M. A. M. Gijs

Giant magnetoresistance of electrodeposited Co/Cu multilayers

Abstract We report on the structural and electrical characterization of electrodeposited Co/Cu multilayers grown in a single electrolyte based on CoSO4 and CuSO4. A high degree of crystallographic orientation and superlattice coherence is found in the growth on (100)- and (111)-oriented substrates. The magnetoresistance (MR), measured in the current-in-plane configuration at room temperature, is dominated by the giant MR effect for Cu-layer thicknesses d Cu ≿ 3 nm and by the anisotropic MR effect for d Cu ≾ 2.5 nm . A maximum of 14% is measured for dCu ≈ 4 nm. No evidence for antiferromagnetic coupling is found. Instead, the giant MR gradually diminishes with decreasing dCu

Perpendicular giant magnetoresistance of Co/Cu multilayers deposited under an angle on grooved substrates

We propose a novel experimental technique for investigating the giant magnetoresistance effect measured with the current perpendicular to the layer plane (the so‐called CPP geometry). Using holographic laser interference nanofabrication techniques and anisotropic etching the surface of semi‐insulating InP substrates is patterned into V‐shaped grooves of 0.2 μm width. Subsequently, a magnetic multilayer can be evaporated under an angle with the substrate normal, naturally resulting in a CPP‐like magnetoresistance configuration. The technique is illustrated for Co/Cu multilayers, for which we present magnetization and magnetoresistance experiments.

Analysis of the critical current density in high-Tc superconducting films

Abstract Current models used to analyse the temperature dependence of the critical current density of high- T c superconductors are reviewed. The relation between the critical current density and the preparation conditions of thin films in the Y-Ba-Cu-O and the Bi(Pb)-Ca-Sr-Cu-O systems is studied. It is found that in thin films made by high-vacuum sputtering, laser ablation or thermal decomposition of metal-organic compounds the critical current can be described by models for granular superconductors. Films made by ultra-high vacuum evaporation are very homogeneous and have a much higher critical current density of 1.8 × 10 11 A/m 2 at 20 K. In this case flux motion determines the critical current density.

1/f noise in anisotropic and giant magnetoresistive elements

Microfabricated magnetoresistive elements based on either the anisotropic or the giant magnetoresistance effect were tested for their frequency dependent resistance noise behavior at room temperature in a dc magnetic field, using a dc sense current. Thermal resistance noise was the dominant noise source above about 10 kHz. At low frequencies the resistance noise was found to be dominated by a 1/f contribution that depends on the applied magnetic field. The 1/f noise is relatively low and field independent when the element is in a saturated state and contains a relatively large and field dependent excess contribution when the magnetic field is in the sensitive field range of the element. The 1/f noise level observed in saturation is comparable to the 1/f noise level found in nonmagnetic metals; the excess noise has a magnetic origin. The variation of the excess noise level with the applied dc magnetic field can be explained qualitatively using a simple model based on thermal excitations of the magnetization direction.

Preparation, patterning, and properties of thin YBa2Cu3O7−δ films

High TC superconducting thin films were prepared on (100) SrTiO3 substrates by dc triode sputtering and subsequent annealing. In these films Hall‐bar structures having a width down to 5 μm were patterned using a reactive ion etching technique. Superconductivity above 77 K was observed. When compared with the original film there is only a small reduction in TC. The critical current density determined by electrical measurements is substantially reduced. On the other hand, the critical current density in the bulk of the grains as measured by the torque on a film is not reduced by the patterning process. It is suggested that superconductor‐normal metal‐superconductor junctions between the grains account for this difference.

Perpendicular giant magnetoresistance of microstructures in Fe/Cr and Co/Cu multilayers (invited)

We discuss the fabrication and microstructuring techniques of pillar structures made of high vacuum sputtered Fe/Cr multilayers and of molecular beam epitaxy evaporated Co/Cu multilayers, for which we measured the giant magnetoresistance effect with the current perpendicular to the multilayer plane from 4 K to 300 K. Using optical lithography and reactive ion etching techniques we obtained structures with a typical height of 0.5 μm and a width ranging between 3 and 10 μm. For both Fe/Cr and Co/Cu multilayers we find an enhanced magnetoresistance with respect to the in‐plane case. The perpendicular magnetoresistance of the Fe/Cr pillars strongly decreases with temperature, while for the Co/Cu samples the temperature dependence is weaker, indicating electron‐magnon scattering processes of different strength.

Critical current as a function of temperature in thin YBa2Cu3O7−δ films

The critical current Ic of several triode‐sputtered and laser‐ablated thin YBa2Cu3O7−δ films is investigated over the temperature range of 5–85 K. Near the critical temperature Tc it is found that Ic∝(1−T/Tc)γ with γ≂1.5–2. The value of γ depends very sensitively on the value determined for Tc. The low‐temperature data, for temperatures below about 0.6Tc, can be well analyzed in terms of a proximity‐effect model. The effective barrier thickness dN between superconducting parts of the samples is of the order of a few nanometers for all samples studied.The critical current Ic of several triode‐sputtered and laser‐ablated thin YBa2Cu3O7−δ films is investigated over the temperature range of 5–85 K. Near the critical temperature Tc it is found that Ic∝(1−T/Tc)γ with γ≂1.5–2. The value of γ depends very sensitively on the value determined for Tc. The low‐temperature data, for temperatures below about 0.6Tc, can be well analyzed in terms of a proximity‐effect model. The effective barrier thickness dN between superconducting parts of the samples is of the order of a few nanometers for all samples studied.

Current‐distribution effects in microstructures for perpendicular magnetoresistance experiments

We present a simple Ohmic model to calculate the current distribution in small pillar‐like structures for perpendicular magnetoresistance experiments. These calculations are consistent with three‐dimensional electrostatic simulations, based on the finite element method, and facilitate the analysis of magnetoresistance data of microstructured multilayers.

General technique for fabricating large arrays of nanowires

Large arrays of parallel metallic nanowires ranging from 20 - 120 nm in width are fabricated using a general and relatively simple technique. Holographic laser interference exposure of photoresist and anisotropic etching are used to pattern the surface of InP(001) substrates into V-shaped grooves of 200 nm period. Subsequently metal is evaporated at an angle onto the V-grooved substrates, naturally resulting in thousands of ultra-narrow metallic wires in parallel. Resistance measurements proof that as-prepared wires are electrically continuous.

New contacting technique for thin film resistance measurements perpendicular to the film plane

Using microlithography, we have fabricated Au thin film structures for resistance measurements with current directed perpendicular to the film plane. We propose a novel contact geometry for accurate measurement of the very low metallic perpendicular resistance, which prevents any disturbing influence from the much larger resistance of the contact leads. Our experimental results are well explained by a classical model and our interpretation is independently confirmed by a three‐dimensional electrostatic calculation based on the finite element method.