Research Topics

Magnetic impurities

STM image of an individual Cobalt atom on a single crystal Cu(100) surface	X-ray absorption spectra and dichroism of Fe, Co, and Ni impurities on K films compared to the bulk metals (insets).

Owing to their reduced atomic coordination, magnetic impurities confined in a two-dimensional environment can be viewed as a bridge between the atomic and solid state, with many of their electronic and magnetic properties determined by competing intra-atomic correlation and band structure effects. The investigation of the adsorption sites, diffusion and nucleation processes of metal atoms on crystalline surfaces offers countless opportunities to tune the adatom-substrate interaction as well as to construct multiatom clusters of controlled shape and dimensions with unusual magnetic properties. Adatoms and clusters may be considered as the precursors of thin films, as the growth of magnetic mono- and multilayers is typically initiated by the deposition of transition-metal atoms from the vapor phase onto a nonmagnetic substrate. Investigating substrate-impurity hybridization and coordination effects thus provides basic understanding and useful guidelines to tailor the magnetization and magnetic anisotropy of films as well as nanoparticles and optimize sensitive interface properties that govern the performances of magnetic storage media and electron transport in spintronic devices. Magnetic impurities further constitute a fundamental ingredient of dilute magnetic semiconductors and oxide materials.

Metal nanoparticles

Magnetic anisotropy energy (K) and orbital magnetic moment (L) of Co nanoparticles self-assembled on Pt(111).	Exchange-biased permalloy dots imaged by photoemission electron microscopy.

The material properties of bulk solids depend on their elemental composition and crystalline structure. Particles whose dimensions are reduced below the micron scale offer two additional handles to control their electronic and magnetic behavior: size and shape. Although obviously related, these two factors play quite a distinct role in governing the interplay of quantum and classical properties. Progress in the growth and characterization methods of supported nanoparticles opens great perspectives to produce regular particle arrays using either bottom-up self-assembly methods or top-down lithography and ion milling techniques. Here, we intend to exploit such methods to synthesize arrays of magnetic metal particles supported on magnetic and nonmagnetic substrates as well as in tunnel barrier junctions, addressing their magnetoelectronic behavior either one-by-one or in homogeneous ensembles.

Molecular-metal interfaces

Self-assembly of metal-phthalocyanines imaged by STM.

The assembly of molecular species deposited on flat or patterned substrates are topics of great current interest, owing to recent fundamental and practical advances in the field of molecular electronics and spintronics. Experiments on single as well as on multitudes of molecules connected in parallel have shown several useful electronic effects, including rectification, switching, and charge storage. The ability to arrange specific functional species into interconnected units to form, e.g., molecular wire arrays, and logic gates on a given substrate is a pre-requisite to construct prototype molecular devices. Molecule-based magnetic materials, on the other hand, display unique properties unavailable in conventional metal/intermetallic and metal-oxide magnets. Molecular self-assembly in two dimensions is investigated here as a route to position a large number of molecules into specific and predictable locations, with the goal of establishing conducting molecular links or robust magnetic coupling. The balance of intermolecular and molecule-surface interactions that governs self-assembly can be tuned by the appropriate choice of functional groups, substrate material and symmetry. In connection with our studies of magnetic impurities, we focus on metal-organic complexes obtained by supramolecular synthesis at surfaces or directly by sublimation in vacuum of molecular crystals, aiming at creating molecules that exhibit paramagnetic or remanent ferro/ferrimagnetic behavior depending on the temperature, substrate material, and choice of ligands.

X-ray detection of magnetic resonance

Element resolved ferromagnetic resonance spectra of Gd-doped yittrium-iron-garnet.

Resonance and relaxation processes underlie our understanding of magnetization dynamics. A fundamental question is where and how angular momentum is transferred from an external electromagnetic field and distributed to the atoms and electrons in a lattice. The interplay of the spin, electron, and lattice degrees of freedom in ferromagnetic materials make this a challenging and microscopically poorly understood issue, which has broad applications in high-frequency magneto-optic and data storage devices. Here, we pursue a novel approach to detect continuous-wave magnetic resonance spectra using polarized x-ray absorption. This technique, exploiting directional dichroism effects, allows us to resolve element-specific ferromagnetic resonance spectra in heterogeneous magnetic compounds such as rare-earth-doped garnets and metal multilayers. Phenomena related to dynamic coupling of elemental moments in alloys and thin films, spin and orbital momentum precession, and angular moment compensation can be investigated. Moreover, x-ray detection of paramagnetic resonance in metal-organic molecular species can be envisaged.

ICN-ALBA Collaboration

Combined STM and XMCD facility at ICN-ALBA

Taking advantage of the proximity of the new spanish synchrotron light source, an important part of our activity is directed towards combining the expertise and facilities of our institute and ALBA. Two of the seven beamlines in phase I of ALBA will be devoted to research in magnetism. PEEM (photoemission microscopy), XMCD and XRMS experimental facilities will be installed in variable polarization beamlines in the soft x-ray range. The XMCD endstation will be equipped with variable magnetic fields up to 7 T , a liquid He cryostat, and UHV environment. Our contribution to the latter consists in the design, set up and maintenance of a dedicated UHV chamber for sample growth and characterization, a prerequisite for the success of many experiments that require fine-tuning of the sample composition and morphology. This chamber has been recently assembled and includes e-beam evaporators, Knudsen cells, a LEED Auger spectrometer, and a variable-temperature STM. It will be available to national and foreign ALBA users for performing XMCD experiments that require in-site sample growth and atomic-scale characterization, pending scientific and technical approval of beamtime proposals.