Daniel Steil

Engineered materials for all-optical helicity-dependent magnetic switching

The possibility of manipulating magnetic systems without applied magnetic fields have attracted growing attention over the past fifteen years. The low-power manipulation of the magnetization, preferably at ultrashort timescales, has become a fundamental challenge with implications for future magnetic information memory and storage technologies. Here we explore the optical manipulation of the magnetization in engineered magnetic materials. We demonstrate that all-optical helicity-dependent switching (AO-HDS) can be observed not only in selected rare earth-transition metal (RE-TM) alloy films but also in a much broader variety of materials, including RE-TM alloys, multilayers and heterostructures. We further show that RE-free Co-Ir-based synthetic ferrimagnetic heterostructures designed to mimic the magnetic properties of RE-TM alloys also exhibit AO-HDS. These results challenge present theories of AO-HDS and provide a pathway to engineering materials for future applications based on all-optical control of magnetic order.

Light-induced magnetization reversal of high-anisotropy TbCo alloy films

Magnetization reversal using circularly polarized light provides a way to control magnetization without any external magnetic field and has the potential to revolutionize magnetic data storage. However, in order to reach ultra-high density data storage, high anisotropy media providing thermal stability are needed. Here, we evidence all-optical magnetization switching for different TbxCo1−x ferrimagnetic alloy compositions using fs- and ps-laser pulses and demonstrate all-optical switching for films with anisotropy fields reaching 6 T corresponding to anisotropy constants of 3 × 106 ergs/cm3. Optical magnetization switching is observed only for alloy compositions where the compensation temperature can be reached through sample heating.

Band structure evolution during the ultrafast ferromagnetic-paramagnetic phase transition in cobalt

Using spin- and time-resolved XUV photoemission, researchers monitor the band structure evolution of Co during its phase transition. The evolution of the electronic band structure of the simple ferromagnets Fe, Co, and Ni during their well-known ferromagnetic-paramagnetic phase transition has been under debate for decades, with no clear and even contradicting experimental observations so far. Using time- and spin-resolved photoelectron spectroscopy, we can make a movie on how the electronic properties change in real time after excitation with an ultrashort laser pulse. This allows us to monitor large transient changes in the spin-resolved electronic band structure of cobalt for the first time. We show that the loss of magnetization is not only found around the Fermi level, where the states are affected by the laser excitation, but also reaches much deeper into the electronic bands. We find that the ferromagnetic-paramagnetic phase transition cannot be explained by a loss of the exchange splitting of the spin-polarized bands but instead shows rapid band mirroring after the excitation, which is a clear signature of extremely efficient ultrafast magnon generation. Our result helps to understand band structure formation in these seemingly simple ferromagnetic systems and gives first clear evidence of the transient processes relevant to femtosecond demagnetization.

Nanoscale magnetic imaging using circularly polarized high-harmonic radiation

We introduce laboratory-scale magneto-optical imaging with sub–50-nm resolution using high-harmonic radiation. This work demonstrates nanoscale magnetic imaging using bright circularly polarized high-harmonic radiation. We utilize the magneto-optical contrast of worm-like magnetic domains in a Co/Pd multilayer structure, obtaining quantitative amplitude and phase maps by lensless imaging. A diffraction-limited spatial resolution of 49 nm is achieved with iterative phase reconstruction enhanced by a holographic mask. Harnessing the exceptional coherence of high harmonics, this approach will facilitate quantitative, element-specific, and spatially resolved studies of ultrafast magnetization dynamics, advancing both fundamental and applied aspects of nanoscale magnetism.

Ultrafast magnetization dynamics in Co-based Heusler compounds with tuned chemical ordering

We have studied thin film samples of Co 2 FeSi and Co 2 MnSi with different degrees of chemical ordering using the time-resolved magneto-optical Kerr effect to elucidate the influence of defects in the crystal structure on magnetization dynamics. Surprisingly, we find that the presence of defects does not influence the optically induced magnetization dynamics on the ultrashort timescale (some 100fs). However, we observe a second demagnetization stage with a timescale of tens of picoseconds in Co 2 MnSi for low chemical ordering; that is, a large number of defects. We interpret this second demagnetization step as originating from scattering of mostly thermalized majority electrons into unoccupied minority defect states.

Impact of local order and stoichiometry on the ultrafast magnetization dynamics of Heusler compounds

Nowadays, a wealth of information on ultrafast magnetization dynamics of thin ferromagnetic films exists in the literature. Information is, however, scarce on bulk single crystals, which may be especially important for the case of multi-sublattice systems. In Heusler compounds, representing prominent examples for such multi-sublattice systems, off-stoichiometry and degree of order can significantly change the magnetic properties of thin films, while bulk single crystals may be generally produced with a much more well-defined stoichiometry and a higher degree of ordering. A careful characterization of the local structure of thin films versus bulk single crystals combined with ultrafast demagnetization studies can, thus, help to understand the impact of stoichiometry and order on ultrafast spin dynamics.Here, we present a comparative study of the structural ordering and magnetization dynamics for thin films and bulk single crystals of the family of Heusler alloys with composition Co2Fe1 − xMnxSi. The local ordering is studied by 59Co nuclear magnetic resonance (NMR) spectroscopy, while the time-resolved magneto-optical Kerr effect gives access to the ultrafast magnetization dynamics. In the NMR studies we find significant differences between bulk single crystals and thin films, both regarding local ordering and stoichiometry. The ultrafast magnetization dynamics, on the other hand, turns out to be mostly unaffected by the observed structural differences, especially on the time scale of some hundreds of femtoseconds. These results confirm hole-mediated spin-flip processes as the main mechanism for ultrafast demagnetization and the robustness of this demagnetization channel against defect states in the minority band gap as well as against the energetic position of the band gap with respect to the Fermi energy. The very small differences observed in the magnetization dynamics on the picosecond time-scale, on the other hand, can be explained by considering the differences in the electronic structure at the Fermi energy and in the heat diffusion of thin films and bulk crystals.

High photon flux and high repetition rate fiber-laser driven HHG

High harmonic generation (HHG) of ultrashort laser pulses is a key process for the generation of coherent extreme ultraviolet to soft X-ray radiation, which is routinely employed in a wide variety of applications [1]. Amongst the large variety photoelectron spectroscopy and microscopy particularly demands for high repetition rate and high photon flux sources [2, 3]. As such, fiber laser driven HHG has emerged as a powerful source to provide the highest photon fluxes in combination with unprecedented repetition rates [4].

Nanoscale magnetic imaging using a compact high-harmonic source

Compact sources based on high harmonic generation (HHG) offer experimental access to ultrafast dynamics, high-resolution imaging and spectroscopy, and also provide for a simultaneous probing of element-specific spin and charge carrier dynamics. Here, we extend the capabilities of compact EUV sources to the nanoscale imaging of magnetic structures by using circularly polarized harmonics in conjunction with Fourier transform holography.

All-optical control of ferromagnetic thin films and nanostructures: Competition between polarized light and applied magnetic field

The interplay of light and magnetism has been a topic of interest since the original observations of Faraday and Kerr where magnetic materials affect the light polarization. While these effects have historically been exploited to use light as a probe of magnetic materials there is increasing research on using polarized light to alter or manipulate magnetism. For instance deterministic magnetic switching without any applied magnetic fields using laser pulses of the circular polarized light has been observed for specific ferrimagnetic materials [1,2]. Very recently, we demonstrated, optical control of ferromagnetic materials ranging from magnetic thin films to multilayers and even granular films (fig.1) being explored for ultra-high-density magnetic recording [3,4]. Our finding shows that optical control of magnetic materials is a much more general phenomenon than previously assumed. These results challenge the current theoretical understanding and will have a major impact on data memory and storage industries via the integration of optical control of ferromagnetic bits. In this presentation we will study in detail the combine effect of applied magnetic field and polarized light. Depending on the light polarization and the applied field direction the two effect can add or cancel each other. The influence of both the helicity and the applied on domain structure is studied for different [Co/Pt] multilayers.

Electronic Scattering Dynamics and Ultrafast Magnetization Dynamics

A dynamical model for Elliott-Yafet type scattering of carriers and its importance for the demagnetization dynamics after ultrafast optical excitation is reviewed. It is pointed out that the demagnetization in 3d ferromagnets as well as recent experimental results on “novel” materials are still in need of a microscopic explanation.