PHYSICAL REVIEW B 86, 165436 (2012) Surface spin waves of fcc cobalt films on Cu(100): High-resolution spectra and comparison to theory J. Rajeswari, 1,3 H. Ibach, 2,3 C. M. Schneider, 1,3 A. T. Costa, 4 D. L. R. Santos, 4 and D. L. Mills 5,* 1 Peter Gr¨ unberg Institut (PGI-6), Forschungszentrum J¨ ulich, 52425 J¨ ulich, Germany 2 Peter Gr¨ unberg Institut (PGI-3), Forschungszentrum J¨ ulich, 52425 J¨ ulich, Germany 3 J¨ ulich Aachen Research Alliance, Germany 4 Instituto de F´ ısica, Universidade Federal Fluminense, 24210-346 Niter´ oi, R. J., Brazil 5 Department of Physics and Astronomy, University of California, Irvine, California 92697, USA (Received 12 May 2012; published 22 October 2012) With the help of a recently developed electron energy-loss spectrometer we have studied the surface spin waves on an eight-monolayer cobalt film deposited on Cu(100) surfaces with unprecedented energy resolution. Standing waves of the bulk of the film are discovered in the region of small wave vectors q ‖ < 0.35 ˚ A −1 . The dispersion of surface spin waves is isotropic even for large q ‖ . Because of the high-energy resolution and the complete characterization of the electron optical properties of the spectrometer reliable data for the linewidth of the surface spin waves are obtained. As a byproduct the dispersion of the Rayleigh surface phonon was measured. Data are compared to theoretical spin wave spectra extracted from calculations of the transverse spin susceptibility based on an ab initio electronic structure that incorporates both the metallic substrate and the magnetic film. The calculation takes fully into account the itinerant nature of the electrons responsible for the magnetic moments. The agreement between theoretical and experimental spin wave energies and linewidths is remarkably good. DOI: 10.1103/PhysRevB.86.165436 PACS number(s): 75.70.Tj, 75.30.Ds I. INTRODUCTION The phenomenon of magnetism is characterized by a complex interplay of competing interactions on different length scales. Breaking the inversion symmetry by a surface may cause additional interactions, which eventually determine the magnetic behavior and give rise to new and unexpected effects. Examples are the perpendicular magnetic surface anisotropy which may reorient the magnetization in ultrathin films to point perpendicular to the film plane, 1 ground states with unexpected antiferromagnetic order, 2 or the formation of extended noncollinear spin structures at surfaces in the presence of the Dzyaloshinskii-Moriya interaction. 3 Surface magnetism therefore provides us with a wealth of different magnetic ground states, which depend critically on the details of the system under investigation. While ground states and static properties of many surface and thin-film systems have been investigated in some detail, less is known about their dynamic behavior and the magnetic excitations such as spin waves which, however, may have a profound influence on magnetic ordering and critical phenomena. This is mostly due to the lack of techniques with an appropriate surface sensitivity. Lately, the development of advanced, specifically designed electron energy-loss spectrometers has enabled experimental investigations of surface spin wave excitations in the high- wave vector regime. 4,5 Early studies were made by Vollmer et al. 6 with spin-resolved energy-loss spectra of fcc cobalt films deposited on Cu(100). A considerable number of experimental studies on surfaces of cobalt and iron films of varying thickness have appeared since then. 7–11 The issues addressed in these papers concerned primarily the dispersion of surface spin waves, the presence of spin waves in ultrathin films down to one-monolayer thickness, 7,8 and the asymmetry of the spin wave dispersion due to spin-orbit coupling (Dzyaloshinskii- Moriya interaction). 9 A characteristic feature of the surface spin waves of cobalt and iron is their short lifetime leading to a broad linewidth which increases with increasing wave vector parallel to the surface q ‖ . For wave vectors close to the boundary of the surface Brillouin zone the width becomes so large that no clear resonance structure is observed in the energy-loss spectrum. 6 The short lifetime of spin waves is due to the itinerant nature of magnetism in 3d transition metals: Spin waves decay rapidly into electron-hole pair excitations involving a spin flip (Stoner excitations). The thereby-caused Landau damping of the spin waves is particularly strong for spin waves in thin films and the surface spin waves of bulk materials since the broken translation symmetry increases the number of available channels for Stoner excitations. Since strong Landau damping leads to a considerable renormalization of the spin wave frequencies Landau damping cannot be considered as a perturbation but is rather to be treated as an integral part of a theoretical description which takes the itinerant nature of 3d transition metals into account. 12–15 This entails that for 3d -metal films energy and linewidth of spin waves are two sides of the same coin and of equal interest. We note in passing that for nickel surfaces Landau damping is so strong that all attempts to observe surface spin waves in the high wave vector regime have failed although bulk spin waves are clearly observed in neutron scattering. 16 So far, reliable data on the linewidth of surface spin waves do not exist, not even for cobalt films which provide for the best-defined spin wave signals. An attempt by Etzkorn et al. was restricted to the high wave vector regime 17,18 because of the comparatively low-energy resolution of their spectrometer. Based on an estimate of the momentum resolution Etzkorn et al. (incorrectly) disregarded the kinematic broadening due to the finite q ‖ range accepted by the energy analyzer. The intrinsic energy width in the medium momentum range was therefore overestimated. As we have shown in a recent technical publication the kinematic broadening can be quite substantial, in particular when the spectrometer operates in a low-resolution mode. 19 165436-1 1098-0121/2012/86(16)/165436(11) ©2012 American Physical Society