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Femtosecond opto-magnetism and novel approaches to ultrafast magnetismat the nanoscale

Periodic Report Summary - FANTOMAS (Femtosecond opto-magnetism and novel approaches to ultrafast magnetismat the nanoscale)

Project Objectives:

The aim of FANTOMAS is to train young scientists in the emerging supra-disciplinary field of 'nano-opto-magnetism', a new scientific area with novel technological opportunities at the junction of coherent nonlinear optics, nanoscience and magnetism, with a potentially high impact on the information society. In the research program we investigate nonthermal effects of light on nanomagnets in order to obtain a comprehensive understanding of the physical mechanisms leading to a highly efficient ultrafast (10 to 12 seconds and faster) optical control of magnetism at the nanoscale.

Work Performed:

1. The ultrafast all-optical magnetisation reversal was investigated in various rare-earth containing ferromagnetic alloys and the range of compositions of GdxFe100-x-yCoy suitable for all-optical magnetisation reversal was established.
2. In order to understand all optical magnetisation reversal results, an atomistic model has been developed. The model has provided an understanding of the thermal processes involved in the opto-magnetic reversal process.
3. Studies on the coupling of light with magnetic semiconductors have been conducted.
4. Time resolved pump-probe investigations of ultrafast photo-induced magnetic anisotropy changes in a cobalt-substituted yttrium iron garnet were performed.
5. A number of methods to obtain nanostructured samples based on gadolinium iron cobalt (GdFeCo) continuous films have been investigated. Among these methods the lift-off and pre-pattering methods were found to be the most promising, i.e. the obtained samples maintained the out-of-plane magnetic anisotropy.
6. The process of epitaxial lift-off (ELO) was adapted to enable us to tune the magnetic anisotropy in gallium manganese arsenide ((Ga,Mn)As) dilute magnetic semi-conductors.
7. A detailed characterisation of a representative Gd23.4Fe73.26Co3.34 sample has been done using the x-ray magnetic circular dichroism (XMCD) technique.

Main Results:

1. All optical magnetisation reversal dynamics and its mechanism in a GdFeCo alloy was investigated using a combination of two advanced experimental and theoretical methods: femtosecond single-shot time-resolved optical imaging of magnetic structures and multiscale modelling beyond the macro-spin approximation, respectively. This combination revealed an ultrafast novel linear path way for magnetisation reversal that does not involve precession but occurs via a strongly non-equilibrium state.
2. A semi-classical model for femtosecond laser induced demagnetisation due to spin polarised excited electron diffusion in the supper-diffusive regime has been developed. The approach treats the finite elapsed time and transport in space between multiple electronic collisions exactly. It also incorporates the presence of multiple metal films in the sample.
3. To fabricate the dilute magnetic semiconductor (Ga,Mn)As with specifically tailored magnetic anisotropies, a new fabrication process called ELO has been developed. This process relies on the high etching selectivity of III-V compounds in diluted hydrofluoric acid. The method has been used to fabricate (Ga,Mn)As samples with a thermally activated transition from hard to easy magnetisation axis perpendicular to the sample plane.
4. The coupling of light with magnetic semiconductors, a photo-induced change of the coercive field, i.e. a photocoercivity effect (PCE), under very low intensity illumination of a low-doped (Ga,Mn)As ferromagnetic semiconductor was observed, with a strong correlation between the PCE and the sample resistivity.
5. On ultrafast magnetisation dynamics in magnetic dielectrics large angle magnetisation precession amplitude has been observed in a Co-substituted ferrimagnetic yttrium iron garnet thin film.

Expected results and impact:

Significant scientific output is expected including potential breakthroughs in the investigation of ultrafast magnetism that may lead to the development of unprecedentedly fast magnetic recording and information processing technology applications.