Ultrafast demagnetization of metals: Collapsed exchange versus collective excitations

We have investigated the magneto-optical response of Fe epitaxial films by femtosecond pump-probe polarimetry in a broad probe spectral region ($1.8\text{–}2.6$ eV). From the extrapolated photoinduced variation of the conductivity tensor, spin and charge dynamics have been disentangled. In particular, the analysis of the off-diagonal tensor element rules out any appreciable modification of the electronic band structure upon laser excitation and suggests that ultrafast demagnetization is determined by collective excitations, i.e., spin fluctuations. Our experimental evidence provides significant insight into the microscopic mechanisms governing the complex spin dynamics of metals in the subpicosecond time scale.

Figure 1

(a) Time-resolved, $p$-pol Kerr angle of the 50 nm Fe(001) film for a probe photon energy of 2.36 eV (solid dots: rotation; open dots: ellipticity). (b) Transient reflectivity (solid dots: $s$-pol projection; open dots: ${45}^{\circ }$-pol). Extrapolated dynamics of (c) the diagonal and (d) the off-diagonal elements of the conductivity tensor (solid dots: absorptive part; open dots: dispersive part).

Figure 2

(a) Transient dynamics of the conductivity tensor [same data of Figs. 1 and 1 on a shorter time windows]: The data are normalized for comparative purposes. (b) Differences between dispersive and absorptive parts of the normalized data shown in (a) (also the differences are normalized). The dashed line is a phenomenological fit. Dynamics of (c) diagonal and (d) off-diagonal elements of $\mathit{}\mathbf{\sigma }$. The lines represent the fitting model.

Figure 3

Calculated Drude-like components of (a) diagonal and (b) off-diagonal elements of the conductivity tensor of Fe assuming a Drude time $\tau =3$ fs (blue solid line: absorptive part; red dashed line: dispersive part).

Figure 4

(a) Diagonal and (b) off-diagonal elements of the conductivity tensor in the complex plane (the abscissa is absorptive, and the ordinate is dispersive) for different probe energies. Each curve represents the path of the element from its unperturbed value ($-1$ ps delay) to 1 ps past the pump pulse (dashed arrows indicate the time direction). (c) and (d) report the data shown in (a) and (b), respectively, after removing the transient electron-phonon components. The large arrow in (c) points towards the origin of the complex plane.

Figure 5

(a) Electronic density of state (DOS) of Fe, from Ref. [50] (solid line: majority spin; dashed line: minority spin). The quantities $n_u+n_d$ and $n_u-n_d$ represent the calculated total charge density and the spin polarization, respectively. (b) Density of state of Fe, assuming a collapsed exchange splitting leading to a 30% reduction of the spin polarization. Calculated absorptive part of (c) diagonal and (d) off-diagonal elements of the $\mathit{}\mathbf{\sigma }$ using the DOS shown in (a) (solid lines) and in (b) (dashed lines). (e) The calculated variation $\Delta {\sigma }_{xy}^a/{\sigma }_{xy}^a$ (line) vs photon energy is compared with the experimental $|\Delta {\sigma }_{xy}/{\sigma }_{xy}|$ (dots).