Sep 21 2020
Powerful ultrashort pulses of X-rays delivered by free-electron X-ray lasers can be utilized to image nanometer-scale objects in just a single shot.
Schematic sketch of the scattering experiment with two competing processes. The soft x-ray beam (blue arrow, from left) hits the magnetic sample (circular area) where it scatters from the microscopic, labyrinth-like magnetization pattern. In this process, an x-ray photon is first absorbed by a Cobalt 3p core electron (a). The resulting excited state can then relax spontaneously (b), emitting a photon in a new direction (purple arrow). This scattered light is recorded as the signal of interest in experiments. However, if another x-ray photon encounters an already excited state, stimulated emission occurs (c). Here, two identical photons are emitted in the direction of the incident beam (blue arrow towards right). This light carries only little information on the sample magnetization and is usually blocked for practical reasons. Image Credit: Max Born Institute (MBI Berlin).
Magnetization patterns can be made visible by tuning the wavelength of the X-ray to an electronic resonance. However, when progressively intense pulses are used, the magnetization image vanishes. The mechanism that causes this loss in resonant magnetic scattering intensity has currently been elucidated.
Similar to flashlight photography, short but powerful flashes of X-rays help record images or X-ray diffraction patterns, which “freeze” movement that is slower than the length of the X-ray pulse.
The benefit of X-rays over visible light is that nanometer-scale objects can be distinguished because of the short wavelength of X-rays. Moreover, if the X-ray wavelength is adjusted that corresponds to specific energies for electronic transitions, special contrast can be obtained, thus enabling, for example, to make the magnetization of different domains inside a visible material.
But the fraction of X-rays dispersed from a magnetic domain pattern reduces when the intensity of the X-ray in the pulse is increased.
Scattering and Magnetization
Although such an effect had been already noticed in the initial images of magnetic domains captured at a free-electron X-ray laser earlier in 2012, a range of different explanations had been proposed to describe this loss of intensity in the scattered X-rays.
At MBI Berlin, a research team along with collaborators from France an Italy, has now accurately recorded the reliance of the resonant magnetic scattering intensity as a function of the intensity of the X-ray incident per unit area (the “fluence”) on a ferromagnetic domain sample.
By integrating a device to detect the intensity of each single shot striking the real sample area, the researchers were able to capture the scattering intensity over three orders of magnitude influence with unparalleled accuracy. This was done despite the intrinsic shot-to-shot differences of the X-ray beam striking the small samples.
The researchers performed experiments with soft X-rays at the FERMI free-electron X-ray laser in Trieste, Italy.
Magnetization is essentially a property that is directly paired with the electrons of a material, which continue the magnetic moment through their orbital motion and spin. For the experiments, the team utilized patterns of ferromagnetic domains that form in cobalt-containing multilayers—a prototypical material that is generally utilized in magnetic scattering experiments at X-ray lasers.
Reduction in Resonant Magnetic Scattering
When interacting with X-rays, the electrons’ population is disrupted and energy levels can be changed. Both kinds of effects could cause a reduction in scattering, either via a temporary reduction of the actual magnetization in the material because of the reshuffling of electrons with a different spin, or by not being able to identify the magnetization anymore due to the change in the energy levels.
It has also been disputed whether the loss in scattering intensity was caused by the onset of stimulated emission at high X-ray fluences administered during a pulse of around 100 fs duration.
In the latter case, the mechanism is attributed to the fact that during stimulated emission, the direction of an emitted photon is replicated from the incident photon. Consequently, the emitted X-ray photon would not play a role in the beam scattered away from the original direction, as shown in the above image.
In the findings presented in the Physical Review Letters journal, the team detonated that while the loss in magnetic scattering in resonance with the Co 2p core levels has been attributed to stimulated emission before, this process is not important for scattering in resonance with the shallower Co 3p core levels.
The experimental data over the whole fluence range has been clearly explained by simply considering the real demagnetization that takes place inside each magnetic domain, which was earlier characterized with laser-based experiments by the MBI team.
Single-shot Experiments
Considering the short lifetime of the Co 3p core levels of around a quarter femtosecond dominated by Auger decay, the hot electrons which were probably produced by the Auger cascade in concert with subsequent electron scattering events resulted in a reshuffling of spin up and spin down electrons briefly quenching the magnetization.
Since this decreased magnetization already manifests itself inside the duration of the X-ray pulses utilized (70 and 120 fs) and lasts for a relatively longer time, the latter part of the X-ray pulse communicates with a domain pattern where the magnetization has truly faded away.
This corresponds with the observation that less reduction of the magnetic scattering is seen when striking the magnetic sample with the same number of X-ray photons within a duration of shorter pulse. On the contrary, the opposite behavior would be anticipated if stimulated emission were the dominant mechanism.
Apart from clarifying the mechanism at work, the study results have significant ramifications for upcoming single-shot experiments on magnetic materials at free-electron X-ray lasers.
Comparable to the situation in structural biology, in which imaging of protein molecules by powerful X-ray laser pulses can be hindered by the destruction of the molecule at the time of the pulse, scientists examining magnetic nanostructures also have to select the fluence and pulse duration wisely in their experiments.
With the fluence reliance of resonant magnetic scattering mapped out, investigators at X-ray lasers currently have a guideline to develop their upcoming experiments accordingly.
Journal Reference:
Schneider, M., et al. (2020) Ultrafast Demagnetization Dominates Fluence Dependence of Magnetic Scattering at Co M Edges. Physical Review Letters. doi.org/10.1103/PhysRevLett.125.127201.