Spin splitting of image-potential states on fcc Fe/Cu(100)

Abstract

The ferromagnetic exchange splitting of the lowest image-potential states has been measured for Fe films on Cu(100). The majority and minority spin contributions were separated using polarization-dependent two-photon photoemission without magnetizing the sample. The results show an increased spin splitting for decreasing film thickness in agreement with the increasing magnetic moment found by band structure calculations. The lifetime of the states is between 3 fs and 15 fs with slightly shorter values for the minority state. PACS: 73.20.At; 79.60.Dp; 79.60.Ht Electrons can be trapped at a metal surface in the potential well formed by the attractive Coulomb-like image force and the repulsive surface barrier, if a gap in the projected bulk band structure exists [1]. A Rydberg series of normally unoccupied states converging towards the vacuum level Evac results with binding energies En En = Evac−0.85 eV/(n+a)2, n = 1, 2, . . . These so called image-potential states have been studied in recent years mainly by inverse photoemission [2] and with higher accuracy by two-photon photoemission (2PPE) [3]. Investigations on a variety of metal surfaces revealed that the binding energy En depends on the position relative to the band gap [4]. This influence is accounted for by introducing the quantum defect a in the above equation. For magnetic materials the energetic position of the bulk band gap differs for the two spin orientations. As a result, also the energy En depends on the spin orientation of the electron in the image-potential state [5]. As evident from the upper equation the spin splitting decreases with increasing quantum number n. For the 3d-transition metals spin splittings for the n= 1 image-potential state of the order of several tens of meV depending on material and surface orientation have been reported [6–9]. Thin films of iron grown epitaxially on Cu(100) provide an interesting system for the study of spin effects of image-potential states. Iron films undergo several structural and magnetic phase transitions with increasing film thickness [10–12]. At coverages below 2 monolayers (ML) molecular beam epitaxy of iron results in a mixture of singleand double-layer islands, i.e. a rough surface overlayer. For higher coverages the growth mode changes to layer-by-layer growth that persists to about 10 ML. Up to 4 ML the films exhibit a distorted fcc structure with ferromagnetic coupling throughout the whole film. In the intermediate range between 5 ML and 10 ML the atoms are arranged in a relaxed fcc phase with a reconstructed surface layer. The magnetic properties are characterized by antiferromagnetic coupling between bulk layers but an enhanced magnetism of the surface layer. In both cases the easy axis of the magnetization exhibits an inplane orientation. At ≈ 10 ML the film structure changes to bcc. The magnetic coupling is again ferromagnetic with the easy axis oriented perpendicular to the surface plane. The influence of this complex magnetic behavior on the n = 1 image-potential state will be discussed here. The present results improve and extend previous work [9] to an increased coverage range and include the photon-energy dependence.