AVS2004 Session MI-TuP: Poster Session

Tuesday, November 16, 2004 4:00 PM in Room Exhibit Hall B
Tuesday Afternoon

Time Period TuP Sessions | Topic MI Sessions | Time Periods | Topics | AVS2004 Schedule

MI-TuP-1 Comparative Studies of Magnetic Phases of the Interfacial Layers for Co/Ge(100) and Co/Ge(111) Films
J.S. Tsay (National Chung Cheng University, Taiwan); C.W. Su (Academia Sinica, Taiwan); C.H. Hwang (Tunghai University, Taiwan); Y.D. Yao (Academia Sinica, Taiwan)
Magnetic phases of the interfacial layers were comparatively investigated for Co/Ge(100) and Co/Ge(111) films thinner than 13 monolayers using surface magneto-optic Kerr effect technique. Co/Ge(100) films show nonferromagnetic behavior up to 12 monolayers at 300 K. After systematic investigations of the magnetic properties upon cryogenic treatments, magnetic phase diagram of the Co/Ge(100) films was established. The boundary between nonferromagnetic and ferromagnetic phases was experimentally determined to be from below 150 K to above 300 K as the cobalt thickness increases from 9 to 14 monolayers. This behavior is consistent with the thickness-dependent scaling law of Curie temperature for a thin film system. As comparing to Co/Ge(111) system, this boundary shifts to higher Co thickness side. Due to the difference of the electronegativities for Co and Ge atoms, Co LMM Auger line shifts to a higher kinetic energy as the Co thickness increases. This gives the spectroscopic evidence of the formation of interfacial compounds. Co/Ge(100) exhibit a diffused 2x1 diffraction pattern within the first monolayer thickness followed by a diffused background for thicker films as observed using low-energy electron diffraction technique. Ordered structure up to several monolayers were observed for Co/Ge(111) films as deposited at 300 K. In additional, the critical exponent βï? in the power law relationship of magnetization for Co/Ge(100) films is about 0.38 that lies close to the value expected by three-dimensional Heisenberg model, while the critical exponent of Co/Ge(111) is close to the value of two-dimensional XY model. These experimental evidences show that the structure of Co/Ge(100) interfacial layers is in a much disordered state. This causes a three-dimensional stacking of subsequently deposited Co atoms and furthermore the different magnetic transition behavior of Co/Ge(100) and Co/Ge(111) films.
MI-TuP-2 Fabrication and Magnetic Property of Co Platelets On Si (111) Surfaces
M.H. Pan, H. Liu, J.Z. Wang, J.F. Jia, Q.-K. Xue (The Chinese Academy of Science, China); J.L. Li, S.Y. Qin, C.-K. Shih (University of Texas at Austin)
Self-organized Co platelets with uniform size and shape were fabricated on Si(111)-7x7 surfaces covered with identical Al nanocluster arrays. The Al nanocluster array not only suppresses reaction between Si and Co, but also enables formation of well-defined Co nano-platelets. These platelets appear as equilateral triangles with fixed orientation and two-monolayer "magic" thickness, and the area of individual plates is quantized in N square units of halves of the 7x7 unit cells. Despite their small volume (a few nm3), these magnetic nanoplateles exhibit unusually high blocking temperature (>100 K).
MI-TuP-3 Surfactant Effects on the Growth of FePt Nanoparticles: Toward Core-Shell Nanomagnets
A.C.S. Samia, X.-M. Lin, J.A. Schlueter, J.S. Jiang, S.D. Bader (Argonne National Laboratory)
Nanosized magnetic materials offer interesting possibilities to investigate fundamental physics and create new technologies in sensors, biomedicine and data storage applications. Particularly, magnetic alloy nanoparticles have attracted great interest due to their potential in ultra high-density recording media applications. Among the different nanomaterials being developed for this application is the FePt system. The high magnetic anisotropy, good chemical stability and resistance to corrosion of this material make it an ideal candidate for permanent magnet applications. Furthermore, monodispersed FePt nanoparticles can be readily obtained from the simultaneous reduction of platinum acetylacetonate and decomposition of iron pentacarbonyl in the presence of organic ligand stabilizers. To date most synthetic work has focused on the use of oleic acid and oleyl amine as passivating surfactants. Using this surfactant combination, spherical FePt nanoparticles in the size range of 3-10 nm have been reported. As prepared, the magnetic nanoparticles are superparamagnetic and requires an annealing step to transform them to a more stable magnetic state. Here we report the effects of other surfactant systems on the particle size and growth of FePt nanoparticles. We will present the effects of oleic acid and trioctylphosphine oxide (TOPO) surfactants on the particle size, size distribution and shape of FePt nanoparticles. By gaining insights on the role of these surfactants in regulating the growth of FePt nanoparticles we are able to synthesize larger FePt nanoparticles. To overcome the superparamagnetic limitation in ferromagnetic nanoparticles we are also developing novel core-shell exchange-spring nanomagnets, which consist of hard magnetic (CoPt, FePt) and soft magnetic components (Co). Such combination results to the interaction of the two phases by exchange coupling that leads to a high magnetic energy product.
MI-TuP-5 Rotational Loss in Exchange Bias Systems and their Modeling
K. Steenbeck, R. Mattheis, M. Diegel (IPHT Jena, Germany)
The rotational loss E in sputtered AF/F systems (AF: IrMn, thickness t = 0 to 13 nm, F: NiFe, CoFe, CoFe/Cu, thickness about 18 nm) is determined in dependence on AF thickness t by torquemetry at high field and at 10 and 300 K. After onset of E at low thickness a huge loss peak occurs at that thickness where exchange bias starts to develop and goes down to a lower and constant value above a critical thickness tc. Our simulations are based on a statistical distribution of coupling energies and include new aspects not considered up to now. For the first time rotational loss can be calculated in (111) textured films and the complete thickness dependence of E(t) can be described. Below tc we consider crystallites with homogeneous AF magnetization, a 3-axial AF anisotropy K and an AF interface net moment which undergoes irreversible switching for critical values j/Kt. This loss disappears for large t. For t above tc we include domain walls parallel to the F/AF interface. Our calculations display that the main loss contribution at that thickness is caused by complete AF 60 ° domain walls created in crystallites with 3-axial anisotropy and strong enough coupling (j above 1.5 σ, σ domain wall energy). Switching processes of partial domain walls in crystallites having their j/σ in a critical interval contribute only at low level. The modelling allows to derive numerical values for the coupling energy per spin, the AF anisotropy constant K and the AF domain wall energy σ.
MI-TuP-6 Nanostructured Zigzag Shaped Magnetic Devices
D.P. Pappas, F.C.S.S. da Silva, W.C. Uhlig, J. Unguris (NIST)
Magnetism in zigzag shaped thin film elements is investigated using scanning electron microscopy with polarization analysis, magneto-transport measurements, and micromagnetic simulations. We find that the angle of magnetization alternates along the length of the element, and is strongly correlated to the corrugated edges. We show that this simple and unique geometry can be used as a natural means of biasing the magnetization relative to the current to form a magnetic field sensor. In this configuration the sensors are primarily sensitive to fields parallel to the applied current. These results can be interpreted in terms of a coherent rotation model of the magnetization. These devices are scalable to nanometer dimensions.
MI-TuP-7 Spin Momentum Transfer Induced Dynamics in Magnetic Nanostructures
W.H. Rippard, M.R. Pufall, S. Kaka, T.J. Silva, S.E. Russek (NIST); J.A. Katine, M. Carey (Hitachi Global Storage Technologies)
We have directly measured high-frequency precessional dynamics induced by a dc current I injected into patterned nanopillar devices and continuous spin-valve structures through a lithographically defined nanocontact. The induced magnetization dynamics have been studied as a function of current, material, as well as applied field strength H and direction. For in plane applied fields, the excitation frequency is found to linearly decrease with applied current, in qualitative agreement with single domain modeling. The excited frequencies vary between 5 GHz and 40 GHz as a function of applied field and frequencies of excitation can be well-described by the Kittel equation, indicating that the excitations are ones with wavelengths much larger than the contact size. As the angle of the field is varied with respect to the film plane, the dynamics become more complicated. Abrupt shifts in the frequency occur with applied current applied and df/dI can vary strongly with I at a given field. These shifts can be either to an increased or decreased frequency, depending on the field strength and angle. Moreover, the frequency of precession can be multivalued several different, non-harmonically related frequencies being measured at a given field and current with each mode having a linewidth <50 MHz. The power output of the devices is a strong function of the direction of the applied field, increasing by roughly two orders of magnitude as the field is varied from in-plane to out-of-plane. In general, the room-temperature linewidths of the excitations are quite small, on the order of 10 to 50 MHz, but also vary with H and I. For certain field geometries linewidths of < 2MHz are measured, leading to oscillations with quality factors >18,000. Single domain modeling based on an LLG equation modified to contain a spin-torque term captures a number, but not all, of the behaviors we observe.
MI-TuP-9 Undercut Nano Contact-hole Fabrication for a Ferromagnetic Vertical Single Electon Transistor and TMR Enhancement in the Coulomb Blockade Regime
S. Haraichi, T. Wada (National Institute of Advanced Industrial Science and Technology, Japan)
Recently, we have fabricated ferromagnetic single electron transistor s (FSET) with nanometer sized vertical magnetic tunnel junctions and observe d a strong TMR enhancement in the Coulomb blockade regime at relatively high temperature. The FSET consists of under layer drain electrodes, interlayer insulating layer with nano contact-holes, and over layer source electrodes, which is fabricated on an SOI substrate whose cap silicon layer acts as the gate electrode. The key issue of the process is the fabrication of undercut nano contact-holes. We use the bilayer SiO2 of high-temperature sputte r layer and low-temperature sputter layer as an interlayer insulating layer. By using the etching rate dependence on sputtering temperature of SiO2 in the electron beam direct lithography process, undercut nano contact-hol es have been successfully fabricated with a minimum diameter of 17 nm. Final ly, we have fabricated a vertical crossbar type FSET and obtained over 100% TMR enhancement at 15 K. This TMR enhancement can be modulated by the gate voltage.
MI-TuP-10 Giant Magneto Resistance (GMR) Effect in Nanoscale Alternating Magnetic/ Non-Magnetic Metallic Multilayer
K.B. Ravi (Birla Institute of Technology, India)
Giant magneto resistance (GMR) effect in nanoscale alternating magnetic/ non-magnetic metallic multilayers has evinced tremendous interest worldwide. Cu/Co Multilayers have been electrolytically deposited directly on to n-Si substrate from single bath, there by eliminating the need of a conducting seed layer. Magneto resistance is very sensitive to the growth conditions and can be destroyed by intermixing magnetic and non-magnetic interfaces and also by poor crystalline quality of the layers. Co and Cu are weakly miscible elements and for the reason they are most likely to yield chemically sharp interfaces. The observed MR value is »1%. Interface characteristics are analyzed using TEM, AFM and EPMA. Aim of the present work is to improve the magneto resistance value in these films by optimizing the deposition conditions and the layer thickness. The studies are going on to find out the effect of Cu layer thickness [by fixing Co layer to 2 nm thick] on MR and also to correlate the interface structure with giant magneto resistance.
MI-TuP-11 Interface Chemistry and Structural Properties of Expitaxial Ultrathin Fe Films on MgO(100)
E.D. Lu, V.K. Lazarov, H.T. Johnson-Steigelman, M. Gajdardziska-Josifovska, P.F. Lyman (University of Wisconsin at Milwaukee)
Epitaxial ultra thin Fe films have been grown successfully on MgO(100)(1x1) at room temperature (RT). The chemistry and structures of the interface during Fe film growth has been investigated by low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS). Prior to Fe deposition, commercial MgO(100) substrates were annealed at 750 °C in either furnace in air or ultra high vacuum (UHV) annealing in order to get well-ordered (1x1) reconstruction. We have found that band offsets toward valence band maximum (VBM) for MgO is 2.6 ± 0.2eV at Fe deposition of 5.0 monolayer (ML). We have also identified chemical states present on the initial stage of Fe deposition with elemental Fe0, Fe2+, and Fe3+ species. After deposition of more than 5.0 ML Fe, the elemental Fe becomes dominant, and finally body-centered cubic (bcc) Fe (100) films can be grown epitaxially on MgO(100)(1x1). Upon annealing for improving quality of the epitaxial Fe films, we have found there is an interdiffusion occurring between the Fe films and the MgO(100) substrates and becoming severe upon annealing to 500 °C.
Time Period TuP Sessions | Topic MI Sessions | Time Periods | Topics | AVS2004 Schedule