We have measured the change in the resistivity of thin films of SrRuO 3 and CaRuO 3 upon introducing point defects by electron irradiation at low temperatures , and we find significant deviations from Matthiessen's rule. For a fixed irradiation dose, the induced change in resistivity decreases with increasing temperature. Moreover, for a fixed temperature, the increase in resistivity with irradiation is found to be sublinear. We suggest that the observed behavior is due to the marked anisotropic scattering of the electrons together with their relatively short mean free path (both characteristic of many metallic oxides including cuprates) which amplify effects related to the Pippard ineffectiveness condition.

A phenomenological approach is presented for the giant magnetoresistance (GMR) of magnetic multilayers oriented in the CPP mode (current perpendicular to the plane of the layers). New results are found for the dependence of the GMR on the number of repeats of the multilayer.

Experimental evidence is presented showing that for magnetic multilayers measured in the CPP mode (current perpendicular to the planes), the magnetic-field dependence of the magnetoresistance MR(H) is dominated by scattering processes in which the electron is scattered by the potential due to pairs of neighboring magnetic layers. It is demonstrated experimentally that curtailing such scattering processes leads to a significant decrease in MR(H). These results are confirmed by a calculation.

The magnetic-field dependence of the magnetoresistance MR(H) has been measured in the CPP mode (current perpendicular to the plane) for multilayers containing three different types of magnetic layers: permalloy Py(80 Angstrom), Co(10 Angstrom), and Co(70 Angstrom). These data clarify the role of the electron mean free path in interpreting the MR(H) curves. A critical discussion is given of the view that denies the role of the electron mean free path for determining the MR(H) curves.

We have measured the change in the resistivity of thin films of SrRuO3 and CaRuO3 upon introducing point defects by electron irradiation at low temperatures, and we find significant negative deviations from Matthiessen's rule. For a fixed irradiation dose, the induced change in resistivity decreases with increasing temperature. Moreover, for a fixed temperature, the increase in resistivity with irradiation is found to be sublinear. We suggest that the observed behavior is due to the marked anisotropic scattering of the electrons together with their relatively short mean free path (both characteristic of many metallic oxides including cuprates) which amplify effects related to the Pippard ineffectiveness condition.

The magnetic-field dependence of the magnetoresistance MR(H) of magnetic multilayers in the CPP mode (current perpendicular to the layers) was recently measured for two samples that differ only in the ordering of the layers: [Fe/Cu/Co/Cu]N and [Fe/Cu]N[Co/Cu]N, where N denotes the number of repeats. Ohm's law states that MR(H) should be the same for these two samples because in a series circuit, the resistance is independent of the ordering of the resistors. However, the measured MR(H) curves were found to be completely different for the two samples. We show that this unexpected result can be explained as follows. The mean free path is sufficiently long that MR(H) is due to the scattering of electrons by pairs of neighbouring magnetic layers, which differ for the two samples. To confirm these ideas, we carried out calculations of MR(H) and obtained quantitative agreement with experiment.

We have carried out an experimental and theoretical study of non-local electron scattering in magnetic multilayers by measuring the magnetoresistance MR(H) in the CPP (current-perpendicular-to-plane) mode for two samples consisting of different magnetic layers (M1, M2) separated by non-magnetic layers (NM). For the two samples, the ordering of the layers was as follows: [M1/NM/M2/NM]N and [M1/NM]N[M2/NM]N. If the non-local character of the electron scattering were unimportant, the two samples would yield identical curves for MR(H) in the CPP mode. However, our measured MR(H) curves are completely different for the two samples. This demonstrates the importance of non-local electron scattering. For our measurements, M1 = Fe(50 Å), M2 = Co(20 Å), NM = Cu(200 Å) for Fe-Co samples, and M1 = Co(10 Å), M2 = Co(60 Å), NM = Cu(200 Å) for the Co-Co samples. To confirm our ideas, we calculated MR(H), including the effect of non-local electron scattering, and obtained quantitative agreement with experiment.

We have measured the field dependence of the magnetoresistance MR(H) in the CPP mode for two types of magnetic multilayers that differ only in the ordering of the magnetic layers: [Co (10 Å)/Cu/Co (60 Å)/Cu]N and [Co (10 Å)/Cu]N [Co (60 Å)/Cu]N, with the Cu layers always being 200 Å. The series resistor model predicts that in the CPP mode, MR(H) is independent of the ordering of the layers. Nevertheless, the MR(H) curves measured were found to be completely different for the two cases. We suggest that the reason for this difference is that the electron mean free path is long enough that the potential that scatters the electrons is due to pairs of neighboring magnetic layers, which differ for the two types of magnetic multilayers.

We measured the exchange coupling strength and magnetoresistance for multilayers of Co/Cu grown by sputtering. The samples have (111) texture with a grain size of about 140 Å. Their mosaic spread, determined by high-angle x-ray rocking curves in triple-axis geometry, is as large as , suggesting the description disordered. The magnetoresistance oscillates as a function of the Cu thickness and reaches 70% at room temperature with near zero remanence at the first antiferromagnetic peak. These results indicate that a sufficient condition for oscillatory exchange coupling with a period of Å is a well defined separation between the magnetic layers. We also measured the exchange coupling strength as a function of the magnetic layer thickness and found no oscillations. The measured saturating magnetic field is accurately described by the `orange-peel' coupling effect.

We have carried out measurements of the magnetic-field dependence of the magnetoresistance [MR(H)] in the current perpendicular to the plane (CPP) mode for magnetic multilayers having the following configurations: [Fe/Cu/Co/Cu](N) and [Fe/Cu](N)[Co/Cu](N). The two configurations had the same number, types, and thicknesses of magnetic and nonmagnetic layers; their only difference lay in the ordering of the magnetic layers. Nevertheless, the measured MR(H) curves are found to be completely different for the two configurations. The implications of these results are discussed for the spin-diffusion length. [S0163-1829(99)00229-5].

The giant magnetoresistance (GMR) was measured for Fe/Au magnetic multilayers in both the CIP and CPP configurations, for a series of samples each having the same thickness for the Fe layers (10 Å) but a different thickness for the Au layers (in the range 20-50 Å). It was found that the GMR is only a few per cent in the CIP configuration, whereas the GMR ranges from 20 to 80% in the CPP configuration. We attribute this order-of-magnitude difference in the values of the GMR for the two configurations to the short electron mean free path, resulting from the relatively high resistivity of these multilayers. A short electron mean free path reduces sharply the value of the GMR in the CIP configuration, while leaving unchanged the value of the GMR in the CPP configuration.

Measurements were performed of the magnetic-field dependence of the magnetization M(B) and the magnetoresistance MR(B) of a Au85Fe15 alloy. It was found that although MR(B) is proportional to the square of M(B) at higher temperatures (above 120 K), a term linear in M(B makes an important contribution to MR(B al lower temperatures. These results are interpreted as evidence for the presence in the alloy of a range of sizes for the: Fe particles which become progressively unblocked (superparamagnetic) as the temperature increases. A calculation based on this interpretation yields quantitative agreement with both the M(B) and MR(BI data at all temperatures.

The magnetization of Co/Au(111) and Co/Cu(111) multilayers grown by molecular-beam epitaxy has been measured. For ultrathin Co layers, superparamagnetic behavior is observed, very similar to that reported previously for granular samples. For somewhat thicker Co layers, hysteresis effects occur, indicating the absence of superparamagnetism. The clear transition seen between these two modes of behavior is attributed to the growth of the Co particles from very small superparamagnetic clusters to larger islands and ultimately to a film, as the thickness of the Co layers is increased. Different magnetic properties are found for the Co/Au(111) and the Co/Cu(111) systems during the transition from granular to continuous layers. An investigation by reflection high-energy electron diffraction suggests that this difference is mainly due to the different growth modes of Co on Au(111) and on Cu(111).

We use our measurements of the magnetization and the magnetoresistance for very small superparamagnetic particles of Co to obtain the low-temperature value of the magnetic anisotropy energy density, C approximate to 3x10(8) erg/cm(3). This is nearly two orders of magnitude larger than the corresponding value for C for bulk Co. The enormous enhancement of C for very small particles of Co is consistent with results previously reported for very small particles of Fe and of FeNi.

A phenomenological theory is presented for the magnetic field dependence of the magnetoresistance (MR) of a granular sample containing very small superparamagnetic particles embedded in a nonmagnetic host. It is found that if the particles are present in a range of sizes, then MR varies almost linearly with the magnetization. The calculated magnetic field dependence of MR is in excellent agreement with recent data for a melt-spun granular sample of CuCo.

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%.

We have studied the magnetic properties of a melt-spun granular sample of Cu87Co13. The Cu matrix contains very fine particles of Co, having a bimodal distribution of sizes: ‘larger’ particles (∼ 300 Å) and very much smaller particles (< 15 Å) with the latter exhibiting superparamagnetism. The magnetisation data indicate a spread of sizes for the smaller superparamagnetic particles, ranging from individual Co atoms up to clusters of more than 100 atoms of Co. Assuming a simple distribution of particle sizes, we calculated the magnetic field dependence of the magnetisation of the superparamagnetic particles and find excellent agreement with experiment.

We have measured the giant magnetoresistance (GMR) and the magnetization of a melt-spun granular sample of ${\mathrm{Cu}}_{87}$${\mathrm{Co}}_{13}$. The Cu matrix contains very small particles of Co that exhibit superparamagnetism. Although the GMR is due to these small superparamagnetic particles, we find that the GMR does not vary quadratically with the magnetization. This unexpected result is attributed to the presence of a range of sizes for the superparamagnetic particles. Assuming a simple distribution of particle sizes, we calculated the magnetic-field dependence of the GMR and find excellent agreement with experiment.

We have prepared a melt-spun granular sample of Cu87Co13 in which the Cu matrix contains very small particles of Co that exhibit superparamagnetism. Both the giant magnetoresistance (GMR) and the magnetisation were measured, and we find that the GMR does not vary quadratically with the magnetisation. This unexpected result is attributed to the presence of a range of sizes for the superparamagnetic particles. Assuming a simple distribution of particle sizes, we calculated the magnetic field dependence of the GMR and find excellent agreement with experiment.

We have deposited impurities (Au, Ge and Cu) at the interface of Co/Cu multilayers grown by MBE. It was found that small fractions of a monolayer of impurities dramatically affect the GMR.