Quantum-well states and spin polarization in thin Ni films on Cu(0 0 1)
Introduction
Quantum confinement of electrons in ultrathin films leads to standing electron waves, denoted as quantum-well (QW) states. QW states in ultrathin magnetic films and, in particular, in sandwich structures with magnetic materials involved are important for understanding and engineering magnetic materials at the atomic level and for their applications in electronic and spintronic devices [1], [2], [3], [4], [5]. For example, magnetic multilayers with nonmagnetic spacer layers exhibit oscillatory exchange coupling [6] and giant magnetoresistance [7], [8]. Spin-polarized QW states are believed to act as mediators for the oscillatory exchange coupling. They are experimentally accessible with photoemission techniques, whereas spin resolution is necessary to deduce their spin dependence.
Ultrathin Ni films on Cu(0 0 1) represent one of the most studied magnetic thin-film systems [9], [10]. They show unique magnetic properties such as unusual spin reorientation transitions. With increasing film thickness, the magnetization changes from in-plane to out-of-plane orientation at about 7–10 monolayers (ML) and back to in-plane at about 40 ML [11], [12], [13], [14]. Only few spin-integrated photoemission studies investigated the electronic structure for Ni films on Cu [15] and Ni/Cu multilayers [16]. Previous work with spin-integrated inverse photoemission dealt with Ni films on Cu(0 0 1) and Ni/Cu/Ni [17] and Cu/Ni/Cu [18] trilayer structures. So far, the main interest was in the electronic states of the Cu layer. For Ni films on Cu(0 0 1), discrete QW states were identified for normal electron incidence, but no spin resolution was used and no dispersion was measured [17]. However, it is the spin polarization of the electronic states that governs the magnetic properties of the thin-film structures and thus determines the magnetic response of possible devices. The spin polarization of the confined electron states with their oscillating density of states at the Fermi level influences the spin transport and the magnetic scattering in nanometer devices.
In the present study, the unoccupied states above the Fermi level of thin Ni films on Cu(0 0 1) have been investigated by spin- and angle-resolved inverse photoemission (IPE) [19], [20]. The spectra of thin Ni films are compared with those of the Cu(0 0 1) substrate and of a thick bulk-like Ni film. For a 6 ML thick Ni film, the exchange splitting of the QW states and their dispersion as a function of the wave vector parallel to the surface have been determined.
Section snippets
Experiment
In our spin-polarized inverse photoemission setup [21], a standard GaAs photocathode emits a beam of low-energy electrons (7–15 eV) with defined energy, momentum and spin. The electron spin polarization amounts to 33%. The spin polarization direction is perpendicular to the electron beam direction and, for normal incidence, parallel to the magnetization direction of a remanently in-plane magnetized sample. After impinging on the sample, the electrons may decay via radiative transitions into
Results
Fig. 1 shows IPE spectra for normal electron incidence on the Cu(0 0 1) substrate and on 6 ML, 10 ML and 100 ML thick Ni films on Cu(0 0 1). For non-ferromagnetic Cu(0 0 1), no spin asymmetry is observed within the experimental statistical error (Fig. 1a). The spectrum exhibits two features, both are well-known from the literature [28]: a bulk-like transition between sp states of Cu at 0.5 eV above the Fermi level and the n = 1 image-potential-induced surface state (IS) at 4.15 eV.
With 6 ML of ferromagnetic
Summary
In summary, the unoccupied electronic states in thin ferromagnetic Ni films on Cu(0 0 1) have been investigated. Three spin-polarized quantum-well features are clearly resolved. For a 6 ML thick Ni film, the exchange splittings of the quantum-well states have been determined. Furthermore, the energy vs. momentum relation of the electronic states has been studied. The dispersion of the quantum-well states as a function of the wave vector parallel to the surface follows the corresponding sp band
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