Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D41: Magnetism in 2 Dimensions |
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Sponsoring Units: DCMP Chair: Jason Crain, National Institute of Standards and Technology Room: Colorado Convention Center 504 |
Monday, March 5, 2007 2:30PM - 2:42PM |
D41.00001: Spin-polarized scanning tunneling spectroscopy of individual magnetic adatoms Yosef Yayon, Victor Brar, Michael Crommie, Lax Senapati, Steve Erwin An important goal in condensed matter physics is the ability to measure the spin-polarization state of a single magnetic atom or impurity. We have used spin-polarized scanning tunneling spectroscopy (SP-STS) to probe the local spin-dependent electronic structure of isolated Fe and Cr adatoms deposited onto magnetic Co islands on a copper surface. Individual Fe and Cr atoms prepared in this way show strong spin-polarized contrast in their dI/dV spectra. The spectra of Fe and Cr adatoms differ in that Fe atoms couple ferromagnetically to the Co islands while Cr adatoms couple antiferromagnetically to the islands. dI/dV spatial mapping reveals spin-dependent contrast in the spatial features of individual Fe and Cr atoms. Density functional theory calculations support our interpretation of the experimental results. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D41.00002: Spin-polarized metastable-atom deexcitation spectroscopy study of Xenon-adsorbed iron surfaces Yasushi Yamauchi, Mitsunori Kurahashi, Taku Suzuki, Xia Sun, Zhongping Wang The electron spin polarization at the interface between nonmagnetic and ferromagnetic medias is one of the essential factors that may alter the spin transport phenomena. To investigate fundamental aspects of induced spin polarization we have examined the adsorbate-covered magnetic surfaces by means of spin polarized metastable-atom deexcitation spectroscopy (SPMDS). Use of spin-polarized metastable helium atoms in triplet states moving at thermal energies gives rise to the ultimate surface sensitivity. Although Xenon can adsorb on surfaces at low temperatures by the van der Waals force, no electron exchange with surfaces, especially no spin interaction, is expected because of its closed shell structure. SPMDS spectra measured for Xenon-adsorbed iron surfaces show three prominent peaks that are the same as those previously reported for other surfaces by D. M. Oro, et al. [Phys. Rev. A 49 (1994) 4703]. Two peaks ($^{2}P_{1/2}$, $^{2}P_{3/2})$ at higher kinetic energies exhibit clear spin asymmetries while the other low energy peak has no appreciable spin asymmetry. The spin asymmetries will be discussed on the basis of spin polarization and deexcitation processes of metastable atoms. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D41.00003: Homochiral magnetism in low-dimensional magnets Stefan Blugel, Marcus Heide, Gustav Bihlmayer Spin structures observed in nanomagnets are commonly explained on the basis of the Heisenberg exchange and the magnetic anisotropy. Electrons propagating in the vicinity of inversion-asymmetric environments such as of surfaces, interfaces or in ultrathin films can give rise to the Dzyaloshinskii-Moriya (DM) interaction, typically unimportant for metals. Surprisingly, there is no hard number known from theory about its strength, as this requires supercomputing at the cutting edge. One deals with long-ranged complex magnetic structures in low-dimensions. Since the DM interaction arises from spin-orbit coupling, each atom of the long range structure has a different electronic environment and previous strategies, e.g.\ applying the generalized Bloch theorem, fail. But if DM is important, the so-far anticipated collinear magnetism become unstable, and homochiral spin structures occur. We developed a perturbative strategy implemented into the FLAPW code {\tt FLEUR} which can cope with this challenge. We show by first-principles calculations based on the vector-spin density formulation of the DFT that the DM interaction is indeed sufficiently strong to compete with the interactions that favor collinear spin alignment. We predict new magnetic phases in thin films which had been overlooked during the past 20 years. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D41.00004: Temperature-induced Domain Shrinking in Dipolar Frustrated Ising Ferromagnet Danilo Pescia, Alessandro Vindigni, Oliver Portmann, Paolo Politi Motivated by recent experimental observations on ultrathin Fe/Cu(001) films, we performed a theoretical analysis of magnetic domain pattern evolution in 2D Dipolar Frustrated Ising Ferromagnet. Due to the competition between long-ranged dipolar interaction and nearest neighbor ferromagnetic exchange interaction, the ground state is given by a succession of saturated domains of positive and negative magnetization, which alternate in a sharp striped pattern of characteristic domain width $L_{gs}$. Close to the Curie temperature $T_C$, the Mean Field theory predicts the occurrence of a cosine modulation with a much smaller spatial period ($L(T_C$)). We found that these two limits are connected continuously in the temperature range $0\le T \le T_C$. But, as translational invariance does not hold, the interplay between thermal fluctuations and the two competing interactions gives rise to a non-trivial magnetization profile at intermediate temperatures. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D41.00005: Controlling Spin-Density Wave Periodicity in Thin Cr$_{1-x }$V$_{x}$ Films Oleg Krupin, Eli Rotenberg, S. D. Kevan Chromium is an itinerant antiferromagnet with a spin-density wave (SDW) ground state driven by a nesting of Fermi surface sheets around the Gamma and H points of the Brillouin zone. Periodicity of the SDW plays an important role in mediating magnetic interactions in magnetic multilayer structures providing a giant magnetoresistance effect and potentially interesting for application in spintronic devices. Therefore control of SDW in thin chromium films is of the high importance. It requires a detailed understanding of phenomena related to stabilization of SDW. We used angle-resolved photoemission to characterize spin-density wave and Fermi surface topology in thin Cr$_{1-x }$V$_{x }$films as a function of the film thickness, temperature, composition and hydrogen surface coverage. A key feature of our results is the ability to control the magnetic structure of thin films of Cr with an external perturbation: balancing the surface energetic interactions favored commensurate state of SDW vs. the energy associated with Fermi surface topology stabilizing SDW incommensurate phase in the bulk. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D41.00006: Manipulating spins using spin-valves of self-assembled molecular wires Vladimir Burtman, Valy Z. Vardeny We studied spin transport using spin-valves of self-assembled monolayer (SAM) devices sandwiched between two ferromagnetic electrodes, namely La0.33Sr0.66MnO3 (LSMO) and Co having different coercive fields. The SAM film contained isolated molecular wires that bond with both electrodes, in an otherwise insulating molecular matrix that bond only with one electrode. The relative resistance change, or magnetoresistance (MR), DR/R between the device resistance with the electrodes magnetizations parallel and anti-parallel to each other serves as a figure of merit and show spin injection through the isolated molecular wires. We found a giant MR of up to 80{\%} at 10K. The MR response was measured at various temperatures and biasing voltages to obtain the complete magneto-transport characteristic properties of the organic spin-valve devices. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D41.00007: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 3:54PM - 4:06PM |
D41.00008: Structural and magnetic properties of ferromagnetic metal-oxide films grown by polymer assisted deposition M. Jain, P. Shukla, M. F. Hundley, A. K. Burrell, T. M. McCleskey, Q. X. Jia Ferromagnetic metal-oxide films such as doped lanthanum manganites have been extensively investigated over the past decade due to their potential applications in different fields. To grow these oxide films, the most widely used approaches are physical vapor deposition, chemical vapor deposition, and chemical solution deposition techniques. One of the challenges in solution-based processes of such oxide films has been to produce high quality multilayer films and at the same time to control the stoichiometry. We describe a solution route called polymer-assisted deposition (PAD) to grow such oxide films. High quality epitaxial single layer and multilayer coated films of La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) and La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) have been grown by PAD. Multilayer is used to effectively take the advantages of both LSMO and LCMO with an aim to achieve large values of magnetoresistance (MR) near room temperature. An MR value as high as -66{\%} at 5 T has been obtained at 295K for the multilayer-coated films with LSMO/LCMO volume ratio of 60/40. The successful growth of epitaxial doped lanthanum manganites with desired properties by PAD shows that PAD is a feasible alternative approach to the growth of high quality metal-oxide films. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D41.00009: Growth, structure and magnetic properties study of CVD cobalt layers Nirmalendu Deo, Harold S. Gamble Chemical vapour deposition (CVD) of cobalt was performed on oxidised silicon wafers at the temperature ranges 300-450 $^{o}$C, in hydrogen ambient, from cobalt tricarbonyl nitrosyl. For deposition of cobalt at 300 $^{o}$C the layer was found to be highly resistive ($\sim $250$\mu \Omega $cm). As the deposition temperature increases the layer resistivity decreases, and at 450 $^{o}$C the layer resistivity was reduced to $\sim $30$\mu \Omega $cm. Thus reduced resistivity is taken as evidence that the cobalt layer is purer. X-ray diffraction of the cobalt layers reveal both hcp and fcc peaks. The AES analysis shows that cobalt layer deposited at 300 $^{o}$C contains 26atom {\%}O, 10atom {\%}N. At higher deposition temperature of 400$^{o}$C and above the impurities was 1{\%} or less as documented by AES. At 300 $^{o}$C deposited cobalt layer the surface looks agglomerated as seen by SEM. At 350 $^{o}$C the grain structure is elongated and at 400 $^{o}$C and above the grain structure changes to hexagonal structure. At this temperature the cobalt phase-change occurs from hcp to fcc. The roughness of cobalt layer is higher in lower deposition temperature but this is only due to higher layer thickness measured by AFM. VSM shows, the saturation of magnetisation (Ms) for layers deposited at 400 $^{o}$C and 450 $^{o}$C is consistent with the bulk value of 1422 emu/cm$^{3}$. As the cobalt deposition temperature increases, the layer coercivity decreases from 705 to 400-Oe. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D41.00010: Combinatorial Exploration of Magnetostriction of Fe$_{1-x-y}$Ga$_{x}$M$_{y}$ Ternary Alloys Jason Hattrick-Simpers, Kyu Sung Jang, Samuel E. Lofland, Noble Woo, Bruce Van Dover, Manfred Wuttig, Ichiro Takeuchi Fe$_{80}$Ga$_{20}$ is a well known magnetostrictive material, which owes its large magnetostriction to a tetragonal distortion of the Fe lattice from a local ordering of Ga clusters. Here we will report on the synthesis and characterization of thin film combinatorial Fe-Ga-Pd and Fe-Ga-Al ternary spread samples. The composition spread samples were synthesized in an ultra high vacuum (10$^{-9}$ Torr) co-sputtering chamber. Magnetic properties were mapped through the use of a room temperature scanning SQUID and a high throughput magneto optical kerr effect (MOKE) system. Magnetostrictive measurements were performed on micromachined cantilever libraries at room temperature. The correlation between magnetic and magnetostrictive properties across the composition phase diagram of the two systems will be discussed. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D41.00011: Memory interference in stage-2 CoCl$_{2}$ graphite intercalation Masatsugu Suzuki, Itsuko Suzuki, Motohiro Matsuura Memory interference effects of aging behavior in stage-2 CoCl$_ {2}$ GIC ($T_{cu}$ = 8.9 K and $T_{cl}$ = 6.9 K)$^{1,2}$ have been studied by low frequency ($f$ = 0.1 Hz) AC magnetic susceptibility and genuine thermoremant magnetization experiments. When the system is aged at multiple stop temperatures ($T_{s}$) for wait times (typically $t_{w} = 3.0 \times 10^{4}$ sec) during a zero-field cooling (ZFC) protocol, the AC magnetic susceptibility exhibits multiple aging holes (dips) at the stop temperatures ($T_{s} < T_{cu}$) on reheating. The depth of the aging hole at $T_{s}$ = 6.0 K is logarithmically proportional to the wait time. The depth of thr aging hole (for the same $t_{w}$) exhibits a local maximum at 6.5 K just below $T_{cl}$. It drastically decreases with increasing temperature and reduces to zero above $T_{cu}$. The genuine thermoremnant magnetization (TRM) measurement also indicates that the memory of the specific spin configurations imprinted at multiple stop temperatures between $T_{cl}$ and $T_ {cu}$ for a wait time during the field-cooled (FC) protocol can be retrieved on reheating. \\ 1. M. Suzuki, I.S. Suzuki, and M. Matsuura, Phys. Rev. B\textbf {73}, 184414 (2006).\\ 2. M. Suzuki, I.S. Suzuki, and M. Matsuura, J. Phys. Condensed Matter in press, Proceeding of HFM 2006, Osaka, Japan (August, 2006). [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D41.00012: Experimental Determination of the Heats of Formation for the Ordered Intermetallics in the Fe-Pt System David Berry, Katayun Barmak, Ysela Chiari Accurate heats of formation are necessary to examine phase stability and to aid in the modeling of phase transitions. However, most reported heats of formation, particularly for intermetallic compounds, are available only as the result of theoretical calculations with little or no experimental verification. For the Fe-Pt system, in which a phase transition from the disordered A1 phase to the ordered L1$_{0}$ phase is of great current interest for application in ultrahigh density magnetic recording media, only a few sets of calculated heats of formation are available, for which there is sizable disagreement. Using non-isothermal differential scanning calorimetry (DSC) of sputter-deposited multilayer thin films, the heats of formation of the ordered intermetallic phases, namely L1$_{2}$ Fe$_{3}$Pt and FePt$_{3}$, and L1$_{0}$ FePt, are measured. These values are then compared with the first principles calculated values available in the literature, where there is good qualitative agreement; however, all of the calculated values have underestimated the total heats of formation. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D41.00013: Resistivity, transverse magnetoresistance and Hall effect induced by electron-surface scattering on thin gold films deposited onto preheated mica substrates under high vacuum Raul C. Munoz, Juan P. Garcia, Ricardo Henriquez, German Kremer, Luis Moraga We report measurements of the resistivity $\rho $, transverse magnetoresistance $\Delta \rho $/$\rho $ and Hall effect carried out on 4 gold films (thickness of 69, 93, 150 and 185 nm) evaporated onto mica substrates under high vacuum, where the signal is primarily determined by electron-surface scattering. The experiments were performed at low temperatures T (4K $\le $ T $\le $ 50K) under high magnetic field strengths \textbf{B} (1.5 T $\le $ B $\le $ 9 T). $\rho $, $\Delta \rho $/$\rho $ and the Hall tangent tan($\theta )$ = E$_{H}$/E$_{x}$ (E$_{H}$ stands for the transverse Hall field, E$_{x}$ for the longitudinal field) depend on film thickness. Sondheimer's theory predicts $\rho $ and tan($\theta )$, but leads to $\Delta \rho $/$\rho $ \textit{one order of magnitude smaller than observed}. Calecki's model predicts $\rho $ and tan($\theta )$, but leads to $\Delta \rho $/$\rho $ \textit{several orders of magnitude smaller than observed}. The failure of current theories to predict all 3 transport coefficients is the first compelling evidence pointing to the need of a new, fresh theory to describe size effects arising from electron-surface scattering in metallic films in the presence of a magnetic field. Work funded by FONDECYT 1040723. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D41.00014: Kapitza Pendulum Effect in Weakly Disordered Continuously Degenerate Magnetic Materials Igor Fomin Limitation of general Larkin-Imry-Ma mechanism of disruption of a long-range order by a weak disorder is discussed. It is shown, that the long-range order can be preserved if not all components of the continuously degenerate order parameter are coupled to the relevant random field. The order parameter in that case has a freedom to adjust to disorder so that the disrupting effect of the disorder is excluded. For a weak disorder energy gain due to the adjustment is greater then the gain at the disruption. The mechanism of adjustment is analogous to that, known for a mechanical pendulum with the vibrating point of suspension (Kapitza pendulum). Physically relevant examples of 3D ferromagnet with a 2D random anisotropy and superfluid $^{3}$He in aerogel are considered. [Preview Abstract] |
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