A. Fognini

Nanoscale spin reversal by non-local angular momentum transfer following ultrafast laser excitation in ferrimagnetic GdFeCo

Ultrafast laser techniques have revealed extraordinary spin dynamics in magnetic materials that equilibrium descriptions of magnetism cannot explain. Particularly important for future applications is understanding non-equilibrium spin dynamics following laser excitation on the nanoscale, yet the limited spatial resolution of optical laser techniques has impeded such nanoscale studies. Here we present ultrafast diffraction experiments with an X-ray laser that probes the nanoscale spin dynamics following optical laser excitation in the ferrimagnetic alloy GdFeCo, which exhibits macroscopic all-optical switching. Our study reveals that GdFeCo displays nanoscale chemical and magnetic inhomogeneities that affect the spin dynamics. In particular, we observe Gd spin reversal in Gd-rich nanoregions within the first picosecond driven by the non-local transfer of angular momentum from larger adjacent Fe-rich nanoregions. These results suggest that a magnetic materials microstructure can be engineered to control transient laser-excited spins, potentially allowing faster (~ 1 ps) spin reversal than in present technologies.

Ultrafast reduction of the total magnetization in iron

Surprisingly, if a ferromagnet is exposed to an ultrafast laser pulse, its apparent magnetization is reduced within less than a picosecond. Up to now, the total magnetization, i.e., the average spin polarization of the whole valence band, was not detectable on a sub-picosecond time scale. Here, we present experimental data, confirming the ultrafast reduction of the total magnetization. Soft x-ray pulses from the free electron laser in Hamburg (FLASH) extract polarized cascade photoelectrons from an iron layer excited by a femtosecond laser pulse. The spin polarization of the emitted electrons is detected by a Mott spin polarimeter.

Fe on W(110), a stable magnetic reference system

Time resolved pump probe experiments with ultra short infrared pump and x-ray photoemission probe pulses require a stable magnetic reference system with reproducible magnetic properties. In search of such a system we found in iron on tungsten an ideal sample. The coercive field of this system remains constant at 12.2±1 Oe between 15 and 25 monolayers. Kerr effect measurements and scanning electron microscopy with polarization analysis images prove that the magnetization switches from single domain to single domain state. Capping with Au increases the coercive field and prevents the Fe layer from deterioration.

The role of space charge in spin-resolved photoemission experiments

Spin-resolved photoemission is one of the most direct ways of measuring the magnetization of a ferromagnet. If all valence band electrons contribute, the measured average spin polarization is proportional to the magnetization. This is even the case if electronic excitations are present, and thus is of particular interest for studying the response of the magnetization to a pump laser pulse. Here, we demonstrate the feasibility of ultrafast spin-resolved photoemission using free electron laser (FEL) radiation and investigate the effect of space charge on the detected spin polarization. The sample is exposed to the radiation of the FEL FLASH in Hamburg. Surprisingly, the measured spin polarization depends on the fluence of the FEL radiation: a higher FEL fluence reduces the measured spin polarization. Space-charge simulations can explain this effect. These findings have consequences for future spin-polarized photoemission experiments using pulsed photon sources.

The influence of the excitation pulse length on ultrafast magnetization dynamics in nickel

The laser-induced demagnetization of a ferromagnet is caused by the temperature of the electron gas as well as the lattice temperature. For long excitation pulses, the two reservoirs are in thermal equilibrium. In contrast to a picosecond laser pulse, a femtosecond pulse causes a non-equilibrium between the electron gas and the lattice. By pump pulse length dependent optical measurements, we find that the magnetodynamics in Ni caused by a picosecond laser pulse can be reconstructed from the response to a femtosecond pulse. The mechanism responsible for demagnetization on the picosecond time scale is therefore contained in the femtosecond demagnetization experiment.

Magnetic pulser and sample holder for time- and spin-resolved photoemission spectroscopy on magnetic materials

A compact coil setup, in conjunction with a high power current pulser, is presented; developed especially for time- and spin-resolved photoemission spectroscopy measuring the sample in magnetic remanence at room temperature. A novel approach is presented where the sample is switched in the stray field of a coil pair. This enables the electrical biasing of the sample without altering the electron trajectories due to field gradients introduced by the coils. The pulser driving the coils reaches a peak power of 1 MW at 1 kA with a switching frequency up to 10 kHz suitable for experiments, for example, with state of the art repetition rates of femtosecond laser systems.

The topografiner with energy analysis

The topografiner technology, developed originally at the National Bureau of Standards (now National Institute of Standards and Technology) with the aim of measuring the surface micro-topography, is less widespread than scanning tunneling microscopy but has a remarkable property: the electrons can escape the tip-surface junction. We have recently used topografiner imaging to map the surface of various metals and semiconductors with (almost) nanometer lateral spatial resolution. In this paper, we describe our attempt to endowing the NIST topografiner with an energy analysis of the electrons escaping the junction, with the aim of performing spectroscopy with nanometer spatial resolution.

Ultrafast demagnetization in iron: Separating effects by their nonlinearity

The laser-driven ultrafast demagnetization effect is one of the long-standing problems in solid-state physics. The time scale is given not only by the transfer of energy, but also by the transport of angular momentum away from the spin system. Through a double-pulse experiment resembling two-dimensional spectroscopy, we separate the different pathways by their nonlinear properties. We find (a) that the loss of magnetization within 400 fs is not affected by the previous excitations (linear process), and (b) we observe a picosecond demagnetization contribution that is strongly affected by the previous excitations. Our experimental approach is useful not only for studying femtosecond spin dynamics, but can also be adapted to other problems in solid-state dynamics.

Free Electron Lasers

Data storage applications of magnetism utilize dynamic processes on sub-nanosecond time scales and on length scales of less than 100 nm. Magnetization dynamics on the sub-picosecond time scale has been observed and may lead the way to novel devices. In order to investigate these processes it is essential to have a method-combining sub-micrometer spatial with sub-picosecond temporal resolution. Ultrafast x-ray pulses now offer this possibility. The radiation of free electron lasers (FELs) consists of sub-picosecond x-rays with unprecedented peak brightness. This chapter explains the basic principles of FELs. The peak intensity offers the possibility of imaging a fluctuating system (for example a ferromagnetic domain structure) within a single pulse. These imaging techniques as well as spectroscopic methods are explained. In order to perform time resolved experiments using large scale facilities it is essential to understand the techniques used to determine the timing between the pump laser and the x-ray probe pulses, a topic laid out.