Ultrafast spin dynamics — experiments and models

Two possible approaches to increase density — heat-assisted magnetic recording and spintronics — both rely on ultrafast manipulation of the carriers of information. A world-class consortium of partners already conducting pioneering research in the field launched the project 'Multiscale modelling of femtosecond spin dynamics' (FEMTOSPIN) to enhance understanding of fast processes.

Recently, optical spin manipulation and magnetisation processes have been shown to be much faster than conventional ones based on current-generated magnetic fields. Models covering a variety of timescales are necessary to support development of related devices. To access the timescale of photons, electrons and spin interactions, one requires time-dependent density function theory (DFT). On the other hand, to compare model outputs to experimental results, one requires mesoscopic continuum models.

FEMTOSPIN is passing information from DFT to the mesoscopic model using atomistic spin models. Experimental work is used to fine-tune models and provide greater insight.

DFT electronic structure calculations are now providing insight into the mechanisms and properties underlying ultrafast magnetisation dynamics. In particular, models are illuminating the role of spin transport in magnetisation changes following application of a 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 an important link to experiments.

Models have shed light on numerous related phenomena. Partners' discovery of thermally induced magnetisation switching (switching by a heat pulse alone without an applied magnetic field) prior to initiation of the project sparked worldwide attention. Now, the team has found the apparent origin of the effect. Further, the models predict such heat-driven reversal to take place in synthetic ferrimagnets consisting of two ferromagnetic layers coupled antiferromagnetically. Experimental testing of this prediction is under way.

FEMTOSPIN is developing critically important multi-scale models of magnetisation phenomena validated by advanced experimental research. Better understanding of the behaviours of spin-ordered materials together with the development of advanced modelling tools will lead to a new generation of ultrafast magnetic information storage and processing.