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

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Abstract
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 Δμ̄. At high precession amplitudes, the existence of Δμ̄ causes a measurable decrease of the Gilbert parameter. At precession-cone angles exceeding 4°, 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 Δμ̄ 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.