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Optica Publishing Group
  • CLEO/Europe and IQEC 2007 Conference Digest
  • (Optica Publishing Group, 2007),
  • paper IE5_4

Femtosecond imaging of the spin dynamics of CoPt3 nanostructures

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Abstract

The study of magnetic nanostructures is a major issue both for condensed matter physics and for information storage technology. In this context, performing time resolved magneto-optical imaging with a high spatial resolution on the ultrafast time scale is of outstanding interest. Concerning the temporal resolution, magneto-optics performed with femtosecond laser pulses appears to be a powerful method for investigating the physical processes involved in the laser induced ultrafast demagnetization as well as the magnetization precession and damping [1,2]. In this work, we report a powerful technique for imaging the spin dynamics of individual CoPt3 submicron dots and thin CoPt3 films with a temporal resolution of 150 fs and a spatial resolution of 300 mn by using femtosecond confocal magneto-optical microscopy. The experimental set-up consists in time resolved Kerr magneto-optical pump probe measurements, where the magnetization dynamics is induced (794 nm) and probed (396 nm) with femtosecond optical pulses delivered by an amplified Titanium-Sapphire laser operating at 5 kHz. Figure la) shows the temporal variation of the differential polar Kerr signal ΔM/M (t) of an individual CoPt3 dot with a diameter of 1 pm at remanent state. The density of excitation is 1 mJcm−2. The dynamics reflects the initial demagnetization during the pump excitation as well as the re-magnetization associated to the equilibrium between the spins and the lattice. It occurs with a time constant τspin.lat = 1 ps. The spatiotemporal imaging has been performed for a fixed delay between the pump and probe pulses for different magnetization state coirfiguration. Figure lb) shows the image of two adjacent 1 μm CoPt3 dots recorded for a pump probe delay t = 300 fs. The image is scarmed with a density of laser excitation of 1 mJcm−2. The corresponding contrast is due to a partial demagnetization of the two dots. The opposite direction of the magnetization for the left dot (white contrast) corresponds a permanent switching performed, before imaging the dots, with pump pulses of 8 mJcm−2 in the presence of a small negative magnetic filed (−100 Oe). Similar conditions are used to perform a pump-probe imaging of a region of a 15 mn thick CoPt3 film (figme 1c). Prior to imaging it, the logo of our institute IPCMS is written by switching the magnetization to the reverse state with a pump fluence of 8 mJcm−2. We will also report the all optical switching of a CoPt3 thin film using circularly polarized light. These results show that it is possible to image the detailed dynamics of high-speed magnetic devices with a simultaneous spatial resolution of a few hundreds of nanometers and a subpicosecond temporal resolution opening the way to studying magnetic nano-devices operating with a bandwidth of a few TeraHertz.

© 2007 IEEE

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