Ultrafast behavior of induced and intrinsic magnetic moments in CoFeB/Pt bilayers probed by element-specific measurements in the extreme ultraviolet spectral range

The ultrafast and element-specific response of magnetic systems containing ferromagnetic $3d$ transition metals and $4d/5d$ heavy metals is of interest both from a fundamental as well as an applied research perspective. However, to date no consensus about the main microscopic processes describing the interplay between intrinsic $3d$ and induced $4d/5d$ magnetic moments upon femtosecond laser excitation exist. In this work we study the ultrafast response of CoFeB/Pt bilayers by probing element-specific, core-to-valence-band transitions in the extreme ultraviolet spectral range using high harmonic radiation. We show that the combination of magnetic scattering simulations and analysis of the energy- and time-dependent magnetic asymmetries allows us to accurately disentangle the element-specific response in spite of overlapping Co and Fe ${\mathit{M}}_{2,3}$ as well as Pt ${\mathit{O}}_{2,3}$ and ${\mathit{N}}_7$ resonances. We find a considerably smaller demagnetization time constant as well as much larger demagnetization amplitudes of the induced moment of Pt compared to the intrinsic moment of CoFeB. Our results are in agreement with enhanced spin-flip probabilities due to the high spin-orbit coupling localized at the heavy metal Pt, as well as with the recently formulated hypothesis that a laser-generated, incoherent magnon population within the ferromagnetic film leads to an overproportional reduction of the induced magnetic moment of Pt.

Figure 1

Schematic of the T-MOKE geometry with an incident angle $\theta ={45}^{\circ }$ of the $p$-polarized XUV radiation. After reflection, the XUV pulses are spectrally dispersed by a flat field concave grating and detected by a CCD camera. An external magnetic field $B$ is aligned perpendicularly to the incoming $p$-polarized XUV light. The sample is excited by laser pulses at $\lambda =800\mathrm{nm}$ in a nearly collinear geometry. The inset shows the depth-dependent absorption profiles of the CoFeB/Pt and CoFeB/MgO/Pt samples.

Figure 2

Measured and calculated static magnetic asymmetry of (a) CoFeB/MgO/Pt and (b) CoFeB/Pt as a function of photon energy. The color-coded dots mark the photon energies of the time-resolved measurements, while the solid lines are static measurements with a continuous HHG spectrum. In (b) we additionally show the calculated magnetic asymmetry stemming from CoFe (dotted line) and Pt (dashed line). The largest relative contributions to the magnetic asymmetry of Pt are not only found around the Pt ${\mathrm{O}}_{2,3}$ and Pt ${\mathrm{N}}_7$ edges, but also at $57\mathrm{eV}$, where contribution of Co and Fe approaches zero, cf. panel (c).

Figure 3

Magnetic asymmetry as a function of time after optical excitation. Harmonics H35 and H39 in panel (a) and (b) are dominated by the dynamics of Fe and Co, respectively. For the CoFeB/Pt system, HHG peaks H33 and H37 [panel (c)] are sensitive to both layers, CoFeB and Pt, and are characterized by a sign change of the time-resolved asymmetry. The asymmetries of the HHG peaks H41 [panel (c)], H43, H45, and H47 [panel (a)] are dominated by Pt contributions and, different from H35 and H39, exhibit a 100% reduction of $A$. For the CoFeB/MgO/Pt sample system, all shown harmonics H33–H47 show the same relative demagnetization amplitude and represent the response of Fe or Co. Harmonic H47, close to the Pt ${\mathrm{N}}_7$ edge, shows no evidence of an increase, i.e., no evidence for tunneling of spin-polarized carriers from the CoFeB to Pt via the MgO layer, cf. panel (f).

Figure 4

Normalized asymmetry $\mathrm{\Delta }A/A_0$ as a function of excitation fluence for three selected time delays, $t=1,5,500$ $\mathrm{ps}$. In panel (a) we show H35 (Fe) and H39 (Co) for the CoFeB/MgO/Pt and in (b) additionally H47 (Pt) for the CoFeB/Pt system. Pt demagnetizes significantly more efficiently and remains out of equilibrium with the CoFeB layer for up to $500\mathrm{ps}$.