D. Greig

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).

Magnetization pinning at a Py/Co interface measured using broadband inductive magnetometry

Broadband ferromagnetic resonance responses for metallic single-layer and bilayer magnetic films with total thicknesses smaller than the microwave magnetic skin depth have been studied. Two different types of microwave stripline transducers were used to excite and detect magnetization precession: a coplanar waveguide and a microstrip line both with characteristic width larger than the free propagation path for traveling spin waves along the film. Both transducers show efficient excitation of higher-order standing spin wave modes across the film thickness in samples 30–91 nm thick. The ratio of amplitudes of the first standing spin wave to the fundamental resonant mode is independent of frequency for single-layer permalloy films. In contrast, we find a strong variation in the amplitudes with frequency for cobalt–Permalloy bilayers and the ratio is strongly dependent on the ordering of layers with respect to a stripline transducer. Most importantly, cavity ferromagnetic resonance measurements on the same sa...

The low temperature thermoelectric power of some palladium and platinum alloys

Abstract The thermoelectric power, S, of a number of palladium and platinum alloys has been measured in the temperature range 2°K to 120°K. For alloys of palladium or platinum with their neighbouring elements it has been possible to correlate the sign of the diffusion component of thermopower Sd in the residual resistance temperature range with the slope of the density of states versus concentration curves. Furthermore, there is excellent numerical agreement between the observed values of Sd/T and those calculated by a method based on the rigid band model for all the platinum-gold alloys investigated and for the palladium-silver alloys apart from one which contained less than 1 % silver. However, the sign of Sd was not predicted correctly for a palladium-platinum alloy. Each of the specimens exhibits a large positive phonon drag component of thermopower which persists to high temperatures and is not greatly reduced on alloying. These two features as well as the positive sign are the result of phonon-induc...

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.

Magnetic multilayers of Fe/Au: role of the electron mean free path

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 A) but a different thickness for the Au layers (in the range 20-50 A). 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.

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.

Exchange anisotropy pinning of a standing spin-wave mode

Standing spin waves in a thin film are used as sensitive probes of interface pinning induced by an antiferromagnet through exchange anisotropy. Using coplanar waveguide ferromagnetic resonance, pinning of the lowest energy spin-wave thickness mode in Ni80Fe20/Ir25Mn75 exchange-biased bilayers was studied for a range of Ir25Mn75 thicknesses. We show that pinning of the standing mode can be used to amplify, relative to the fundamental resonance, frequency shifts associated with exchange bias. The shifts provide a unique “fingerprint” of the exchange bias and can be interpreted in terms of an effective ferromagnetic film thickness and ferromagnet-antiferromagnet interface anisotropy. Thermal effects are studied for ultrathin antiferromagnetic Ir25Mn75 thicknesses, and the onset of bias is correlated with changes in the pinning fields. The pinning strength magnitude is found to grow with cooling of the sample, while the effective ferromagnetic film thickness simultaneously decreases. These results suggest that exchange bias involves some deformation of magnetic order in the interface region.

Giant magnetoresistance and super-paramagnetism in Co/Au multilayers

Abstract We present results for the magnetoresistance and magnetisation of MBE grown Co/Au multilayers with very thin Co thicknesses. These multilayers exhibit superparamagnetic properties similar to the non-multilayer granular systems when the Co layer is discontinuous. By controlling the growth rate, it was found that we can vary the Co island size and therefore the blocking temperature of superparamagnetic particles.

The electrical resistivity of transition metals at high temperatures

Abstract The electrical resistivity of transition metals at high temperatures is surprisingly insensitive to the details of the electronic states and structure. We suggest that this behaviour originates in the ‘tightly-bound’ nature of d-orbitals and on examining the matrix elements for scattering between states we conclude that ‘single site’ approximations are not likely to be appropriate for the calculation of the transport properties of structurally-disordered transition metals.

The lattice thermal conductivity of some palladium and platinum alloys

Abstract The thermal conductivity of a number of palladium-silver, platinum-gold and platinum-iridium alloys has been measured in the temperature range 2°K to 120°K. Below about 30°K the lattice thermal conductivity is an order of magnitude smaller than in noble metal alloys of similar concentrations. The low values are explained in terms of the strong electron-phonon scattering which is present in these metals, although it is concluded that it is impossible to decide at present whether the important electronic transitions are s to d or d to d. At higher temperatures, measurements from transition metal alloys are largely independent of solute concentration, and tend to the values observed in noble metal alloys. It is inferred that at ∼100°K, the lattice resistance arises mainly from phonon-phonon U-processes.