This chapter will take you through a simple introduction to transport theory covering the Boltzmann equation, the Fuchs—Sondheimer model for thin films, the normal magnetoresistance and quantum interference effects in metals with strong electron scattering. At the end of the chapter we will also introduce you to a number of the basic techniques involved in electron transport measurements. All of this is by way of introduction to basic transport properties common to all metals. In later chapters these ideas will be developed and applied to systems in which spin dependent transport is important.

We calculate the conductivity of two-dimensional hybrid multilayer films formed by the self-assembly of an ordered liquid crystalline array of molecular columns on an ultra-thin MBE grown metal layer. Electrons in the metal interact with the molecular overlayer, forming new energy bands which have novel transport properties. Because of the short mean free path encountered in these structures due to strong Coulomb and surface roughness scattering, conductivity changes occur where the molecular columns bond strongly to the metal surface. The local conductivities are calculated using Kubo–Greenwood formula for the hybrid system, and the global conductivity is obtained via a many-body diffusion model. The model includes both the general effects of a dielectric overlayer on electron scattering in the metal, and the specific effect of real and virtual electron capture by the molecular columns. The frequency-dependent conductivity is calculated for a range of columnar lengths and external gate fields.

We have carried out measurements of the magnetoresistance MR(H) in the CPP (Current Perpendicular to the Plane) mode for two types of magnetic multilayers which have different layer ordering. The series resistor model predicts that CPP MR(H) is independent of the ordering of the layers. Nevertheless, the measured MR(H) curves were found to be completely different for the following two configurations:[Co(1nm)/Cu(20nm)/Co(6nm)/Cu(20nm)]*N and [Co(1nm)/Cu(20nm)]*N[Co(6nm)/Cu(20nm)]*N showing that the above model is incorrect. We have carried out a calculation showing that these results can be explained quantitatively in terms of the non-local character of the electron scattering, without the need to invoke spin-flip scattering or a short spin diffusion length.

This paper presents an account of the application of medium-energy ion scattering (MEIS) to the investigation of thin-film metallic multilayers grown using molecular-beam epitaxy. MEIS can provide high resolution compositional and structural information as a function of depth in the near surface region (0-250 Angstrom); these parameters are inextricably linked with the magnetic properties exhibited by materials of this type. Amongst the information available from MEIS is the accurate determination of the layer spacings, structural information from individual layers (even at thicknesses close to a monolayer), and high sensitivity to disorder in the layers. MEIS therefore provides additional information above that provided by in situ reflection high-energy electron diffraction monitoring during growth and ex situ x-ray diffraction measurements so that it represents an ideal complementary technique for the analysis of thin-film magnetic multilayer materials of this type. An Au/Fe multilayer sample of a type previously shown to exhibit giant magnetoresistance (GMR) was analyzed. Individual gold layers were clearly resolved and a measurement of the bilayer spacing obtained; this parameter determines the magnitude of the exchange coupling and GMR. Au/Fe/Au trilayer samples grown on both MgO(100) and sapphire(11 (2) over bar 0) substrates were also analyzed for a series of Fe layer thicknesses between 2 and 16 Angstrom. The MgO(100) grown samples showed unusually high second-layer Au signal consistent with atomic layer spacings in the Fe layers that lead to enhanced illumination of the second-layer Au. This effect could be modeled using bcc(100) layer spacings thus confirming the structure to be bcc(100) Fe between fcc(100) Au layers. In the sapphire-grown samples, twinned fcc(111) structure was observed in the individually resolved Au and Fe layers. The amplitude of the Fe blocking features was reduced with increasing Fe layer thickness indicating a reduction in crystallinity until for the highest thickness there was little indication of structure within the layer. The maximum layer thickness for fcc(lll) Fe growth was seen to lie between 8 and 16 Angstrom.

We present results for the specific heat of a Nd1/2Sr1/2MnO3 crystal. The continuous phase transition from paramagnet to metallic ferromagnet is observed at 250K and a first order transition from metallic ferromagnet to the insulating charge ordered state is observed at 150K in zero field. Hysteresis is observed on warming and cooling, which increases in a magnetic field. The magnetic field is seen to drive the low temperature charged ordered state into the ferromagnetic state through a first order transition.

We have determined a maximum MR of 22% in melt-spun Cu70Cu30 after annealing at 450��C for 1 hr. Two types of Co-rich precipitates were found in the as-spun ribbon but these do not appear to contribute to the MR. On annealing, a very fine distribution of Co-rich precipitates forms at temperatures above 300��C which are responsible for the high MR developed in this material.

We have grown epitaxial Co/Cu multilayer samples at different temperatures using MBE on sapphire (1120)0 substrates with a 30 Å layer of Nb as a buffer, and find a remarkable correlation between the sharpness of X-ray rocking curves and the magnitude of the GMR. Whereas the peak GMR for our previous samples grown on GaAs was never greater than 26%, the maximum GMR of samples grown on sapphire under optimal growth conditions was as high as 50%.

Results are presented of the magnetoresistance of MBE-grown (111) Co/Cu multilayers measured with the current perpendicular to the plane of the layers (CPP). Although for measurements made with the more common geometry of current in the plane of the layers (CIP) there are large differences between the results on samples made by sputtering and those prepared by MBE, for these new CPP data the results on samples made by the two techniques are very much alike. For copper layers with thicknesses between 0.9nm to 6nm the magnetoresistance shows oscillations with copper thickness that were almost non-existent in the earlier CIP data. At the second peak the magnetoresistance in the CPP geometry is an order of magnitude greater than that in the CIP configuration. Although the interfaces in these samples have been shown to be very sharp, they appear to form a mosaic structure with the antiferromagnetic regions embedded in a ferromagnetic structure. It is argued that for CIP measurements the GMR is greatly reduced by these ferromagnetic correlations over lengthscales long compared to the electron mean free path. For CPP measurements, on the other hand, it is the spin diffusion length that is the determining factor with the mean free path no longer a key parameter and with values of the GMR virtually independent of the growth process.

The specific heat of twinned and detwinned single crystal samples of YBa2Cu3O7−δ have been measured with magnetic fields up to 8 tesla applied parallel to the c-axes of the crystals. The samples measured had transition temperatures around 90K and transition widths from 0.3 K to 2 K. The data exhibits scaling behaviour characteristic of the 3-dimensional-X-Y model with critical exponents consistent with those observed in superfluid4He. We then compare the scaling properties of the specific heat in the different samples and look for the ‘universal’ nature of the scaling function.

Amorphous alloys of the binary system CaAl are known to have highly unusual electron transport properties with resistivities up to 450μΣcm and a Hall coefficient that deviates from free electron values at Ca concentrations higher than 45 atomic percent. For amorphous CaMg alloys, on the other hand, the resistivity is very much less and this great difference between the two sets of alloys is not fully understood. We report on the correlation of photoemission and transport measurements made on two sets of amorphous CaAl and CaMg alloys prepared by magnetron sputtering in such a way that we could carry out both sets of measurements within the same UHV system. A special feature of the measurements was that the electrical resistivity was also measured in-situ using a specially designed 4-point probe to check for amorphicity and to compare with transport experiments carried out elsewhere. Photoemission studies were carried out in the energy range 15–50 eV with tuneable synchrotron radiation enabling us to examine the Ca 3p-3d photoemission resonance in detail. The main result from the present series of experiments was that whereas in the CaAl alloys the Fermi edge developed a shoulder at high concentrations of Al, this feature was completely absent in CaMg. At the same time the intensity of the Ca 3p-3d photoemission resonance revealed the presence of d-states in both sets of alloys, indicating that the presence of these dstates cannot, by itself, explain the high resistivity of CaAl.

Results for the temperature and magnetic-field dependence of melt-spun Cu87Co13 are presented and discussed. The magnetoresistance of the as-spun sample exhibits superparamagnetic behavior with a magnetoresistance of 18% at 0.4 K and in a field of 80 kOe. The magnetic-field dependence of the magnetoresistance is approximately proportional to the Langevin function. The microstructure of the sample was investigated by transmission electron microscopy and it appears that the giant magnetoresistance is due to the presence of small Co precipitates with an approximate mean diameter of 12 Å and a mean separation of 35 Å.

Results for the temperature and magnetic-field dependence of melt-spun Cu87Co13 are presented and discussed. The magnetoresistance of the as-spun sample exhibits superparamagnetic behavior with a magnetoresistance of 18% at 0.4 K and in a field of 80 kOe. The magnetic-field dependence of the magnetoresistance is approximately proportional to the Langevin function. The microstructure of the sample was investigated by transmission electron microscopy and it appears that the giant magnetoresistance is due to the presence of small Co precipitates with an approximate mean diameter of 12 angstrom and a mean separation of 35 angstrom.

We have observed oscillations with non-magnetic spacer layer thickness in the saturation fields and characteristic exchange fields of Co/Cu multilayers grown by MBE. The specimens consisted of a set of samples in which the thickness of copper layers took a series of values between 5Å and 20Å, while the cobalt layer thickness was set at 15Å throughout. The multilayers were grown epitaxially on GaAs (110) with Ge, Co (110) and Au in the buffer region, resulting in Co/Cu bilayers in the (111) orientation. Although oscillations were observed in these magnetic measurements, no oscillations were observed in the magnetoresistance itself. These observations are somewhat at odds with similar measurements made on samples prepared by sputtering. We conclude with a discussion involving exchange coupling, in-plane anisotropy and defects in the magnetic structure in an attempt to account for these conflicting results.

We argue that the point made by Aubin and Gagnon concerning the thermopower-signal measurement is not relevant in our case. We correctly operated our lock-in in the two-phase mode.Received 7 January 1992DOI:https://doi.org/10.1103/PhysRevB.47.15321©1993 American Physical Society