10.06.2013

FLASH makes magnetisation invisible

Surprising effect discovered at the free-electron laser

An international team of scientists observed a new effect when they carried out material magnetization experiments at DESY’s free-electron laser FLASH: the extremely intensive X-ray light of FLASH makes the magnetization of materials invisible within a few femtoseconds. The working group now presented their research results in the scientific journal Physical Review Letters.

Detector images of the resonant magnetic scattering of a cobalt/platinum multilayer system. (a) Scattering image from the amount of 1000 FEL pulses with low intensity (7,5mJ/cm2). (b) Scattering image of a single FEL pulse with an intensity of 5J/cm2. Although this scattering image was taken with only one third less light, its scattered light intensity is much less. The magnetic scattering signal broke down. The strong signal above right in (b) is due to the damage of the sample.

Resonant magnetic X-ray scattering is an established method to investigate the magnetic properties of materials at conventional synchrotron radiation sources. Thus, it is possible to show for example the single magnetic domains – fields, in which the magnetization is oriented to a certain direction. A research team from DESY, the Center for Free-Electron Laser Science CFEL, Technische Universität Berlin, Helmholtz-Zentrum Berlin, the University of Hamburg and Université Pierre et Marie Curie (Paris) now also uses this experimental method at the X-ray laser FLASH. With its ultra-short and brilliant X-ray flashes, this facility is ideal to experimentally trace ultra-fast magnetisation changes. The flashes were excited by an additional optical laser and were detected in an FEL beam which was not extremely focussed and therefore not very intensive.

However, when the experimenters introduced their sample of cobalt and platinum layers into a strongly focussed beam, they were confronted with a big surprise: above a certain X-ray intensity, magnetisation could not be measured anymore; the magnetic diffraction image of the sample disappeared almost completely. “The nature and intensity of the diffraction changed instantaneously,” said Leonard Müller, lead author of the study. “The interaction of the X-ray light excites the electrons of the material in such a strong way that the diffraction image fades out and eventually disappears.” It is remarkable that this happens without an additional excitation. The scientists conclude that the X-ray flash does not only measure the sample but changes it at the same time, and this takes place on a time-scale which obviously must be shorter than the flash that already is incredibly short.

The material’s magnetic properties are determined by its system of electrons. Its spins and thus its magnetic moments align and produce a magnetisation of the whole material. This magnetic structure can be made visible when irradiating it with X-ray light of a specific wavelength, or energy. The light’s energy must be selected in such a way that it lifts electrons up into a specific energy level; only in this case, resonant magnetic scattering will occur. “In simulations, we calculated that the X-ray light flowing at five Joule per square centimetre, as it was used in the experiments, changes the electronic structure of the sample in less than ten femtoseconds,” said Beata Ziaja from the CFEL theory division. “This changes the energy levels of the magnetic atoms, and the magnetic elements of the scattering collapse.” At FLASH, the light flashes currently have a length of 100 femtoseconds (a tenth of a trillionth of a second) and they are very appropriate to show atomic systems in motion. However, when investigating electronic properties, it is necessary to take into consideration the influence of laser light, since the electronic system reacts more rapidly than the atomic system. This is not only true for the current measurements at magnetic systems but presumably also for other systems that are investigated with resonant scattering, i.e. superconductors, the scientists explained. For this purpose, Müller suggests a systematic investigation of the influence of FEL light on the electronic structure of relevant materials. This would provide a completely new insight into the fundamental and ultrafast interaction processes of X-ray light and electronic structure.

“Of course, we would also want to investigate the performance of the electronic system when it is incited with an X-ray flash with a length of only one femtosecond,” the research team said. However, this pulse length cannot be generated by FLASH.  Therefore, these results are a challenge for the scientists responsible for development and operation of future FEL sources.