Spin-wave modes in antiparallel magnetized ferromagnetic double layers

For the first time the spin-wave-mode spectra of a symmetric dipolar coupled ferromagnetic double-layer system have been measured and compared in both the parallel and the antiparallel magnetized state by means of inelastic light scattering by use of a Brillouin-type spectrometer. Two symmetric Permalloy (${\mathrm{Ni}}_{80}$ ${\mathrm{Fe}}_{20}$) films of 25 nm thickness have been prepared with equivalent easy axes of an M-induced uniaxial anisotropy, but with different coercivities, $H_{C1}$=9.2 Oe and $H_{C2}$=30.4 Oe. By proper choice of remagnetizing fields applied along the easy axes, the parallel magnetized state (PM), $M_1$+$M_2$=2M, and antiparallel magnetized state (APM), $M_1$-$M_2$=0, could be realized. For both states the attributed spin-wave modes have been studied and compared with theoretical predictions. In particular, the predicted asymmetric, high-energy spin-wave modes of the APM state lying in the microwave frequency range (15 GHz) were shown to exist, to agree well with theory, and to be switched on and off reversibly by relatively small remagnetizing fields (15 Oe).