We have applied a polarized light optical microscope in reflective mode with a Bi-substituted yttrium-iron-garnet indicator film with in-plane anisotropy for visualization of the magnetostatic fields produced by nanostructured magnetic CoNiCu/Cu electrodeposited multilayers. By analysis of the magneto-optical stray field image, detailed information is obtained not only on the as-grown multilayer magnetic structure but on its change during the magnetization reversal processes. The influences of crystal lattice defects and nonuniformity of the nonmagnetic spacers thicknesses on the domain wall nucleation and motion are studied. Peculiarities of the re-magnetization of antiferromagnetically exchange coupled multilayers are discussed, including real-time observations of domain wall creep in a constant applied field.

The magnetic properties of heavy rare earth silicide GdSi2-x thin films are investigated as a function of the annealing temperature of the films. Resistivity measurements reveal in the two films annealed at high and low temperatures, but for a short time, the existence of two transition temperatures corresponding to the presence of an ordered and a disordered structure. In the film annealed at high temperature for a long time, only one transition temperature occurs. It corresponds to a magnetic structure transformation.

We show that the Kubo formula can be used to calculate the non-local electrical conductivity of layered systems from first principles. We use the Layer Korringa Kohn Rostoker method to calculate the electronic structure and the Green function of Co/Cu/Co trilayers within the local density approximation to density functional theory. This Green function is used to calculate the conductivity through the Kubo formula for both majority and minority spins and for alignment and anti-alignment of the Co moments on either side of the Cu spacer layer. This allows us to determine the giant magnetoresistance from first principles. We investigate three possibilities for the scattering in Co/Cu/Co: (1) equal electron lifetimes for Cu, majority spin Co, and minority spin Co, (2) equal electron lifetimes for majority and minority Co, weaker scattering in Cu and spin dependent interfacial scattering, (3) electron lifetimes for majority and minority spin cobalt proportional to their Fermi energy densities of states and spin dependent interfacial scattering.

The electronic structure of magnetic multilayers is expected to play an important role in determining their transport properties. We explain how the conductance through a ballistic point contact is related to simple geometrical projections of the Fermi surface. The essential physics is first discussed for simple model systems and then realistic results for magnetic metallic multilayers based on first principles band structure calculations are presented. The electronic structure is shown to make an important contribution to the perpendicular giant magnetoresistance.

The giant magnetoresistance in magnetic superlattices for the current perpendicular to and in the layer planes is studied within a unified semi-classical approach that is based on the Boltzman equation with exact boundary conditions for the spin-dependent distribution functions of conduction electrons. We show that the main differences between the in-plane and perpendicular-to-plane magnetoresistance result from the fact that they originate from different interface processes responsible for spin-dependent scattering. A correlation between the giant magnetoresistance and the superlattice magnetization is also discussed and it is shown that its study has much potential for yielding information about properties of spin-dependent scattering in magnetic superlattices.

We study the magnetoresistance of [Fe/Cr]30/MgO superlattices grown by molecular beam epitaxy at a various magnetic field directions. The theory of the orientation dependence of the effect is developed. It is shown that the magnetic field strength dependence of magnetoresistance can be calculated for arbitrary orientation of magnetic field if this dependence is known for in-plane and perpendicular-to-plane magnetic fields. It is noted that the magnetization curve can be obtained by making use of the results of the magnetoresistance measurements.

We use the model of free electrons with random point scatterers (FERPS) to calculate the electrical conductivity and giant magnetoresistance (GMR) for FeCr multilayer systems and compare our results with the experimental values. Our analysis suggests that the primary cause of the GMR in FeCr systems is regions of interdiffusion near the interfaces. We find that in the samples analyzed, these regions of interdiffusion occupy about 8.5Å of the magnetic layer near each interface.

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.

Co-Cu-NiFe-Cu weakly coupled multilayers, were patterned into stripes with dimensions from 1×6μm2 to 2×20μm2. After proper biasing using a DC field created by an adjacent bias conductor, these structures show large linear field spans (±1500e), with MR values up to 6%. These can be used for current or position sensor applications. Sensor response to AC fields was studied up to 1Mhz, using the bias conductors as sources of the AC field. We find that the signal output is reduced above 10 to 100kHz. For comparison, sensors were also fabricated out Co-Cu AF-coupled multilayer (3×100μm2 structures) needed for field detection up to several kOe.

We present magnetoresistance (MR) data of high-vacuum magnetron sputtered NiFe/Cu multilayers (NiFe=Ni80Fe20) grown on Si(100) substrates with a Cu buffer layer and compare these with earlier results on Co/Cu(100) multilayers [1]. Measured MR values are directly proportional to the antiferromagnetically coupled fraction in the multilayers. Extrapolating to full antiparallel alignment, we can make a reliable comparison of the MR with the magnetoresistance model of Levy, Zhang, and Fert [2,3]. For the NiFe/Cu multilayers the deduced spin-asymmetry parameters are for interface and bulk scattering, respectively. Although much smaller than in our Co/Cu multilayers, , it is still the spin dependence of the interface scattering that is the main cause for the large MR values.

The effect of doping the magnetic layer on the magnetoresistance and saturation field of epitaxial, sputter-deposited FeCr/Cr(001) and CoCu/Cu(001) multilayers was examined. FeCr/Cr films with a composition of [14 Å Fe1−xCrx(001)/8 Å Cr(001)]50 where x was varied from 0 to 0.5 and CoCu/Cu films with a composition of [8 Å Co1−xCux(001)/21 Å Cu(001)]40 where x was varied from 0 to 0.2 were deposited onto single crystal MgO(001). The bilayer period and epitaxial orientation of these films were determined by x-ray diffraction. In the FeCr/Cr system the room temperature magnetoresistance was constant at approximately 31% for x≤0.2 and decreased for larger Cr concentrations. The spin-dependent scattering (Δp) is increased over the x=0 value for x=0.1 and x=0.2. The magnetic field required to saturate these multilayers decreases linearly with increasing Cr concentration. The net result is that the sensitivity of these films is increased by Cr doping of the Fe layer. In the CoCu/Cu system the room temperature magnetoresistance, saturation field, and saturation magnetization decrease with the addition of Cu to the Co layer. In contrast to the FeCr/Cr system Δp does not increase with doping of the ferromagnetic layer. However, as in the FeCr/Cr films the sensitivity of these multilayers is increased with respect to that of the x=0 CoCu/Cu multilayer.

We report on the first studies of the giant magnetoresistance and oscillatory coupling in epitaxial Fe/Cr(111) multilayers. A series of samples were grown on hydrogen terminated Si(111) substrates at room temperature by UHV electron beam evaporation; with the thickness of Fe layer fixed at 30 Å, and the thickness of Cr layer varied from 10–47 Å. Giant magnetoresistance (GMR) is observed at 4.2 Å in these samples, with a maximum value of 13% for a Cr layer thickness of 13 Å. The associated oscillatory coupling is comparable to that reported in other crystallographic orientations in terms of both its period and phase.

Ferromagnetic/ferromagnetic Ni/Co multilayers were prepared by DC-magnetron sputtering with component layer thicknesses ranging from 40 Å down to 5 Å. Structural characterizations by x-ray diffractometry show a well-defined compositional modulation along the film growth direction and a preferred (111) crystalline orientation. A longitudinal magnetoresistance ΔR/R over 2.7% with a sensitivity of ~0.11%/Oe was measured at room temperature in small fields less than 20 Oe. The highest room temperature sensitivity obtained in this system was 0.16%/Oe. Magnetoresistive sensitivity was found to vary inversely with the number of bilayers in the multilayers. The magnetic anisotropy of the films as determined by MOKE magnetometry is correlated to the magnetoresistance and indicative of an AMR effect.

This paper presents the results of giant magnetoresistance (GMR), magnetic and microstructural investigations of sputter-deposited Co/Cu multilayers. The multilayers were designed to be at the second maximum of the oscillatory exchange coupling and were deposited onto a range of ion beam irradiated (111) Si substrates giving a transition from a random orientation to a highly oriented (111) texture. The randomly oriented multilayers exhibited mixed coupling and ∼20% GMR, which had components arising from irreversible and reversible magnetization changes, whereas the highly oriented (111) multilayers were almost completely ferromagnetically coupled and showed very little GMR.

We have investigated the surface roughness and the grain size in giant magnetoresistance (GMR) spin valve multilayers of the general type: FeMn/Ni80Fe20Co/Cu/Co/Ni80Fe20 on glass and aluminum oxide substrates by scanning tunneling microscopy (STM). The two substrates give very similar results. These polycrystalline GMR multilayers have a tendency to exhibit larger grain size and increased roughness with increasing thickness of the metal layers. Samples deposited at a low substrate temperature (150 K) exhibit smaller grains and less roughness. Valleys between the dome-shaped individual grainsare the dominant form of roughness. This roughness contributes to the ferromagnetic, magnetostatic coupling in these films, an effect termed “orange peel” coupling by Néel. We have calculated the strength of this coupling, based on our STM images, and obtain values generally within about 20% of the experimental values. It appears likely that the ferromagnetic coupling generally attributed to so-called “pinholes” in the Cu when the Cu film thickness is too small is actually “orange peel” coupling caused by these valleys.

We have studied the effect of an artificially intermixed region grown at the interfaces of Co/Cu spin valves with uncoupled layers. Two different structures are used: exchange-biased spin valves and engineered spin valves in which two layers are antiferromagnetically coupled and a third layer, on top of this system, is not coupled to the other two. It is shown that structural effects, induced by variation of the deposition parameters and by the intermixing can play an important role. Since the present study uses sputtered layers an intrinsic initial intermixing of 4-5Å is already present. For both types of spin valves Gp, ΔG and ΔR/R all show a gradual decrease when the nominal thickness of the total intermixed region is enlarged from 0 to 36Å. Also when the initial degree of intermixing is decreased by sputtering at higher Arpressure, Gp, ΔG and ΔR/R still show a gradual decrease as a function of intermixed layer thickness. Combined with the fact that there is no difference between an intermixed region of thickness t at one Co/Cu interface or intermixed regions of thickness t/2 at two interfaces, this indicates that the electron scattering in the intermixed region is predominantly spin independent, although this region preserves a magnetic moment.

In order to improve the thermal stability of magnetic multilayer “spin valve” structures, we have measured the magnetic and magnetoresistive properties of a number of samples with the general structure of NiO/Co/Cu/Co/Cu/Co/NiO as a function of annealing time at 250 °C. The magnetoresistance (MR) of the samples annealed in air decreases proportionally to the square root of the annealing time. For samples annealed in a vacuum, the decrease in magnetoresistance is reduced, but not eliminated. Magnetometry of a vacuum annealed NiO/Co/NiO sample shows a magnetization reduction and a coercivity increase which suggest oxidation of the NiO-biased “outer” Co layers of the spin valve structure. For increasing NiObiased Co layer thickness, we show enhanced thermal stability and even increasing MR with annealing time for samples with the thickest outer Co layers.

NiFeCo/Cu multilayers fabricated by an improved magnetic field sputtering were investigated in order to achieve the soft GMR (giant magnetoresistance) with a high sensitivity at low magnetic fields. A magnetic field was applied to the film during sputter-deposition, and its field direction was changed alternately from layer to layer. Such an alternate field sputtering is called hereafter layer-by-layer magnetic field sputtering. The best GMR characteristics (large MR at low magnetic fields) were achieved when the angle between the directions of magnetic field applied to neighboring two magnetic layers was 90°. As one of the speculation, it has been considered that the result is attributed to the induced composite magnetic anisotropy which causes the magnetization to occur more dominantly by spin rotation than by domain wall movement.

We have investigated magnetoresistance properties of (100) epitaxial, (11) textured and polycrystalline spin valve heterostructures of the form Ni80Fe20/Cu/NiNi80Fe20/Fe50Mn50 on (100) Si substrates by ultra high vacuum (UHV) ion beam sputtering at room temperature. Magnetoresistance was measured as a function of Cu interlayer thickness (ti) with 10 Å ≤ti ≤ 100 Å and the maximum was found at 20 Å in the case of (100) epitaxial spin valves. Highly (11) textured spin valves with heterostructure configurations similar to the (100) spin valves were found to have a slightly lower magnetoresistance than the (100) heterostructures, but the functional dependence of the magnetoresistance on ti was very similar.

Interface mixing during the sputtering process by energetic neutral bombardment was found to significantly affect the magnetoresistance. Samples were made under various sputtering conditions (gas pressure, ion beam energy, target and substrate configuration) that could enhance or suppress high energy neutral bombardment of the growing film surface. Samples made under the conditions that suppressed neutral bombardment showed higher magnetoresistance and more abrupt interfaces as confirmed by small angle X-ray diffraction analysis of interface mixing by energetic neutral bombardment during sputter deposition.

Early experiments to define oscillations in the CIP magnetoresistance (CIP-MR) of Ag/Co analogous to those for tcu < 5nm in Cu/Co were unsuccessful. The MR in this region was very small. Later experiments by Araki using thin (0.6nm ) Co layers produced much larger MRs and lead us to look at the MRs of similar samples more closely. We conclude that the large MR of such samples is associated with the discontinuous nature of the Co layers. The object of the present paper is to combine the high MR associated with the thin Co layers with the field dependence governed by the magnetization reversal in thick, and magnetically soft permalloy (Py) layers. We have measured the CIP-MR of sputtered samples of the [Co(O.4 nm)/Ag(tAg)/Py(tpy)/Ag(tAg)]×15 system with tAg ranging from 1.05 to 4nm and with tpy = 2 or 4nm. We obtain MRs at 4.2K as large as 35% in less than 100e with slopes as high as 5%/Oe. With CPP measurements slopes as high as 10%/Oe have been obtained. Squid magnetometer measurements indicate that, as the temperature increases, there is a crossover to superparamagnetic behaviour and a resulting gross deterioration of the MR slopes at room temperature. Efforts to increase the room temperature sensitivity are described. Detailed measurements of the CPP-MR of the [Co(0.4nm)/Ag(4nm)/Py(tpy)/Ag(4nm)]x20 series of multilayers are consistent with a two spin band model modified to take account of the granular nature of the Co.