ALBERT

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 magnetic and magnetotransport properties of several series of sandwiches consisting of two ferromagnetic layers (Ni, Co, Ni80Fe20) separated by a noble metal (Cu, Ag, Au) are described. In order to vary the relative orientation of the magnetizations of the two ferromagnets, one of them was constrained by exchange anisotropy (e.g., NiFe/Fe50Mn50). The ferromagnetic layers are magnetically soft and not coupled antiparallel, giving very large changes of resistance at low fields. At room temperature relative changes ΔR/R of 4.1% in 10 Oe for Si/Ta 50 A(ring)/NiFe 62 A(ring)/Cu 22 A(ring)/NiFe 40 A(ring)/FeMn 70 A(ring)/Ta 50 A(ring) and 8.7% in 20 Oe has been obtained for a structure based on Co/Cu/Co layers. The magnetoresistance versus the thickness of the ferromagnetic layer shows a broad peak near 80 A(ring) for Ni, Co and NiFe, demonstrating the importance of bulk rather than interfacial spin-dependent scattering, in contrast to Fe/Cr multilayers. The magnetoresistance decreases exponentially with increasing interlayer (Cu and Au) thickness, indicating that the magnetoresistance is due to the exchange of polarized electrons from one ferromagnetic layer to the other. The variation with Ag interlayer thickness is different for structural reasons.

Notes: The magnetic properties of Co/Pt multilayer films prepared by dc sputtering have been investigated using ferromagnetic resonance and a vibrating sample magnetometer. The thickness of the Co layer is constant at 25 A(ring) while the thickness of the Pt layer varies from 0.9 to 18 A(ring). The saturation magnetization per unit volume of Co is nearly constant for all the films and close to that of bulk Co. In the ferromagnetic resonance spectra, the presence of two resonance modes having an amplitude ratio that varies substantially with the Pt thickness is strong evidence that a significant coupling between cobalt layers exists even at 18 A(ring) of Pt. For Pt thicknesses greater than 3.6 A(ring), a uniaxial perpendicular magnetic anisotropy field component increases with Pt thickness and saturates at about 9.0 A(ring). If the induced anisotropy energy is attributed to a surface anisotropy energy at the Co/Pt interface, it has a value Ks of approximately 0.57 erg/cm2. The temperature dependence of the magnetic hysteresis loop and the ferromagnetic resonance spectrum indicate an increase in the uniaxial anisotropy field with decreasing temperature.

Notes: Lattice and magnetic x-ray diffraction from a 2000 A(ring) thick film of Dy, sandwiched by LaF3 films on a GaAs(111) substrate, are reported. The structure was grown by molecular beam epitaxy with the c axis of the Dy parallel to the LaF3 c axis and GaAs [111] axis. We find that the c-axis lattice constant of the Dy has a similar temperature dependence to bulk Dy from room temperature to about 110 K, but below this, the film is different from bulk. The transition to ferromagnetic ordering at ∼86 K exhibits temperature hysteresis which is also evident in the magnetic x-ray scattering and SQUID magnetometry data. This hysteresis may arise from a combination of the strain-energy barrier accompanying the transition and magnetic inhomogeneities in the film. The temperature dependence of the magnetic wave vector is qualitatively similar to bulk, although a weaker dependence is observed below ∼110 K, which is possibly caused by magnetoelastic effects. The magnetic coherence length (310 A(ring)) is significantly shorter than the structural coherence length (730 A(ring)).

Notes: Lattice and magnetic x-ray diffraction from a 2000 A(ring) thick film of Dy, sandwiched by LaF3 films on a GaAs(111) substrate, are reported. The structure was grown by molecular beam epitaxy with the c axis of the Dy parallel to the LaF3 c axis and GaAs [111] axis. We find that the c-axis lattice constant of the Dy has a similar temperature dependence to bulk Dy from room temperature to about 110 K, but below this, the film is different from bulk. The transition to ferromagnetic ordering at ∼86 K exhibits temperature hysteresis which is also evident in the magnetic x-ray scattering and SQUID magnetometry data. This hysteresis may arise from a combination of the strain-energy barrier accompanying the transition and magnetic inhomogeneities in the film. The temperature dependence of the magnetic wave vector is qualitatively similar to bulk, although a weaker dependence is observed below ∼110 K, which is possibly caused by magnetoelastic effects. The magnetic coherence length (310 A(ring)) is significantly shorter than the structural coherence length (730 A(ring)).

Notes: Exchange biased magnetic tunnel junction (MTJ) structures are shown to have useful properties for forming magnetic memory storage elements in a novel cross-point architecture. MTJ elements have been developed which exhibit very large magnetoresistive (MR) values exceeding 40% at room temperature, with specific resistance values ranging down to as little as ∼60 Ω(μm)2, and with MR values enhanced by moderate thermal treatments. Large MR values are observed in magnetic elements with areas as small as 0.17 (μm)2. The magnetic field dependent current–voltage characteristics of an MTJ element integrated with a silicon diode are analyzed to extract the MR properties of the MTJ element itself. © 1999 American Institute of Physics.

Notes: Various uncoupled, magnetoresistive multilayer systems show quasiequilibrium resistance noise associated with ||dR(H)/dH||. The size of this 1/f noise, particularly in comparison with Johnson noise in various frequency ranges, is discussed for all the different systems. All the observed samples exhibit resistive Barkhausen noise, allowing the measurement of domain sizes very similar to those observed in coupled samples. In addition, the asymmetric hysteresis of the Barkhausen noise in uncoupled multilayers suggests the same type of hysteretic domain structure observed in antiferromagnetically coupled multilayers. © 1996 American Institute of Physics.

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: 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.