatures, but grows by more than an order of magnitude as the temperature is lowered, indicating that it is associated with enhanced spin scattering at low temperatures. The degree of asymmetry is sample specific, being larger in some samples and smaller in other nominally identical samples. Furthermore, we find that the asymmetric component of RsId is sensitive to the particular configuration of contact leads used in a four terminal measurement. We also find that the asymmetry depends on the size of the wire, being generally larger for shorter and narrower wires and disappearing entirely for very long samples. These observations are indicative of the mesoscopic nature of this phenomenon. The samples in this experiment were patterned onto oxidized silicon substrates by conventional e-beam lithography techniques. After thermal deposition of 99.999% Au, the samples were implanted with Fe ions at energies and dosages calculated to give impurity concentrations of 0.2 and 0.4 at. % [12]. All samples of one concentration were fabricated and ion-implanted at the same time to ensure uniform film properties. The inset of Fig. 1(a) shows a schematic of one of the samples. The thickness of the films was 33 nm, and the sheet resistance Rh was ,1 V at 4.2 K after ion implantation. The samples were measured in 4 He and 3 He cryostats (for the 0.4 at. % samples)

We report on the low-temperature resistivity ρ(T) of AuFe spin-glass wires with a thickness around 20 nm and a linewidth varying between 300 μm and 150 nm. The observed linewidth dependence of ρ(T) is due to the contribution of the disorder-enhanced electron-electron interaction, while the spin-glass contribution is independent of linewidth. However, a quantitative analysis of the ρ(T) data reveals a substantial suppression of both the amplitude of the spin-glass resistivity and the interaction strength between magnetic impurities when compared to bulk AuFe alloys. This may indicate the presence of a size effect resulting from the finite spin-glass thickness.

In recent years, a level of nanoscale fabrication has been achieved where dimensions and energy scales are such that quantum effects are dominant and can even be controlled in a variety of condensed matter systems. In this paper, several examples are given to illustrate the influence of nanostructuring on the physical properties of superconducting and normal magnetic systems. For superconductors we will discuss quantum interference effects in mesoscopic single loops and multiloop structures and the confinement of flux lines by a lattice of artificial submicron antidots. For magnetic layers we will analyze the size dependence of the Kondo and spin-glass effects.

Recent advances in electron beam lithography have allowed to prepare high-quality metal structures with submicrometer dimensions. At low temperatures the size of these structures can become smaller than the coherence length over which the electrons maintain their phase memory. Therefore, a mesoscopic regime can be reached, where the quantum interference between conduction electron waves becomes important. The interference effects can be conveniently tuned by applying a perpendicular magnetic field. Using a lift-off technique we have prepared various submicrometer Al and Au structures. In the superconducting Al loops the Ginzburg-Landau coherence length, which diverges near Tc, corresponds to the relevant characteristic length scale. Due to their mesoscopic sample size the Al loops behave as superconducting quantum interference devices (SQUIDs), even when no artificial Josephson junctions are present in the branches of the loop. The mesoscopic confinement of the superconducting wave function causes in addition anomalous Little-Parks oscillations of Tc when a magnetic field is applied. We have been able to further reduce the linewidth of the mesoscopic sample by using the tip of a scanning tunneling microscope (STM) for local exposure of a very thin Langmuir-Blodgett layer used as an electron sensitive resist.

The puzzling appearance of a large resistance peak in the superconducting state near ${\mathit{T}}_{\mathit{c}}$ of mesoscopic superconducting wires is analyzed. It is shown that this resistance anomaly can be explained in terms of thermal fluctuations producing phase slips of the superconducting order parameter in quasi-one-dimensional wires. Good quantitative agreement with the experimental observations is obtained within the framework of the modified Langer-Ambegaokar and McCumber-Halperin models.

The superconducting field (H)-temperature (T) phase boundary has been measured in mesoscopic Al samples of different topology: lines, open and filld squares, which were made under the same conditions from the same material. These samples clearly show different superconducting H-T phase boundaries which are nicely reproducing the predictions of the theoretical calculations made for their particular confinement geometries. The confinement of the flux lines by the lattice of the submicrometer holes has been studied in the Pb/Ge multilayers. A substantial enhancement of the critical current j(c), has been achieved. Sharp integer and rational matching peaks in the j(c)(H) curve are observed. The possibility of the ''quantum design'' of the superconducting critical parameters (H-c(T) and j(c)(T, H)) of the mesoscopic and nanostructured superconductors by optimizing the confinement geometry for the superconducting condensate and for the flux lines has been demonstrated.

Quantum interference phenomena have been studied in superconducting Al loops with dimensions smaller than the temperature dependent coherence length ξ(T). Besides the classical Little-Parks (LP) effect, anomalous resistance oscillations have been observed whose amplitude shows an unusual magnetic field and current dependence at low flux quanta. The magnetic field (H) - critical temperature (Tc) phase boundary has been measured for different transport currents jt. A clear field enhancement of Tc is observed at low current densities. The amplitude of the field enhancement of Tc is strongly influenced by the transport current through the loop. Higher jt values induce a transition from the Tc(H) boundary with LP oscillations to a new regime without oscillations. This transition is governed by the interplay between the characteristic superconducting length scale ξ(T), the loop circumference 2πr and the de Broglie wavelength λD ∝ (1/jt), tuned by the transport current.

Giant magneto resistance has been observed in MBE grown Ag single crystals, loaded with Co clusters by ion implantation. Mossbauer spectroscopy with 57Co on a polycrystalline Ag foil implanted with Co clusters is indicative of volume expanded Co clusters.

A scanning tunneling microscope (STM) has been used to locally expose Langmuir–Blodgett layers of a negative electron beam resist (ω‐tricosenoic acid) on top of a thin gold film. The STM operates in a dry nitrogen atmosphere at a voltage difference of about −10 V between the electrochemically etched Pt‐Ir tip and the gold surface. After development in ethanol, the unexposed areas of the gold film are removed by argon ion milling. Gold wires with a linewidth down to 15 nm have been prepared. Electrical transport measurements confirm the homogeneity and the one‐dimensional metallic character at low temperatures of the gold structures.

When transition metal impurities are introduced into noble metals, the scattering of the conduction electrons at the impurity spins leads to important changes of their low temperature properties [1]. For small concentrations below ≃ 100 ppm of the magnetic dopant, the Kondo effect gives rise to a logarithmic increase of the resistivity ρ(T). Below the Kondo temperature T K a compensation cloud of conduction electrons is formed around the impurity spins which gradually screens their magnetic moment. At higher concentrations (∼ lat.%) the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the randomly distributed impurity spins competes with the Kondo effect and causes a freezing of the impurity spins into a disordered configuration called a spin glass. This is reflected by the appearance of a typical broad maximum in ρ(T) since the spin scattering rate again decreases at the lowest temperatures due to the freezing process.

We have employed electron heating experiments, and noise thermometry to perform quantitative measurements of the thermopower in mesoscopic samples. This new measuring technique allows to detect finite size effects in the thermopower of narrow AuFe wires with an Fe concentration ranging from 50 to 3000 ppm. The size effects emerge when reducing the width of the wires below ≅300 nm. Our observations can be understood in terms of a magnetic anisotropy which affects the spins close to the surface of the sample. The spin glass freezing at lower temperatures suppresses the size effects.

We report on intrinsic time-dependent conductance fluctuations observed in mesoscopic AuFe spin glass wires. These dynamical fluctuations have a 1/f-like spectrum and appear below the measured spin glass freezing temperature of our samples. The dependence of the fluctuation amplitude on temperature, magnetic field, voltage and Fe concentration allows a consistent interpretation in terms of quantum interference effects which are sensitive to the slowly fluctuating spin configuration., Comment: 4 pages, 4 figures

We have developed a new lithographic patterning technique with the STM, which allows to pattern thin evaporated Au films at the nanometer scale. The STM tip is used to expose a very thin layer of ω-tricosenoic acid, which has been deposited on top of the Au films using the Langmuir-Blodgett technique and acts as an electron sensitive, negative resist. We have fabricated narrow Au lines with a width down to 15 nm and we have checked that our STM lithography does not degrade the metallic properties of the Au films. We have also performed detailed four-terminal magnetoconductance measurements of Au lines which interconnect large contact pads. These measurements confirm the presence of pronounced quantum interference effects in our mesoscopic, quasi one-dimensional fineline structures.

Thin evaporated gold films have been patterned by locally exposing a Lang-muir-Blodgett layer of an electron beam resist with the tip of an STM. The 12 nm thick resist layer consists of 4 monolayers of ω-tricosenoic acid. The STM operates in a dry nitrogen atmosphere at a voltage difference of about —10V between the electrochemically etched Pt-Ir tip and the Au film. The ω-tricosenoic acid layer acts as a negative resist and the unexposed areas can be removed by argon milling. We have written submicrometer line structures which can be connected to larger predefined contact pads. Measurements of the temperature dependence of the electrical resistance confirm the metallic character of the Au lines. The low-field magnetoresistanee is in agreement with the theory for the one-dimensional weak electron localization. While linewidths down to 15 nm can be achieved, our STM patterning also allows to prepare larger arrays of nanoscale loops and dots.

We have measured the differential resistance R=dV/dI of mesoscopic AuFe wires as a function of temperature T and dc current bias I. As a function of I, the low temperature resistance R(I) shows a maximum, reflecting a similar maximum in R(T) which is due to the onset of spin-glass order in these films. However, R(I) is asymmetric in I, this asymmetry being related to the presence of magnetic impurities in the AuFe films. The degree of asymmetry is sample-specific and is sensitive to the four terminal measurement configuration used to measure the sample. We believe the asymmetry is related to anomalous spin-scattering in the sample.

We present a new lithographic technique with a scannning tunneling microscope (STM), allowing us to pattern thin evaporated Au films at the nanometer scale. The electron beam produced by the STM tip is used to expose a very thin layer of ω‐tricosenoic acid. Only four monolayers of the acid, which acts as an electron sensitive, negative resist, are deposited on top of the Au films using the Langmuir–Blodgett technique. We have verified the influence of the exposure conditions on the quality of the fabricated fine‐line structures with a linewidth down to 15 nm. We can also measure the electrical properties of narrow Au lines which interconnect large predefined contact pads. As expected, the low‐temperature magnetoresistance is strongly influenced by quantum interference effects.