Interfacial Magnetism of Eu/Gd(0001) Studied by Magnetic Circular Dichroism in Photoemission
Physical Review Letters
We report on the fabrication of an atomically flat heteromagnetic rare-earth interface of 1-ML-Eu͑6 3 6͒͞Gd(0001), and its element-specific magnetic analysis by means of magnetic dichroism in photoemission, revealing a high net Eu magnetization at low temperatures. This magnetic Eu phase is due to a strong positive interlayer exchange coupling across the interface which overrules the weak negative intralayer coupling between Eu spins in the hexagonal two-dimensional lattice.
... [S0031-9007(98)05556-2] PACS numbers: 75.70.Cn, 78.20.Ls, 79.60.Dp Magnetic coupling in layered metallic structures has become a key issue in thin-film magnetism since the observation of oscillatory exchange coupling across nonferromagnetic spacer layers [1, 2] . Although this phenomenon was first discovered in rare-earth (RE) superlattices , most studies today deal with transition metal (TM) systems because of their technological relevance to magnetic storage devices. The present theoretical understanding of TM multilayers has been developed on the basis of detailed observations revealing, e.g., short-period oscillations in interlayer coupling strength  as well as 90 ± coupling . For these key findings, the fabrication of high-quality interfaces and the experimental access to atomic layers right at the interface has been essential. In metallic RE systems, due to the localized nature of the 4f electrons even in the solid phase, the exchange coupling is well described by the Heisenberg Hamiltonian H 2JS i S j , where J denotes the effective coupling strength between the localized 4f-spin moments S i and S j . The exchange coupling in RE metals is indirect, mediated by the 6s and 5d conduction electrons ; it is oscillatory and usually described in a Ruderman-Kittel-Kasuya-Yosida picture. Yet, since the 1980s, little progress has been made in understanding the coupling in RE superlattices. This is due mainly to the high intermiscibility of all trivalent RE metals, which has prevented preparation of atomically sharp interfaces [1,6]. Thus, a magnetic analysis of atomic layers right at a RE interface has not even been attempted so far. As a consequence, the present-day picture of RE interfaces is still based on the first semiempirical model assuming ideally flat interfaces  . In the present Letter, we report on the fabrication of an atomically flat heteromagnetic RE interface near a surface and its magnetic analysis by means of magnetic circular dichroism (MCD) in photoemission (PE). The Eu͞Gd interface, a text-book example of a Heisenberg system with spin-only localized 4f moments, is found to form a thermodynamically stable hexagonal two-dimensional lattice. The temperature dependence of the interface magnetization is monitored by MCD in PE, using 4f PE spectra. By comparison with a layer-dependent mean-field calculation, this allows for the first time to determine the strength of interlayer ͑J Ќ ͒ and intralayer ͑J jj ͒ exchange coupling for this RE interface. The PE experiments were performed at the U2-FSGM crossed-undulator beam line of the Berliner Elektronenspeicherring für Synchrotronstrahlung (BESSY), providing soft x rays with Х45% circular polarization around hn 48 eV . The light was incident at an angle of 15 ± with respect to the film plane, and the photoelectrons were collected around surface normal with a hemispherical electron-energy analyzer. For x-ray absorption (XA) measurements the SX700͞III monochromator at BESSY  was employed. The XA spectra were recorded in the total-electron-yield mode for different angles of light incidence with respect to the sample surface. 100-Å-thick Gd(0001) films, monitored by a quartz microbalance, were grown epitaxially on W(110) at 270 K by vapor-phase deposition of high-purity Gd metal evaporated from an electron-beam heated Mo crucible. The base pressure in the experimental chamber was 3 3 10 211 mbar, rising to 2 3 10 210 mbar during evaporation. Subsequent annealing at 630 K led to a wellordered Gd(0001) surface. Thin Eu films were deposited onto the Gd film at 300 K in an analogous way. A 1 monolayer (ML) Eu coverage, Q 0 , could be calibrated via the 4f-binding energy as well as the shape of the Eu-4f PE spectrum, since the 7 F 6 binding energy passes through a minimum of ͑2.45 6 0.02͒ eV at Q 0 . For Q . Q 0 , two additional features, growing in intensity with Q, appeared in the PE spectrum, one at each side of the multiplet, which can be assigned to interface and surface Eu atoms, respectively. An analogous behavior had been reported for Yb͞Mo(110)  . The films were magnetized remanently in plane at 25 K by short field pulses of about 2000 Oe produced by a nearby electromagnet. 0031-9007͞98͞80(10)͞2221(4)$15.00