Spin dynamics in ferroic materials

The work presented in my thesis delivers new insights into the magnetoelectric coupling in strain mediated artificial multiferroics, and into laser-induced magnetisation processes in ferromagnetic and ferrimagnetic materials. The work mainly consists of three independent experimental results.
Firstly, I have investigated magnetoelectric coupling in Ni nanopatterned islands deposited on a PMN-PT ferroelectric single crystal. The results constitute the first experimental proof of a 90° uniform magnetisation rotation by the application of an electric field. Since the rotation of the magnetisation is complete, the corresponding magnetoelectric coupling coefficient is among the highest measured so far. I have found that the multidomain structure of the ferroelectric single crystal leads to a complex strain-mediated magnetoelectric coupling. This suggests that realising the full magnetoelectric stack at the nanoscale, in order to achieve a single domain configuration in the ferroelectric as well as in the ferromagnet, is of primary importance not only to fulfil large scale integration requirements but also to achieve a reliable magnetisation manipulation by an electric field. Since the electric field induced magnetisation reorientation is related to the strain generated in the ferroelectric as the polarisation switches, the ultimate speed of the reorientation process is limited by the propagation of ferroelectric domain walls which is significantly slower than ferromagnetic switching.
Next, I have investigated laser-induced magnetisation switching in nanopatterned GdFeCo nanostructures. Heat pulse induced switching was observed in nanostructures at different length scales down to a 200 nm out-of-plane domain located in the centre of a 400 nm wide nanostructure. Due to the structuring process, all nanostructures had in-plane edge domains that are found to play no particular role in the efficiency of the switching thanks to the strength of the driving force by which the switching occurs.
Lastly, I developed a novel experimental set-up that allows one to obtain a real time measurement of the transient magnetisation state induced by the excitation of a magnetic sample with an ultrashort near infrared (IR) pulse. My approach relies on ultrashort extreme ultraviolet (XUV) pulses from the free electron laser FLASH and an off-axis Fresnel zone plate to obtain a spatial encoding of the XUV-IR delay over a time window of 1500 fs. Using a single pump-probe event, we succeeded in capturing the ultrafast demagnetisation process of a cobalt thin layer with a time resolution comparable to laser based repetitive pump-probe experiments. The analysis of the statistical distribution of the demagnetisation time obtained from subsequent shots, suggests that no stochastic contribution are present in the early time period of the demagnetisation process.
The results presented in my thesis stress the importance of further investigating magnetisation processes at nanometer length scales and femtosecond time scales. Although this two aspects may seems apparently unrelated, the velocity of electrons in an itinerant ferromagnet is approximately 1 nm/fs. In this regard, further novel and exciting results are expected from experiments combining reciprocal space techniques with ultrashort pulses from free electron lasers in the soft x-ray regime.