Ultrafast electron and spin dynamics

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Forschungszentrum Jülich Institut für Festkörperforschung

- Institut für Elektronische Eigenschaften -

Ultrafast electron and spin dynamics

The ultimate limit of switching a magnetic moment or spin depends on the interactions between electron system, spin system, and lattice. The microscopic excitation and relaxation mechanisms involve a time scale far below one picosecond.

In our studies we are addressing different classes of materials where dynamical phenomena are particularly interesting from theoretical and practical points of view. One example is materials combining different type of magnetic ordering such as ferro-, ferri- antiferromagnets. The aim is to investigate the influence of the type of magnetic ordering on spin and charge dynamics. Another example is materials with high degree of spin polarization at the Fermi level such as half-metallic alloys. These materials are attractive to be used as spin injectors and spin-filtering elements. The presence spin degree of freedom due to large difference in density of states for spin up and spin down channels opens a new way to probe mechanisms driving spin and charge dynamics in metals. To do that we are employing an approach based on spin-selective optical excitation. Along with these studies our research is focused on investigations of strongly correlated electron systems. The effect of electron correlation is important, for example, in some transition metal oxides and responsible for a set of interesting phenomena the most prominent of which is the metal-isolator phase transition. The investigation of correlated electron systems is an important issue because opens new perspectives to manipulate spin and charge dynamics through cooperative phenomena.

pump

probe

h 2 h

± H

h

Fig.1 Principle of linear and nonlinear pump-probe time-resolved measurements

Another very intriguing class of materials is exchange coupled structures displaying unidirectional magnetic anisotropy. In spite of much progress in theoretical modeling and experimental studies achieved in the last years the spin and carrier dynamics in these materials was scarcely studied and needs to be investigated. Particularly interesting basic aspects we are addressing is dynamics of ultrafast unpinning in the vicinity of the critical points such as blocking, Curie or Neel temperatures. It is know that the important role in the settling of exchange bias plays structural and magnetic properties of AFM/FM interface. That implies the necessity of using interface sensitive methods. Two complementary optical methods are being used in our experiments. These are the time-resolved linear pump-probe method based on measuring linear magneto-optical Kerr effect and nonlinear one based on the second harmonic generation measurements (Fig. 1). The last one is known as a powerful tool to probe electronic and magnetic properties of surfaces and interfaces. Combination of too methods provides us information about volume and interfacial dynamical properties of biased structures. The investigations of ultrafast phenomena are exploiting ultrafast laser sources.

Our experimental facilities include:

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• Ti/Saphire laser, pulse duration 100 fs, pulse energy 1nJ, λ=740- 850 nm

• Optical regenerative amplifier, pulse duration 120 fs, pulse energy 1mJ

• Frequency doubler λ=370- 425 nm

• Precision mechanical delay line with 1µm resolution