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Spin polarized electron energy loss spectroscopy (SPEELS)

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Introduction

spin waves are collective excitations in magnetic solids. The properties of spin waves depend strongly on their wavelength (or wave vector). For the long wavelength the resulting spin precession has a very low frequency and the spin waves can be treated classically. Ferromagnetic resonance (FMR) and Brillouin light scattering (BLS) experiments provide information about the long wavelength spin waves in ultrathin magnetic films and nanostructures. If the wavelength is comparable to the lattice constant, the spin waves are governed by the microscopic exchange coupling and a quantum mechanical description is needed. Therefore, experimental information on these short wavelength (large wave vector) spin waves in ultrathin films is highly desired and may lead to fundamentally new insights into the spin dynamics in reduced dimensions in the future.

Up to now, the spin polarized electron energy loss spectroscopy (SPEELS) is the only technique, which can be used to measure the dispersion of such short wavelength spin waves in ultrathin films and nanostructures.

The First Experiment

For the first time Kirschner's group [1] in Max-Planck institute of Microstructure Physics [2] showed that the signature of the large wave vector spin waves can be detected by spin polarized electron energy loss spectroscopy (SPEELS) [1] [2]. Later, with a better momentum resolution, the spin wave dispersion was fully measured in 8 ML fcc Co film on Cu(001) [3] and 8 ML hcp Co on W(110) [4], respectively. Those spin waves were obtained up to the surface Brillouin zone (SBZ) at the energy range about few hundreds of meV.


Basic Principle of the Experiment

Fig. 1. A schematic representation of the SPEELS experiment.

In the spin polarized electron energy loss spectroscopy experiment, a spin-polarized electron beam created by a photocathode, with a well-defined energy (usually between 3 and 30 eV), is scattered from a ferromagnetic surface. The intensity of the back-scattered electrons is analyzed with respect to their energy to determine the energy- and wave vector- transferred to the sample. Furthermore, the intensity of the scattered electron beam, for the two possible orientations of the incoming electron spin, parallel and antiparallel with respect the sample magnetization, is recorded [5]. A schematic representation of the SPEELS experiment and the scattering process taking place in this experiment is given in Fig.1.

Fig. 2. Series of SPEELS spectra of 2 ML Fe on W(110).

An example of their recent work is the investigation of 1 and 2 monolayer Fe films grown on W(110) and measured at 120 K and 300 K, respectively [6] [7]. The corresponding (normalized) energy loss spectra of a 2 monolayer sample are shown in Fig. 1 for different wave vector transfers. The spin wave excitation appears as a well-defined peak in the loss spectrum. The total angular momentum conservation during the scattering process prohibits the excitation of spin waves for incoming electrons having a majority spin character (I↑). However, since the spins of electrons of the sample are not aligned perfectly in the remanence state, a much weaker spin wave peak shows up in I↑ channel. The sensitivity of SPEELS is great, it is also sensitive to the vibrational excitation (phonons) in the system (in Fig. 2 the peaks at 70 meV and 130 meV are vibrational loss features of O and H, respectively). One can avoid the non-spin-dependent effects by taking the difference of the spectra ΔI = I↓ - I↑, shown in Fig 2(c).

By plotting the peak position (energy) versus wave vector transfer one obtains the dispersion on the spin waves and the full spin wave. The dispersion of the Fe films has been measured up to the surface Brillouin zone (SBZ) boundary along Fe[001] direction [8].


References

  1. ^ M. Plihal, D. L. Mills, and J. Kirschner, Phys. Rev. Lett. 82, 2579 (1999).
  2. ^ H. Ibach, D. Bruchmann, R. Vollmer, M. Etzkorn, P. S. Anil Kumar, and J. Kirschner, Rev. Sci. Instrum. 74 4089 (2003).
  3. ^ R. Vollmer, M. Etzkorn, P. S. Anil Kumar, H. Ibach, and J. Kirschner, Phys. Rev. Lett. 91, 147201 (2003).
  4. ^ M. Etzkorn, P. S. Anil Kumar, W.X. Tang, Y. Zhang, and J. Kirschner, Phys. Rev. B 72, 184420 (2005).
  5. ^ R. Vollmer, M. Etzkorn, P. S. Anil Kumar, H. Ibach, and J. Kirschner, Phys. Rev. Lett. 91, 147201 (2003).
  6. ^ W. X. Tang, Y. Zhang, I. Tudosa, J. Prokop, M. Etzkorn, and J. Kirschner, Phys. Rev. Lett. 99, 087202 (2007).
  7. ^ J. Prokop, W. X. Tang, Y. Zhang, I. Tudosa, T. R. F. Peixoto, Kh. Zakeri, and J. Kirschner, Phys. Rev. Lett. 102, 177206 (2009).
  8. ^ W. X. Tang, Y. Zhang, I. Tudosa, J. Prokop, M. Etzkorn, and J. Kirschner, Phys. Rev. Lett. 99, 087202 (2007).
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