Effect of interfaces on Gilbert damping and ferromagnetic resonance linewidth in magnetic multilayers

In magnetic multilayers, the presence of sharp interfaces causes a local increase of the interaction between spin waves and conduction electrons. This leads to an increase of the Gilbert spin-damping parameter near an interface. In turn, the ferromagnetic-resonance linewidth is increased over its value in single-layer films. In addition, the precession of magnetic spins during ferromagnetic resonance induces conduction-electron transitions from the spin-up to the spin-down band. As a result, the spin-up Fermi level differs from the spin-down Fermi level by an amount Delta<(<mu>)over bar>. At high precession amplitudes, the existence of Delta<(<mu>)over bar> causes a measurable decrease of the Gilbert parameter. At precession-cone angles exceeding 4 degrees, the Gilbert parameter returns nearly to its single-layer value. Ferromagnetic-resonance line shapes are predicted to be non-Lorentzian, narrower and sharper near the top. This line-narrowing effect increases with increasing microwave power. The effect of Delta<(<mu>)over bar> spreads into the entire multilayer, so that precession in one magnetic layer can cause a reduction or increase of the Gilbert parameter in other layers, or even a spontaneous spin precession in these layers. (C) 2001 American Institute of Physics.