Exploring laser-induced interlayer spin transfer by an all-optical method

We investigate the influence of spin currents during ultrafast laser-induced demagnetization of magnetic bilayer structures by a new all-optical method to measure material- and/or depth-resolved magnetization dynamics. By describing the magneto-optical response of the bilayers in the complex Kerr plane, it is shown that the material-specific magnetization dynamics of the individual layers can be measured by a marginal adjustment to any conventional time-resolved magneto-optical Kerr effect setup. We use this technique to trace superdiffusive spin currents in magnetic Ni/Fe bilayers, providing new insight on its importance to ultrafast laser-induced demagnetization.

Figure 1

(a) Example of a Kerr vector $\Phi$ of a single magnetic layer in the plane spanned by the ellipticity $\varepsilon$ and rotation $\theta$. (b) Example of the total Kerr vector ${\Phi }_{\mathrm{tot}}$ of a sample consisting of two individual layers with individual Kerr vectors ${\Phi }_1$ and ${\Phi }_2$. On performing a measurement by projecting on the $P$ axis, which is orthogonal to ${\Phi }_1$, only ${\Phi }_2$ turns up in the magneto-optical signal.

Figure 2

Double modulation TR-MOKE setup tailored for material and/or depth sensitivity by the addition of a quarter wave plate. See text for details.

Figure 3

(a) and (b) show the basic concept of the experiments performed in this article. Fs laser pulses induce spin currents between Ni and Fe. By measuring the magnetization dynamics of the Ni and Fe layer for parallel (a) and antiparallel (b) alignment, the contributions of these spin currents to the demagnetization process can be determined. (c) Typical branch of a hysteresis loop for the bilayer under investigation. The direction of the magnetization in the Ni and Fe layers are denoted by the arrows, displaying four different configurations of the bilayer structure.

Figure 4

(a) Measured values of $V_1\mathrm{f}$ for Fe and Ni as obtained from fitting hysteresis loops as a function of $\alpha$. The lines are fits to Eq. (1). In (b) one branch of a hysteresis loop is shown for $\alpha =-7^{\circ }$, showing only a contribution from Fe to the MO contrast. In (c) the same is shown at $\alpha =-{68}^{\circ }$, where only a contribution from Ni is seen.

Figure 5

Demagnetization traces of Ni (a) and Fe (b) for the P and AP configuration of the magnetic bilayer. The lines are fits with an analytical solution of the three temperature model [28], while the upper bounds for the errors due to mixing of the Ni and Fe signals are indicated by the shaded areas. For Ni the time for maximum demagnetization is indicated by vertical bars. While for Ni a significant change in the demagnetization properties is observed between the P and AP configuration, the traces for Fe are almost identical.