Toward ultrafast magnetic depth profiling using time-resolved x-ray resonant magnetic reflectivity

Abstract

During the last two decades, a variety of models have been developed to explain the ultrafast quenching of magnetization following femtosecond optical excitation. These models can be classified into two broad categories, relying either on a local or a non-local transfer of angular momentum. The acquisition of the magnetic depth profiles with femtosecond resolution, using time-resolved x-ray resonant magnetic reflectivity, can distinguish local and non-local effects. Here, we demonstrate the feasibility of this technique in a pump–probe geometry using a custom-built reflectometer at the FLASH2 free-electron laser (FEL). Although FLASH2 is limited to the production of photons with a fundamental wavelength of 4 nm ( ), we were able to probe close to the Fe L 3 edge ( ) of a magnetic thin film employing the third harmonic of the FEL. Our approach allows us to extract structural and magnetic asymmetry signals revealing two dynamics on different time scales which underpin a non-homogeneous loss of magnetization and a significant dilation of 2 Å of the layer thickness followed by oscillations. Future analysis of the data will pave the way to a full quantitative description of the transient magnetic depth profile combining femtosecond with nanometer resolution, which will provide further insight into the microscopic mechanisms underlying ultrafast demagnetization.

Magneto-optical detection of non-collinear magnetization states in ferromagnetic multilayers

“…However, being an element specific magnetometry technique is a constraint for many multilayer structures, as layer dependent magnetometry can only be achieved with XMCD if the magnetic layers have different elemental compositions. Other x-ray-based techniques, such as x-ray magnetooptical Kerr effect or x-ray magnetic reflectivity, are presently being developed to serve as powerful tools for investigating magnetization depth profiles of specific magnetic elements in thin films and multilayer [16,19,20] given their potential to achieve sub-nanometer depth resolution. However, these techniques have not yet demonstrated their full potential and furthermore, their data analysis will depend on magnetization depth profile models.…”

Ultrafast laser-induced magneto-optical changes in resonant magnetic x-ray reflectivity

“…Soft-x-ray resonant magnetic reflectivity (XRMR) is a powerful method for the investigation of magnetic properties of thin films and surfaces that can provide at the same time information about the structure and morphology of the sample as well as magnetization depth profiles [17][18][19][20][21] and has already been also used successfully for the study of ultrafast demagnetization by magnetic circular dichroism [22][23][24][25]. For the study of ultrafast processes in samples on crystalline substrates or with buried layers, it typically provides much higher signals than totalelectron-yield detection of x-ray absorption, which is important considering the often very low average fluence of sources of pulsed x rays.…”

Ultrafast element- and depth-resolved magnetization dynamics probed by transverse magneto-optical Kerr effect spectroscopy in the soft x-ray range

“…While common optical and transport-based magnetometry can only provide an indirect access to the spatial dependence of the relevant non-local spin dynamics, ultrafast resonant magnetic soft x-ray scattering [16] is a unique technique which combines magnetic contrast with element selectivity and a potential for depth resolution. This has been exploited in first steps towards ultrafast magnetization depth profiling [17,18], however, so far without being able to provide a quantitative analysis of the spatially inhomogeneous evolution of the magnetization with sub-picosecond temporal resolution. One challenge in order to exploit resonant magnetic scattering for nanoscale magnetization profiling is related to the complex interplay of angleand/or photon energy-dependent absorption, refraction, interlayer reflections, and interference effects.…”