Laser induced magnetisation in femtoseconds
Using numerical simulations at the atomistic level has provided valuable insights into ultrafast magnetisation processes. The results have offered a basic formalism to understand the physics of the excitation of ferromagnetic materials with laser pulses on the timescale of a few femtoseconds. However, many important questions also arose. Within the EU-funded FEMTOSPIN (Multiscale modelling of femtosecond spin dynamics) project, world-leading research groups in Germany, Hungary, Spain, Sweden and the United Kingdom developed new theoretical models of ultrafast processes. In parallel, innovative experiments by researchers in Germany, the Netherlands and the United Kingdom provided a detailed test of the model predictions. The aim was to gain a better understanding of the physical processes underpinning the development of all-optical technology for data storage by hard drives. Ultrafast manipulation of carriers of information has the potential not just for extremely high recording densities, but also for significantly faster data rates. This part of the project was overseen by the industrial partner. Models covering a variety of timescales are necessary to support the development of such all-optical technology. To access the timescale of photons, electrons and spin interactions, time-dependent density function theory (DFT) was needed. To compare model outputs to experimental results, mesoscopic continuum models were required. DFT electronic structure calculations illuminated the role of spin transport in magnetisation changes following application of an ultrashort laser pulse. Electronic structure calculations are then linked mathematically to classical atomistic spin models. Together, these feed into the large-scale macrospin models that form the missing link to experiments. This multiscale approach shed light on numerous related phenomena including the discovery of the origin of thermally induced magnetisation switching by the FEMTOSPIN team. Furthermore, models predicted that heat-driven reversal takes place in synthetic ferrimagnets consisting of two ferromagnetic layers coupled antiferromagnetically. Experiments confirmed this prediction. FEMTOSPIN has developed a multiscale approach to modelling magnetisation phenomena that was validated by experimental research. A better understanding of the behaviours of spin-ordered materials offered by the development of advanced modelling tools will lead to a new generation of ultrafast magnetic information storage devices.
Keywords
Ultrafast magnetism, atomistic simulations, electronic structure, FEMTOSPIN, multiscale modelling, recording density, DFT
