M.J. Walker

The giant magnetoresistance of Co/Cu superlattices grown by MBE

Abstract We report the first observation of a giant magnetoresistance (MR) in Co/Cu superlattices grown by MBE. The maximum value of the MR is - 26% and this is found in a specimen for which the copper layers are about 7 A thick and which includes a thin 10 A layer of gold in the buffer region between the substrate and the superlattice. From RHEED and X-ray diffraction it is shown that the metallic layers in the specimens grown in this way are extremely flat and that the orientation of the copper is (111).

Perpendicular resistance of Co/Cu multilayers prepared by molecular beam epitaxy

Abstract The magnetoresistance (MR) of layered metal systems with the current in a direction perpendicular to the plane of the layers (CPP) is studied. The technique for measuring the resulting very small resistances utilises a SQUID to act as a high-precision current comparator. The samples are Co/Cu multilayers grown by molecular beam epitaxy (MBE) in ultrahigh vacuum (UHV). Perpendicular resistance measurements are reported for Nb/Cu/X/Cu/Nb where X is the Co/Cu multilayer. The Co is fixed at a nominal value of 1.5 nm while the Cu is varied between 1 and 5 nm. These measurements show large oscillations in the CPP-MR as the Cu thickness in increased. A comparison is made between MBE and sputtered samples.

Systematic study of molecular beam epitaxy growth and magnetic properties of Fe on Au(111)

We have grown a series of Fe samples on Au(111) by molecular beam epitaxy. Fe was grown on Au at 30 °C and shows clear reflection high energy electron diffraction (RHEED) oscillations up to 8 monolayers, suggesting layer-by-layer growth on Au. A combined study of RHEED, medium energy ion scattering, and x-ray diffraction scans has strongly indicated that Fe initially grew as fcc(111) on Au(111) below the thickness of 3 monolayer and for the subsequent growth the Fe bcc(110) structure begins to form. Magneto-optical Kerr effect measurement has shown that the magnetization easy axis lies in the plane of the film for the thickness range from 3 to 20 A and there is a perpendicular anisotropy when the thickness of Fe is about one monolayer.

The role of interface impurities in Co/Cu giant magnetoresistance multilayers

Abstract 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.

Optical and magneto-optical properties of MBE grown Co/Au multilayers

Abstract The optical and magneto-optical properties of Co/Au multilayers, grown by molecular beam epitaxy (MBE), have been examined in the wavelength range 300 to 900 nm using ellipsometry and normal incidence Kerr polarimetry. The dispersion of fundamental optical and magneto-optical constants and the complex Kerr rotation are discussed in terms of the interaction of electromagnetic radiation with a multilayered structure. This is done on the basis of a single equivalent layer approach for dealing with multilayered media using values for the optical constants of cobalt and gold measured on single films of these materials grown by MBE. It is suggested that a restriction of the mean free path of the conduction electrons in the gold layers, of the order of the individual sub-layer thickness, accounts for some aspects of the observed optical spectra.

Interface scattering and the giant magnetoresistance of MBE-grown Co/Cu superlattices

A study has been carried out of the effect of interface scattering on the magnitude of the giant magnetoresistance (MR) of MBE-grown Co/Cu superlattices. The authors find that increasing interface scattering by annealing the Co/Cu superlattice leads to a progressive decrease in the magnitude of the MR. In contrast to their results, it was recently reported that annealing increases the MR for Fe/Cr superlattices. An explanation is presented in terms of the spin dependence of interface scattering which accounts both for their data for the Co/Cu system, as well as for the opposite results obtained for the Fe/Cr system.

The effect of non-local electron scattering on the current-perpendicular-to-plane-mode magnetoresistance of magnetic multilayers

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 A), M2 = Co(20 A), NM = Cu(200 A) for Fe-Co samples, and M1 = Co(10 A), M2 = Co(60 A), NM = Cu(200 A) 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.

Enhanced magnetoresistance of Co/Cu(111) wedge superlattices grown by MBE

Abstract We have grown epitaxial Co/Cu multilayer samples at different temperatures using MBE on sapphire (11 2 0)0 substrates with a 30 A 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%.

Giant magnetoresistance of dissymmetrical Co/Au multilayers

Abstract Results are presented for the magnetoresistance (MR) of sapphire/Nb 3 /Cu 3 /Au 8 /(Co/Au 8 ) n dissymmetrical multilayers built by alternating a 0.3 nm discontinuous Co layer with a 0.7 nm continuous one. The observed enhanced MR is related to a higher spin scattering asymmetry for the granular Co layers.

Exchange-coupled MBE-grown Co/Cu/Co(111) trilayers

Abstract It is shown that the high-field curvature in the in-plane and polar magnetization curves of exchange-coupled MBE-grown Co/Cu/Co(111) structures can be accurately fitted by assuming a sizeable biquadratic coupling in addition to the bilinear coupling, and the existence of a large areal fraction of ferromagnetic inclusions. Measurements on a sample with a single Co layer confirm that the curvature does not originate from the individual Co layers. Analysis of two Co/Cu/Co(111) trilayers with different CoCu interface qualities, as measured by NMR, shows significantly higher bilinear and biquadratic coupling strengths for the trilayer with the better defined CoCu interfaces. While NMR indicates reasonably good structural quality on a short lateral length scale, polar Kerr measurements as a function of interlayer thickness, and polarized neutron reflection measurements, indicate the existence of structural imperfections on a long lateral length scale.