1. Scientific Motivation and Background
- Investigation of the Eigen excitations of magnetic microparticles on the picosecond timescale - important in order to develop new magnetic storage media or ultrafast switches
The rapid development of the density and the data rate in modern storage media like hard disk drives is leading to very fast magnetization processes. These may be nanosecond or even sub-nanosecond write head operations on a media disk, divided into microscopic magnetic substructures. Other possible future applications may be realized in the field of magnetic logic operations in new devices.
Whereas intense studies have led to a better understanding of the magnetic equilibrium state in such inhomogeneously magnetized microstructures [1-3], the investigation of the magnetization dynamics, e. g. the response of these magnetic structures to an excitation of a short excitation field pulse (< 200ps) is one of the most striking challenges in state of the art solid state physics. Currently these phenomena as vortex dynamics, domain wall dynamics and the dymamics of the microscale domains itself are being tackled by using different new time resolved, x-ray microscopic techniques [4-6].
2. Experimental approaches and aim
- Usage of magnetic imaging by means of PEEM (electron emission) and STXM (x-ray emission) in combination with stroboscopic pump and probe techniques at different synchrotron radiation facilities (ALS, SLS)
- Local modification (e. g. of the core) of the magnetic properties of the samples by means of FIB (at FZD)
To obtain lateral and time resolved information about the magnetization dynamics of magnetic microparticles we use basically two different special synchrotron based x-ray microscopes. The first technique is the so called photoemission electron microscope (PEEM), located at the Swiss Light Source (SLS), Paul Scherrer Institut (PSI), Switzerland. The compound in question is excited by synchrotron x-rays and consequently electrons from the inner core shells are emitted these are analyzed by an electron microscope which produces an image of the sample. The second technique is scanning transmission x-ray microscopy (STXM), used to make x-ray images and x-ray absorption spectra (XAS) of thin samples in transmission. We will use the STXM at the Advanced Light Source (ALS), Lawrence Berkeley Laboratory (LBNL), USA. These experiments are based upon the x-ray magnetic circular dichroism (XMCD) leading to the possibility of magnetic imaging. Furrthermore, for both setups a stroboscopic pump and probe technique is used to enable time-dependent studies. Pump steps down to ~20 ps are possible in order to get very valuable information about the temporal evolution of the magnetization. It has been demonstrated by J. Fassbender et al. that Cr ion implantation in permalloy thin films results in a decrease of the saturation magnetization, the Curie temperature and the magnetic anisotropy, on the other side the magnetic damping behaviour is increased . Similar results can be obtained by means of focussed ion beam (FIB) implantation with Co ions. By adjusting the ion fluence local areas, e. g. the core of the sample, can be magnetically modified. FIB is done in close collaboration with Division of Process Technology (FWIP).
The main goal of this project is to investigate the dynamic Eigen excitations by means of complementary time resolved PEEM and STXM in different magnetic microstructures. These are stepwise modified by means of FIB from a vortex to a modified vortex and finally to a magnetic ring structure.
3. First Results and Outlook
The figure below shows the time dependent series obtained on a 5*5 µm permalloy square by means of time-resolved PEEM. An analysis has been made by means of the local Fourier transforms extracted from the time resolved dichroic image series at selected frequencies. The top row shows the amplitude map of a 5 micron square and the second row the phase map, respectively. The lower halve shows the same for a 5 micron square with a whole in the center which has been prepared by focused ion beam. On the topographic grayscale micrographs on the left (sum of left and right helicity), the hole can be seen. There are clearly some differences in the local maps as well in the average spectra (not shown), and the effects have to be investigated more closely in the near future.
Figure 1: Local Fourier transforms extracted from the time resolved dichroic image series at selected frequencies.
 R. P. Cowburn, D. K. Koltsov, A. O. Adeyeye, M. E. Welland, D. M. Tricker,
Phys. Rev. Lett. 83, 1042 (1999).
 T. Shinjo, T. Okuno, R. Hassdorf, K. Shigeto, T. Ono,
Science 289, 930 (2000).
 A. Wachowiak, J. Wiebe, M. Bode, O. Pietzsch, M. Morgenstern, R. Wiesendanger,
Science 298, 577 (2002).
 S.-B. Choe, Y. Acremann, A. Scholl, A. Bauer, A. Doran,J. Stoehr, H. A. Padmore,
Science 304, 420 (2004).
 H. Stoll, A. Puzic, B. van Waeyenberge, P. Fischer, J. Raabe, M. Buess, T. Haug, R. Höllinger, C. Back, D. Weiss, G. Denbeaux,
Appl. Phys. Lett. 83 , 3328 (2004)
 J. Raabe, C. Quitmann, C. H. Back, F. Nolting, S. Johnson, C. Buehler,
Phys. Rev. Lett. 94, 217204 (2005)
 J. Fassbender, J. McCord, M. Weisheit, R. Mattheis,
Intermag 2005 Nagoya.