Electron energy loss spectroscopy of spin waves (magnons) in ultra-thin films
Wave packets of spin waves offer the unique capability to transport a quantum bit, the spin, without the transport of charge or mass. In this context, high-momentum / high-energy spin waves are of particular interest as they permit a spin confinement within a few nanometers. However, many fundamental questions need be solved before spin waves in the high momentum regime may eventually be employed in a future “Terahertz Magnonics”. A major obstacle for a better understanding of the properties of spin waves of high momentum and high-energy is that conventional methods for studying spin waves fail in the required momentum / energy regime. Furthermore, they do not allow the study of systems with small dimensions. Here the technique of choice is inelastic electron scattering: "monochromatic" electrons are scattered from the surface of a sample and the backscattered electrons carry the information on the surface excitation spectrum (Fig. 1).
Fig. 1: Scheme for inelastic electron scattering from surfaces
Based on a long-standing tradition in the design and application of advanced electron energy loss spectrometers in Jülich we have developed an instrument which enables the investigation of spin waves in ultra-thin ferromagnetic films to be carried out. [1, 2]
Figure 2: Top-view scheme of the electron energy loss spectrometer showing the monochromators and lens systems
Despite the low cross section for electron / solid exchange scattering, we are now in a position to observe spin waves in ultra-thin films with hitherto unprecedented energy resolution down to 4meV [3-5]. The figure below shows a sample spectrum with spin wave excitation and annihilation peaks for a 15 atom layer thick cobalt film deposited on a copper crystal. The spin wave spectrum is composed of three contributions corresponding to various eigenmodes of the film. The small vibration peak is due to a minute trace of carbon, far below the detection limit of most common surface analysis tools such as Auger-spectroscopy.
Fig. 3: High resolution spectrum of spin waves in a cobalt film of 15 atom layers thickness.
 H. Ibach, J. Rajeswari, C. M. Schneider, Rev. Sci. Instrum. 82 (2011) 123904.
 H. Ibach, J. Rajeswari, J. Electr. Spectros. Rel. Phenom. 185 (2012) 61.
 J. Rajeswari, H. Ibach, C. M. Schneider, A. T. Costa, D. L. R. Santos, D. L. Mills, Phys. Rev. B 86 (2012) 165436.
 J. Rajeswari, H. Ibach, C. M. Schneider, Phys. Rev. B 87 (2013) 235415.
 J. Rajeswari, H. Ibach, C. M. Schneider, EPL 101 (2013) 17003.