Ultrafast magnetization dynamics in metallic heterostructures consists of a combination of local demagnetization in the ferromagnetic constituent and spin-dependent transport contributions within and in between the constituents. Separation of these local and non-local contributions is essential to obtain microscopic understanding and for potential applications of the underlying microscopic processes. By comparing the ultrafast changes of the polarization rotation and ellipticity in the magneto-optical Kerr effect (MOKE) we observe a time-dependent magnetization profile M (z, t) in Co/Cu(001) films by exploiting the effective depth sensitivity of the method. By analyzing the spatio-temporal correlation of these profiles we find that on time scales before hot electron thermalization (< 100 fs) the transient magnetization of Co films is governed by spin-dependent transport effects, while after hot electron thermalization (> 200 fs) local spin-flip processes dominate.

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We present a computer simulation model for the investigation of electron promotion processes in atomic collision cascades in metals. The model combines molecular dynamics and molecular orbital calculations to describe the formation of hot electrons in close atomic collisions. We apply this model to a set of collision cascades initiated by the impact of a 5-keV silver atom onto an Ag͑111͒ surface. The calculations show that about 15% of the bombarding energy originally introduced into the solid is dissipated into the generation of hot electrons in close collisions. Furthermore, we find that the nascent excitation energy spectrum closely resembles a power law f͑E exc ͒ ϰ E exc −␦ with exponents ␦ Ϸ 2-3.

The influence of the projectile impact angle on secondary ion formation was studied using a computer simulation model applied to the bombardment of an amorphous silver crystal by 5-keV Ag atoms. The model employs a molecular dynamics (MD) scheme for the description of particle dynamics within the atomic collision cascade. The electronic degree of freedom is treated within the framework of a free electron gas model incorporating kinetic excitation by electronic friction and electron promotion. Transport of the excitation energy away from the spot of generation is treated by a diffusive approach. In combination with a rate equation model for electronic charge transfer an individual ionization probability α + is assigned to each sputtered particle. The results reveal that the average ionization probability of sputtered atoms increases upon the transition from normal to oblique incidence. The dependence of α + on the emission velocity of ejected atoms is traced back to the temporal structure of the excitation profile induced after projectile impact.

We present a molecular dynamics (MD) based computer simulation model for particle bombardment of metal surfaces. In addition to the description of the atomic collision cascade initiated by the particle impact, our model incorporates the electronic degree of freedom of the target and therefore is capable of simultaneously predicting secondary ion formation in sputtering as well as ion-bombardment-induced kinetic electron emission (KEE). Hence, our simulation concept may be regarded as an approach to close the gap between classical MD simulations usually excluding electronic effects and ab initio many-body quantum mechanics breaking down for system sizes needed to describe atomic collision cascades. In this study, we apply our model to the keV self-bombardment of a (111)-oriented silver surface in order to predict kinetic electron emission yields.

The ionization probabilities of particles emitted from a silver (111) surface after bombardment with 5-keV silver atoms are calculated by means of a hybrid computer simulation model combining molecular dynamics with kinetic electronic excitation. The simulations produce a four-dimensional electron excitation profile parameterized by an effective electron temperature T e ! r; t À Á which is then used to calculate an individual ionization probability a + for each sputtered atom. The results demonstrate the significant role of the time structure of the electron temperature profile.

In order to investigate the different role of kinetic and potential projectile energy for secondary ion formation, the authors have measured the ionization probability of indium atoms sputtered from a clean indium surface under irradiation with rare gas (Xeq+) ions of different charge states q at the same kinetic impact energy of 20 keV. In this energy range, the kinetic energy of the projectile is predominantly deposited via nuclear stopping, leading to a collision-dominated sputtering process. The authors find that the ionization probability increases significantly if a highly charged ion is used as a projectile, where the ionization energy becomes comparable to or even exceeds the kinetic energy, indicating that a higher level of electronic substrate excitation induced by the potential energy stored in the projectile can boost the secondary ion formation process. This experimental result is discussed in terms of microscopic model calculations describing the secondary ion formation process. At the same time, the authors observe a significant change of the emission velocity distribution of the sputtered particles, leading to a pronounced low-energy contribution at higher projectile charge states. It is shown that this “potential sputtering” contribution strongly depends on surface chemistry even under conditions where the surface is dynamically cleaned by interleaved 5 keV Ar+ ion bombardment.

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