Enhanced atomic-scale spin contrast due to spin friction.

Atom manipulation with the magnetic tip of a scanning tunneling microscope is a versatile technique to construct and investigate well-defined atomic spin arrangements. Here we explore the possibility of using a magnetic adatom as a local probe to image surface spin textures. As a model system we choose a Néel state with 120° between neighboring magnetic moments. Close to the threshold of manipulation, the adatom resides in the threefold, magnetically frustrated hollow sites, and consequently no magnetic signal is detected in manipulation images. At smaller tip-adatom distances, however, the adatom is moved towards the magnetically active bridge sites and due to the exchange force of the tip the manipulation process becomes spin dependent. In this way the adatom can be used as an amplifying probe for the surface spin texture.

Magnetization reversal of individual Co nanoislands.

We investigate the magnetization reversal of individual Co islands on Cu(111) in the size range of N=700 to 18,000 atoms by spin-polarized scanning tunneling microscopy at 8 K. The switching field H(sw) changes with island size in a nonmonotonic manner: it increases with island size and reaches a maximum value of 2.4 T at N=5500 atoms, and it decreases for larger islands. We extract the energy barrier for magnetization reversal as a function of island size. The maximum H(sw) corresponds to an energy barrier of 1 eV. Our results elucidate a crossover of the magnetization reversal from an exchange-spring behavior to domain wall formation with increasing size at around 7500 atoms.

Electronic states of moiré modulated Cu films.

We examined by low-energy electron diffraction and scanning tunneling microscopy the surface of thin Cu films on Pt(111). The Cu/Pt lattice mismatch induces a moiré modulation for films from 3 to about 10 ML thickness. We used angle-resolved photoemission spectroscopy to examine the effects of this structural modulation on the electronic states of the system. A series of hexagonal- and trigonal-like constant energy contours is found in the proximity of the Cu(111) zone boundaries. These electronic patterns are generated by Cu sp-quantum well state replicas, originating from multiple points of the reciprocal lattice associated with the moiré superstructure. Layer-dependent strain relaxation and hybridization with the substrate bands concur to determine the dispersion and energy position of the Cu Shockley surface state.

Atomic-scale engineering of magnetic anisotropy of nanostructures through interfaces and interlines.

The central goals of nanoscale magnetic materials science are the self-assembly of the smallest structure exhibiting ferromagnetic hysteresis at room temperature, and the assembly of these structures into the highest density patterns. The focus has been on chemically ordered alloys combining magnetic 3d elements with polarizable 5d elements having high spin-orbit coupling and thus yielding the desired large magneto-crystalline anisotropy. The chemical synthesis of nanoparticles of these alloys yields disordered phases requiring annealing to transform them to the high-anisotropy L1(0) structure. Despite considerable efforts, so far only part of the nanoparticles can be transformed without coalescence. Here we present an alternative approach to homogeneous alloys, namely the creation of nanostructures with atomically sharp bimetallic interfaces and interlines. They exhibit unexpectedly high magnetization reversal energy with values and directions of the easy magnetization axes strongly depending on chemistry and texture. We find significant deviations from the expected behaviour for commonly used element combinations. Ab-initio calculations reproduce these results and unravel their origin.

Impurity-induced local magnetism and density of states in the superconducting state of YBa2Cu3O7.

17O NMR is used to probe the local influence of nonmagnetic Zn and magnetic Ni impurities in the superconducting state of optimally doped high TC YBa2Cu3O7. Zn and Ni induce a staggered paramagnetic polarization, similar to that evidenced above TC, with a typical extension xi=3 cell units for Zn and xi>or=3 for Ni. In addition, Zn is observed to induce a local density of states near the Fermi energy in its neighborhood, which also decays over about 3 cell units. Its magnitude decreases sharply with increasing temperature. This allows direct comparison with the STM observations done in BiSCCO.

Reversible H-induced switching of the magnetic easy axis in Ni/Cu(001) thin films.

A reversible switching of the easy axis of magnetization for Ni on Cu(001) from in plane to out of plane is found by changing the partial pressure of hydrogen in the gas phase around the sample, allowing even for oscillations of the magnetization direction. A quantitative low-energy electron diffraction study of the diffracted intensity versus electron energy [I(E)] shows that the hydrogen-induced spin reorientation transition is accompanied by changes of the tetragonal distortion of the topmost Ni layer upon hydrogen adsorption. Surprisingly, the orientation switch to perpendicular to the surface comes with a relaxation, i.e., reduction of the film's tetragonal distortion rather than its amplification.

Absence of static phase separation in the high T(c) cuprate YBa(2)Cu(3)O(6+y).

We use 89Y NMR in YBa(2)Cu(3)O(6+y) in order to evaluate with high sensitivity the distribution of hole content p in the CuO2 planes. For y=1 and y=0.6, this hole doping distribution is found narrow with a full width at half maximum smaller than Deltap=0.025. This rules out any large static phase separation between underdoped and optimally doped regions in contrast with the one observed by STM in Bi2212 and by NQR in LaSrCuO. This establishes that static electronic phase separation is not a generic feature of the cuprates.