Coherent Optical Phonons and Parametrically Coupled Magnons Induced by Femtosecond Laser Excitation of the Gd(0001) Surface

Coherent spin dynamics in the THz domain coupled to a coherent phonon is observed in the time-resolved second harmonic response of the Gd(0001) ferromagnetic metal surface. An LO phonon of 2.9 THz is excited by a transient charge displacement at the surface caused by resonant absorption of a fs laser pulse in the exchange-split surface state. This lattice vibration modulates the interlayer distance inducing a coherent variation of the exchange interaction between spins in adjacent layers. The resulting magnetization dynamics is considered as optical magnon wave packets coupled to the phonon.

Figure 1

Valence electron states in Gd(0001) taken from photoemission ○, inverse photoemission (solid line), and scanning tunneling spectroscopy data above the Fermi level (solid line, filled) [14]. The exchange-split surface state (filled area) appears around the Fermi level in addition to bulk bands. Indicated are the two main absorption channels for 1.52 eV pump photons and the SH probing scheme.

Figure 2

(a) Time dependence of the even (upper panel) and odd SH (lower panel) response measured in pump-probe mode for a 20 nm Gd(0001) film at 90 K using $815\mathrm{n}\mathrm{m}/35\mathrm{f}\mathrm{s}$ laser pulses. Plotted are the quantities defined in Eqs. (3, 4). The change of the linear reflectivity is displayed for comparison in the upper panel (thin solid line). The inset shows the experimental scheme of transversal geometry, where the magnetization is oriented perpendicular to the plane of incidence. (b) Coherent part of the even and odd SH fields extracted from (a) after smoothing [21] and subtracting the incoherent background. Solid lines represent a fit to the data. The inset shows the corresponding Fourier transformation.

Figure 3

(a) Quenching of the even SH oscillations by Y overlayers measured at 800 nm. (b) Photoemission spectra of bare Gd(0001) and covered by nominally 1 and 3 ML coverage (photon energy 36  eV).

Figure 4

(a) Temperature dependence of the static magnetic SH contrast $(I^{\uparrow }-I^{\downarrow })/(I^{\uparrow }+I^{\downarrow })$ and scaled initial amplitudes of the oscillating contributions of ${\Delta }_{\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n}}$ and ${\Delta }_{\mathrm{o}\mathrm{d}\mathrm{d}}$ determined from time-dependent data [cf. Fig. 2b], measured at 800 nm at different temperatures. The dashed line is to guide the eye. (b) Illustration of a half cycle of coupled lattice and spin oscillations for the magnetic moments of two adjacent Gd atoms in the surface ($S_1$) and the subsurface layer ($S_2$), separated by the interlayer distance $d_{12}$ that oscillates with amplitude $\delta$. The initial increase of $J$, indicated by a maximum of ${\Delta }_{\mathrm{o}\mathrm{d}\mathrm{d}}$ in Fig. 2b, is a consequence of the initial decrease in $d_{12}$ leading to a larger value of $J$ [15].