1. Spin dynamics in magnetic alloys and multilayers using femtosecond laser high harmonics

The next important step in understanding of spin dynamics in magnetic alloys and multilayers requires examination of optically induced transients with femtosecond time resolution and element-selectivity. Requirements for such measurements were only recently met by strong progress in laser-based extreme ultraviolet light (XUV) sources generating femtosecond pulses with photon energies reaching up to 72 eV. Such high-energy photons are produced when an intense, laser pulse is focused into a collection of gas atoms, generating high-order harmonics of the fundamental light. Tuning the XUV probe beam to a chosen absorption edge, results in a resonant increase of the signal for the corresponding element.
In our recent experiments, we combined element selectivity with femtosecond time resolution to study magnetic response of Ni/Ru/Fe multilayers. By exciting the multilayer with near infrared laser light we observed the evolution of magnetization response in the Ni and Fe layers simultaneously but separately using synchronized XUV probe pulses (see the Figure below this section). Following the excitation, we detected a fluence-dependent magnetization quenching. Unexpectedly, we also observed magnetization enhancement in the Fe layer for parallel alignment of Fe and Ni magnetization. We ascribed the observed response to the optically generated superdiffusive spin currents between the layers (see Rudolf et al., Nature Communication 2012) – a very new physical phenomenon described theoretically only a few months before the experimental observation.

 

2. Ultrafast magnetization dynamics in magnetic thin films, complex alloys and multilayers measured using visible light

In order to investigate the time evolution of the average magnetization of a thin film after optical excitation, we employ an all-visible pump-probe setup. The magnetic response is probed at a central wavelength of 400nm (3eV) using the magneto-optical Kerr effect (MOKE) in longitudinal or polar geometry, while the excitation of the electron system is carried out at a central wavelength of 800nm (1.5eV).

A femtosecond laser amplifier system with pulse energies of up to 1mJ, a repetition rate of 1kHz and pulse durations down to 50fs is used as a light source for the experiment. The mirror alignment can be easily switched between polar and longitudinal geometries to address either the out-of-plane or in-plane components of the sample magnetization.

The experiment can study optically-induced dynamics with a temporal resolution below 100fs and is designed to provide important complementary information to our IR- pump XUV-probe experiments. It provides access to femtosecond magnetization dynamics in the valence and conduction bands of 3d transition metals and their alloys, magnetic multilayer systems, as well as novel rare earth–transition metal materials.

 

3. All-optical switching in magnetic superlattices

It has been known for some time that magnetic moment of a certain ferrimagnetic alloys can be completely reversed using ultrafast pulses of circularly polarized light without an external magnetic field [3.1]. Although this effect has been first observed in relatively complex ferrimagnetic alloys more recent discovery shows that also ferromagnetic superlattices can be switched by light [3.2]. This extraordinary discovery opens door to their applications in novel magnetic memories and spintronics applications.

In our project we grow ferromagnetic superlattices on silicon and glass substrates for reflection and transmission geometries, respectively and investigate their switching depending on material and laser beam properties. A femtosecond laser oscillator and a quarter wave plate are used as a light source for the experiments (see Figure below). The response of the region swept by laser beam is analyzed using either Kerr or Faraday imaging or magnetic force microscopy. The example of the optically switched magnetic sample (darker lines marked by arrows) is shown in the figure below as inset.

[3.1] C. D. Stanciu et al., “All-optical magnetic recording with circularly polarized light“, Phys. Rev. Lett. 99, 4 (2007)
[3.2] C.-H. Lambert et al., “All-optical control of ferromagnetic thin films and nanostructures“, Science 345, 6202 (2014)

4. Magnetic resonant scattering of high-order laser harmonics in the extreme ultraviolet spectral range

Ultrashort pulsed light sources in the extreme ultraviolet (XUV) spectral range (20-250eV) provide a unique possibility to study laser-induced spin dynamics in nanometer-sized magnetic domains. Therefore, we set up a resonant magnetic scattering (RMS) experiment using a laser-driven HHG source. Our experimental approach includes a multilayer mirror pair as a monochromator and a CCD camera to detect the scattered photons in transmission geometry. The resulting image is directly related to the domain structure. With the advantage of the high coherence and superior time resolution of the laser-generated high-order harmonics, RMS allows jitter-free pump-probe studies of the dynamics of magnetic domain quenching with femtosecond temporal and nanometer lateral resolution. Additionally, element-selectivity can be also employed in this method to support time-resolved investigations of complex alloys and multilayer systems.