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Notes: The M2,3 edge of Rh in Co-Rh alloys shows measurable x-ray magnetic circular dichroism. Such dichroism is not present, however, for analogous alloys of Co-Ru. The induced Ru magnetic moment, if any, is thus demonstrated to be significantly smaller than the induced Rh moment in otherwise similar Co alloys.

Notes: The magnetic structures which occur in (Co/Ni81Fe19/Co)/Cu multilayer films showing giant magnetoresistance have been investigated using electron microscopy. Rather similar fine domains, with sub-μm dimensions, were found in films comprising 14 and 6 magnetic layers. Whilst the observed structure depended greatly on the magnetic history of the sample, a combination of differential phase contrast imaging and low angle diffraction allowed an estimate to be made of the extent to which neighboring magnetic layers were aligned antiparallel to each other. For both samples typically two layers were found to have parallel alignment leading to the possibility that departures from the expected antiferromagnetic behavior are more prevalent at the surfaces rather than in the bulk of the multilayer.

Notes: The structural changes that accompany the development of GMR (giant magnetoresistance) at low ((approximately-less-than)10 Oe) fields in annealed magnetic multilayers are of current interest because of potential applications of such structures in sensors. In this paper we report a study of the development of GMR in [111]-oriented multilayers comprising ferromagnetic films of a mixture of Ag and permalloy (NixFe1−x, x∼0.8) alternating with Ag spacer films. The multilayers were grown by molecular beam epitaxy (MBE) on Pt(111) seed films on sapphire (0001) substrates at temperatures in the range 20 to 200 °C. The structure of the multilayers was investigated using x-ray diffraction and electron microscopy. For a series of multilayers grown with nominally identical ferromagnetic and spacer layer thicknesses, the magnetoresistance is found to be strongly dependent on both growth temperature and subsequent annealing temperature. The multilayers exhibited a negative magnetoresistance in the as-grown state which more than doubled when the growth temperature was increased from 20 to 100 °C. However, the highest magnetoresistance (peak 5.6%; maximum slope 0.4% per Oe) was obtained by annealing (at 400 °C) multilayers grown at 100 °C. Transmission electron microscopy studies of such multilayers showed no evidence for discontinuities or penetration of the ferromagnetic films by Ag along grain boundaries. Thus, we conclude that discontinuous or granular multilayers with complete phase separation are not necessary for GMR with low saturation fields.

Notes: The structural and magnetic properties of [111]-oriented multilayers comprising ferromagnetic films of Permalloy-silver alternating with Ag spacer films are described. The multilayers are grown by molecular-beam epitaxy on Pt(111) seed films on sapphire (0001) substrates at temperatures in the range 25–175 °C. For a series of multilayers with similar bilayer periods ((approximately-equal-to)50 A(ring)) the magnetoresistance (MR) is found to be strongly dependent on both growth temperature and subsequent annealing temperature. The multilayers exhibit a negative magnetoresistance in the as-grown state which more than doubles when the growth temperature is increased from 25 to 100 °C; however, the highest MR (peak 5.6%; maximum slope 0.4% per Oe) is obtained by annealing (at 400 °C) multilayers grown at 100 °C. The primary effects of annealing are an improvement of structural order, partial segregation of Ag from the ferromagnetic films into adjacent Ag films, a slight decrease in laminar order, and a reduction in long-wavelength roughness of the multilayer interfaces. No evidence is found for discontinuities in the magnetic layers with the highest MR.

Notes: Uncertainties in the application of the 〈Lz〉/〈Sz〉 sum rule to experimental spectra of V, Cr, Mn, Fe, Co, and Ni are discussed. An important contribution to these uncertainties is the possible presence of dichroism due to diffuse magnetic moments, which are known to exist in Fe, Co, and Ni.

Notes: The existence of oscillatory interlayer exchange coupling of ferromagnetic layers via (111)-oriented noble metal spacer layers is controversial. We present evidence from magnetic and giant magnetoresistance studies for well-defined antiferromagnetic interlayer coupling in single crystalline (111) permalloy/Au multilayers. Four oscillations in the coupling are observed as the Au spacer layer thickness is increased. The oscillation period is (approximately-equal-to)10 A(ring) which is significantly shorter than the period of (approximately-equal-to)11.5 A(ring) predicted in Ruderman–Kittel–Kasuya–Yosida based models. Similar oscillatory interlayer coupling is found in polycrystalline permalloy/Au multilayers prepared by dc magnetron sputtering. The interlayer coupling strength is significantly weaker in the polycrystalline as compared to the (111)-oriented crystalline samples. In both cases the coupling strength is weaker than in comparable structures containing Ag, for which the coupling is weaker than in similar structures containing Cu. The weakness of the antiferromagnetic interlayer coupling via Au leads to very low saturation fields, lower than for all other noble and transition metals. Indeed, the saturation fields are as low as just a few Oersted for sufficiently thick Au layers. Consequently, we find giant magnetoresistance values of (approximately-equal-to)1%/Oe or greater at room temperature in polycrystalline permalloy/Au multilayers. These values are the highest values yet reported in multilayer structures and are comparable to or greater than those recently reported in discontinuous permalloy/Ag multilayers.

Notes: The spin structure of antiferromagnetic Fe/Cr multilayers has been investigated by polarized neutron reflectivity. Measurements were taken on freshly sputtered films as well as films annealed at different temperatures. For annealing temperatures up to 350 °C adjacent Fe layers were found to remain antiferromagnetic, but the coupling strength gradually decreases. In multilayers annealed at higher temperature both antiferro- and ferromagnetic phases are present. In all cases the magnetoresistance is proportional to the amount of antiferromagnetism. The presence of off-specular, diffuse scattering around the antiferromagnetic Bragg peak indicates that the magnetic domains are laterally limited; however, their size is not correlated directly to the magnetoresistance.

Notes: We have studied the magnetothermopower S(H,T) for a series of Co/Cu multilayers with a constant Co layer thickness of 10 A(ring) but with Cu layer thickness varying between 9 and 21 A(ring). The thermopower is negative, and its magnitude increases with magnetic field. We also found the magnetothermopower to be linear in the conductance for all samples. In two samples studied, the linear relationship holds from room temperature down to 50 K, with the proportionality constant linear in temperature. The zero-field thermopower is roughly the same for all samples despite different Cu layer thickness. This behavior can be understood from the spin-dependent interface scattering model.

Notes: Recently the interlayer exchange coupling strength in Co/Ru multilayered structures was found to oscillate between ferromagnetic and antiferromagnetic coupling as a function of the Ru layer thickness.1,2 However, in the ferromagnetic coupling regimes, the determination of the interlayer coupling constant, A12, cannot be performed using standard magnetometry methods. Here we demonstrate that Brillouin light scattering from thermally activated spin waves in multilayered structures is applicable for the determination of the interlayer exchange coupling strength both in the ferromagnetic and antiferromagnetic regimes.2 In multilayered structures consisting of alternating magnetic and nonmagnetic layers, dipolar spin-wave modes exist within each magnetic layer (so-called Damon–Eshbach modes), which couple across the intervening nonmagnetic layer. Due to the coupling between the magnetic layers, which is dipolar as well as of exchange type, the spin-wave modes form a band of collective spin-wave excitations.3–5 Two different types of collective modes exist: (i) The so-called stack surface mode, for which the spins of all magnetic layers precess in phase. The frequency of this mode is independent of any exchange coupling, but is sensitive to the net magnetization of the multilayer stack. (ii) The collective bulk modes. Their frequencies depend both on the interlayer exchange constant as well as on the layer-to-layer distribution of the directions of the magnetization.6,7 In addition, in the regime of large antiferromagnetic coupling, a new collective spin-wave mode is found in theoretical investigations, which is reminiscent of the "optic'' high-frequency spin-wave mode of antiferromagnetic bulk material.6,7 This mode goes soft with decreasing canting angle between neighboring magnetic layers.The spin-wave frequencies, and therefore A12, are found to oscillate as a function of the Ru layer thickness in the Co/Ru multilayers with a period of 11.5 A(ring) and in the permalloy/Ru multilayered system with a period of 12 A(ring). In comparison to the Co/Ru multilayers we find for the permalloy/Ru multilayers characteristic differences: First, the amplitude of the oscillation is smaller by a factor of two compared to the Co/Ru system. This effect may be attributed to the reduced saturation magnetization of permalloy. Second, we find evidence for an additional, short-period oscillation with the first minimum in the spin-wave frequencies, i.e., correspondingly in A12, at dRu=8 A(ring). Its periodicity is estimated as between 5–8 A(ring). To our knowledge this is first evidence for the presence of a short-period oscillation in a sputtered multilayered system. From the data, however, the decay in oscillation amplitude cannot be extracted due to the rather small number of observed oscillations and the comparably large error in the determination of A12. A more detailed presentation of these data is reported elsewhere.8

Notes: The microstructure and magnetic properties of phase-separated Fe-Ag and Co-Ag granular alloy films, grown epitaxially on (001) NaCl substrates and exhibiting giant magnetoresistance (GMR), have been investigated. Surprisingly, two Fe-Ag films of similar composition grown under identical conditions are found to have substantially different microstructures yet display similar GMR. The microstructure of the films is characterized by Fe-rich or Co-rich regions, respectively, 350–700 nm in extent, surrounded by a Ag-rich matrix. Within the Ag-rich regions, the Fe concentration varies from 20 to 25 at % and the Co concentration is (approximately-equal-to)16 at %. Within these regions essentially pure fcc Co particles and bcc Fe particles are in parallel and rotated epitaxial alignment respectively with the fcc silver matrix. The Co and Fe particles are (approximately-equal-to)15–25 A(ring) in diameter. It is these small particles which most likely account for the giant magnetoresistance exhibited by these alloys. This suggests that a size distribution of magnetic particles, sharply peaked at the optimum size with limited bulk segregation, might give rise to larger GMR values.