The magnetism of 1-ML-thick films of Fe x Co 1−x on Pt͑111͒ was investigated both experimentally, by x-ray magnetic circular dichroism and magneto-optical Kerr effect measurements, and theoretically, by firstprinciples electronic structure calculations, as a function of the film chemical composition. The calculated Fe and Co spin moments are only weakly dependent on the composition and close to 3 B / atom and 2 B / atom, respectively. This trend is also seen in the experimental data, except for pure Fe, where an effective spin moment of only S eff = ͑1.2Ϯ 0.2͒ B / atom was measured. On the other hand, both the orbital moment and the magnetic anisotropy energy show a strong composition dependence with maxima close to the Fe 0.5 Co 0.5 stoichiometry. The experiment, in agreement with theory, gives a maximum magnetic anisotropy energy of 0.5 meV/atom, which is more than 2 orders of magnitude larger than the value observed in bulk bcc FeCo and close to that observed for the L1 0 phase of FePt. The calculations clearly demonstrate that this composition dependence is the result of a fine tuning in the occupation number of the d x 2 −y 2 and d xy orbitals due to the Fe-Co electronic hybridization.

We report a new multinuclei based nuclear magnetic resonance method which allows us to image the staggered polarization induced by nonmagnetic Li impurities in underdoped ͑O 6.6 ͒ and slightly overdoped ͑O 7 ͒ YBa 2 Cu 3 O 6+y above T C . The spatial extension of the polarization imp approximately follows a Curie law, increasing up to six lattice constants at 80 K at O 6.6 in the pseudogap regime. Near optimal doping, the staggered magnetization has the same shape, with imp reduced by a factor 2. imp is argued to reveal the intrinsic magnetic correlation length of the pure system. It is found to display a smooth evolution through the pseudogap regime.

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.

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.