Surface Physics
Time of flight spectra

(a) Time-of-flight spectra corresponding to the scattering of Ne ions of the given energies at a Cu(100) surface

Diffraction pattern

(b) Diffraction pattern produced by the scattering of low-energy electrons (220 eV) on Cu(100)

* Both experiments have been performed at the UHV-LEIS

Scientific activities at the Surface beamline concentrate on the growth and characterization of thin films of materials and the study of their surfaces and interfaces. Many technologically relevant materials find applications in the form of thin films.

An example is the field of information storage in magnetic media. It is expected not only to increase the capacity of conventional devices like hard disks, but also to develop new elements of spintronics that will allow new informatic arquitectures based on magnetic RAM (MRAM) or magnetic tunnel junctions (MTJ). For this aim, it is essential to control properties like the magnetic anisotropy, which can depend in a sensitive way on composition, morphology and structure of the films.

When system dimensions approach the nanometer scale, physical properties become increasingly determined by those of the surfaces and interfaces. In addition, their sensitivity to structural defects increases. For example, the magnetic properties of bulk Fe are known to depend strongly on the lattice parameter [1]. Due to this and to the number of different structural phases caused by epitaxial growth, the magnetic behaviour of ultrathin epitaxial films of Fe on Cu(100) is very complex [2]. It is therefore of enormous importance to perform a detailed characterization using techniques sensitive to the surface, in order to understand the properties of the systems and to be able to tailor them according to specific necessities.

The UHV‐surfaces beamline at CMAM contains a powerful set of facilities for the growth of thin epitaxial films by Molecular Beam Epitaxy (MBE) and in-situ sample characterization using several experimental techniques. These can be classified in techniques for characterization of thin films, among them the standard ion‐beam techniques that use the high‐energy ions provided by the CMAM accelerator [Rutherford backscattering spectroscopy (RBS), elastic recoil detection (ERDA), etc.] and methods for surface characterization [low‐energy ion scattering (LEIS), low‐energy electron diffraction (LEED) and Auger electron spectroscopy (AES)]. A quite unique experimental facility is the LEIS‐ToF system for surface structure determinations. Here the sample is bombarded by a chopped beam of noble gas ions (He, Ar, Ne...) with energies typically in the range 2‐6 keV. Time‐of‐Flight (ToF) spectra of scattered and recoiled particles are recorded. Azimuthal or polar scans of the intensity of scattered or recoiled particles can be obtained from the spectra by rotating the sample.  In this way,  using ion fluences of less than 1012 ions/cm2, surface structure determinations can be performed with minimal sample damage [3]. This represents a surface-sensitive technique complementary to LEED in many aspects.

This combination of growth facilities and analysis techniques allows the study of many materials interesting for their magnetic applications. Among them, materials containing rare-earths show peculiar magnetic properties. Compounds between transition metals and rare earths can combine high ordering temperatures with high magnetic anisotropies. For example, the systems Nd-Fe and Sm-Co contain the magnetically hardest materials known and are promising candidates for approaching the superparamagnetic limit in magnetic nanostructures. Another class of materials with interesting magnetic properties are rare-earth oxides; for example, EuO is a ferromagnetic semiconductor showing a transition with an associated change of several orders of magnitude in the resisitivity [4].  Effects relevant for spintronics have been observed in surface phases of other rare-earth oxides, like the Rashba splitting in GdO 5. Also transition-metal oxides like CrO2, a ferromagnetic half-metal [6], can show full spin polarization of the conduction electrons. Structural characterizations of thin films of these materials have been rather scarce. Therefore, it is expected that the structural information provided by the techniques available at this experimental system will lead to significant advances in the understanding of their magnetic properties.

[1] V.L. Moruzzi, P.M. Markus and J. Kübler, Phys. Rev. B 39, 6957 (1989)
[2] S. Müller, P. Bayer, C. Reischl, K. Heinz, B. Feldmann, H. Zillgen and M. Wuttig, Phys. Rev. Lett. 74, 765 (1995)
[3] S.Y. Grachev, D.M. Borsa and D.O. Boerma, Surf. Sci. 516, 159 (2002)
[4] P.G. Steeneken, L.H. Tjeng, I. Elfimov, G.A. Sawatzky, G. Ghiringhelli, N.B. Brookes and D.-J. Huang, Phys. Rev. Lett. 88, 047201 (2002)
[5] O. Krupin, G. Bihlmayer, K. S4tarke, S. Gorovikov, J.E. Prieto, K. Döbrich, S. Blügel and G. Kaindl, Phys. Rev. B 71, 201403(R) (2005)
[6] J.M. D. Coey and M. Venkatesan, J. Appl. Phys. 91, 8345 (2002)