We investigate the electronic and magnetic properties of single Fe, Co, and Ni atoms and clusters on monolayer graphene (MLG) on SiC(0001) by means of scanning tunneling microscopy (STM), x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), and ab initio calculations. STM reveals different adsorption sites for Ni and Co adatoms. XMCD proves Fe and Co adatoms to be paramagnetic and to exhibit an out-of-plane easy axis in agreement with theory. In contrast, we experimentally find a nonmagnetic ground state for Ni monomers while an increasing cluster size leads to sizeable magnetic moments. These observations are well reproduced by our calculations and reveal the importance of hybridization effects and intra-atomic charge transfer for the properties of adatoms and clusters on MLG.

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We studied tetragonally distorted Fe(1-x)Co(x) alloy films on Rh(001), which show a strong perpendicular anisotropy in a wide thickness and composition range. Analyzing x-ray magnetic circular dichroism spectra at the L_(3,2) edges we found a dependence of the Co magnetic orbital moment on the chemical composition of the Fe(1-x)Co(x) alloy films, with a maximum at x=0.6. For this composition, we observed an out-of-plane easy axis of magnetization at room temperature for film thickness up to 15 monolayers. Since both the magnetic orbital moment and the anisotropy energy show similar composition dependence, it confirms that both quantities are directly related. Our experiments show that the adjustment of the Fermi level by a proper choice of the alloy composition is decisive for the large magnetic orbital moment and for a large magnetic anisotropy in a tetragonally distorted lattice.

International audienceVery large average velocities, up to 600 m/s, have been found for domain-wall motion driven by 3-ns-long pulses of electric current in zero magnetic field in the NiFe layer of 200-nm-wide NiFe/Cu/Co nanowires. For longer pulses, the domain-wall motion is strongly hindered by pinning potentials. Dipolar interactions between the NiFe and Co layers caused by anisotropy inhomogeneities have been identified as the most important among the different potential sources of DW pinning. The domain-wall velocities increase with current density, but a substantial drop is observed at current densities above 4×10^11 A/m

Current-induced magnetic domain wall motion at zero magnetic field is observed in the permalloy layer of a spin-valve-based nanostripe using photoemission electron microscopy. The domain wall movement is hampered by pinning sites, but in between them high domain wall velocities (exceeding 150 m/s) are obtained for current densities well below 10 12 A/m 2 , suggesting that these trilayer systems are promising for applications in domain wall devices in case of well controlled pinning positions. Vertical spin currents in these structures provide a potential explanation for the increase in domain wall velocity at low current densities.Moving magnetic domain walls using electric currents via spin-torque effects rather than using a magnetic field is one of the recent exciting developments in spintronics 1 . Since the prediction of spin-torque effects 2 , many experimental 3,4,5,6,7,8,9,10,11,12,13,14,15 and theoretical 16,17,18 works have been dedicated to the study of currentinduced domain wall motion (CIDM). Besides fundamental investigations, the use of domain walls in logic 19 and memory 20 devices has already been proposed. Low current densities and high domain wall (DW) velocities at zero magnetic field are required for future applications.Direct evidence of CIDM at zero field has been reported for several nanostripe systems, including permalloy (FeNi) 6,11 , magnetic semiconductors 9 and systems with perpendicular magnetization 7,12,14,15 . For the commonly used FeNi system, the critical current densities are not much below 10 12 A/m 2 at zero magnetic field 8,11 , associated with DW velocities going from some m/s up to about 100 m/s 11 . Much lower critical currents are found for magnetic semiconductors like GaMnAs (about 1 × 10 9 A/m 2 ) because of the low magnetic moments, but the observed DW velocities are small (< 1 m/s) 9 . Moreover, these materials are not ferromagnetic at room temperature. Low current density values are also found in spin-valve-based nanostripes with either in-plane 3,10 or perpendicular anisotropy 7 . Additionally, transport measurements in FeNi/Cu/Co trilayers show CIDM induced by subnanosecond current pulses 5 , indirectly indicating high DW velocities in such spin-valve-based systems. In this work, we show that in these systems CIDM at zero magnetic field can take place with high DW velocities (exceeding 150 m/s) at current densities well below 10 12 A/m 2 . These high velocities are observed only in certain regions of the nanostripes, where domain wall pinning is limited. Currents perpendicular to the plane in the vicinity of the DW are probably partly responsible for this increase in efficiency, which makes the trilayer systems possible candidates for spintronic applications based on CIDM if pinning can be controlled.We observed domain wall motion in the FeNi layer of 400 nm wide FeNi (5 nm)/Cu (8 nm)/Co (7 nm)/ CoO (3 nm) nanostripes, by using Photoemission Electron Microscopy (PEEM) combined with X-ray Magnetic Circular Dichroism (XMCD) 21 . The Cu spacer layer is chosen to ...