Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A42: Artificially Structured or Self-Assembled Magnetic Materials - I |
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Sponsoring Units: GMAG Chair: William Pratt, Michigan State University Room: LACC 150B |
Monday, March 21, 2005 8:00AM - 8:12AM |
A42.00001: Large scale and high density regular array of magnetic quantum dots by nanosphere lithography Jun Yuan, Peiweng Wu, Wei Peng, Xinling Tuo, Xiaogong Wang Magnetic quantum dots are single domain magnetic particles which have attracted intense study recently for their fundamental and technological properties. Large scale preparation of monodiapersed and similar shaped magnetic quantum dots remains a challenge at high area density. X-ray lithography is expansive to set up and have limited lateral resolution for nanometer sized magnetic quantum dots. Electron beam lithography technique is also high cost and inefficient tool. This has prompted a number of alternative methods based on self-organized structure such as self-assembled diblock copolymer (Science, Vol. 290, 2000). In this paper, we report the preparation of high density patterned magnetic dots of varying size and shape by nanosphere lithography. Although nanosphere lithography has been widely used to produce mesoscopic scale dot arrays, we find that one can not directly scale them down without modification because of the conformal deformation of polystyrene spheres we have employed. We have used ion beam modification technique to controllably reopen the pore structure, alloying size and shape selective deposition of regular array of magnetic nanoparticles between 20-50 nm in size. The magnetic and electrical characterization of the large scale array has been carried out by SQUID, VSM and magnetic force microscopy and the result will be reported as a function of structural, material and processing parameters. [Preview Abstract] |
Monday, March 21, 2005 8:12AM - 8:24AM |
A42.00002: Fabrication and structural characterization of ordered magnetic nanodot arrays over large area Chang-Peng Li, Igor V. Roshchin, Xavier Batlle, Ivan K. Schuller Self-assembly of nanopores in anodized alumina is of much interest as a controlled fabrication method of magnetic nanostructures for fundamental studies and potential magnetic recording applications. Up to 10 micron thick Al films are e-beam evaporated on N-type Si substrate for porous alumina mask fabrication. By controlling anodization conditions, hexagonally ordered pores with 8-125 nm diameter and 20-160 nm periodicity are formed over $\sim $1 cm$^{2}$ area. SEM and AFM characterization shows that the pores are distributed within $\sim $10{\%} standard deviation from the mean value. Fe magnetic nanodot arrays are fabricated by subsequent e-beam evaporation of Fe and mask lift-off. The smallest dot array fabricated this way is 44 nm, which corresponds to 0.4 Tbit/in$^{2}$ density. The nanodot periodicity is confirmed by small angle neutron scattering measurements. For nanoscale exchange bias studies, Fe/FeF$_{2}$ bilayer nanodot array are prepared using low angle Ar ion etching instead of the lift-off. [Preview Abstract] |
Monday, March 21, 2005 8:24AM - 8:36AM |
A42.00003: Studies of Lithographically Defined Geometrical Frustrated Magnetic Networks Ruifang Wang, K.C. Ku, R. Freitas, G. Xiang, O. Maksimov, N. Samarth, P. Schiffer, M.S. Lund, C. Leighton We used electron beam lithography to pattern Permalloy and MnAs thin films into honeycomb, triangular, and Kagome lattices of separated ferromagnetic nanodots. In these lattices, the dot size is around 100nm with varying separation. Micromagnetic simulation suggests that these lattices can experience magnetic frustration because of the incompatibility of the lattice symmetry and the dipole interaction among the ferromagnetic dots. These lattices have been studied by MFM, both under external field and in the remnant state. This research is supported by a grant from Army Research Office Grant (DAAD19-03-1-0236) and MRSEC at Penn State University. [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 8:48AM |
A42.00004: Structural and magnetic characterization of Co/NiFe dot arrays using soft x-ray resonant magnetic scattering Yongseong Choi, D.R. Lee, J.W. Freeland, G. Srajer, V. Metlushko The field dependence of Co/NiFe dot arrays was studied in a layer-selective way, and structural and magnetic characterizations were conducted. Using x-ray resonant magnetic scattering (measured at the Ni/Co $L_3$ absorption edges), element-specific hysteresis loops were obtained from the NiFe and Co layers of patterned dot arrays. One dot-array sample was grown with an oxide layer between the NiFe and Co layers, and another was grown without it. The two arrays show different field dependence of NiFe magnetization. These results were compared with the hysteresis loops from single-layered NiFe and Co dot arrays. For structural and magnetic characterization of the dot arrays, soft x-ray resonant magnetic scattering measurements in reflectivity mode were performed with circularly polarized x-rays. The work at Argonne is supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. W-31-109-ENG-38. [Preview Abstract] |
Monday, March 21, 2005 8:48AM - 9:00AM |
A42.00005: Tailoring size effects on the exchange bias of ferromagnetic-antiferromagnetic nanodots Vincent Baltz, Jordi Sort, Stefan Landis, Bernard Rodmacq, Bernard Dieny The dependences of the exchange bias effects on the antiferromagnetic (AFM) and ferromagnetic (FM) layers thicknesses have been investigated in continuous films and sub-100 nm dots composed of NiFe-IrMn bilayers. The nanostructures were prepared by sputtering the materials on prepatterned Si square dots. At room temperature, HE in continuous films decreases as the AFM layer thickness (tIrMn) increases, whereas HE in the nanodots remains rather constant. As a result, it is possible to either enhance or reduce HE in the nanostructures, with respect to continuous films, by varying tIrMn. Such a behavior is not observed when varying the FM layer thickness. An enhancement of the coercivity and a reduction of the blocking temperature in the dots are also observed. These effects are ascribed to the 3D confinement of the AFM spin structure and the concomitant enhanced thermal activation effects in the nanostructures. The influence of the dot size and temperature, together with atomistic simulation results, will also be presented. [Preview Abstract] |
Monday, March 21, 2005 9:00AM - 9:12AM |
A42.00006: Spin waves in perpendicularly magnetized nanoscale permalloy dots Mark Zimmerman, Chengtao Yu, Michael Pechan, Jordan Katine, Liesl Folks, Matthew Carey Square arrays of permalloy dots, 100 nm, 200 nm, 500 nm, and 1000 nm in diameter and 40 nm thickness, were created using electron-beam lithography. Ferromagnetic resonance measurements were made at 9.7 GHz with magnetic field was applied out of plane. In addition to the usual uniform mode, a manifold of lower field spin wave (SW) modes were observed for each sample. The spacing between the SW modes exhibits an inverse relationship with the dot diameter. Micromagnetic simulations performed using the open source program OOMMF, are in agreement with experimental results. The SW mode geometry is consistent with ``drum head'' type modes described by Bessel functions as described recently in other dot structures [1]. Supported by US DOE FG02-86ER45281 (MU). \begin{enumerate} \item G. N. Kakazei, et. al., APL, 85, 443 (2004). \end{enumerate} [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:24AM |
A42.00007: Ferromagnetic resonance force microscopy investigations of micron size permalloy dots Tim Mewes, Gleb Kakazei, Vidya Bhallamudi, Razvan Chirla, Palash Banerjee, Yuri Obukhov, Philip E. Wigen, P. Chris Hammel, Sharat Batra Low temperature ferromagnetic resonance force microscopy has been used to investigate the dynamic magnetic properties of permalloy disc arrays. Two dimensional scans of the sample at various external magnetic fields reveal the influence of the tip field and the array structure on the FMRFM-signal. Due to the high force sensitivity and the large signal to noise ratio of the ferromagnetic resonance force microscope individual dots with a 1.5\,$\mu m$ diameter and a center to center distance of 1.8\,$\mu m$ are readily resolved. [Preview Abstract] |
Monday, March 21, 2005 9:24AM - 9:36AM |
A42.00008: Magnetism, NMR spectra, and optical transformations in Nafion with paramagnetic nano-structure E.M. Levin, Q. Chen, S.L. Bud'ko, K. Schmidt-Rohr The bulk magnetization and the $^{19}$F NMR spectra of the ionomer Nafion as-received and doped with Mn$^{2+}$, Co$^{2+}$, Fe$^{2+}$, and Fe$^{3+ }$paramagnetic ions have been studied, with and without treatment in 1H-1,2,4-triazole. As-received Nafion is diamagnetic at 300 K but below 10 K it shows a small paramagnetic ``tail'' indicating the presence of a small amount of paramagnetic centers. Nafion doped with Mn, Co, and Fe ions shows clear paramagnetic behavior at 300 K, which depends on the type and amount of doping ion and treatment in triazole. $^{19}$F NMR spectra at 13 kHz magic angle spinning show differential increases in linewidth and spinning sidebands intensities. The observed changes in magnetism are determined by the spin state of the paramagnetic ions, while NMR reflects dipole-dipole and Fermi contact interactions between them and nearby fluorines of Nafion. Doping of Nafion with Fe and Co and treatment in triazole can strongly affect its color. The most pronounced effect is the temperature- induced reversible transformation of colorless Nafion to violet (Fe doped) and the reverse phenomenon (Co doped) observed simultaneously with the magnetic transformation just below room temperature. All observed phenomena are discussed in terms of the paramagnetic ions in the nano- structure of the Nafion matrix. [Preview Abstract] |
Monday, March 21, 2005 9:36AM - 9:48AM |
A42.00009: Nanoparticles studied by magnetic speckles Karine Chesnel, Steve Kevan, Jeffrey Kortright, Eric Fullerton, Shouheng Sun, Kannan Krishnan Magnetic nanoparticles self assemblies are promising for advanced permanent magnetic applications [1]. The recent development of Soft X-Ray Resonant Magnetic Scattering (SXRMS) provides a very good tool to study magnetic order and reversal processes in such nanostructures. The chemical selectivity and the polarization sensitivity allows to probe the magnetic configuration, as shown by recent studies on superparamagnetic Co particle assemblies [2]. Moreover, by using coherent light and 2D detection one can obtain remarkable speckle patterns that are related to the local magnetic distribution [3,4]. In this work, we present spectroscopy measurments in circular polarization as well as SXRMS measurement performed in transmission geometry (small angle scattering) on Co and Fe3O4 nanoparticles assemblies. We recorded magnetic speckles in linear polarization at specific points tempertaure and magetic field. By studying the cross-correlation between the speckles patterns, we can measure the statistical evolution of the microscopic magnetic distribution through the superparamagnetic transition. [1] H. Zeng et al. Nature \textbf{240}, 395 (2002) [2] J.B. Kortright \textit{et al.}, Phys. Rev. B \textbf{70}, (2004) in press. [3] K.Chesnel \textit{et al}., Phys. Rev. B \textbf{66}, 172404 (2002) [4] M. S. Pierce \textit{et al.}, Phys. Rev. Lett. \textbf{90}, 175502 (2003). [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:00AM |
A42.00010: Quantitative studies of the vortex state in sub-100 nm magnetic nanodots. Igor V. Roshchin, Chang-Peng Li, Xavier Batlle, S. Roy, S. K. Sinha, S. Park, R. Pynn, M. R. Fitzsimmons, J. Mejia Lopez, D. Altbir, A. H. Romero, F. Ott, M. Viret, Ivan K. Schuller Magnetism at nanoscale, when the size of the structures is comparable to or smaller than the ferromagnetic domain size, offers a great potential for new physics. Specifics of magnetic reversal in such structures are important for the high-density magnetic memory. Sub-100 nm magnetic dots are fabricated using self-assembled nanopores in anodized alumina [1]. Magnetization measurements performed using SQUID magnetometer indicate transition from a vortex to a single domain state for the Fe dots. This transition is studied as a function of the dot size and magnetic field. Monte Carlo and micromagnetic simulations confirm the experimental observations. Virgin curves measured at various temperatures indicate thermally activated vortex annihilation and nucleation. Quantitative analysis of the polarized neutron reflectometry in small angle geometry yields the vortex core size of $\sim $14 nm, in a good agreement with the 13 nm obtained from the simulations. Work supported by AFOSR. [1] Kai Liu et al., Applied Physics Letters \textbf{81}, 4434 (2002). [Preview Abstract] |
Monday, March 21, 2005 10:00AM - 10:12AM |
A42.00011: Frustrated Vortices V. Rose, H. Ibach, V. Metlushko, S.-H. Chung, K. Buchanan, M. Grimsditch, V. Novosad, A. Hoffmann, S.D. Bader Patterning soft magnetic materials into ring structures can give rise to a vortex state of the magnetization. When two rings interact strongly, i.e., through direct contact, the vortex states should have opposite chiralities. Thus, for three interacting rings there is an obvious frustration between the magnetic states. We have fabricated isolated and contiguous arrays of permalloy rings, with diameters of 1--4~$\mu$m, widths of 0.2--1.8~$\mu$m, and thickness of 15~nm. Their field dependent magnetization was investigated with magnetic force microscopy and magneto-optical Kerr effect measurements, accompanied by micromagnetic simulations. Generally, in isolated rings the magnetization reverses via nucleation and annihilation of a vortex state. However, in the case of three interconnected rings the magnetization reversal depends on the direction of the applied field. With the field along the midpoints of two of the rings the magnetization changes gradually, while for the field tangential to the connection between two of the rings vortices develop with opposite chiralities and the third ring remains in an onion state. [Preview Abstract] |
Monday, March 21, 2005 10:12AM - 10:24AM |
A42.00012: Exchange Biased Vortices J. Sort, J. Nogu\'es, M.D. Bar\'o, S.-H. Chung, K. Buchanan, M. Grimsditch, V. Novosad, A. Hoffmann, B. Dieny Soft ferromagnetic discs with submicrometer diameter can reverse their magnetization via nucleation and annihilation of a vortex state. We prepared 400 nm diameter discs of Ni$_{80}$Fe$_{20}$/IrMn bilayers, where the exchange coupling between the ferromagnetic Ni$_{80}$Fe$_{20}$ and the antiferromagnetic IrMn modifies the magnetization reversal. Annealing of the Ni$_{80}$Fe$_{20}$/IrMn discs in an applied field establishes an exchange bias along the direction of the magnetic field during annealing. Magneto-optic Kerr effect measurements with the field applied along the exchange bias direction reveal a typical vortex hysteresis loop, which is now shifted with respect to zero field. Magnetic force microscopy in applied fields confirm that the reversal is via a vortex state. When the applied field is rotated with respect to the exchange bias direction the nucleation and annihilation fields reduce slightly, until at a critical angle of $\approx 80^{\circ}$, beyond which no vortex nucleation is observed. Micromagnetic simulations show that beyond the critical angle the magnetization reversal occurs via rotation of a C-state. [Preview Abstract] |
Monday, March 21, 2005 10:24AM - 10:36AM |
A42.00013: Theoretical analysis of the transmission Phase Shift of a Quantum Dot in presence of Kondo correlations Andres Jerez, Pavel Vitushinsky, Mireille Lavagna We study the effects of Kondo correlations on the transmission phase shift of a quantum dot coupled to two leads. Experimental determination of the phase shift made by embedding a quantum dot in one of the arms of an Aharonov-Bohm interferometer leads to a value of the phase which differs from the well-known theoretical predictions. We propose here a theoretical interpretation of these results based on Bethe Ansatz calculations combined with the scattering theory. Quantitative agreement is obtained with experimental results both in the unitary limit and the weak Kondo coupling regimes. [Preview Abstract] |
Monday, March 21, 2005 10:36AM - 10:48AM |
A42.00014: Kondo Effect in an Exactly Solvable Double Dot System with Singlet Groundstate Robert Konik We analyze transport through a double dot system connected to two external leads. Each dot is treated as possessing a single active level. With symmetric couplings to the leads, only a single effective lead is available to interact with the double dot. We model the system through a generalization of the Anderson model which is exactly solvable via the Bethe ansatz provided certain, not particularly restrictive, constraints are placed upon the dot parameters. From this analysis we see that the zero temperature dot system at its particle-hole symmetric point favors singlet formation. Using exact solvability, we further determine how this double dot singlet evolves under increasing temperature and magnetic field together with changes of the gate voltage moving the dot-lead system away from particle-hole symmetry. We are able to analyze both the resulting transport signatures of this evolution and the effective energy scale governing the changes. [Preview Abstract] |
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