We report on angle-resolved photoemission studies of the electronic pi states of high-quality epitaxial graphene layers on a Ni(111) surface. In this system the electron binding energy of the pi states shows a strong dependence on the magnetization reversal of the Ni film. The observed extraordinarily large energy shift up to 225 meV of the graphene-derived pi band peak position for opposite magnetization directions is attributed to a manifestation of the Rashba interaction between spin-polarized electrons in the pi band and the large effective electric field at the graphene/Ni interface. Our findings show that an electron spin in the graphene layer can be manipulated in a controlled way and have important implications for graphene-based spintronic devices.

We present an innovative approach to the production of single-crystal iron oxide nanorings employing a solution-based route. Single-crystal hematite (alpha-Fe2O3) nanorings were synthesized using a double anion-assisted hydrothermal method (involving phosphate and sulfate ions), which can be divided into two stages: (1) formation of capsule-shaped alpha-Fe2O3 nanoparticles and (2) preferential dissolution along the long dimension of the elongated nanoparticles (the c axis of alpha-Fe2O3) to form nanorings. The shape of the nanorings is mainly regulated by the adsorption of phosphate ions on faces parallel to c axis of alpha-Fe2O3 during the nanocrystal growth, and the hollow structure is given by the preferential dissolution of the alpha-Fe2O3 along the c axis due to the strong coordination of the sulfate ions. By varying the ratios of phosphate and sulfate ions to ferric ions, we were able to control the size, morphology, and surface architecture to produce a variety of three-dimensional hollow nanostructures. These can then be converted to magnetite (Fe3O4) and maghemite (gamma-Fe2O3) by a reduction or reduction-oxidation process while preserving the same morphology. The structures and magnetic properties of these single-crystal alpha-Fe2O3, Fe3O4, and gamma-Fe2O3 nanorings were characterized by various analytical techniques. Employing off-axis electron holography, we observed the classical single-vortex magnetic state in the thin magnetite nanorings, while the thicker rings displayed an intriguing three-dimensional magnetic configuration. This work provides an easily scaled-up method for preparing tailor-made iron oxide nanorings that could meet the demands of a variety of applications ranging from medicine to magnetoelectronics.

Direct observations of current-induced domain-wall propagation by spin-polarized scanning electron microscopy are reported. Current pulses move head-to-head as well as tail-to-tail walls in submicrometer Fe20Ni80 wires in the direction of the electron flow, and a decay of the wall velocity with the number of injected current pulses is observed. High-resolution images of the domain walls reveal that the wall spin structure is transformed from a vortex to a transverse configuration with subsequent pulse injections. The change in spin structure is directly correlated with the decay of the velocity.

In a combined numerical and experimental study, we demonstrate that current pulses of different polarity can reversibly and controllably displace a magnetic domain wall (DW) in submicrometer permalloy (NiFe) ring structures. The critical current densities for DW displacement are correlated with the specific spin structure of the DWs and are compared to results of micromagnetic simulations including a spin-torque term. Using a notch, an attractive local pinning potential is created for the DW resulting in a highly reproducible spin structure of the DW, critical for reliable current-induced switching.PACS numbers: 72.15. Gd, 75.60.Ch, 75.60.Ej, 85.70.Kh Switching by domain wall motion [1] induced by spinpolarized currents rather than by external fields is a promising approach to the switching of magnetic nanostructures, since it entails simple fabrication processes without the need for strip lines, combined with the possibility of achieving fast and reproducible switching [2 -6]. The current-induced magnetization switching mechanism has been shown to be able to switch multilayer giant magnetoresistance structures [7] and to reverse simple single layer elements as observed for L-shaped elements [2,3], for ring-shaped [4] and straight [5] structures with constrictions, and also in multilayer wires [6]. This currentinduced domain wall motion is due to a spin-torque effect, where the electrons transfer angular momentum to the domain wall when passing through it, pushing it in the direction of the electron flow [8]. Since the original paper by Berger [8], a number of different theories have been suggested that treat the interaction between the spinpolarized current and the magnetization in the ballistic limit [9,10] or in the diffusive limit [9,11]. For wide inplane domain walls with widths of hundreds of nanometers [12] the spins of the electrons are expected to follow the magnetization adiabatically, and thus the diffusive description is expected to apply. While there are a number of experimental results, no direct quantitative comparison to the theoretical predictions has been made available.In this Letter we demonstrate that different types of head-to-head domain walls present in ring structures can be reversibly and controllably displaced by spin-polarized current pulses. Direct comparison with the results of our micromagnetic simulations that include a diffusive spintorque term allows us to determine to what extent the experimentally observed effects can be described by a purely diffusive spin-torque theory.Rings are a particularly apt geometry to investigate the effect of pulses on domain walls, since head-to-head walls with different spin structures can be obtained for different ring geometries [12]. For thin film rings, transverse walls

The spin-dependent electronic structure of thin epitaxial films of magnetite, Fe 3 O 4 (111), has been investigated at room temperature by means of spin-, energy-, and angle-resolved photoemission spectroscopy. Near the Fermi energy E F a spin polarization of Ϫ(80Ϯ5)% is found. The spin-resolved photoemission spectra for binding energies between 1.5 eV and E F show good agreement with spin-split band energies from densityfunctional calculations.PACS number͑s͒: 75.70. Ak, 75.50.Bb, 75.70.Cn, 79.60.Bm The materials class of half-metallic ferromagnets ͑HMF's͒ has attracted renewed interest recently in the search for efficient spin polarizers in spin electronics.1 The intriguing feature of metallic conductivity for one spin component and semiconducting behavior for the other was in most cases theoretically predicted on the basis of electron band structure calculations. An experimental struggle extended over many years and is still ongoing to convincingly verify the truly intrinsic spin-dependent electronic structure of HMF's and consequently the high-spin polarization at the Fermi energy E F . The use of surface-sensitive measurements like spinpolarized photoemission, tunneling into superconductors, or superconducting point-contact spectroscopy imposed severe constraints in addition to problems with sample stoichiometry and homogeneity. In many cases the preparation of highquality thin films was indispensable instead of bulk single crystals which are believed to be superior. Thus, from the first theoretical prediction of, e.g., HMF behavior in Heusler alloys in 1983, 2 it took almost two decades to find evidence for spin polarization values at E F which come close to the expected ones.3 However, problems with the stoichiometry and especially surface composition of the films used are prevailing. [3][4][5] Besides the Heusler alloys, the majority of HMF's have been identified among transition metal oxides on the basis of the local spin-density approximation ͑LSDA͒ to the densityfunctional theory. Predictions have been made for Fe 3 O 4 , 6,7 CrO 2 , 8,9 manganites, 10,11 and the double perovskite Sr 2 FeMoO 6 .12 Only recently, values of the spin polarization of over 90% near E F were found for CrO 2 at 1.8 K using superconducting point-contact spectroscopy, 13,14 although values of 95% had been obtained earlier at 300 K at binding energies of 2 eV below E F using spin-polarized photoemission. 15 The most straightforward evidence of a minority spin gap and a concomitant 95% spin polarization near E F was obtained in La 0.7 Sr 0.3 MnO 3 at 40 K by means of spin-polarized photoemission spectroscopy. 16In this paper we present experimental evidence for the half-metallic ferromagnetic state of magnetite (Fe 3 O 4 ) by means of spin-and angle-resolved vacuum ultraviolet ͑VUV, hϭ21.2 eV͒ photoemission spectroscopy. Using epitaxial Fe 3 O 4 (111) films we obtain at room temperature a negative spin polarization of Ϫ(80Ϯ5)% at E F . This value agrees within 6% with the magnetization at 300 K of a thin Fe 3 O 4 film. 17 M...

Here we report the photoemission studies of intercalation process of Fe underneath graphene layer on Ni(111). The process of intercalation was monitored via XPS of corresponding core levels and UPS of the graphene-derived π states in the valence band. fcc-Fe films with thickness of 2-5 monolayers at the interface between graphene and Ni(111) form epitaxial magnetic layer passivated from the reactive environment, like for example oxygen gas.

Current induced domain wall (DW) depinning of a narrow DW in out of plane magnetized ðPt=CoÞ 3 =Pt multilayer elements is studied by magnetotransport. We find that for conventional measurements Joule heating effects conceal the real spin torque efficiency and so we use a measurement scheme at a constant sample temperature to unambiguously extract the spin torque contribution. From the variation of the depinning magnetic field with the current pulse amplitude we directly deduce the large nonadiabaticity factor in this material and we find that its amplitude is consistent with a momentum transfer mechanism. DOI: 10.1103/PhysRevLett.101.216601 PACS numbers: 72.25.Ba, 75.60.Ch, 75.75.+a The recent discovery that a spin-polarized current can displace a domain wall (DW) through the spin transfer from conduction electrons to the local magnetization [1] has opened up an alternative approach to manipulate magnetization. Current induced domain wall motion (CIDM) has been investigated experimentally so far in detail in permalloy (Py; Ni 80 Fe 20 ) nanowires characterized by wide DWs (>100 nm) where the spin of a conduction electron is expected to follow adiabatically the magnetization direction as the electron passes across the DW [1,2]. A key question that has been raised is whether the spin transfer effect contains nonadiabatic contributions due to spin relaxation or nonadiabatic transport [2][3][4][5][6]. It was predicted [3,7] that from the efficiency of the spin transfer effect, which is measured by probing the dependence of the DW propagation magnetic field on the injected current, the nonadiabaticity can be deduced. However, in Py nanowires, the complicated 2D spin structures of the DWs prevent direct comparison to 1D models and a meaningful comparison to full 2D micromagnetic simulations is only possible if the exact spin structure during pulse injection is known, which is generally not the case. In particular, the wall deformations and transformations that have been observed [8] can render the results impossible to interpret in terms of the nonadiabaticity.To obtain simple DW spin structures, out-of-plane magnetized materials with a strong uniaxial anisotropy can be used where the simple Bloch or Néel DW spin structure is more apt for an analysis using an analytical 1D model including the nonadiabatic torque terms. In addition, a larger nonadiabaticity is expected in these materials due to the larger magnetization gradient for such narrow DWs [2,4,9]. This larger nonadiabaticity may explain the high efficiency of the current induced DW depinning reported recently in such materials [10,11]. However, another major obstacle for the determination of the nonadiabaticity from the dependence of the DW depinning magnetic field on current is that Joule heating strongly affects the thermally activated DW depinning. For experiments carried out at a constant cryostat temperature, it is thus hard to extract directly the contribution from the spin transfer torque.In this Letter we probe CIDM in out-of-plane magnetized ðPt=C...

In situ prepared Fe 3 O 4 ͑100͒ thin films were studied by means of scanning tunneling microscopy ͑STM͒ and spin-polarized photoelectron spectroscopy ͑SP-PES͒. The atomically resolved ͑ ͱ 2 ϫ ͱ 2͒R45°wavelike surface atomic structure observed by STM is explained based on density functional theory ͑DFT͒ and ab initio atomistic thermodynamics calculations as a laterally distorted surface layer containing octahedral iron and oxygen, referred to as a modified B layer. The work-function value of the Fe 3 O 4 ͑100͒ surface extracted from the cutoff of the photoelectron spectra is in good agreement with that predicted from DFT. On the Fe 3 O 4 ͑100͒ surface both the SP-PES measurements and the DFT results show a strong reduction of the spin polarization at the Fermi level ͑E F ͒ compared to the bulk density of states. The nature of the states in the majority band gap of the Fe 3 O 4 surface layer is analyzed.