Unifying ultrafast demagnetization and intrinsic Gilbert damping in Co/Ni bilayers with electronic relaxation near the Fermi surface

Abstract

The ability to controllably manipulate the laser-induced ultrafast magnetic dynamics is a prerequisite for future high-speed spintronic devices. The optimization of devices requires the controllability of the ultrafast demagnetization time ${\ensuremath{\tau}}_{M}$ and intrinsic Gilbert damping ${\ensuremath{\alpha}}_{\mathrm{intr}}$. In previous attempts to establish a relationship between ${\ensuremath{\tau}}_{M}$ and ${\ensuremath{\alpha}}_{\mathrm{intr}}$, the rare-earth doping of a permalloy film with two different demagnetization mechanisms was not a suitable candidate. Here, we choose Co/Ni bilayers to investigate the relations between ${\ensuremath{\tau}}_{M}$ and ${\ensuremath{\alpha}}_{\mathrm{intr}}$ by means of the time-resolved magneto-optical Kerr effect (TR-MOKE) via adjusting the thickness of the Ni layers, and obtain an approximately proportional relation between these two parameters. The remarkable agreement between the TR-MOKE experiment and the prediction of a breathing Fermi-surface model confirms that a large Elliott-Yafet spin-mixing parameter ${b}^{2}$ is relevant to the strong spin-orbital coupling at the Co/Ni interface. More importantly, a proportional relation between ${\ensuremath{\tau}}_{M}$ and ${\ensuremath{\alpha}}_{\mathrm{intr}}$ in such metallic films or heterostructures with electronic relaxation near the Fermi surface suggests the local spin-flip scattering dominates the mechanism of ultrafast demagnetization, otherwise the spin-current mechanism dominates. It is an effective method to distinguish the dominant contributions to ultrafast magnetic quenching in metallic heterostructures by simultaneously investigating both the ultrafast demagnetization time and Gilbert damping. Our work can open an avenue to manipulate the magnitude and efficiency of terahertz emission in metallic heterostructures such as perpendicular magnetic anisotropic Ta/Pt/Co/Ni/Pt/Ta multilayers, and then it has an immediate implication for the design of high-frequency spintronic devices.