Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A7: Focus Session: Patterned Magnetic Nanostructures |
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Sponsoring Units: GMAG DMP Chair: Olle Heinonen, Argonne National Laboratory Room: 106 |
Monday, March 3, 2014 8:00AM - 8:36AM |
A7.00001: Controlled Magnetic Reversal and Frustration in Artificial Quasicrystals Invited Speaker: Vinayak Bhat Recent studies of ferromagnetic (FM) antidot arrays have been restricted to simple periodic lattices (square, triangular, etc.). We have fabricated artificial FM quasicrystals (AFQ), which are \textit{\textbf{aperiodic}} antidot lattices that are self-similar, retain definite rotational symmetry, and consist of a multiply-connected network of permalloy film segments. We focus on Penrose P2 tilings (P2T) constructed from film segments of two lengths (d$_{\mathrm{1}}=$ 810 nm $-$1618 nm, d$_{\mathrm{2}} =$ 500 nm$-1$ $\mu$ m), width W $\approx $ 100 nm, and thickness t $=$ 25 nm [1]. Static and dynamic magnetizations were studied using DC magnetometry, broadband (BB) FMR, and micromagnetic simulations (MS). Reproducible ``knee'' anomalies observed in the hysteretic, low-field DC magnetization M(H,T) signal a series of abrupt transitions between ordered magnetization textures, concluding in a smooth evolution into a saturated state. Numerous FMR mode signatures quantitatively reproduce in opposite DC field sweeps in the near-saturated regime, which suggests pinning of the magnetization parallel to the AD edges and confinement of domain walls at P2T vertices control segment polarization and reversal. Novel ``asymmetric'' modes, defined by their presence on only one side of the field origin in a given sweep, are observed only in the reversal regime, and accompany knee anomalies in M(H,T). MS agree with experimental DC hysteresis loops and FMR spectra, and indicate that systematic control of magnetic reversal and domain wall motion can be achieved via tiling design, offering a new paradigm of \textbf{magnonic quasicrystals}. AFQ also behave as novel artificial spin ice systems that exhibit non-stochastic switching due to their aperiodicity and inequivalent pattern vertices. MS indicate pinned Dirac monopoles and confined magnetic avalanches exist in AFQ. \\[4pt] [1] V. S. Bhat \textit{et al.}, Phys. Rev. Lett. \textbf{111}, 077201 (2013). [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A7.00002: Size-tuned Highly-ordered Magnetic Nanodot Arrays via ALD-Assisted Block Copolymer Nanolithography Srinivas Polisetty, Chun-Hao Lin, Wayne L. Gladfelter, Marc H. Hillmyer, Chris Leighton Block copolymer nanolithography of large-area well-ordered magnetic nanostructures is now possible \textit{via} a variety of approaches and holds considerable appeal for fundamental science and for bit patterned recording media. Here, we demonstrate a non-lift-off damascene-type approach [1] combined with low temperature atomic layer deposition (ALD) of a conformal ZnO layer to provide size-controlled magnetic nanodots. Perpendicularly-aligned nonporous templates were achieved by solvent annealing polystyrene-$b$-polylactide (PS-PLA) films. Low temperature ALD was then used to conformally coat the template with a ZnO layer of variable thickness to systematically reduce the pore diameter. Our damascene-type non-lift-off process [1] was then used to synthesize Ni$_{\mathrm{80}}$Fe$_{\mathrm{20}}$ dot arrays from such templates, achieving tunable dot diameters (6-30 nm) and controlled dot height (by Ar milling time). Magnetic measurements were used as a probe of island volume, good agreement being obtained between simple calculations, imaging, and blocking temperature measurements. The results demonstrate a simple route to size control from a fixed polymer template, enabling detailed studies of separation-dependent inter-dot magnetic interactions for example. \\[4pt] [1] Baruth, \textit{et al.}, \textit{ACS Appl. Mater. Interfaces} \textbf{3}, 3472 (2011). [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A7.00003: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 9:00AM - 9:12AM |
A7.00004: Concave nanomagnetic triangles: size and shape effect on the anisotropy and magnetization switching Alexander Kozhanov, Terence Fisher, Alexander Anferov, Igor Eremin, Ivan Vasilyevskiy Single domain nanomagnets are essential for magnetic memory and non-volatile logic applications. Recently a non-volatile logic device based on the triangular nanomagnet was proposed. Dependent on the triangle's shape and dimensions ``Y'' or ``buckle'' magnetization alignment ground states are defined by configurational anisotropy. Triangle shape distortions such as corner rounding results in preferable ``buckle'' ground state not favorable for nonvolatile logic applications. In this work we investigate the effect of triangle dimensions and shape on the configurational anisotropy and magnetization ground state profile. 50$\mu $m $\times$ 50$\mu $m arrays of 50-500nm aside equilateral permalloy triangles capped with Al layer were fabricated. Arrays of triangles with different corner radius, amount of concavity and vertex extrusion were fabricated. Field modulated MOKE technique was used to characterize triangle anisotropy. Micromagnetic simulations accompanied the experimental results and were used to investigate the ground states magnetization alignment, energy profile and switching dynamics. We demonstrate that triangle shape variations can be used to effectively manipulate its anisotropy profile, ground state stability and switching times. We map the ``Y'' and ``buckle'' triangle ground state diagram in the concavity-corner rounding-vortex extrusion parameter space. The investigated triangle shapes are assessed for the non-volatile logic applications. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A7.00005: Collective spin waves in reconfigurable artificial crystals and magnonic meta-materials Invited Speaker: Dirk Grundler Periodically nanopatterned ferromagnets have generated great interest in the research field of magnonics in that they support spin-wave (SW) nanochannels, allow for multi-directional emission of short-wavelength SWs via the grating coupler effect and form artificial crystals for SWs (magnons) in the GHz frequency regime. Allowed SW minibands and forbidden frequency gaps are not just tailored by the geometrical and material parameters, but reflect decisively the periodic order of the nanomagnets' remanent magnetization. Thereby a further degree of freedom is offered for controlling wave phenomena in solids compared to photonics and plasmonics. We investigated such so-called reconfigurable magnonic crystals (MCs) consisting of a one-dimensional (1D) array of permalloy nanostripes that allow one to vary the Brillouin zone boundaries, forbidden frequency gaps and number of SW minibands in one-and-the same device. When excited by a microwave antenna, an unexpected metamaterial property was found in that both reciprocal and nonreciprocal SW excitation occurred depending on the parallel and antiparallel alignment of magnetic moments in neighboring stripes. Such excitation characteristics are not found in natural materials. Switching an individual stripe from parallel to antiparallel magnetization in an otherwise saturated 1D MC modified the transmitted SW amplitude considerably offering SW control on the nanoscale. Combined with the grating coupler effect, periodically nanopatterned ferromagnets are expected to provide interesting building blocks for magnonic applications aiming at transmitting and processing information at microwave frequencies with spin waves. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A7.00006: Effects of dipolar interactions in magnetic nanoparticle systems Sergiu Ruta, Ondrej Hovorka, Roy Chantrell Understanding the effects of magnetostatic interactions in magnetic nanoparticle systems is of importance in magnetic recording, biomedical applications such as in hyperthermia cancer treatment, or for sensing approaches in biology and chemistry, for example. In this talk we discuss the macroscopic and microscopic effects of dipole-dipole interactions in three-dimensional assemblies of magnetic nanoparticles in various spatial arrangements, including the BCC, FCC, or randomized lattices. Our study is based on the kinetic Monte-Carlo modelling and concentrates on exploring the effect of the particle arrangement, distributions of particle volumes and anisotropy axes, and the role of thermal effects on the overall behaviour of hysteresis loops, ZFC/FC temperature scans and the magnetization decay data computed during the relaxation to equilibrium. In the case of the FCC lattice we find a counter-intuitive effect where increasing the interaction strength enhances/suppresses the hysteresis loop coercivity at high/low temperatures. The analysis of the domain pattern formation and pair correlation functions suggests for the observed behaviour to be a result of the phenomenon of frustration. We also discuss the possibility of observing the super-ferromagnetic phases on similar syste [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A7.00007: Effect of dipolar nanoparticle interaction on transverse magnetic susceptibility: particle pair model Elizabeth Plowman, Ondrej Hovorka, Gennady Friedman Determining nanoparticle dipolar interactions from experimental measurement of magnetic moments is a classical inverse problem in magnetism. It is important in a variety of applications including magnetic information storage and Magnetic Particle Imaging (MPI). Historically, magnetic moment relaxation has been used to characterize system parameters including dipolar interactions. However, the results are sensitive to particle size distribution. We demonstrate that dipolar coupling strength in a nanoparticle-pair can be determined from transverse magnetic susceptibility, a readily measured parameter. Moreover, we demonstrate that this method is insensitive to particle size, rendering it more robust for real-world experiments. We present both analytical and numerical models for transient and steady-state transverse magnetic susceptibility and resulting interaction strength of our two-particle system. In the analytical model master equation is employed. The particles are assumed to be immobile and the set of possible states is discrete. In the numerical models both master equation and Landau-Lifshitz-Gilbert dynamics are employed. In these models random particle anisotropy directions are taken into account. The results of each model are compared. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A7.00008: The role of geometrical symmetry on thermally activated processes in clusters of interacting dipolar moments Ondrej Hovorka, Joe Barker, Gary Friedman, Roy Chantrell Thermally activated magnetization decay is studied in ensembles of clusters of interacting dipolar moments by applying the master-equation formalism, as a model of thermal relaxation in systems of interacting single-domain ferromagnetic nanoparticles. Solving the associated master-equation reveals a breakdown of the energy barrier picture depending on the geometrical symmetry of structures. Deviations are most pronounced for reduced symmetry and result in a strong interaction dependence of relaxation rates on the memory of initialization of an ensemble. Developed is a simple two-state system description of an ensemble, which accounts for the observed anomalies. These results follow from a semi-analytical treatment, and are fully supported by kinetic Monte-Carlo simulations. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A7.00009: Synthesis of Fe Nanowires via a Novel Approach Kinjal H. Gandha, Kevin Elkins, J. Ping Liu Iron nanowires with high magnetization and high coercive force were fabricated via reduction of as-synthesized $\alpha $-FeOOH nanowires. Thermal treatment is used to facilitate subsequent phase transformation from the precursor to $\alpha $-Fe phase. Increasing reduction time and temperature leads to agglomeration and sintering of the nanowires. By using fluid bed technique and by adjusting the reaction temperature, time and gas component in the process of heat treatment, $\alpha $-Fe nanowires with length of 200nm$-$300nm and diameter of 20nm$-$30nm were prepared. The iron nanowires have a coercive force of 628 Oe and saturation magnetization of 197 emu/g at room temperature. This novel process is effective to produce iron nanowires with well controlled morphology and composition. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A7.00010: Energy minimization to optimize information lifetime in arrays of nanowires Eugenio Vogel, Eduardo Cisternas Magnetic nanowires trapped in the alumina membrane used to produce them can be used to store information at the nanoscale (symbols, barcodes, etc.). This is achieved by inscribing a ferromagnetic domain over the randomly oriented magnetizations as left by the fabrication process. This is achieved by a powerful magnetic tip which is able to overturn wire magnetization of sectors with a few wire diameters across. As the tip is withdrawn the ferromagnetic symbols prevail and wires in the sector interact repulsively so the overall energy is increased. This is a factor of instability for the stored information. In the present paper we investigate ways of minimizing this repulsive energy but still preserving the information stored by the original symbol at its original scale. The inscription of an opposite ferromagnetic band is a possible technique to minimize the repulsive energy (JMMM 337 (2013) 74-78). Application of this stabilization technique to different symbols is discussed. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A7.00011: Structural and Magnetic Characterizations of Y$_{\mathrm{x}}$Co$_{\mathrm{y}}$ Nanowires Bishnu Dahal, Keshab Sapkota, Rajendra Dulal, Parshu Gyawali, Ian L. Pegg, John Philip Nanowires of YxCo$_{\mathrm{y}}$ (Y$_{2}$Co$_{17}$, YCo$_{3}$ and YCo$_{5})$ are grown using electrospinning technique and by annealing at high temperature. The size of the nanowires varies from 80 -- 300 nm in diameter. Structural analyses show that Y$_{2}$Co$_{17}$ exhibits rhombohedral crystal structure while YCo$_{5}$ displays hexagonal crystal structure. The as-grown nanowires are polycrystalline in nature with an average grain size of 40 nm. YCo$_{3}$ nanowires are amorphous in nature. All the Y$_{\mathrm{x}}$Co$_{\mathrm{y}}$ nanowires are found to be strong ferromagnetic materials as reported in the bulk system. The observed coercivity of the Y$_{\mathrm{x}}$Co$_{\mathrm{y}}$ nanowires is low, typically around 500 Oe in comparison to the large coercivity observed in YCo nanoparictles [Preview Abstract] |
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