Epitaxial Gd(0001) has been investigated with spin-polarized low-energy-electron diffraction and the magneto-optic Kerr effect. The ferromagnetic critical temperature of the surface layer is found to lie up to 22 K above the bulk Curie point, demonstrating the existence of surface-enhanced magnetic order. Furthermore, spin-resolved photoemission spectroscopy reveals that the 4/ spins of the surface are not ferromagnetically coupled to the bulk moments.The concept of universality in the field of phase transitions has stimulated renewed interest for this mature field of solid state physics, because a variety of apparently very different phenomena can be described by the same theoretical principles. In this sense, for example, presetting criticality and pure surface-enhanced transitions are essentially the same phenomenon. 1 On the other hand, advances in experimental techniques today allow the crosschecking of such far-reaching predictions. Magnetic systems as model systems have always been appealing to experimentalists and theorists. Recently systems with free surfaces have been investigated extensively both analytically 2 " 4 and by Monte Carlo simulations. 5 For systems exhibiting a continuous bulk phase transition the critical behavior is found to be related to the ratio of J\/Jb of the parallel coupling Ji between surface spins in the top layer and that J b between bulk spins. For values of J x below a certain critical value J Xc the surface and the bulk will have the same critical temperature, while for J x > J Xc a pure "surface transition" has been predicted to occur. In such a case the surface spins undergo a critical ordering transition in the presence of disordered bulk spins. 5 One purpose of this Letter is to report the first direct observation of such a critical surface transition on epitaxial Gd(0001) by means of the surface-sensitive technique of spin-polarized low-energy-electron diffraction (SPLEED). For an in situ comparison with the corresponding bulk transition the magneto-optical Kerr effect was used. An interaction which is not included in the above model is that case when the perpendicular coupling between the topmost surface layer and the layer(s) below is different from J b . It is a further goal of this paper to investigate this question by means of spin-polarized photoemission. In fact, we find for the first time that the Af spins of the surface atoms of Gd(0001) undergo a so-called "magnetic surface reconstruction." 6 This observation has become possible by our making use of the surface-induced binding-energy shift of the 4/levels. 7 " 9The SPLEED technique as well as the spin-,

The magnetic ordering at the surface of epitaxial Cr overlayers on Fed 00) is studied by spinpolarized electron-energy-loss spectroscopy. The exchange asymmetry oscillates with the thickness of the Cr overlayer with a period of about two atomic layers, proving directly that the surface Cr layer has a net ferromagnetic moment and that successive layers order antiferromagnetically. The exchange asymmetry is predominantly due to spin-flip scattering. The spin-flip spectrum broadens toward lower energy with increasing thickness, suggesting that Cr may have a surface-enhanced magnetic moment.

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