Spin Wave Signature in the Spin Polarized Electron Energy Loss Spectrum of Ultrathin Fe Films: Theory and Experiment

Abstract

We present theoretical studies based on the use of realistic electronic structures, which conclude that in the spin polarized electron energy loss spectrum of Fe and of ultrathin Fe films a strong signature of spin waves should appear for energy losses in the range of 250 meV and below. New experimental data we present show that indeed the spin asymmetry in the loss spectrum increases dramatically in this regime, as expected from its presence. [S0031-9007(99)08710-4] PACS numbers: 75.30.Ds, 75.50.Bb Currently there is great interest in ultrathin ferromagnetic films and, more generally, in the outer surface layers of magnetic materials. Through their study, we may test our ability to predict electronic structures and magnetic properties of new, artificially synthesized materials and to extend our knowledge of bulk magnetism to the very different surface environment. One may also explore magnetism in two dimensions and the transition to three. With ultrathin films incorporated into multilayers, exciting applications in magnetic recording have been realized, and new applications to magnetic storage are envisioned. Such applications require full knowledge of the response characteristics of ultrathin ferromagnetic films. Finally, while the remarks above and the present paper place their primary focus on ultrathin ferromagnetic films, we note that in many other systems of current interest magnetism in reduced dimensionality plays a key role. The high temperature superconductors provide an example. Thus, new experimental methods which explore the magnetism of ultrathin films and surfaces are of fundamental interest. In this paper, we present theoretical calculations and the first data which show that electron energy loss spectroscopy in its spin polarized version (SPEELS) can probe short wavelength spin waves in ultrathin magnetic films. This is thus a new technique which may be employed to probe the nature of spin fluctuations at the surface of diverse magnetic materials, and in thin films of such materials.Theory in the area of ultrathin film and surface magnetism has focused almost entirely on ground state properties of the ultrathin films [1]. Electronic band structures from such studies do approximate the quasiparticle energies; these can be compared with data taken by techniques such as inverse photoemission [2].In addition, such films possess a spectrum of spin excitations which control their dynamic response. Also, these enter importantly into the analysis of other phenomena. For instance, the spin dependence of the inelastic mean free path of excited electrons is controlled by their scattering from spin excitations. This enters centrally into the analysis of diverse spin polarized electron spectros-copies and also transport in such materials. Low lying spin waves in ultrathin films have been studied both by ferromagnetic resonance [3] and by Brillouin light scattering [4]. Both methods excite only modes with very long wavelengths compared to a lattice constant. Thus, they probe only properties of the film th...

Spin-polarized electron energy loss spectroscopy on Fe(100) thin films grown on Ag(100)

“…Spin-polarized electron energy loss spectroscopy (SPEELS) developed in the 1980s and has become a valuable technique for probing Stoner excitations [1][2][3][4][5][6][7][8] and more recently spin waves [9]. In so-called spin flip exchange scattering, an incident electron of a given spin occupies an empty state of the target material and an electron of opposite spin is excited and detected.…”

Spin-waves in thin films with Dzyaloshinskii-Moriya interaction

“…5 can be determined by neutron scattering and SPEELS which measure the magnetization as a function of T of films of few atomic layers. 16,17 Finally, we note that though we use a rather large value of D for the presentation clarity, the formalism given above works for any finite value of D. Experimental techniques mentioned above are able to detect SW mode separations even for very small D. The typical SW energies range from a few (acoustic modes) to several hundreds of meV (optical modes). In this range of energy, the number of modes is given by the number of layers as seen in Fig.…”

“…The smaller the number of layers, the larger the level separation, making it easier for observation of SW modes in very thin films by neutron scattering and SPEELS. 16,17 …”

“…16 For SW higher-energy modes, there has been an experimental breakthrough with the spin polarized electron energy loss spectroscopy (SPEELS): this technique allows us to detect very high-energy surface magnons, up to 240 meV in ultrathin ferromagnetic films. 17 Second, for the surface effect on the SW spectrum and the surface magnetization, the neutron scattering can be used for a complete determination of the magnetic properties of a system at finite T. 16 Third, the dependence of T c on the film thickness shown in Fig. 5 can be determined by neutron scattering and SPEELS which measure the magnetization as a function of T of films of few atomic layers.…”

Interaction between magnetostatic waves and neutrons in magnetic thin films

“…Among the most commonly used experimental probes, one can distinguish small wave-vector techniques (ferromagnetic resonance [1], Brillouin light scattering [1]) which are very accurate, allowing to measure ultra-thin films, and inelastic neutron diffraction working at high wave-vector transfer but which is usually restricted to bulk samples. In order to combine the strengths of those techniques, recent studies have been devoted to the scattering of neutrons [2] or electrons [3] by surface spin waves. In this article, we explore the interaction of a neutron at grazing incidence with spin waves in a permalloy film.…”