VOLUME 58, NUMBER 5 PHYSICAL REVIEW LETTERS 2 FEBRUARY 1987 Ferromagnetic Order and the Critical Exponent y for a Gd Monolayer: An Electron-Spin-Resonance Study M. Fade and K. Baberschke Institut fiir Atom- und Festk'drperphysik, Freie Universitdt Berlin, D-1000 Berlin 33, Federal Republic of Germany (Received 18 September 1986) A monolayer of Gd(0001) on W(l 10) is measured by electron-spin resonance from r = 240 to 360 K in UHV. The ferromagnetic Curie temperature lies ~ 20 K below the bulk Curie point. The measured temperature dependence of the static susceptibility—deduced from the ESR intensity—follows a power law X—t~ y with y « 1.8 for a monolayer and y~ 1.25 for an 80-A film. This agrees well with the theoretical y of 2D and 3D Ising systems. The experiment represents the first in situ UHV ESR study with full surface analysis allowing the measurement of magnetic phase transitions on single-crystal sur- faces. PACS numbers: 75.50.Cc, 75.70.Dp, 76.30.Kg The critical behavior of the paramagnetic susceptibili- ty and the magnetization of thin surface layers has been the subject of many investigations. 1 Gadolinium is a prominent candidate for such studies. 2 " 5 Many ques- tions concerning the magnetic behavior of a thin Gd film are still open to be answered: Does a Gd monolayer (ML) show a ferromagnetic phase transition at all? If it does, will the transition temperature TQ S be shifted to higher or lower temperatures? Is it possible to detect finite-size effects in thin Gd films, i.e., is there a thickness-dependent shift of Tc s ? What is the difference between a magnetic monolayer [i.e., Gd(0001) on W(l 10)] and a semi-infinite system (i.e., the surface lay- er of Gd on Gd metal)? Answers to these questions con- tribute to the theoretical understanding of the local- moment spin-spin coupling and of the influence of the local-moment-conduction-electron interaction. Since the ferromagnetic properties of rare-earth-transition- metal compounds are essentially determined by those in- teractions, the answers to the above questions have direct applications to the "engineering" of thin-film ferromag- nets. First studies on polycrystalline Gd films prepared in ultrahigh vacuum (UHV) by Rau 2 (using electron- capture spectroscopy) and by Cerri, Mauri, and Lando- lt 3 (using spin-resolved photoemission) showed a devia- tion of the film magnetization from the bulk one and a shift of the surface-layer ordering temperature to higher values than the bulk Curie temperature 7"c/> = 292.5 K. The magnetic ordering of the topmost layear of a 140-A epitaxial Gd(0001) film on W(110) 22 K above T Ch was reported by Campagna and co-workers. 4a They mea- sured in zero applied field using spin-polarized low- energy electron diffraction (SPLEED) and spin-resolved photoemission. A Gd monolayer on Fe(100) was investi- gated by Taborelli et al. 5 using spin-polarized Auger- electron spectroscopy (SPAES). In the present work we will demonstrate the usefulness of UHV ESR in the field of surface magnetism. As has been shown, 6 ' 7 the high sensitivity of ESR (10 12 spins) allows us to detect a fraction of a monolayer Gd on a metal surface in the paramagnetic regime. The fer- romagnetic resonance (FMR) which was also recorded here (below TQ S ) will be discussed elsewhere. While the FMR of ultrathin magnetic layers (Fe, Ni, CO) has been reported previously, 8 " 10 the present work is the first ESR measurement of a well-characterized magnetic monolayer far above the transition temperature. At present, UHV ESR seems to be the only technique able to collect magnetic data above TQ S for a clean magnetic ML. Even the torque measurements of the surface mag- netization of Gradmann 11 are restricted to the magneti- cally ordered state. It is known 4b ' 12 that a Gd(0001) ML grows epitaxially on W(l 10), provided that extreme care is taken to clean the substrate and the evaporant. The growth modus is controlled by Auger-electron spectroscopy (AES) and LEED. Figure 1 (a) shows the linear increase of the Gd(138/140 eV) peak amplitude till the monolayer (defined as an adsorbate coverage 0A =1.0) is complete after an evaporation time of 16 min. Longer evaporation times lead to a Stranski-Krastanov-type growth modus (formation of Gd islands on one or two epitaxial layers) in agreement with Ref. 12. We chose deliberately a cov- erage of 6A =0.8 (i.e., 80% of a monolayer) to have at most one monolayer or less on the surfce (approximately 40 mm 2 ). A sample with 0 A = 16 and a 80-A thick film were also measured. For the latter thickness it is as- sumed that the film has formed a smooth surface again, and a LEED structure is detected. 45 After preparation and characterization of the epitaxial Gd film, the sample is moved in situ into a quartz finger of the UHV chamber. 6,7 Rotation of the sample allows ESR experi- ments with the Zeeman field H applied perpendicular and parallel to the surface plane. Experimental ESR recorder traces at approximately 50 K above the ordering temperature are shown in Fig. 1(b). First, we see that the ESR is sensitive to -^ of a © 1987 The American Physical Society 511