C. D. Potter

Giant magnetoresistance in Fe/Cr superlattices with very thin Fe layers

Carefully tailored Fe/Cr epitaxial superlattices with extremely thin Fe layers have been grown on MgO(100) by molecular beam epitaxy. The low‐angle x‐ray spectra reveal the presence of sharp interfaces down to an Fe layer thickness of a few monolayers. An [Fe(4.5 A)/Cr(12 A)]50 superlattice shows a 220% magnetoresistance at 1.5 K, and a saturation field of 110 kOe. A further decrease of the Fe layer thickness produces a drastic decrease in the magnetoresistance.

On the Fe thickness dependence of the giant magnetoresistance in epitaxial Fe/Cr superlattices

Abstract We analysed the transport properties of Fe/Cr(100) superlattices as a function of the Fe thickness. We find two magnetic inactive atomic layers and interpret the magnetoresistance with the quasi-classical formalism based on the Boltzmann kinetic equation.

The superconducting proximity effect in Nb/Fe multilayers

Abstract We prepared Nb/Fe multilayers with varying Nb or Fe thickness by means of MBE. The x-ray diffraction patterns show good layering quality. Critical field measurements as a function of temperature and angle demonstrate that very thin Fe layers (≈ 12 A) decouple the Nb layers completely thus leading to 2D behaviour of the multilayers. The dependence of T c as a function of Nb thickness and the critical fields are well described by the theoretical model derived for the proximity effect in superconductor/ferromagnet multilayers.

Influence of different kinds of interface roughness on the giant magnetoresistance in Fe/Cr superlattices

Abstract We present a detailed study of the influence of the interface properties on the amplitude of the GMR in Fe/Cr superlattices, including changes of the coupling, a synchrotron X-ray diffraction analysis and a theoretical interpretation within the quasi-classical formalism. With increasing amplitude of the interface roughness we observe a reduction of the GMR.

Electric transport properties of epitaxial Fe and Cr films with very low intralayer scattering

The low-temperature transport properties of epitaxial Fe and Cr films grown on MgO(1 0 0) substrates by molecular beam epitaxy are characterised by extremely low intralayer resistivities, indicating a very small concentration of defects. This makes such films particularly suitable for studies of the influence of the interface scattering on the transport properties of Fe/Cr superlattices.

Enhanced pinning and matching effects in superconducting films with a lattice of submicrometer holes

Abstract Magnetization studies have been carried out on single Pb films with a square lattice of submicrometer holes. These submicrometer hole patterns create the possibility to trap an integer number, as well as a certain rational number of flux lines per hole corresponding to a regular organization of flux lines inside the structure. The stable flux line configurations lead to the appearance of sharp peaks in the magnetization M ( H ⊥ ).

Magnetoresistance and magnetization oscillations in Fe/Cr/Fe trilayers

The 2‐ML (monolayer) oscillation period has been observed in the magnetization as well as in the magnetoresistance of Fe/Cr/Fe trilayers. Kerr effect measurements were performed in order to verify the periodicity and determine the kind of the coupling between the Fe layers. The magnetoresistance loops show characteristic steps at magnetic field values at which the size of the magnetization changes.

Anomalous field-dependence of the critical-current and flux-creep rate in Pb/Ge multilayers

Abstract Critical current densities J c and normalized flux creep rate S (≡ dln M /dln t ) have been measured as a function of the perpendicular field H ⊥ in low- T c Pb/Ge multilayers, which can be used to model the superconducting properties of the layered high- T c cuprates. By taking a proper Ge separator thickness a field-induced crossover between two different pinning regimes can be observed. This transition leads to an anomalous behavior of J c ( H ⊥ ) and S ( H ⊥ ) in Pb/Ge multilayers, while a sing le Pb film demonstrates a monotonic suppression of J c by H ⊥ . These data have been analyzed in the framework of the collective pinning (CP) theory of magnetically coupled superconducting planes: the J c ( H ⊥ ) anomaly is interpreted as a crossover from three-dimensional (3D) flux lines in low fields to two-dimensional (2D) pancake vortices in higher fields. This interpretation has also been confirmed by the suppression of the J c ( H ⊥ ) anomaly in the Pb/Ge multilayers with a square lattice of holes, forming additional defects for the confinement of the flux lines and for the expansion of the 3D regime.

Critical fields of W/Si multilayers

Abstract Amorphous W/Si multilayers, with single layer thickness varying between 10 and 100 A, have been prepared by UHV electron beam evaporation. The superconducting samples ( d w T c d w >30 A) have a positive coefficient. With decreasing temperature, the parallel critical field H c2∥ ( T ) shows a dimensional crossover which depends on the ratio between the total multilayer thickness and the in-plane superconducting coherence length. The amorphous character of the layers enables one to prepare continuous and smooth W layers with thickness down to d w =10 A . These ultrathin layers give rise to very high values of the parallel critical fields.

Dimensional crossover in superconductor/spin-glass multilayers

We have prepared epitaxial superconductor/spin-glass multilayers, V/AgFe. The spin-glass layers were made by coevaporation of Ag and Fe resulting in a AgFe system having 1 atomic%Fe. We measured critical temperatures and critical fields for multilayers with constant V thickness TV=14 nm and 35 nm, and varying AgFe thickness 1–8 nm. The critical fields as a function of temperature and angle show that a AgFe thickness of 4 nm is able to decouple the V layers in the films with tV-14 nm. The decoupling is not observed in in the multilayers with tV=35 nm, which have a higher Tc. This suggests that decoupling of the V layers occurs only when Tc becomes smaller than the spinglass temperature.