AVS1997 Session MI+NS-TuA: Thin Films and Nanostructures

Tuesday, October 21, 1997 2:00 PM in Room J3

Tuesday Afternoon

Start	Invited?	Item
2:00 PM	Invited	MI+NS-TuA-1 General Approach to the Epitaxial Growth of Epitaxial Rare-Earth Transition Metal Thin Films and Heterostructures E.E. Fullerton, J.S. Jiang, C.H. Sowers, J.E. Pearson, S.D. Bader (Argonne National Laboratory); X. Wu (Northern Illinois University) We have grown a variety of epitaxial rare-earth-transition-metal (RE-TM) thin films and heterostructures by sputtering onto single-crystal MgO substrates coated with epitaxial W and Cr buffer layers ¹. The W and Cr(100), (211) and (110) buffer layers are epitaxially grown onto MgO(100), (110) and (111), respectively. The RE-TM films are deposited by cosputtering from elemental sources (RE=Nd, Sm, Pr, Dy, Ho and TM=Fe, Co) at substrate temperatures of 400-800°C. By proper choice of the buffer layer, the phase and orientation of RE-TM films can be controlled. Examples to be highlighted include SmFe₁₂(001), Sm₂Co₇(100) and (110), and Sm₂Co₁₇(110) films and MFM and AFM characterizations. Magnetic measurements of the SmFe₁₂ films demonstrate the strong out-of-plane, uniaxial anisotropy of 10 T expected for c-axis growth. The Sm-Co(100) films exhibit uniaxial in-plane anisotropies of ~20-25 T and room-temperature coercive fields that increase to 4.1 T as the film thickness decreases to 7.5 nm. The 3-T coercivities of the Sm-Co(110) films are independent of thickness and attributed to the twinned microstructure of Sm2Co7 films grown onto Cr and W(100) buffer layers. We have incorporated these films into hard-soft magnetic heterostructures (e.g. Sm2Co7/Co(110) superlattices and Sm2Co7(100)/TM bilayers) to study the switching behavior of exchange-coupled two-phase magnets. These results suggest the use of epitaxial TM buffer layers provides a general approach for the synthesis of single-phase RE-TM films. Work supported by U.S. DOE, BES-MS, No. W-31-109-ENG-38. ¹E. E. Fullerton et al., Appl. Phys. Lett. 69, 2438 (1996).
2:40 PM		MI+NS-TuA-3 Microstructure of MBE-grown FePt (110)-oriented Films. R.F.C. Farrow, R.F. Marks, D.K. Weller, J.-U. Thiele (IBM Almaden Research Center); David J. Smith, M.R. McCartney (Arizona State University) The growth and microstructure of FePt films is of current interest since FePt, near the equiatomic composition, forms the L1₀ chemically-ordered phase which has an extremely high magnetic anisotropy. We have shown earlier ¹ that FePt films grown by MBE on Pt (110) / MgO(110), at substrate temperatures near 500°C , have a high degree (S=0.84) of chemical ordering with the easy magnetic axis along the in-plane MgO [001] direction. Here we show that films grown at much lower temperature (300°C ) have partial chemical ordering (S=0.4), the same overall epitaxial crystallographic orientation, two-fold in-plane magnetic anisotropy but a smaller anisotropy constant. MBE growth and in situ characterization of the films by RHEED is described. Magnetic measurements were made by the magneto-optical Kerr effect, Kerr spectroscopy and torque magnetometry. Transmission electron microscopy revealed that the chemical ordering was inhomogeneous and varied over length scales from one to a few tens of nm. Magnetic inhomogeneities were imaged by electron holography and correlated with structural and chemical inhomogeneities in the films. ¹R.F.C. Farrow, D. Weller, R.F. Marks, M.F. Toney, A. Cebollada, G.R. Harp, J. Appl. Phys. 79(8), 5967 (1996).
3:00 PM		MI+NS-TuA-4 Large Perpendicular Magnetic Anisotropy in Co₃Pt Chemically-Ordered Alloy Films D.K. Weller, R.F.C. Farrow (IBM Almaden Research Center); G.R. Harp (Ohio University); M.F. Toney (IBM Almaden Research Center) We have previously reported a new ordered alloy in the Co-Pt phase diagram at compositions close to Co(75at%)Pt(25at%) ¹. This phase has a close packed hexagonal structure with a composition modulation of ABAB... planes following the sequence Co/CoPt/Co/CoPt/Co... and can be stabilized in MBE growth at elevated temperatures of about 400 deg C on sapphire (0001) substrates. Besides extraordinary magneto-optical properties ² this ordered phase shows also strong perpendicular magnetic anisotropy with anisotropy fields ranging up to 42 kOe at a magnetization of 1050 emu/cc (K₁ = 2.2 10⁷ erg/cm^c). Anisotropy constants were measured with standard torque magnetometry and a Kerr method. The anisotropy has a strong growth temperature dependence and follows closely the trend earlier observed in magneto-optical Kerr spectra ³. Films with close to 100% perpendicular remanence have been generated. ¹G.R. Harp, D. Weller, T.A. Rabedeau, R.F.C. Farrow, and M.F. Toney, Phys. Rev. Lett. 71, 2493 (1993).
3:20 PM		MI+NS-TuA-5 GMR Behavior of Granular Fe Implanted Ag Thin Films J.C. Soares, J.G. Marques, C.M. Jesus (University of Lisboa, Portugal); L.M. Redondo, M.F. Da Silva (Instituto Tecnologico e Nuclear, Portugal); M.M. Pereira de Azevedo, J.A. Mendes, M.S. Rogalski, J.B. Sousa (University of Porto, Portugal) GMR behavior was observed, for the first time, to our knowledge, in thin Ag films with granular Fe structure obtained by ion implantation. The films were deposited by both pulsed laser ablation and evaporation onto Si substrates from 99.99 at.% pure Ag targets at a base pressure of 5x10^-9 mbar. The thicknesses of the deposited films (2000 - 3000 Å) were determined with a profilometer and confirmed by Rutherford Backscattering Spectroscopy (RBS). The films were implanted with Fe⁺ ions, at doses of 1-6x10¹⁶ at/cm² and 150-180 keV energy, using a High Fluence Ion Implanter. Moessbauer conversion electron spectroscopy (CEMS) was used as a local probe for some films of the series, which were implanted with ⁵⁷Fe, and shows the formation of large and small Fe clusters, coexisting with isolated Fe atoms. Magnetization data reveals a dominant superparamagnetic behavior of the clusters for low Fe doses, and an increasing ferromagnetic contribution as the dose increases, which is consistent with the CEMS results. A detailed study of the magnetoresistance from 4.5 K up to 300 K was made using magnetic fields up to 16 T. We observe an increase in the GMR effect associated with the enhancement of the implanted dose and correspondingly higher Fe concentration in the films. The magnetoresistive behavior is discussed in terms of the structural RBS and Moessbauer data, combined with the magnetic properties of the granular films.
3:40 PM	Invited	MI+NS-TuA-6 Terahertz Spin Precession and Coherence in Magnetic Quantum Structures D.D. Awschalom, S.A. Crooker (University of California, Santa Barbara); F. Flack, N. Samarth (Pennsylvania State University) The fundamental understanding of coherent electronic processes in quantum systems plays an important role in the development of magneto-electronics that rely on the quantum mechanical aspects of electronic states in condensed matter. A relatively unexplored area is the study of coherent processes that involve the electronic spin degrees of freedom in mesoscopic structures. We describe an experimental effort aimed at elucidating the effects of quantum confinement, reduced dimensionality, and interfacial geometries on coherent electronic spin dynamics in "spin engineered" semiconductor nanostructures containing magnetic ions. Femtosecond-resolved measurements of induced Faraday rotation in "spin-engineered" ZnCdMnSe digital magnetic semiconductor quantum wells are used to track the transfer of angular momentum from a circularly polarized photon pulse to charge carriers and eventually to a sublattice of magnetic ions. In transverse magnetic fields, terahertz spin precession of photoinjected electrons is observed, accompanied by a rapid spin-relaxation of holes. By separating the dynamics of electrons and holes using the effects of quantum confinement, this technique allows the spin relaxation of each carrier species to be clearly resolved in a range of applied fields and temperatures. Further, the experiment directly shows a coherent transfer of angular momentum from the transient exchange field of the holes to the magnetic sublattice through the ultrafast rotation of the local Mn moments. The perturbed ions undergo free-induction decay at microwave frequencies, enabling time-domain all-optical electron spin resonance measurements in magnetic nanostructures. In contrast to more conventional spin resonance techniques, this experiment inherently allows magnetic resonance measurements with both field-tunable spectroscopic and temporal resolution in nanometer-scale geometries.
4:20 PM		MI+NS-TuA-8 Linear and Nonlinear Magneto-Optical Properties of Regular Arrays of Magnetic Nanodots F. Bentivegna (Universiteit Nijmegen, The Netherlands); A. Carl (Gerhard-Mercator Universitaet Duisburg, Germany); E. Jurdik, A. Kirilyuk, Th. Rasing (Universiteit Nijmegen, The Netherlands); E.F. Wassermann, M. Thielen, S. Kirsch, H. Weinforth (Gerhard-Mercator Universitaet Duisburg, Germany) Large scale periodic magnetic nanostructures are deposited on a glass substrate by electron beam evaporation of magnetic materials onto periodically patterned photoresist masks. The patterning of the masks requires an interference lithography process followed by selective etching. The preparation results in a regular array of nanoscaled, flat magnetic cylinders, with a dot diameter and a period of the array essentially controlled by the wavelength of the light used for the photolithography. Monolithic (Fe or Co) as well as multilayered (Co/Pt) arrays of dots were deposited, with dot diameter of about 0.8 µm and a periodicity of about 1 µm. The structural characterization of such samples can be achieved by electron or atomic force microscopy. For the magnetic characterization, we used magneto-optical techniques. The well-known linear magneto-optical Kerr effect (MOKE) gives informations on the bulk magnetic properties of the structures. Its second-order equivalent, the nonlinear magneto-optical Kerr effect (NOMOKE), is particularly sensitive for symmetry breaking interfaces. Therefore, in contrast to MOKE, NOMOKE is strongly interface sensitive and is particularly useful to study nanostructures. We present and compare here the results obtained with both techniques and the informations they give on the magnetic properties of the samples. The strict periodicity of the array allows us to study the linear and nonlinear magneto-optical effects also in the diffracted beams, in addition to the specular one.
4:40 PM		MI+NS-TuA-9 Fabrication and Domain Imaging of Iron Magnetic Nanowire Arrays D.A. Tulchinsky, M.H. Kelley, J.J. McClellan, R. Gupta, R.J. Celotta (National Institute of Standards & Technology) In spite of the fundamental and technological interests in 0-D and 1-D magnetic systems, the fabrication and measurement of large uniform arrays of magnetic nanostructures remains a challenge. Utilizing a evaporation/shadowing technique, we have fabricated and studied the domain structure of magnetic ~100nm wide iron wires with aspect ratios of 1:1500. The magnetic stripes were fabricated by starting with the corrugated surface produced by Cr atoms laser-focused in a one-dimensional standing wave [1]. The Cr was deposited as a periodic array of ~0.15mm long lines extending in the transverse direction for ~1mm. The Cr lines have a spacing of 213nm, a peak height of ~36nm, and a FWHM of ~80nm. Magnetic nanowires were then fabricated by evaporating iron at a grazing-elevation, perpendicular to the Cr lines. The 1000's of iron lines produced form a regular array of magnetic stripes which are 20-40nm thick, spaced every 213nm, ~100nm wide, and are ~0.15mm long with aspect ratios of ~1:1500. Individual magnetic domains were then imaged by utilizing the high resolution capability of scanning electron microscopy with polarization analysis (SEMPA). The elongated domains observed are ~100nm wide with average length ~16µm, indicating that the domains have a high average aspect ratio near 1:160. Few domain reversals were seen and magnetization perpendicular to the long axis of the wires was also not observed. This work partially supported by the Office of Naval Research and by the National Research Council Postdoctoral Research Associateship Program. [1] J. J. McClelland, R. E. Scholten, E. C. Palm, and R. J. Celotta, Science 262, 877 (1993).
5:00 PM		MI+NS-TuA-10 Room-Temperature Dipole-Ferromagnetism in Linear-Self-Assembling Mesoscopic Fe Particle Arrays. A. Sugawara (Japan Advanced Institute for Science and Technology, Japan); M.R. Scheinfein (Arizona State University) Quasi one-dimensional, nanometer diameter Fe particle arrays have been prepared by self-organized shadow growth on regularly faceted, SiO covered NaCl (110) surfaces. The particle size and spacing can be modified by adjusting the template annealing temperature and Fe evaporation temperature. The SiO underlayer prevents any epitaxial relationship between the islands and the NaCl, while an SiO overlayer prevents oxidation. The magnetic properties were measured ex-situ with the surface magneto-optic Kerr effect. A room temperature dipole-ferromagnetic phase was observed for linear arrays of Fe particles with radii larger than 2.5 nm. Magnetization curves indicate easy-axis alignment along the rows of particles. Remanence and coercivity were strong functions of particle diameter and linear island density. Experimental results are compared with Monte Carlo micromagnetics calculations, which, with no free parameters, reproduce the experimental observations.