Following ultrafast magnetization dynamics in depth

In current information technology, functional magnetic devices typically consist of stacks of thin layers of magnetic and nonmagnetic materials, each only about one nanometer thick. The stacking, choice of atomic species, and the resulting interfaces between the layers are key to the particular function, for example as realized in the giant magnetoresistance read heads in all magnetic hard drives. Over the last years, it was shown that ultrashort laser pulses down to the femtosecond range (1 femtosecond = 10^-15 s) can effectively and very fast manipulate the magnetization in a material, allowing a transient change or even permanent reversal of the magnetization state. While these effects have been predominantly studied in simple model systems, future applications will require an understanding of magnetization dynamics in more complex structures with nanometer-scale heterogeneity.

Researchers from Max Born Institute in Berlin together with their colleagues from Leibniz-Institut für Kristallzüchtung, Leibniz-Institut für Analytische Wissenschaften and Helmholtz-Zentrum Berlin have now demonstrated a novel technique that allows resolving the spatiotemporal evolution of laser-induced spin dynamics within a complex magnetic heterostructure on the femto- and picosecond timescale. Using ultrashort soft X-ray pulses of about 8 nanometer wavelength generated by a broadband laboratory-scale source based on High-Harmonic-Generation (HHG), they were able to follow the magnetization depth profile evolving within a 10-nanometer thin ferrimagnetic iron-gadolinium (FeGd) layer after it was hit by a femtosecond infrared (IR) laser pulse. The basic sensitivity to the magnetization stems from the transverse magneto-optical Kerr effect (T-MOKE) which leads to a magnetization-dependent reflectivity in combination with being element-specific. To obtain depth information within the structure, the team developed the following approach: When the wavelength of the radiation is close to an atomic resonance, its penetration depth into the material strongly changes. How far certain spectral components of the broadband soft X-ray pulse can "look" into the material thus depends on their exact wavelength. Consequently, this depth-information can be retrieved via the spectral changes observed after reflection. The magnetization profile at each point in time is determined by fitting the measured T-MOKE spectra with calculated spectra obtained from magnetic scattering simulations .

In the experiment, the 27-femtosecond short IR laser pulse triggering the changes in the magnetization was incident on the tantalum layer covering the actual magnetic FeGd layer. In the first few hundreds of femtoseconds, a homogeneous demagnetization of the FeGd layer was observed. To their surprise, however, the scientists found that at later times of around one picosecond, the reduction of the magnetization due to the laser pulse was strongest on the side of the FeGd layer not facing the incident laser pulse. Transiently, an inhomogeneous magnetization profile forms, reflecting enhanced demagnetization at the interface towards the thin platinum layer underneath. Based on the timescale of the evolving magnetization gradient, the responsible microscopic processes could be identified: Contrary to initial expectations, a significant influence due to ultrafast spin transport phenomena across the interface could be ruled out, as this would lead to magnetization gradients already within the first hundreds of femtoseconds. Instead, the observed effect arises due to heat injection from the buried platinum layer into the magnetic layer. The platinum absorbs the IR laser pulse much stronger than the other layers in the heterostructure and therefore acts as a localized internal heat source.

The approach demonstrated by the researchers allows following the evolution of magnetization profiles with femtosecond temporal and nanometer spatial resolution within the so far difficult-to-access depth of a sample. Thus, it paves a way to testing fundamental theoretical predictions in ultrafast magnetism as well as studying laser-induced spin and heat transport phenomena in device-relevant geometries.