Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A34: Focus Session: Nanomagnetism -- Nanostructured Films |
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Sponsoring Units: DMP GMAG Chair: Kirill Bolotin, Vanderbilt University Room: E144 |
Monday, March 15, 2010 8:00AM - 8:12AM |
A34.00001: Field Manipulation of Magnetization States in Nanoscale Ferromagnetic Rings Through Geometric Design Tianyu Yang, Abby Goldman, Moureen Kemei, Kathy Aidala, Mark Tuominen In this work, conduct a systematic study of field evolution of small magnetic rings with the aim of determining stable magnetized states as a function of geometric parameters. Such rings may be a foundation for novel data storage applications. In these experiments, ferromagnetic cobalt and permalloy nanorings, with diameter in range of 300 nm to 800 nm, are fabricated by electron beam lithography, development, thin-film deposition and lift-off. Magnetic force microscopy (MFM) is used to observe magnetic states of nanorings having different geometry. We will discuss the different magnetic configurations that result when an in-plane magnetic field, either homogeneous or inhomogeneous on a scale compared to the dimension of the ring, is used to manipulate the magnetization. [Preview Abstract] |
Monday, March 15, 2010 8:12AM - 8:24AM |
A34.00002: Magnetic Property in large array cobalt antidot thin film using polymer-assisted nanosphere lithography Wei-Li Lee, Chi-Chih Ho, Yung-Wu Hsieh, Wen-Tau Juan, Keng-Hui Lin We have developed a new method to prepare monolayer of close- packed nanospheres (NSs) over large area onto a substrate of any kind utilizing polymer bridging effect. The NSs packing domain can be as large as 1 cmx1 cm which is demonstrated from its diffraction pattern. It was then used as a template to fabricate series of cobalt antidot thin films with different antidot diameter ranging from 100nm to 180nm. Because of the good periodicity and less defects in our nanostructured samples, we would be able to not only qualitatively study their magnetic properties but also quantitatively. As the antidot diameter increases, the surface to bulk volume fraction increases and the surface magnetism becomes more prominent. We found a systematic increase in magnetic coercivity with the antidote diameter, while the saturation magnetization drops at large antidote diameter. Detailed analysis and their implication will be discussed. [Preview Abstract] |
Monday, March 15, 2010 8:24AM - 8:36AM |
A34.00003: Magnetic Triangular Dot and Antidot Arrays from Self-Assembly Lithography P. Panyajirawut, M.S. Rzchowski Well-ordered two dimensional sub-micron triangular dots arrays of Ni/Co multilayers have been fabricated using the nanosphere lithography technique, with an additional oxygen plasma etch to tailor the size of the polystyrene spheres. Using self-assembled close-packed monolayers of polystyrene spheres as deposition masks, the magnetic heterostructure is deposited through the interstitial areas to form triangular anitdot arrays. We perform longitudinal MOKE (Magneto-Optic Kerr Effect) measurement on patterned arrays with a spatial resolution of 100 $\mu $m. We present the room temperature magnetization hysteresis loop measurements of Ni/Co multilayers triangular-shaped and antidot arrays. The magnetic measurements reveal that, although the unpatterned multilayer has magnetic anisotropy induced during the sputter deposition, the anisotropy is much reduced after patterning the continuous film. The patterned dot arrays have increased coercivity relative to that of the unpatterned film. This could be from to the changes of multidomain formation during magnetization reversal for the patterned dots. [Preview Abstract] |
Monday, March 15, 2010 8:36AM - 8:48AM |
A34.00004: Imaging the three dimensional magnetic nanostructure of Co/Pd multilayer media Benjamin McMorran, Paul Morrow, Daniel Pierce, John Unguris, Randy Dumas, Kai Liu Multilayers of Co and Pd (or Pt) are well known for magnetization that points out-of-plane, perpendicular to the surface, but local measurements of the in-plane magnetization remain elusive due to a scarcity of measurement techniques. We used SEM with polarization analysis (SEMPA) to study both continuous films and patterned dots made of pressure-graded Co/Pd multilayer media. Images of all three vector components of the remanent surface magnetization show a rich variety of three dimensional magnetic structures in patterned 2 $\mu $m diameter dots. Labyrinth magnetic domains polarized in the up (+z) and down (-z) directions act as sources and sinks, respectively, for the in-plane magnetization vector field. Larger out-of-plane magnetic domains coincide with the presence of in-plane vortices. In some dots, the distribution of magnetization angles is concentrated along the surface of a cone. These results show that in-plane magnetization in real Co/Pd multilayers plays an important role in the domain configuration of the patterned films. [Preview Abstract] |
Monday, March 15, 2010 8:48AM - 9:00AM |
A34.00005: E-beam lithography patterning of Co/Pd multilayer pillars below 50 nm in diameter Ryan Comes, Richard Kasica, Gerard Henein, Jiwei Lu, Stuart Wolf Co/Pd multilayer superlattice structures are among the most popular thin film systems for use in future bit patterned media. Multilayer Ta(9 nm)/Pd(7)/{\{}Co(0.3)/Pd(0.3){\}}x15 films were deposited on a Si(111) substrate by combined RF and DC magnetron sputtering. The films were characterized by x-ray reflectivity, x-ray diffraction, and a vibrating-sample magnetometer and found to exhibit perpendicular anisotropy. Additional films were made with a 30 nm sputtered nanocrystalline Si capping layer which serves as an etch stop [1]. These films were patterned into pillars with diameters of less than 50 nm using e-beam lithography. We present results of the lithographic process and analysis of the Co/Pd nanopillars which are formed. \\[4pt] [1] Jung-Sub Wi, et al. ``Electron beam lithography of Co/Pd multilayer with hydrogen silsesquioxane and amorphous Si intermediate layer.'' J. Vac. Sci. Technol. B, 24(6), 2616, Nov/Dec 2006. [Preview Abstract] |
Monday, March 15, 2010 9:00AM - 9:12AM |
A34.00006: Microscopic return point memory in Co/Pd multilayer films Keoki Seu, Run Su, Daniel Parks, Sujoy Roy, Erik Shipton, Eric Fullerton, Stephen Kevan We have conducted resonant x-ray scattering with a coherent beam on Co/Pd, a system which exhibits perpendicular magnetization. The system forms labyrinth domains and we collect a pure magnetic signal in our transmission geometry. The energy of the incident X-rays is tuned to the Co L$_3$ edge and the coherence is established with a 10 um pinhole. The return and complementary point memory, defined by the correlation between the speckle pattern at whole loops and half-loops respectively, depends on the applied field and is a maximum near initial magnetization reversal, indicating that the system has strong memory during nucleation. As the field is increased, the memory decreases and reaches a minimum as the magnetization begins to saturate. This indicates that the system follows a large path in configuration space as the labyrinths evolve with the applied field. [Preview Abstract] |
Monday, March 15, 2010 9:12AM - 9:24AM |
A34.00007: Magnetic switching due to hydrogen absorption in Co/Pd multilayers Kineshma Munbodh, Felio Perez, Samuel Ducatman, David Lederman Co/Pd multilayers with cobalt thicknesses ranging from 2.5 {\AA} to 12 {\AA} and palladium thicknesses ranging from 6.5 {\AA} to 15 {\AA} have been fabricated by d. c. sputtering on Al$_{2}$O$_{3}$ (110) in an argon atmosphere. The morphological and structural characterizations revealed smooth surfaces, layered structure and highly oriented growth in the [111] direction. The magnetic and electronic transport properties were measured in a hydrogen and helium atmosphere at room temperature using a vibrating sample magnetometer and a four-point technique a with current-in-plane configuration, respectively. All samples exhibited significant changes on the magnetic and transport properties as a function of hydrogen absorption. These preliminary results show that these devices may be used effectively as corrosion resistant hydrogen sensors and hydrogen storage devices. [Preview Abstract] |
Monday, March 15, 2010 9:24AM - 9:36AM |
A34.00008: Effect of interactions on edge property measurements in magnetic multilayers Robert McMichael, Meng Zhu The edges of patterned thin films are important, especially in magnetic nanostructures. In previous work, it has been shown that the magnetic properties of film edges in Ni$_{80}$Fe$_{20}$ (Py) stripe arrays can be measured with a precision of a few percent using the ferromagnetic resonance (FMR) of localized edge modes. In this work, we extend this measurement technique to multilayer films, showing the effects of interactions between edge modes in the magnetic layers. We fabricate magnetic multilayer stripes consisting of 10 nm Py / $x$ Cu / 20 nm Py, where $x$ ranges from 1 nm to 20 nm, and we find that the edge saturation fields of both Py layers increase as the spacer is reduced, indicating enhanced magnetostatic interactions. An approximate analytical model based on the static dipolar interactions is used to simulate experimental and micromagnetic model data. [Preview Abstract] |
Monday, March 15, 2010 9:36AM - 9:48AM |
A34.00009: Transition States and the Energy Barrier to Magnetization Reversal of Thin Film Nanomagnets with Perpendicular Anisotropy Gabriel Chaves-O'Flynn, Daniel Bedau, Eric Vanden-Eijnden, Daniel Stein, Andrew Kent We use the String Method [1] in conjunction with the micromagnetics OOMMF package to calculate the energy barrier for magnetization reversal of square thin film nanomagnets with perpendicular anisotropy. The lowest energy state consists of out of plane magnetization configurations. A field applied perpendicular to the plane lifts the degeneracy between the states. The effect of the element size and the consequences of breaking the square symmetry are investigated. We find that the transition state is not uniform: it starts with a localized nucleation, which expands to complete the reversal. The field dependence of the energy barrier is compared to that of macrospin model, and nonuniform reversal is shown to be the preferred transition configuration, providing a lower energy barrier to reversal. This result indicates the limits of the macrospin model. We present the dependence on the energy barrier on the exchange constant and simulation cell size. [1] W. E, W. Ren, E. Vanden-Eijnden, J. Chem. Phys. 126, 164103 (2007) [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:00AM |
A34.00010: Interlayer coupling enhanced by the interface roughness: A perturbative method Ching-hao Chang, Tzay-ming Hong Previous experiment on Fe/Cr/Fe$\Delta$001$\Theta$ trilayers reported a surprising observation that the interlayer exchange coupling could be enhanced drastically by the bombardment of irradiation even at low fluences. We propose that it is due to the resonant states in the spacer made possible when the topography of both interfaces is correlated and exhibits prominent Fourier components. A systematic procedure is developed to handle the interface roughness and predict on how to optimize the interlayer coupling. This method can be extended to bridge the gap between theories and experiments in other heterojunctions. [Preview Abstract] |
Monday, March 15, 2010 10:00AM - 10:12AM |
A34.00011: Magnetization profiles in Fe/SmCo Spring Magnets with Graded Interfaces Y.H. Liu, J.S. Jiang, S.G.E. te Velthuis, H. Ambaye, A. Parizzi To understand the improved effectiveness of the exchange coupling in Fe/SmCo spring magnets with a graded interface, we have determined the magnetic structure with Polarized Neutron Reflectometry (PNR). PNR confirms that the Fe/SmCo interface is greatly intermixed. Magnetic hysteresis curves show well-separated coercive fields for the soft phase $H_C^S$ and the hard phase $H_C^H$. Large spin-flip (SF) scattering reveals a twisted magnetic structure at $H_C^S$. SF scattering initially decreases slightly when $H$ increases, then drops dramatically when $H> H_C^H$. This suggests that the twisted region first becomes more narrow and then suddenly diminishes once the SmCo layer switches. Combining 1D micro-magnetic simulations and PNR, we find that the SmCo layer has a much lower average in-plane anisotropy than the bulk. The scattering along the recoil loops strongly depends on the starting field, but in all cases a collinear magnetic structure is indicated at low fields. Overall, the magnetization reversal is mostly reversible rotation for $H < H_C^H$, but an abrupt irreversible switching happens when $H > H_C^H$. [Preview Abstract] |
Monday, March 15, 2010 10:12AM - 10:24AM |
A34.00012: Magnetic properties of ferromagnetic-antiferromagnetic bi-layers with different spin configuration Wondong Kim, J. Wu, J. Park, E. Jin, Z.Q. Qiu We investigated the effect of different spin direction of anti-ferromagnetic layer on the magnetic properties of ferromagnetic layer in Fe-NiO bi-layer system. We prepared the clean MgO(001) surface half-covered with 20 nm Ag films and the other half uncovered. Then NiO wedge layer was grown on the substrate, and added 8 monolayer Fe layers on the wedge layer. We examined the magnetic properties of the bi-layer system using the surface magnetic optical Kerr effect(SMOKE) and X-ray magnetic linear dichroism(XMLD). From SMOKE measurement we observed the coercivity enhancement due to the set-up of anti-ferromagnetic order of NiO films in both of the Fe/NiO/MgO(001) and Fe/NiO/Ag/MgO(001) system. The most remarkable result in our observation is that the coercivity enhancement of Fe/NiO/Ag/MgO(001) is much larger than that of Fe/NiO/MgO(001). XMLD experiments confirmed the out-of-plane spin direction of NiO layers in Fe/NiO/MgO(001) and in-plane spin-direction of NiO layers in Fe/NiO/Ag/MgO(001) We conclude that the in-plane NiO spins have a much stronger effect on the magnetic anisotropy of the in-plane Fe magnetization than the out-of-plane NiO spins. [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 10:36AM |
A34.00013: Mechanism of giant exchange bias in a rare earth superlattice M.R. Fitzsimmons, C. Dufour, K. Dumesnil, J. Dou, M. Pechan, J.A. Borchers, M. Laver After cooling a DyFe$_{2}$/YFe$_{2}$ superlattice [3 nm DyFe$_{2}$/12 nm YFe$_{2}$ repeated 22 times] to 12 K in a 1 T field, which aligns the Fe-spins parallel to the field, the magnetization vs. field curve of the superlattice was dramatically shifted along the \textit{magnetization and applied field} \textit{axes}. The exchange bias was -2 \textit{Tesla}. We developed a one dimensional spin-chain-model that completely explains the polarized neutron reflectometry, magnetometry and X-ray magnetic circular dichroism data. Two in-plane domain configurations were identified in the model. Both configurations contribute to the extraordinarily large exchange bias of the DyFe$_{2}$/YFe$_{2}$ superlattice. Until just recently, we lacked direct evidence for the existence of the domain configurations; however, SANS measurements of the thin film superlattice show compelling proof that magnetization reversal is accompanied by formation of small domains in the sample plane. The domain scattering exhibits a pronounced enhancement at the exchange bias field, and remarkably, is a minimum at the cooling field rather than at high (7 T) fields. [Preview Abstract] |
Monday, March 15, 2010 10:36AM - 10:48AM |
A34.00014: From exchange coupling to magnetic memory: how domains remember at nanoscale Karine Chesnel, Joseph Nelson, Eric Fullerton, Matt Carey, Steve Kevan Magnetic memory, the ability of a material to remember its magnetic domain configuration throughout magnetization, offers potential technological interest for the data storage industry. One way to quantify the magnetic memory is to use Coherent X-ray Resonant Magnetic Scattering (XRMS), at synchrotron facilities. The light is tuned to resonant edges to optimize the magneto-optical contrast. When illuminated by coherent beam, the sample produces speckle patterns. Our approach is to cross-correlate patterns recorded at different field values throughout the magnetization cycle, and at different temperatures. We have studied the return point memory (RPM) that characterizes the memory after a full cycle, and developed a q-selective correlation analysis to study the spatial dependency of the memory. We will give here an overview of different type of memory behaviors, first showing disorder induced memory in thin CoPt films and influence of roughness, then demonstrating the ability to control the magnetic memory by inducing exchange bias (1). We will see how the local exchange couplings pin the magnetic domain in the ferromagnetic layer and lead the large memory enhancement at different spatial scales and under different field cooling conditions. (1) K.Chesnel et al, PRB \textbf{78}, 132409 (2008) [Preview Abstract] |
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