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From Attosecond Magnetism towards Ultrafast Spin Photonics

Final Report Summary - ATOMAG (From Attosecond Magnetism towards Ultrafast Spin Photonics)

The project ATOMAG is dedicated to the study of the interaction of short light pulses with magnetic materials. It is important both for understanding fundamental concepts in modern magnetism and for applications in magnetic recording, imaging purposes as well as in Spintronics or Spin-Photonics. As the optical pulses used to study the magnetic state of matter have a duration of a few femtoseconds (1 fs = 10-15 s), this research field is at the frontier of Ultrafast Optics and Magnetism which we also name Femtomagnetism. In the original project we had put forward three simple questions as guidelines that would keep us progressing over the years:
1. How fast can one modify and control the magnetization of a magnetic system, and more generally what are the basic mechanisms intervening in the magnetization dynamics induced by laser pulses?
2. What is the role and essence of the coherent interaction between light and spins?
3. How far spin-photonics can bring us to the real world of data acquisition and storage? In other words: can one use photons to manipulate efficiently the magnetic state of matter?
The achievement of the project required developing several new scientific apparatus and methodologies for investigating the dynamics of the magnetization in materials after they are perturbed by laser pulses. The most demanding and time consuming was the generation of soft Xrays by High Harmonic Generation (HHG) in a gas jet, typically photons with energy 20-70 eV, and to use them for probing metals in a specific magnetic field environment. This HHG beam facility is now operational and experiments are being performed. They allow measuring the dynamics of cobalt films doped with rare earth magnetic impurities. The precession of the magnetization is strongly affected by such doping and also by the temperature reached after laser excitation. These results have strong implications for magnetic switching as well the transport of spins in spintronic devices.
Another instrumental approach that we developed is to use short acoustic pulses to manipulate the magnetization, a new methodology which we name Ultrafast Magneto-Acoustics. It consists in generating picosecond acoustic pulses, a technique employed since the 1990ies, to modify the magneto-elastic properties of a metallic film. From the local perturbation of the effective magnetic field a motion of precession is resulting. It can be further controlled by a sequence of delayed acoustic pulses to the point that the precession of the magnetization is either amplified or completely suppressed. Therefore it is possible to manipulate the magnetization at a large distance (acoustic pulse propagation of several microns), in a spatially confined environment (typically the wavelength of light), and of course with a temporal accuracy that is given by the femtosecond laser pulses. This approach is very original as only the angular magnetic moment is perturbed with no heating in the region of interest.
In another spirit, we have been able to determine how fast one can control the magnetization in granular films of FePt that might be used for recording in the coming generation of hard drives functioning on the principle of Heat Assisted Magnetic Recording (HAMR). Our experiments performed in ultimate conditions of magnetic field (7 T at room temperature), laser duration (50 fs), low energy density (a few tens of µJ/cm2) have revealed that one can efficiently perform HAMR switching in a few picoseconds.
An important fundamental achievement concerns Coherent Magnetism. The main idea is that the initial phase of the laser pulses can be imprinted in magnetic materials. Therefore, via the spin-orbit interaction, the nonlinear magneto-optical properties depend on an external magnetic field. We performed many experiments in metals and dielectrics and developed theoretical approaches for that purposes. The results turned out to surpass our expectations as they are of great interest for applications. Generally speaking, several types of diffractive magnetic devices can be made that lead to a whole new field once implemented in microstructures: Integrated Magneto-Optical Circuitry.
At the term of this 5-year ATOMAG project we can safely claim that our initial vision was carried out fruitfully. Exploring the coupling between spins and photons at the femtosecond scale is opening a large field that will further impact several areas of Physics: a) condensed matter physics via the fundamental study of dynamical processes in magnetic materials and nanostructures, b) spintronics and spin-photonics via the realization of magnetic media and devices controllable by light or by acoustic pulses, c) fundamental prospects in relativistic electrodynamics when considering the spin-photon interaction in ultimate temporal and intensity conditions. The project was achieved thanks to intense works of the PI’s team as well as to collaborations with colleagues whom I warmly thank here. For those interested in following this field of Femtomagnetsim I recommend the international conferences “Ultrafast Magnetism Conference”, held every two years, which I could initiate in 2013 in Strasbourg thanks to the ERC advanced Grant ATOMAG.