Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D22: Focus Session: Magnetic Nanopatterns |
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Sponsoring Units: GMAG DMP Chair: Laura Lewis, Brookhaven National Laboratory Room: Baltimore Convention Center 319 |
Monday, March 13, 2006 2:30PM - 2:42PM |
D22.00001: Magnetization reversal process and domain wall resistance in a water drop shape ring D.C. Chen, D.P. Chiang, Y.D. Yao Patterned permalloy (Ni80-Fe20) materials have been fabricated by e-beam lithography in the shape of water drop ring. A tip is intentionally added into ring as geometrical defects to interrupt the continuity of magnetization reversal process, in order to create domain wall. Image from Magnetic force microscopy (MFM) with real- time external field confirmed this domain structure. As a result of magneto- resistance (MR) measurement, the ratio of MR is about 0.137 $\sim $ 0.233{\%} and 0.23 $\sim $ 0.71{\%} at sweeping angles of samples and sweeping external field, respectively. The ratio of the change in the electric resistance which is measured by I-V curve is just about the value of domain wall MR ratio which is measured by sweeping angles and external field. In summary, we have successfully demonstrated that the domain wall motion along the direction of perimeter in a ferromagnetic ring at its onion state; and the critical field to form onion state is near 200 Oe and the lowest field which can still drag the domain wall is between 100 and 50 Oe. [Preview Abstract] |
Monday, March 13, 2006 2:42PM - 2:54PM |
D22.00002: Arrays of Ultrasmall Nanoscopic Ferromagnetic Rings Deepak Singh, Hongqi Xiang, Robert Krotkov, Ting Xu, Qijun Xiao, Thomas Russell, Mark Tuominen There has been a considerable amount of recent interest in the magnetic properties of ultrasmall ferromagnetic rings. This is largely motivated by the unique stability of the vortex state of magnetization and its relevance in future data storage elements. In this work, we discuss experimental research on ultra small cobalt nano rings. We fabricate arrays of ultra small rings using a technique involving self-assembled diblock copolymer templates, glancing angle evaporation and ion beam etching. The nanoscopic rings have an outer diameter of 13 nm and inner diameter of 5 nm. The magnetization properties of these arrays are measured in both parallel and perpendicular field orientations in SQUID magnetometer. We compare these measurements to analytical calculations for different magnetic configurations that take into account the competition between exchange energy, Zeeman energy and magnetostatic energy. Based on analytical calculation and magnetic measurement we find that for such ultrasmall ring structures vortex states are the dominant remanent magnetic states. [Preview Abstract] |
Monday, March 13, 2006 2:54PM - 3:06PM |
D22.00003: Reversal mechanism of patterned ferromagnetic elements Yimei Zhu, J.W. Lau, V.V. Volkov Understanding of magnetization reversal dynamics in patterned ferromagnetic elements, or building blocks, is crucial to the development of modern magnetic storage media and spintronic devices. Using state-of-the-art, field-emission transmission electron microscope (TEM) equipped with a custom-made objective lens (the only one of its kind in the world) that allows high-resolution magnetic imaging and electron holography, we study the reversal mechanism and hysteresis behavior of patterned rings, squares, and ellipses with different aspect ratios of Permalloy and Co thin-films. By quantitative analysis of the local magnetization, we measure magnetic properties and shape effect of individual elements, and compare experimental observations with theoretical calculations. We explain why a parent state of the elements, isolated or in an array environment, can result in different low-energy ground states, depending on the switching rate, and how the energy barrier to the vortex nucleation can be measured. New results using a magnetic force microscope built into a TEM stage to locally induce magnetic field gradient and to change magnetic polarization of the elements to study element-element interaction and magnetic reversal will be also reported. [Preview Abstract] |
Monday, March 13, 2006 3:06PM - 3:42PM |
D22.00004: Magnetic nanorings and manipulation of nanowires Invited Speaker: The properties of nanoscale entities, such as nanorings and nanowires, and the response of such entities to external fields are dictated by their geometrical shapes and sizes, which can be manipulated by fabrication. We have developed a method for fabricating a large number of nanorings ($10^{10}$) of different sizes in the range of 100 nm and ring cross sections. During magnetic reversal, both the vortex state and the rotating onion state appear with different proportions, which depend on the ring diameter, ring cross section, and the profile of the ring cross section. In the case of nanowires in suspension, the large aspect ratio of the nanowires can be exploited for manipulation despite extremely small Reynolds numbers of $10^{-5}$. Using AC electric field applied to microelectrodes, both magnetic and non-magnetic nanowires can be efficiently assembled into desired patterns. We also demonstrate rotation of nanowires with precisely controlled rotation speed and chirality, as well as an electrically driven nanowire micromotor a few $\micron $ in size. In collaboration with F. Q. Zhu, D. L. Fan, O. Tchernyshyov, R. C. Cammarata (Johns Hopkins University) and X. C. Zhu and J. G. Zhu (Carnegie-Mellon University). [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 3:54PM |
D22.00005: Bistability and controllable magnetic switching of 100-nm asymmetric ferromagnetic nanorings F.Q. Zhu, Gia-Wei Chern, Oleg Tchernyshyov, C.L. Chien Nanorings can acquire the vortex state with flux closure and zero stray field. We have developed a method for fabricating a large number of nanorings over a macroscopic area. However, for 100-nm nanorings, the nanorings can acquire both the rotating onion state and the vortex state during magnetic reversal with comparable probability. In this work, we report the fabrication and properties of asymmetric nanorings, whose thickness and width vary along the circumference. In contrast to symmetric nanorings, the percentage of vortex formation in asymmetric nanorings can be controlled by the direction of the magnetic field. When the field is along the asymmetry axis, nearly every nanoring can acquire the vortex state. The introduction of asymmetry in the nanorings allows full vortex formation without losing the virtue of small dimension, high stability and high areal density. We have also developed a theoretical model to calculate the dependence of domain wall energy on the local width and thickness of the nanorings to account for the enhancement of vortex state in asymmetric nanorings. [Preview Abstract] |
Monday, March 13, 2006 3:54PM - 4:06PM |
D22.00006: Self-Assembled Cobalt Nanodots along Al$_{2}$O$_{3}$ (0001) step edges Jorge Espinosa, David Lederman Ultrathin films of Co with thicknesses of 0.80 monolayers (ML) and 0.20 ML were grown on Al$_{2}$O$_{3}$ (0001) via molecular beam epitaxy at room temperature. \textit{In situ} non-contact atomic force microscopy images obtained after annealing the 0.80 ML film in-situ at 400 $^{\circ}$C revealed the formation of nano-dots distributed randomly on the sapphire surface with an average diameter of approximately 18 nm and a height of 0.4 nm. The annealing of the 0.20 ML film showed the formation of nanodots with diameters of 16 nm for the film annealed at 500 $^{\circ}$C and 28 nm for an annealing temperature of 500 $^{\circ}$C. In general, the nanodots increase their sizes with the annealing temperature and they move towards the top of the sapphire step edges, indicating a certain degree of self organization. [Preview Abstract] |
Monday, March 13, 2006 4:06PM - 4:18PM |
D22.00007: Interaction effects in heterostructures of nanoscale magnetic particles and magnetic thin films Jens Mueller, Stephan von Molnar, Yuzo Ohno, Hideo Ohno Elongated magnetic nanoparticles attract continuing attention because of potential technological applications. STM assisted CVD is an advantageous fabrication technique for exact positioning of individual Fe particles (diameter 5 -- 20 nm) on different substrate materials. A first step towards an application of these small and local magnetic flux sources to intentionally influence and investigate other materials is to study structures of magnetic particles grown onto an underlying magnetic film. This enhances interaction effects between adjacent particles. Also, the particles alter the magnetic domain structure of the thin film making the transport properties of the latter sensitive to the magnetization state of the particles grown on top leading to a distinct switching effect in the magnetoresistance that persists up to room temperature. The interaction effects in such magnetic particles/thin-film heterostructures are further investigated in a device with a single magnetic nanoparticle and an underlying film of the concentrated magnetic semiconductor EuS. The magnetic interaction of this system can be tuned by sweeping the temperature through T$_{C}$ of EuS. We discuss the heterostructure's magnetization behavior in terms of different magnetic interaction strengths. [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:30PM |
D22.00008: Perpendicular Anisotropy in Cobalt-nanodots on Rutile TiO2(110) N. Widjaja, Jiandi Zhang, C.G. Zhou, Minghu Pan, E.W. Plummer, J. Shen We report the structural and magnetic properties of self-assembled Co-nanodots on rutile TiO$_{2}$(110) substrates. Co-dots with different coverage density were prepared inside a UHV chamber (base pressure $<$ 1x10$^{-10}$ Torr) by thermally evaporating Co-source on the well ordered rutile TiO$_{2}$(110) surface. The size of the Co-dots can be tuned by changing the coverage density and the dose rate of Co independently. The in-situ STM imaging indicates that the size of the Co-dots ranges from 0.5 nm to 1 nm. The magnetization of the systems was measured ex-situ using a Quantum Design SQUID magnetometer after they were capped with NaCl. The magnetization as a function of field measured at various temperatures for 2$<$T$<$250K reveals significant perpendicular anisotropy. The origin of the anisotropy is explained in terms of the competition between magnetocrystalline anisotropy and shape anisotropy of the nanodots. [Preview Abstract] |
Monday, March 13, 2006 4:30PM - 4:42PM |
D22.00009: Geometrical frustration in a regular lattice of nanoscale ferromagnetic islands: artificial spin ice R.F. Wang, C. Nisoli, R.S. Freitas, J. Li, W. McConville, B.J. Cooley, N. Samarth, V.H. Crespi, P. Schiffer, M.S. Lund, C. Leighton We studied frustration in lithographically defined lattices of permalloy islands with lattice geometry designed so that the dipolar interactions are analogous to those of the spin ice materials. These islands are approximately 80 nm by 220 nm laterally and 25 nm in thickness, and the array lattice constant ranges from 320 nm to 880 nm. Our MFM measurements demonstrated that the single domain magnetic moment of individual islands had short range correlations but no long range correlations. The correlations showed behavior consistent with the ice rules for this system, which decreased with increased spacing between the islands. Research was supported by the Army Research Office.\newline \newline Reference:\newline R. F. Wang et al. (Nature, in press). [Preview Abstract] |
Monday, March 13, 2006 4:42PM - 4:54PM |
D22.00010: ``Live'' Surface Ferromagnetism in Fe Dot Multilayers on Cu(111) Maria Torija, An Ping Li, Charles Guan, Ward Plummer, Jian Shen We investigate the crossover behavior from two-dimensional (2D) to three dimensional (3D) in multilayers of magnetic nanodots grown by stacking 2D Fe nanodot assemblies on Cu(111) single crystal substrate with a Cu spacing layer. Using in-situ magneto-optical Kerr effect, we have observed a striking ferromagnetic to spin-glass phase transition with increasing number of Fe dot layers. The topmost layer of the Fe dots survives the phase transition and remains ferromagnetic. This unusual surface ferromagnetism is likely caused by a stronger surface state-mediated intralayer dot coupling which is stronger than the interlayer dot coupling, as confirmed by the fact that the critical temperature of the surface ferromagnetism is considerably higher than that of the bulk spin glass phase in the system. [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:06PM |
D22.00011: Fabrication and magnetism of sub-100 nm exchange-biased magnetic dot arrays and porous networks. Chang-Peng Li, Igor V. Roshchin, Zhi-Pan Li, Ivan K. Schuller Studies of exchange bias (EB) at nanoscale, when the structure size is comparable to magnetic domain sizes, can offer new insights for the mechanism of EB. For these studies, sub-100 nm ferromagnetic (Ni)-antiferromagnetic (FeF$_{2})$ dots and porous network bilayers are fabricated using self-assembled porous alumina masks. The size and periodicity of the nanopatterns are controlled by anodization conditions and pore widening time. Magnetization of the field cooled dots and networks is measured at 10 K by SQUID magnetometry and magneto-optical Kerr effect (MOKE), respectively. For the dots, the EB field is found to decrease as the dot diameter decreases. For example, with Ni/FeF$_{2}$ dot diameter decreasing from 98 nm to 52 nm, the EB field decreases from 640 Oe to 240 Oe. For networks with the constant pore periodicity, an increase in pore diameter from 50 nm to 70 nm results in a decrease of EB field from 150 Oe to 90 Oe. The results suggest that the EB decreases as the magnetic structure size decreases, regardless of its actual shape. [Preview Abstract] |
Monday, March 13, 2006 5:06PM - 5:18PM |
D22.00012: Domain walls resistance study in magnetic thin films by means of thickness modulation Wei-Li Lee, F.Q. Zhu, V. Thampy, C.L. Chien The manner with which the magnetic domain wall (DW) affects the transport properties of a ferromagnet remains an interesting but difficult topic. Experimental extraction of magnetoresistance (MR) of DWs is challenging because of other dominating contributions especially the anisotropic magnetoresistance (AMR). We have explored one way to extract DW resistance by creating structures with built-in thickness modulation (THM). By capitalizing on the thickness dependence of coercivity, the domain size and hence the number of DWs can be controlled through the period of THM. In a previous work, we used lithography to pattern a 50 nm thick Co film, 5$\mu$m in width and 60$\mu$m in length. We then used a focused ion beam to create grooves transverse to its length with depth $\sim$ 8 nm and period $\sim$ 500 nm. Magnetic force microscopy reveals an induced magnetic anisotropy along the groove direction. By applying magnetic field along various directions and taking in account the AMR contributions, we have deduced that the DW resistance in Co is positive with a value of $\sim$ 29.3 m$\Omega$ corresponding to $\sim$ 0.14\% MR. In the present work, the structure is prepared in different way and geometry so that current parallel to walls and current perpendicular to walls DW resistance can be both obtained from the experiment. The latest results and comparison with theoretical models will be discussed. [Preview Abstract] |
Monday, March 13, 2006 5:18PM - 5:30PM |
D22.00013: Ferromagnetic Nanocrystals of Antiferromagnetic FeGe C.G. Zeng, Maria Varela, Paul Kent, Markus Eisenbach, George Malcolm Stocks, Maria Torija, Jian Shen, Hanno Weitering Epitaxial nanocrystals of FeGe have been stabilized on Ge(111). The nanocrystals assume a quasi one-dimensional shape as they grow exclusively along the $<$1$\mathop 1\limits^- $0$>$ direction of the Ge(111) substrate, culminating in the monoclinic modification of FeGe. The uni-directional growth results from a close match between the Ge-atom spacing along $<$1\={ }10$>$Ge and monoclinic b-axis of FeGe. Whereas monoclinic FeGe is antiferromagnetic in the bulk, the nanocrystals are surpisingly strong ferromagnets below $\sim $200 K with an average magnetic moment of 0.8 $\mu _{B}$ per Fe atom. Density functional calculations demonstrate that volume ferromagnetism is stabilized predominantly by compressive strain normal to the growth direction of the nanocrystals. [Preview Abstract] |
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