Oscillatory interlayer exchange coupling of Co/Ru and permalloy/Ru multilayers investigated by Brillouin light scattering (abstract)

Abstract

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 Å and in the permalloy/Ru multilayered system with a period of 12 Å. 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 Å. Its periodicity is estimated as between 5–8 Å. 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