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EXCHANGE Report Summary

Project ID: 339813
Funded under: FP7-IDEAS-ERC
Country: Netherlands

Periodic Report Summary 2 - EXCHANGE (Magnetism at the time and length scale of the Exchange interaction)

Excitation by femtosecond lasers has revealed extraordinary spin dynamics in magnetic materials, that cannot be explained by equilibrium descriptions of magnetism. This has led to a number of fundamental questions that challenge existing theories of magnetism, based on thermodynamics: Are the short range interactions between spins, expressed through the exchange constant, actually constant? How does long range magnetic order emerge while the short range interactions themselves are evolving in time? How to describe the strongly nonlinear dynamics of a spin system so far from equilibrium? These questions are not only of fundamental interest: future magnetic storage and possibly logic will depend on our knowledge of the fundamentals of ultrafast magnetism.

By developing and exploiting a number of novel experimental and theoretical approaches, such as time-resolved single shot magneto-optical imaging, fs-Xray scattering and holographic imaging, THz emission spectroscopy and multiscale modelling, we have succeeded to make major progress in this exciting area over the last years:

1.Using plasmonic antennas we achieved nanoscale confinement of the area where optical switching can occur while the magnetic switching around and below the antenna was
imaged using resonant X-ray holography and magnetic circular
dichroism. By using a multiscale approach, i.e., first-principles density functional theory combined with atomistic spin dynamics, we demonstrated the very intricate structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd and Fe atomic concentrations at the nanoscale level. The emergence of nanoscale vortex like nanoscale spin structures was studied numerically and showed the possibility of creating chiral skyrmions at GHz rates.

2. Using photoemission electron microscopy with x-ray magnetic circular dichroism as a contrast mechanism, new insights into the all-optical magnetization switching (AOS) phenomenon in GdFe-based rare-earth transition- metal ferrimagnetic alloys are provided. This was done on a single shot basis, also providing insight into the reproducibility and statistics of the effect.

3. Using the examples of laser-induced spin-reorientation phase transitions in TmFeO3 and ErFeO3 orthoferrites, we have demonstrated that terahertz emission spectroscopy can obtain novel information about ultrafast laser-induced modification of the exchange constant and the subsequent spin dynamics, which is not accessible by more common all-optical methods. The power of the method is evidenced by the fact that, in addition to the expected quasi-ferromagnetic and quasi-antiferromagnetic modes of the iron sublattices, terahertz emission spectroscopy enables detection of a resonance optically excited at an unexpected frequency of 0.3–0.35 THz.
In addition to these specific results, our work has inspired a strong effort worldwide in the area of optical control of magnetism using femtosecond laser pulses, where many groups in the USA, France, Germany, the UK and in other places have developed activities in this area.

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