Papers

AVS2001 Session MI+SS-FrM: Magnetic Thin Films and Surfaces II

Friday, November 2, 2001 8:20 AM in Room 110

Friday Morning

Start	Invited?	Item
8:20 AM		MI+SS-FrM-1 Novel Spin Structures in Fe₃O₄/CoO and Fe₃O₄/NiO Superlattices Y. Ijiri (Oberlin College); J.A. Borchers, R.W. Erwin, S.H. Lee, K.V. O'Donovan (National Institute of Standards and Technology); P.J. van der Zaag, L.F. Feiner, R.M. Wolf (Philips Research Laboratories); D.M. Lind, P.G. Ivanov (Florida State University) Using polarized neutron scattering methods, we have probed the magnetic ordering in MBE-grown Fe₃O₄/CoO and Fe₃O₄/NiO superlattices. Despite significant differences between the spinel ferrite and the rock salt monoxides, it is possible to grow high-quality epitaxial structures for these materials as a result of good oxygen sublattice matching. We have observed for these superlattices substantial changes in the spin structures for both the ferrimagnetic Fe₃O₄ and the antiferromagnetic monoxide (CoO or NiO). In particular, we have found new magnetic easy axes along with significant differences in the polarization characteristics for these thin films. The unusual structures are discussed in terms of the role of strain and exchange coupling between disparate magnetic materials.
9:00 AM		MI+SS-FrM-3 Surface Structure and Phase/Orientation Control of Manganese Nitride Grown by Molecular Beam Epitaxy H. Yang, H.A.H. Al-Brithen, A.R. Smith (Ohio University); R.L. Cappelletti, J.A. Borchers, M.D. Vaudin (National Institute of Standards and Technology) We have investigated the growth of manganese nitride on MgO(001) substrates using molecular beam epitaxy (MBE) and have studied the surfaces using scanning tunneling microscopy (STM). Manganese nitride has many bulk phases (labeled θ, η, ζ, and ε). Using MBE, we can individually select these phases by controlling the growth parameters. For example, at low Mn flux, we obtain the N-rich θ phase (MnN), which has fct structure;¹ but at increased Mn flux, we obtain the less N-rich η-phase (Mn3N2) which is also fct but includes ordered arrays of N vacancies, according to a model proposed by Kreiner and Jacobs.² Neutron scattering confirms that the Mn moments are aligned in a layered antiferromagnetic arrangement. By adjusting the growth parameters, we are able to control not only the phase, but also the crystalline orientation. At low Mn flux, the η-phase has its c-axis perpendicular to the growth surface (η1). But at yet higher Mn flux, the c-axis is oriented parallel to the surface (η2), a consequence being two equivalent domains, D1 and D2, at 90° to each other. These domains are evident during growth via RHEED, which shows two closely spaced 1st-order streaks due to the fct structure. Also, 1/3-order lines are observed due to the periodic vacancy planes which are normal to the surface. STM images following growth clearly reveal the two domains at the η2 surface. Row structures corresponding to the vacancy planes are clearly observed. Atomic resolution images show enhancement for the Mn atoms at the intersections of the surface and vacancy planes. This is likely related to the fact that these Mn have fewer N neighbors compared to other surface Mn atoms. This work is supported by NSF. ¹ Suzuki et al., J. Alloys and Compounds 306, 66 (2000), ² Kreiner and Jacobs, J. Alloys and Compounds 183, 345 (1992).
9:20 AM		MI+SS-FrM-4 Mesoscopic Magnetic Structures Grown by Self-organization J. Kirschner (Max-Planck-Institut fuer Mikrostrukturphysik, Germany) Mesoscopic magnetic structures play an increasingly important role in magnetic storage technology, magnetic sensors, non-volatile random access memories, and "magneto-electronics" in general. Lithography-type processes for making such structures, though having been quite successful, will not be addressed in this talk. Rather, typical surface science approaches, involving adsorption, surface diffusion, epitaxial growth phenomena, and self-organisation will be exploited to produce and characterize mesoscopic magnetic structures. For example, magnetic wires may be made by step edge decoration on stepped single crystal surfaces by tuning surface diffusion. They form chains of long segments, connected by weak links. These structures resemble Ising chains, with magnetic properties determined by a gradual approach to thermodynamic equilibrium. Magnetic dots may be created by exploiting localized adsorption on reconstructed surfaces. It will be demonstrated, how magnetic pillars with a height-to-diameter ration of 2:1 can be grown on such a template and that such structures may perhaps overcome the "superparamagnetic barrier" in magnetic storage technology.
10:00 AM		MI+SS-FrM-6 Direct Observation of Orbital Kondo Resonance on the Cr(001) Surface O.Yu. Kolesnychenko, R. de Kort, M.I. Katsnelson, A.I. Lichtenstein, H. van Kempen (University of Nijmegen, The Netherlands) Scanning Tunneling Microscopy (STM) is an excellent tool to explore many-body phenomena, such as the formation of the Kondo resonance¹. In addition to``classical'' Kondo effect, many-electron resonances have been theoretically predicted for scattering centers with non-spin degrees of freedom. Here, we will present the first direct evidences for the existence of orbital Kondo resonance on a transition metal surface. Low-Temperature STM and STS investigations were performed on the atomically clean Cr(001). The Cr(001) surface was produced by cleavage of a 99.99% Cr single crystal in situ at 4K. As we found out, the cleavage of Cr single crystals produce atomically flat and clean (001) surfaces. The STS investigations of the Cr(001) surfaces showed a very narrow resonance at 26 meV above the Fermi level. We found that at bias voltages corresponding to the resonance energy a cross-like depressions centered around impurities appears. This cross-like feature is a fingerprint of the orbital symmetry of the resonance analogous, for example, to the visualization of a superconducting gap near a zinc atom². Although the resonance in the Cr(001) surface density of states has been observed previously at room temperature³ and was interpreted as a one-electron surface state, we are going to present additional experimental data which strongly indicate that the observed state on the Cr(001) surface is a many-electron orbital Kondo resonance which is formed by two degenerate spin-split d_xz, d_yz surface states. We also carried out calculations for the periodic degenerated Anderson model which confirm the existence of the orbital Kondo resonance on the Cr(001) surface. ¹ ¹ H.C. Madhaven, et.al., Nature 403, 512 (2000). ² S.H. Pan. et.al., Nature 403, 746 (2000). ³ J.A. Stroscio, et.al., Phys. Rev. Lett. 75, 2960 (1996).
10:20 AM		MI+SS-FrM-7 High Dipolar Magnetic Moment Observed on Ni/Cu(111) Nanostructures by Magnetic Circular X-ray Dichroism C. Boeglin, S. Stanescu, S. Cherifi (IPCMS-CNRS, France); A. Barbier (CEA/DRFMC, France); N.B. Brookes (ESRF, France); P. Ohresser (LURE-CNRS, France); J.P. Deville (IPCMS-CNRS, France) The correlation between ultrathin film growth, structure and morphology and the induced magnetic properties is of fundamental interest in order to improve the theoretical understanding of magnetic properties in ultra-thin films. We report here growth and morphology studies at the first stages of growth of Ni on a Cu(111) single crystal substrate. This work has been performed by in-situ Auger, RHEED and STM at room temperature. The morphology shows in the early stages of the growth that nickel induces 10-30 nm large triangular islands monolayer in height. In addition stripes are formed at the step edges via a step flow growth mode. The particular shape of the oriented triangular islands has been examined by STM and segregation of Cu atoms could be evidenced on top of the Ni islands after 0.5 ML deposition. It is shown that the Ni morphology and the Cu diffusion both have a strong influence on the magnetic properties. Correlations have be evidenced by performing in-situ Magnetic Circular X-ray Dichroism studies on Ni/Cu(111) ultra-thin films. The evolution of the island size during the growth can be correlated with the evolution of the orbital magnetic moment whereas strong dipolar magnetic moments are related to the formation of a second Cu/Ni interface. Moreover, the in-plane orbital magnetic moment anisotropy has been measured and related to magnetocrystalline effects in the film.
10:40 AM		MI+SS-FrM-8 Relating Magnetic and Structural Changes of Thin FeNi Alloy Films on Cu(100) M. Hochstrasser, J.G. Tobin (Lawrence Livermore National Laboratory); S.A. Morton, G.D. Waddill (University of Missouri-Rolla); F.O. Schumann (Freie Universität Berlin, Germany); N.A.R. Gilmann, R.F. Willis (The Pennsylvania State University) At a concentration of around 65% Fe, bulk FeNi alloys exhibit the "Invar effect", a sudden arresting of the Wigner-Seitz cell volume and a zero expansion coefficient. Simultaneously, the crystal structure changes from face-centered cubic to body-centered cubic while the Curie temperature goes to zero. This structural transformation can be arrested in ultrathin alloys films grown epitaxially on a Cu(001) substrate. Theoretical work predicts that the fcc phase can exist in two possible states: a ferromagnetic high volume state or a antiferromagnetic low volume state (2γ state model) and a volume change between the paramagnetic and the high spin state of ~7%, and 1% change between a non-collinear equilibrium state and the high spin state. Experimental work shows a lattice expansion increasing linearly up to 3% at 65% Fe content followed by a sudden relaxation of 2% with increasing Fe content. The initial volume increase is associated with increasing magnetization/magnetic moment & spin alignment in the Ni-rich alloys. As the alloy is cooled below T_C (or a strong external magnetic field is applied), an increasing alignment of the magnetic moments causes the nearest-neighbor spins to push apart producing an internal pressure which expands the lattice. With increasing Fe content, this effect increases due to the increasing number of Fe nearest neighbors with the larger atomic magnetic moments. Eventually, a critical limit is reached (~65% Fe), when a magnetic/lattice volume instability develops. W ith x-ray magnetic dichroism the changes in the elemental magnetic moments were tracked. Spin polarized photoemission studies record a sudden decrease in the "mean-field" exchange splitting of the d-states with increasing Fe content through the critical "Invar transition". Angle-resolved photoemission imaging of states at the Fermi level reveal a much smaller splitting of the sp-states, which also tracks the changing magnetization with changing composition.
11:00 AM		MI+SS-FrM-9 The Structure of Ferromagnetic Ultrathin Fe Films on Cu(100) is not fcc A. Biedermann, R. Tscheliessnig, M. Schmid, P. Varga (TU Vienna, Austria) Ultrathin Fe films on Cu(100) are an ideal model system to study martensitic fcc-bcc phase transitions in Fe.¹ Ultrathin Fe films on Cu are also distinguished by the appearance of a novel ferromagnetic phase which showed distinct fcc-like features in p revious electron diffraction experiments. By means of scanning tunneling microscopy we were able to reveal this phase as a novel "nanomartensitic" phase with the Fe atoms forming localy a bcc like structure ². This shows that the assumption of a ferromagnetic fcc phase is not necessary to explain the ferromagnetism observed in these films. ¹ A. Biedermann, M. Schmid, and P. Varga, Phys. Rev. Lett. 86 (2001) 464-67 ² A. Biedermann, Rupert TschelieÃŸnig,M. Schmid, and P. Varga, Phys. Rev. Lett., submitted
11:20 AM		MI+SS-FrM-10 Angle Resolved Auger Spectra of Ultrathin Fe on Gd (0001) J.P. Nibarger, D.P. Pappas (National Institute of Standards and Technology) The in-plane to out-of-plane spin-reorientation phase transition of ultrathin Fe on Gd (0001) has been measured recently.¹ Theoretical work has indicated the need for six-fold symmetry of the ultrathin Fe film in order to fit existing data on the spin-reorientation phase transition.² Fe films consisting of 1.5 atomic layers on bulk Gd do no exhibit any long range ordering as determined by low energy electron diffraction. Angle resolved Auger electron spectroscopy (ARAES) will be used because it is sensitive to local order on the atomic scale and will determine the local symmetry of the Fe atoms. ARAES spectra will be shown that demonstrate the extent of six-fold symmetry in these films. ¹ C.S. Arnold et al., Phys. Rev. Lett. 83, 3305 (1999). ² A.P. Popov, private communication.
11:40 AM		MI+SS-FrM-11 Transverse Magneto-optical Kerr-effect in the Soft X-ray Regime at Iron and Cobalt Films on W(110) J. Bansmann, V. Senz, A. Kleibert (University of Rostock, Germany) Tunable soft X-ray radiation opens the possibility for investigating element-specifically the magnetic properties of thin films, islands, and nanoparticles on surfaces. Well-known techniques are magnetic dichroism in photoemission (MDAD) and X-ray magnetic circular dichroism in photoabsorption (XMCD). However, magnetic phenomena can also be studied using the transverse Magneto-optical Kerr effect (T-MOKE) at the core levels of ferromagnetic materials using linearly polarized radiation. We will present new data using T-MOKE at iron and cobalt films and self-organized islands on W(110). For recording hysteresis curves an external electromagnet has been applied to the setup. Close to the core levels of e.g., iron and cobalt, the reflectivity and the Kerr rotation is strongly enhanced by resonant forward scattering. We could observe huge intensities in reflexion and intensity asymmetries of 50% at Fe and Co films of less than 6ML. The experimental results will be compared to recent calculations. When annealing Fe(110) films on W(110) a well oriented Fe island structure can be created. Our experimental data clearly show a rotation of the easy axis with respect to thin films which depends on the original coverage before annealing and on the temperature during thermal treatment. In the case of cobalt on W(110) we have investigated the magnetic properties of fcc- and hcp-cobalt films on clean and modified W(110) surfaces. Although the direction of the easy magnetization axis does not change using different cobalt structures, the remanent magnetization and coercive forces have clearly changed.