Just as in flashlight photography, short yet intense flashes of x-rays allow to record images or x-ray diffraction patterns which “freeze” motion that is slower than the duration of the x-ray pulse. The advantage of x-rays over visible light is that nanometer scale objects can be discerned due to the short wavelength of x-rays. Furthermore, if the wavelength of the x-rays is tuned corresponding to particular energies for electronic transitions, one can obtain unique contrast, allowing for example to make the magnetization of different domains within a material visible. The fraction of x-rays scattered from a magnetic domain pattern, however, decreases when the x-ray intensity in the pulse is increased. While this effect had been observed already in the very first images of magnetic domains recorded at a free electron x-ray laser in 2012, a variety of different explanations had been put forward to explain this loss in scattered x-ray intensity.
A team of researchers from MBI Berlin, together with colleagues from Italy and France, has now precisely recorded the dependence of the resonant magnetic scattering intensity as a function of the x-ray intensity incident per unit area ( the “fluence”) on a ferromagnetic domain sample. Via integration of a device to detect the intensity of every single shot hitting the actual sample area, they were able record the scattering intensity over three orders of magnitude in fluence with unprecedented precision, in spite of the intrinsic shot-to-shot variations of the x-ray beam hitting the tiny samples. The experiments with soft x-rays were carried out at the FERMI free-electron x-ray laser in Trieste, Italy.
Magnetization is a property directly coupled to the electrons of a material, which make up the magnetic moment via their spin and orbital motion. For their experiments, the researchers used patterns of ferromagnetic domains forming in cobalt-containing multilayers, a prototypical material often used in magnetic scattering experiments at x-ray lasers. In the interaction with x-rays, the population of electrons is disturbed and energy levels can be altered. Both effects could lead to a reduction in scattering, either through a transient reduction of the actual magnetization in the material due to the reshuffling of electrons with different spin, or by not being able to detect the magnetization anymore because of the shift in the energy levels. Furthermore, it has been debated whether the onset of stimulated emission at high x-ray fluences administered during a pulse of about 100 femtoseconds duration can be responsible for the loss in scattering intensity. The mechanism in the latter case is due to the fact that in stimulated emission, the direction of an emitted photon is copied from the incident photon. As a result, the emitted x-ray photon would not contribute to the beam scattered away from the original direction, as sketched in Fig.1.