Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session M14: Focus Session: Patterned Magnetic Nanostructures |
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Sponsoring Units: GMAG DMP Chair: Frances Hellman, University of California, Berkeley Room: 316 |
Wednesday, March 20, 2013 8:00AM - 8:12AM |
M14.00001: Magnetic ToF GISANS on self-assembled nanoparticles Artur Glavic, Elisabeth Josten, Oleg Petracic, Valeria Lauter Nanoparticle superlattices can be considered as novel type of materials with controllable electronic, optical and magnetic properties. Their building blocks are nanoparticles (or ``nanocrystals'') from a metallic, metal-oxide, or semiconducting material or hybrid between different materials. Using self-assembling techniques it is possible to create a large amount of highly ordered 3D structures, which we have investigated for their structural and magnetic properties. The lateral ordering is quantified using electron microscopy and grazing incidence small angle X-ray scattering (GISAXS) [1,2,4]. The macroscopic magnetic behavior and correlations are investigated by superconducting quantum interference device (SQUID) magnetometry [1,3]. Utilizing the time of flight (ToF) magnetism reflectometer at SNS the magnetic correlations have been studied with polarized GISANS and PNR. \\[4pt] [1] M. J. Benitez et al., J. Phys.: Condens. Matter 23, 126003 (2011).\\[0pt] [2] G. A. Badini Confalonieri et al., Nanotechnology 22, 285608 (2011).\\[0pt] [3] A. Ebbing et al., Phys. Rev. B. 84, 012405 (2011).\\[0pt] [4] D. Mishra et al., Nanotechnology 23, 055707 (2012).\\[0pt] [5] E. Josten et al. (unpublished). [Preview Abstract] |
Wednesday, March 20, 2013 8:12AM - 8:24AM |
M14.00002: Anomalous Magnetoresistance Effect in Topographical Nanoengineered Material Deepak Singh, Stefan Dickert, Rukshan Thantirige, Mark Tuominen Recent developments in nanofabrication allow for the engineering of a broad range of topographical materials with strong implications in spin caloritronics of condensed matter physics. We have applied the top down approach to create a series of nanoengineered materials, which consists of locally hexagonal periodic array of Co dots (12 nm in diameter and 3 nm in thickness, with a periodicity of 28 nm) in direct multidirectional contact with encapsulating thin layer of polycrystalline Cu film (15-30 nm). The electrical transport measurements on the nanoengineered materials unveiled a host of interesting properties that includes the giant thermal hysteresis, which is onset above the room temperature, and anomalous magnetoresistance (MR) behavior. The thermal hysteresis exhibits strong magnetic field dependence, applied perpendicular to the substrate. The most unusual behavior, perhaps, is manifested by MR oscillations, which occur only in the initial field scan in a very unusual temperature range of 100 K \textless T \textless 200 K. The qualitative interpretation of the experimental results suggests that the spin-orbit-type coupling between giant localized moments in periodic sites and the surrounding conduction electrons play important role in the anomalous MR oscillation. [Preview Abstract] |
Wednesday, March 20, 2013 8:24AM - 8:36AM |
M14.00003: Magnetic Dipole Interaction on a Square Lattice Hartmut Zabel, Melanie Ewerlin, Derya Demirbas, Frank Bruessing, Florian Kronast We have studied interactions and phase transitions of circular magnetic islands with dipole character on a square lattice. By lithographic means we have prepared square patterns of periodicity 300 nm decorated with circular islands of 150 nm diameter using Pd0.87Fe0.13 as magnetic alloy. Below the Curie temperature of 260 K each island is in a ferromagnetic, single domain state with dipolar character and zero in-plane anisotropy. Below a second transition temperature the dipoles start to interact. MOKE measurements show a characteristic change in the magnetic hysteresis for temperatures below 160 K with increasing coercivity for decreasing temperatures. Furthermore, below the second transition the in-plane hysteresis becomes anisotropic, having an easy axis along [10] direction and a hard axis along [11] direction. SPEEM experiments at BESSY II of the HZB with circularly polarized incident photons tuned to the Fe L3 - edge show clearly the development of dipolar chains below the second phase transition that increase in length with decreasing temperature. Neighbouring chains are found to be oriented parallel as well as antiparallel. [Preview Abstract] |
Wednesday, March 20, 2013 8:36AM - 8:48AM |
M14.00004: Real-time imaging of magnetic-field gradient directed self-assembly of magnetic nanoparticles into patterns using magnetic recording media Thomas Crawford, Matthew Chapman, Longfei Ye, O. T. Mefford We employ enormous magnetic field gradients at the surface of disk drive media to self-assemble ferrite nanoparticles from a colloidal fluid onto the medium surface. Thus we ``nanomanufacture'' a user-programmed and magnetically-recorded pattern with demonstrated 25 nm precision. Using a low-noise CCD camera for bright-field microscopy with a 40x water dipping lens, we demonstrate real-time optical imaging of the pattern formation. By introducing concentrated ferrofluid to a water solution covering the recording medium, we observe both diffusion of the ferrofluid as well as self-assembly of nanoaprticles onto the magnetic field pattern recorded on the disk. The average intensity of the nanoparticle pattern increases exponentially and then saturates, while the overall brightness of the image decreases exponentially, over both patterned and unpatterned regions. These results hint at interesting nanoparticle dynamics during the initial ferrofluid diffusion and after the nanoparticle assembly process occurs on the disk medium surface. We suggest real-time optical microscopy can help explain the dynamics of colloidal magnetic nanoparticles in the presence of extreme magnetic field gradients which are not employed in typical magnetophoretic assembly. [Preview Abstract] |
Wednesday, March 20, 2013 8:48AM - 9:00AM |
M14.00005: Self-assembly of magnetic nanoparticles in a liquid-crystalline media Jose Amaral, R. Andosca, A.L. Rodarte, C.G.L. Ferri, S. Ghosh We investigate the self-assembly of magnetic Fe$_{3}$O$_{4}$ nanoparticles (NPs) dispersed in a liquid crystal (LC) matrix. The NP assembly is driven by the temperature-induced transition of the LC from the isotropic to the nematic phase. Using magneto-optical Kerr effect (MOKE) and polarized optical miscroscopy, we observe that the NPs are mostly expelled into the isotropic regions, finally ending up clustered around LC defect points when the transition is complete. We use NPs with diameters between 10-30 nm and the concentration of NPs in the LC media range from 0.02{\%} to 0.2{\%} by weight. We find that the resulting NP assemblies exhibit superparamagnetic and ferrimagnetic behavior, depending on their sizes. [Preview Abstract] |
Wednesday, March 20, 2013 9:00AM - 9:12AM |
M14.00006: Toward Dynamic Control over Ordered Nanoparticle Monolayer Fabrication by Electrophoretic Deposition James Dickerson, Isabel Gonzalo-Juan, Alex Krejci A primary challenges to the implementation of nanoparticles into device applications is the rapid production of densely packed, ordered films of these materials. The ordered arrangement of the nanomaterials is required for applications that rely on the collective interactions of the constituents or on the high density of the materials for information storage or surface protection. Rapid fabrication is a manufacturing demand to reduce operation costs and to streamline production. We have achieved a substantial milestone toward the mass production of macroscopic monolayers and thin films of colloidal nanocrystals on various substrates, including conducting metals and doped-semiconducting substrates. Our approach combines the advantages of liquid-phase, colloidal suspension approaches with the superior deposition rate, size scalability, and cost effective features of electrophoretic deposition (EPD) to achieve monolayer-by-monolayer deposition control over nanocrystal films with various degrees of internal order. Such work has the potential for the fabrication of industrial scale quantities and surface areas of these colloidal solids. Our recent research activities have demonstrated film formation with titanium dioxide nanoparticles and core/shell iron oxide nanoparticles. [Preview Abstract] |
Wednesday, March 20, 2013 9:12AM - 9:48AM |
M14.00007: Magnetic Nanostructures by Templated Self Assembly Invited Speaker: Caroline Ross Self assembly techniques provide a route to the rapid synthesis of nanostructures whose long range order and registration can be controlled by pre-patterning the substrate lithographically. This presentation will focus on two processes. First, masks made from templated block copolymer films are used for patterning of metallic magnetic films and multilayers into arrays of lines or dots with feature sizes of 10 nm and above. Second, codeposition of spinel and perovskite oxide phases leads to epitaxial thin film nanocomposites in which ferrimagnetic cobalt ferrite pillars are embedded in a ferroelectric bismuth orthoferrite matrix. The pillars form a regular array when templated by pits of pitch 60 nm and above, and have a strong magnetoelastic anisotropy. Magnetic properties of the resulting nanostructured materials are described. [Preview Abstract] |
Wednesday, March 20, 2013 9:48AM - 10:00AM |
M14.00008: Assembly and manipulation of planar ordered magnetic micro-bead clusters M. Prikockis, A. Chen, T. Byvank, G. Vieira, R. Sooryakumar The driving forces for many complex systems in nature often rely on the competition and cooperation between interacting simple components. These natural systems yield a framework to develop artificial phenomena and devices. In this vein we have investigated interacting micrometer sized beads containing superparamagnetic particles where competing deterministic and stochastic forces are tuned to create ordered clusters that are then maneuvered in a cooperative manner. Ferromagnetic microwires patterned on a silicon surface are utilized to regulate the magnetic interactions by confining the fluid-borne beads to a planar surface. Oriented weak external magnetic fields yield repulsive inter-particle forces that compete with local forces directed toward trap sites whose locations are determined by the underlying magnetic microwire pattern. The self-assembled ordered ``clusters'' of interacting dipolar beads are also subject to observable Brownian fluctuations. The geometrical order and inter-bead spacing within individual clusters are magnetically tuned, while entire clusters can be transported to nearby traps and reform into predictable shapes upon arrival. These features offer the potential for interesting engineering and biophysics studies. [Preview Abstract] |
Wednesday, March 20, 2013 10:00AM - 10:12AM |
M14.00009: Characterization of barium hexaferrite thick films deposited by aerosol deposition method Scooter Johnson, Shu-Fan Cheng, Ming-Jen Pan, Fritz Kub, Charles Eddy We present results on the first deposition of nano-crystalline barium hexaferrite (BaFe$_{\mathrm{12}}$O$_{\mathrm{19}})$ (BaM) powder onto copper, silicon, and sapphire substrates using the aerosol deposition method (ADM). BaM is an important magnetic compound with many applications, including, permanent magnets, magnetic recording, and components in electronic circuits. Advantages of the ADM include the ability to form up to hundreds of microns thick, dense ceramic films at room temperature at high deposition rate on a variety of substrates. Deposition is achieved by creating a pressure gradient that accelerates particles in the aerosol to high velocity. Upon impact with the target the particles fracture and embed. Continual deposition forms the thick compacted film. Scanning electron microscopy and profilometry suggest that the film is compact and well adhered to the substrate surface. We compare magnetization curves of the raw nano-crystalline powder, pressed sintered powder, and deposited film. Our typical values of magnetic saturation are about 60 emu/g, coercive field 2 kOe, remnant magnetization 30 emu/g, and squareness 0.5. The similarity between the deposited films suggests comparable deposition quality across this range of substrate hardness. The reduction in remnance and saturation compared with the powder may suggest a more random orientation of moments and an increase in fracturing of the particles. We conclude with preliminary attempts to magnetically align particles during deposition. [Preview Abstract] |
Wednesday, March 20, 2013 10:12AM - 10:24AM |
M14.00010: FMR Study of Quasicrystalline Arrays of Antidots in Permalloy Films Vinayak Bhat, Justin Woods, Barry Farmer, Lance De Long, Todd Hastings, Joseph Sklenar, John Ketterson We have used electron beam lithography to pattern permalloy films of thickness 25 nm with \textbf{\textit{quasiperiodic, }}five-fold rotationally symmetric Penrose tilings of antidots (AD). Two samples were fabricated with AD kites and darts having long (d$_{\mathrm{1}})$ and short edges (d$_{\mathrm{2}})$ equal to 1620 nm or 810 nm, and 1000 nm or 500 nm, respectively, with fixed Py line width of 100 nm. We have studied broad-band (RF frequencies 10 MHz \textless f \textless 15 GHz, DC applied fields -3.5 kOe \textless H \textless 3.5 kOe) and narrow-band FMR (f $=$ 9.7 GHz, 0 \textless H \textless 8 kOe) for various angles between the in-plane DC field and the array edge. BBFMR spectra for f \textless 4 GHz exhibit rich, highly reproducible structure, in spite of low-field (\textbar H\textbar \textless 500 Oe) hysteresis, including a \textbf{\textit{frequency-independent}} (implying localized) mode near H $=$ 0 Oe. Both low-field FMR data and dynamic simulations exhibit two-fold rotational symmetry instead of the expected five-fold symmetry, which we attribute to an unsaturated state. Higher-field (\textbar H\textbar \textless 12 kOe) simulations exhibit ten-fold rotational symmetry, which we attribute to the symmetry of the demagnetization fields. [Preview Abstract] |
Wednesday, March 20, 2013 10:24AM - 10:36AM |
M14.00011: Magnetization Reversal Study of Geometrically Frustrated, Quasiperiodic Antidot Arrays Justin Woods, Vinayak Bhat, Barry Farmer, Lance De Long, Todd Hastings, Joseph Sklenar, John Ketterson We have used electron beam lithography to pattern quasiperiodic AD arrays in permalloy films of thickness 25 nm. Two five-fold rotationally symmetric Penrose tilings were fabricated with AD kites and darts having long (d1) and short edges (d2) equal to 1620 nm or 810 nm, and 1000 nm or 500 nm, respectively, with fixed Py line width of 100 nm. Two eight-fold Ammann tilings were patterned with square and rhomboid AD of edge lengths of 1000 nm or 2000 nm, resp. Magnetization reversal was studied at various angles between the in-plane, applied DC magnetic field H and the quasiperiodic array. We observed very reproducible hysteresis curves with low-field anomalies not present in our previous studies of periodic, square arrays of square-, circular- and diamond-shaped AD; e.g., for the Penrose tilings, we observed four reproducible knee anomalies (both for 81 \textless H \textless 331 Oe, and for -19 \textgreater H \textgreater -71 Oe). Micromagnetic simulations exhibit systematic evolution of domain walls (DW) in the hysteretic regime due to DW pinning by edges of the quasicrystalline pattern, which correlates DW evolution with observed features in magnetic hysteresis. [Preview Abstract] |
Wednesday, March 20, 2013 10:36AM - 10:48AM |
M14.00012: Observation of Novel Low-Field FMR modes in Permalloy Antidot Arrays Lance De Long, Vinayak Bhat, Barry Farmer, Justin Woods, Todd Hastings, Joseph Sklenar, John Ketterson Permalloy films of thickness 23 nm were patterned with square arrays of square antidots (AD) with feature size D $=$ 120 nm, and lattice constants d $=$ 200, 300, 500 and 700 nm (total sample area $=$ 2 mm x 2mm), using electron beam lithography. Our broad-band (frequencies f $=$ 10 MHz-15 GHz) and narrow-band (9.7 GHz) FMR measurements of even dilute (D/d \textless \textless 1) AD lattices (ADL) reveal remarkably reproducible absorption spectra in the low-frequency, hysteretic regime in which disordered domain wall (DW) patterns and unsaturated magnetization textures are expected for unpatterned films, but in the present case are strongly affected by the periodic ADL. Other modes in the saturated regime exhibit strong dependence on the angle between the applied DC field H and the ADL axes, as confirmed by our micromagnetic simulations. Novel modes are observed at DC fields above that of the uniform mode, which simulations indicate are localized at AD edges. Other novel modes are observed for DC fields below that of the uniform mode, which simulated power and phase maps indicate are confined to ADL interstices oriented parallel to H. These results show even dilute AD concentrations can effect strong control of DW evolution. [Preview Abstract] |
Wednesday, March 20, 2013 10:48AM - 11:00AM |
M14.00013: Micromagnetic Simulations of Quasiperiodic (Penrose Tiling) Antidot Arrays Barry Farmer, Vinayak Bhat, Justin Woods, Lance De Long, Todd Hastings, Joseph Sklenar, John Ketterson We have performed static and dynamic micromagnetic simulations of permalloy antidots (AD) patterned on quasiperiodic arrays of 25 nm film thickness. Two Penrose tilings (five-fold rotationally symmetric) were simulated with AD kites and darts with long (d$_{\mathrm{1}})$ and short edges (d$_{\mathrm{2}})$ equal to 1620 nm or 810 nm, and 1000 nm or 500 nm, respectively, and fixed Py line width of 100 nm. Two Ammann tilings were patterned with square and rhomboid AD of edge lengths 1000 nm or 2000 nm, and line width of 100 nm. Our simulations exhibit FMR modes not previously predicted; for example, power and phase maps for Penrose tilings exhibit three bulk modes (at angles $\varphi \quad =$ 0$^{\mathrm{\thinspace o}}$, 72$^{\mathrm{\thinspace o}}$ and 144$^{\mathrm{o}}$ with respect to in-plane applied DC field H) and two edge modes ($\varphi \quad =$ 72$^{\mathrm{\thinspace o}}$ and 144$^{\mathrm{o}})$ for H $=$ 1.2 kOe. Static micromagnetic simulations exhibit highly repeatable evolution of domain walls (DW) with apparent long-range order in the \textbf{\textit{hysteretic regime}}. We attribute this remarkable reproducibility in a \textbf{\textit{geometrically frustrated, aperiodic system}} to magnetic reversal controlled by DW pinning by AD edges. [Preview Abstract] |
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