Evidence for collective exchange modes in Co/Pd multilayers observed by Brillouin light scattering (abstract)

Abstract

Recently, exchange-dominated collective spin-wave excitations have been predicted for multilayered structures consisting of alternating magnetic and nonmagnetic layers.1 These types of collective excitations exist for nonzero propagation wave vectors if the magnetic layers are exchange coupled across the spacer layers.1 The modes are the collective counterpart to the optic dipolar-exchange mode observed in magnetic double layers by Vohl, Barnás and Grünberg.2 For a multilayer with N periods of bilayer thickness Λ there exist N modes which, in the absence of exchange coupling, form the well-known band of dipolar collective spin-wave excitations. The modes are dominantly surface-mode-like at the upper band edge and mostly bulk-mode-like at the lower band edge. With the onset of exchange coupling the stack bulk modes are shifted to higher frequencies and become exchange-mode-like in character. In the full exchange coupling limit these modes become the so-called standing spin waves of the total multilayer stack, and their frequencies depend on the stack thickness NΛ. The stack surface mode in this limit becomes the surface Damon–Eshbach mode of a single film of thickness NΛ. Brillouin light-scattering experiments have been performed on magnetron-sputtered Co/Pd multilayers.3,4 The number of atomic layers (AL) is the same in each Co and Pd layer. The number of periods was chosen to be N=8 and 30, with Λ varying between 4 and 140 Å. In the case of N=30 a strong increase of spin-wave frequencies for all but the stack surface mode is observed for d<8 AL. Decreasing d0 increases the interlayer exchange coupling and the band of collective bulklike modes increases in frequency to become a band of exchange-dominated modes. Depending on the strength of the coupling (i.e., spacer layer thickness), this band can have frequencies higher than that of the stack surface mode. From the crossing regime the maximum exchange coupling length is estimated to be 10 Å. For samples with N=8, collective exchange modes were not observed. Model calculations show that for N=8 the expected frequency increase near the onset of interlayer exchange coupling is much steeper than in the N=30 case, which might prohibit their experimental observation. Also, the cross section is reduced by a factor of 4 due to the smaller total stack thickness.