Ultrafast spin transfer and its impact on the electronic structure

Abstract

Optically induced intersite spin transfer (OISTR) promises manipulation of spin systems within the ultimate time limit of laser excitation. Following its prediction, signatures of ultrafast spin transfer between oppositely aligned spin sublattices have been observed in magnetic alloys and multilayers. However, it is known neither from theory nor from experiment whether the band structure immediately follows the ultrafast change in spin polarization or whether the exchange split bands remain rigid. We show that ultrafast spin transfer occurs even in ferromagnetic gadolinium metal. Charge transfer between localized surface and extended valence-band states leads to a decrease of the surface spin polarization. This synchronously alters the exchange splitting of the bulk valence bands during laser excitation. Moreover, the onset of demagnetization can be tuned by over 200 fs by changing the temperature-dependent spin mixing. Our results show a promising route to ultrafast control of the magnetization, widening the impact and applicability of OISTR.

Fig. 1.. Optically induced spin transfer.

OISTR at the Gd(0001) surface occurs between bulk-like (5d6s)³ valence bands and occupied majority ( $d_{z^2}^{\uparrow }$ ) and unoccupied minority ( $d_{z^2}^{\downarrow }$ ) spin components of the Gd surface state. (A) For temperatures T → 0, states are highly spin-polarized and optical excitation leads to spin transfer from surface to bulk states. (B) When T approaches the Curie temperature T_C, spin mixing leads to band mirroring and diminishes OISTR.

Fig. 2.. Electron dynamics and transient band structure near the Γ point of Gd(0001).

Photon energies of pump and probe pulses were 0.95 and 34.2 eV, respectively. The NIR pump pulses were s-polarized, and the VUV probe pulses were p-polarized. The dashed vertical line marks zero delay. The white lines indicate the peak positions and are the fitting results of Fig. 4. Optical excitation leads to a depletion of the majority spin surface-state $d_{{\mathrm{z}}^2}^{\uparrow }$ as well as an increase of hot electrons above the Fermi level E_F. Upon laser excitation, $d_{{\mathrm{z}}^2}^{\uparrow }$ shifts upward to lower binding energy, while the minority and majortity spin bulk bands (d^↓ and d^↑) shift initially downward to higher binding energy. The oscillations in binding energy at negative pump-probe delay are of no magnetic origin (see Results).

Fig. 3.. Population dynamics.

Buildup and decay of the hot electron population above the Fermi level (integrated intensity in the range of E − E_F = 0.5 to 1.0 eV, black circles) as well as depletion and population recovery of the occupied majority spin surface state $d_{z^2}^{\uparrow }$ at $\overline{\mathrm{\Gamma }}$ (purple circles, intensity of peak maximum). The solid lines are fits to the data (see ”OISTR between surface and bulk states“ section and fig. S1).

Fig. 4.. Valence-band dynamics.

(A) Binding energy of the $5d_{z^2}^{\uparrow }$ surface state and the minority and majority spin bulk bands (5d^↓ and 5d^↑) as a function of pump-probe delay. Error bars show two standard deviations. The dashed horizontal lines are the average of the binding energies at negative pump-probe delay. The solid lines are double-exponential fits to the data. Time constants τ₁ and τ₂ are listed in Table 1. For more details, see the Supplementary Materials, section E. During optical excitation (cross-correlation indicated by gray area), the binding energy of the majority spin surface state decreases, while that of the majority spin bulk band increases. The opposite shift of the majority spin bands reflects the pump pulse–induced spin transfer from surface to bulk (see Fig. 1). In line, the $5d_{z^2}^{\downarrow }$ minority spin valence band shows a slowed down response reaching maximal binding energy not until 150-fs pump-probe delay (dashed-dotted vertical line). (B) Transient exchange spitting Δ_ex of the bulk bands. We observe a small but significant increase of Δ_ex within the first 100 fs after optical excitation (error bars show two standard deviations; see also fig. S4). With a time constant of 370 ± 50 fs, electron and phonon subsystems equilibrate (see fig. S6). On a comparable time scale the binding energies of the valence bands shift leading to a ∼10% reduced bulk exchange splitting which characterizes the transiently demagnetized state.

Fig. 5.. Transient electronic structure of Gd for different sample temperatures and spin mixing.

(A) Binding energies of the minority (red) and majority (blue) spin valence-band components as a function of pump-probe delay. (B) Corresponding evolution of the exchange splitting. The single exponential fits (solid lines) yield similar decay constants for all three temperatures, but the exchange splitting changes by 250 ± 10, 190 ± 12, and 170 ± 24 meV for 60, 100, and 200 K, respectively. Temporal overlap of pump and probe pulses (i.e., absolute delay zero) was verified by the jump in the electron temperature (gray). Error bars are, for the sake of clarity, only included for the last data point and show two standard deviations.