Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W4: Focus Session: Domain Dynamics |
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Sponsoring Units: GMAG Chair: John Cumings, University of Maryland Room: 112/110 |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W4.00001: Thermal effects in artificial spin ice Jasper Drisko, Stephen Daunheimer, John Cumings Frustrated systems, typically characterized by competing interactions that cannot all be simultaneously satisfied, are ubiquitous in nature and display many rich phenomena and novel physics. Artificial spin ices (ASIs), arrays of lithographically patterned Ising-like single-domain magnetic nanostructures, are highly tunable systems that have proven to be a novel method for studying the effects of frustration and associated phenomena. Recently, thermal activation of ASI systems has been demonstrated [1, 2], introducing the spontaneous reversal of individual magnets and allowing for new explorations of novel phase transitions and phenomena using these systems. We have fabricated ASI samples made from thin film FePd$_{\mathrm{3}}$, which possesses simultaneously a high magnetic moment and a relatively low Curie temperature, and we investigate them using Lorentz Transmission Electron Microscopy. We observe thermally activated reversal of individual magnets when they are heated close to the Curie temperature of the film. Our analysis of Kagome spin ice arrays reveals signatures of competing interactions between vortex formation on the edges of the structures and magnetic charge ordering within the theoretically predicted Kagome ice-II state [3]. These observations are consistent with the emergent frustration of these systems. \\[4pt] [1] S. Zhang et al., \textit{Nature }\textbf{500}, 553-557 (2013) \\[0pt] [2] A. Farhan et al., \textit{Nature Phys}. \textbf{9}, 375-382 (2013) \\[0pt] [3] G.-W. Chern et al., \textit{Phys. Rev. Lett. }\textbf{106}, 207202 (2011) [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W4.00002: Role of Cu in Intrinsic Exchange Bias in FeMn Igor V. Roshchin, Pavel N. Lapa, Dogan Kaya, E. Skoropata, J. Van Lierop, H. Belliveau, P. Jayathilaka, Casey W. Miller We report studies of the role of Cu in the exchange bias (EB) in FeMn. The multilayers of Ta(50 {\AA})/[FeMn(50$-$450 {\AA})/Cu(50 {\AA} )]$_{10}$/Ta(50 {\AA}) exhibit EB while Ta(50 {\AA})/[FeMn(50$-$450 {\AA})/Ta(50 {\AA})]$_{10}$ show no EB. This ``\textit{intrinsic}'' EB occurs between pinned and unpinned uncompensated magnetization (UM) in FeMn. The unpinned UM is distributed uniformly throughout FeMn [1]. Since the magnitude of H$_{e}$ scales with the inverse thickness of FeMn, from Malozemoff's model for the bilayer EB systems, the pinned magnetization should be located near the interface of FeMn. This is consistent with the required for the EB presence of Cu. To test if Cu diffuses into FeMn, M\"{o}ssbauer spectroscopy is performed using the naturally occurring $^{57}$Fe in FeMn. The spectra of the samples consisting of one FeMn layer (50 and 150 {\AA}) with and without Cu show two components: One corresponds to the ``bulk'' FeMn, while the other is attributed to Fe-rich areas of FeMn. These areas are likely to be the source of the unpinned UM. No measurable difference in these spectra for the samples with and without Cu indicates that Cu is unlikely to diffuse into FeMn, contrary to the previously proposed hypothesis [1].\\[4pt] [1] D. Kaya \textit{et al.}, J. Appl. Phys., \textbf{113}, 17D717 (2013). \newline [2] A. P. Malozemoff, Phys. Rev. B \textbf{35}, 3679 (1987), \textit{ibid}., \textbf{37}, 7673 (1988). [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W4.00003: Anomalous magnetization dynamics in artificial spin ice S.K. Mishra, V.S. Bhat, D.H. Parks, J.T. Lee, X. Shi, L.E. DeLong, S.D. Kevan, S. Roy Modern nanotechnology permits one to mimic bulk spin ice crystals with ordered arrays of ferromagnetic nano-islands, which constitute a method for designing fully controlled model systems for studies of frustrated magnetic interactions in two dimensions. Thermal fluctuations within a highly degenerate ground state excite topological magnetic charges, but the nature of the field-dependent equilibrium states and their magnetic dynamics has remained elusive. Here we present results of time dependent, coherent speckle intensity and X-ray correlation spectroscopy (XPCS) in a diffraction geometry, which yields information concerning the dynamics of emergent topological charge defects in a square artificial spin ice. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W4.00004: Simulations of magnetic reversal in continuously distorted artificial spin ice lattices Barry Farmer, Vinayak Bhat, Justin Woods, J. Todd Hastings, Lance De Long Artificial spin ice (ASI) systems consist of lithographically patterned ferromagnetic segments that behave as Ising spins. The honeycomb lattice is an ASI analogue of the Kagom\'{e} spin ice lattice found in bulk pyrochlore crystals. We have developed a method to continuously distort the honeycomb lattice such that the pattern vertex spacings follow a Fibonacci chain sequence. The distortions break the rotational symmetry of the honeycomb lattice and alter the segment orientations and lengths such that all vertices retain three-fold coordination, but are no longer equivalent. We have performed micromagnetic simulations (OOMMF) of magnetization reversal for many samples having different strengths of distortion, and found the kinetics of magnetic reversal to be dramatically slowed, and avalanches (sequential switching of neighboring segments) shortened by only small deviations from perfect honeycomb symmetry. The coercivity increases as the distortion is strengthened, which is consistent with the retarded reversal. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W4.00005: Study of dipolar interaction between nano-disks Megha Chadha, Stephanie K. Walton, Katharina Zeissler, David M. Burn, Solveig Felton, Lesley F. Cohen, Will R. Branford Ferromagnetic nano-dot arrays are interesting for data storage applications, but as the density of disks becomes high the dipolar interactions between disks become strong. In this work we study lithographically prepared arrays of densely packed single domain perm-alloy nano-disks where the dipolar correlations are significant. We study the collective magnetic array properties for different array geometries and varying disk separation and explore the effect of magnetic frustration in these systems. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W4.00006: Control and influence of domain wall chirality in Artificial Spin Ice Stephanie Walton, Katharina Zeissler, Sam Ladak, Dan Read, Tolek Tyliszczak, Lesley Cohen, Will Branford Artificial Spin Ice, comprising ferromagnetic nanobars arranged in a honeycomb geometry, is a directly imageable frustrated system which has demonstrated rich Physics. Its magnetic reversal is mediated by domain wall propagation in the presence of external magnetic fields. These domain walls carry magnetic charge and have a distinct structure or ``chirality,'' namely up or down in the transverse domain wall regime or clockwise or anticlockwise in the vortex domain wall regime. In this talk, both experimental Scanning Transmission X-ray Microscopy and micromagnetic simulations which suggest that the domain wall performs a non-random walk through Artificial Spin Ice due to its chirality are presented. In addition, the role of Walker Breakdown in both the transverse and vortex domain wall regimes is discussed. Furthermore, modes of controlling and measuring domain wall chirality are explored. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W4.00007: Electrical transport measurements on honeycomb artificial spin ice. Katharina Zeissler, Megha Chadha, Lesley Cohen, Will Branford Artificial spin ice is a macroscopic playground for magnetically frustrated systems. We have previously shown that in a cobalt honeycomb artificial spin ice composed of 1 micron long nanowires there are unusual features in the magnetotransport below 50K. Here we explore the low temperature transport of equivalent artificial spin ice structures fabricated from permalloy. We discuss the extent to which the phenomenon is generic to the honeycomb artificial spin ice geometry and the effect of changing the constituent material on the onset temperature and the magnitude of the magnetotransport effect. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W4.00008: Electronic Transport Study of Connected Artificial Kagome Spin Ice D.W. Rench, B.L. Le, P.E. Lammert, R. Misra, V.H. Crespi, N. Samarth, P. Schiffer We present experimental and computational results of magnetotransport in connected ferromagnetic nanowire arrays (connected artificial spin ice). We probed the artificial kagome spin ice lattice using AC transport techniques as a function of applied magnetic field strength and angle and compared these results to calculated transport properties based on OOMMF computational modeling. We find that many of the transport properties observed experimentally can be described in a simple manner using the Anistropic Magnetoresistance (AMR) model for individual nanowires and then calculating the net resistance using classical circuit analogues. Supported by the US Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under grant number DE-SC0005313. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W4.00009: Dynamic Magneto-Optical Kerr Imaging of Perpendicular Anisotropy Artificial Spin Ice Geometries Robert Fraleigh, Paul Lammert, Vin Crespi, Nitin Samarth, Ian Gilbert, Peter Schiffer We present a spatially resolved magneto-optical Kerr imaging study on the magnetization reversal, as a function of applied field, of patterned arrays of perpendicular anisotropy single domain islands. Patterns are made of large collections of CoPt multilayer islands with frustrated (Kagome, triangular) and unfrustrated (square, hexagonal) geometries. Field induced switching is imaged with a Kerr imaging apparatus equipped with an objective lens that allows for diffraction limited spatial resolution as low as 250nm and imaging acquisition as fast as 12 frames/second. The magnetization reversal process is probed by varying lattice spacing, geometry, and artificial defects in the patterned arrays. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W4.00010: Edge mode spectroscopy and imaging for film edge properties in magnetic nanostructures Invited Speaker: Robert McMichael Lithography is an act of violence. Often, films are almost entirely obliterated by patterning, leaving only nanostructures behind with film edges that have borne the brunt of the damage, edges that carry with them the scars of energetic ion bombardment, reactive ions, liftoff and exposure to ambient conditions. In this talk, I will present a variation on ferromagnetic resonance force microscopy that can provide insight into the magnetic properties of film edges in magnetic nanostructures. The method relies on the non-uniformity of the magnetic field in patterned-film nanostructures that are magnetized in-plane, specifically, the low-field regions that form near where the magnetization is directed normal to the edge. In these regions, localized precession forms as trapped spin wave modes, and the resonance condition of these modes serves as an indicator of the edge properties. I will present modeling and measurements on a 500~nm diameter, 25~nm thick Permalloy disk to illustrate the method. Micromagnetic modeling of this disk predicts a main mode that is nearly uniform across the sample and three localized edge modes with higher resonance fields. The spectra measured with various tip positions and mode imaging are consistent with the modeling results. In addition to a strong center mode, three distinct edge modes are observed when the tip is near the disk edge. For a symmetric disk, the modeling predicts that the edge mode resonances are identical on the two opposite edges. However, the measured edge mode resonances on opposite edges of the disk are detected at different resonance fields, suggesting inhomogeneity of the edge properties. By rotating the applied field, we control the position of the localized edge mode along the edge of the disk and confirm that the edge mode resonance field has a strong angular dependence, showing that edge mode properties can vary significantly in a nominally circular disk. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W4.00011: FMR Study of an Eightfold Artificial Quasicrystal Lance De Long, Vinayak Bhat, Joseph Sklenar, Barry Farmer, Justin Woods, John Ketterson, Todd Hastings We have performed DC magnetization, and broad-band and narrowband FMR measurements on eightfold-rotationally-symmetric artificial quasicrystals. Permalloy films of thickness 25 nm were patterned with 1$^{\mathrm{st}}$ and 4$^{\mathrm{th}}$ generation \textbf{\textit{Ammann tilings}} (AT) [1] using standard electron beam lithography. The AT can be viewed as an antidot lattice of squares and rhombi whose edges are film segments of length 1000 nm (7 $\mu $m), and width 130 nm (910 nm), respectively, in 4$^{\mathrm{th}}$ (1$^{\mathrm{st}})$ generation AT. In spite of clear DC magnetization hysteresis in the low-field regime, we observed remarkably sharp and reproducible FMR spectra (including both the low-field-reversal and the saturated regimes) that strongly reflected the geometry of the AT. The applied DC field \textbf{H} could be oriented in-plane at an angle $\varphi $ with respect to a AT reference axis. Our FMR spectra exhibit the expected eight-fold symmetry of the AT for experimentally accessible RF frequencies (7 to 18.5 GHz). Static and dynamic micromagnetic simulations were in good agreement with our experimental FMR spectra. \\[4pt] [1] B. Gr\"{u}nbaum and G. C. Shephard, \textit{Tilings and Patterns} (Freemann, New York, 1986). [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W4.00012: Simultaneously generated spin waves in the magnetostatic backward volume wave and magnetostatic surface wave configuration in a cross-like structure Jason Liu, Grant Riley, Ferran Macia, Andrew Kent, Kristen Buchanan Spin waves, or magnons, in laterally confined microstrips have attracted a great deal of attention recently due to their potential for magnonic logic applications. Previous experimental work on spin wave propagation in metallic magnetic nanowires has focused on the magnetostatic surface wave (MSSW) configuration where the static magnetic field is applied in-plane, perpendicular to the nanowire, because they can be excited relatively easily by an antenna. Spin wave propagation in the less studied magnetostatic backward volume wave (MSBVW) configuration where the magnetization direction is along the nanowire is, however, also of interest because spin waves can propagate without the need for an external field in this geometry. In this work, micro-Brillouin light scattering (micro-BLS) was used to investigate the generation of propagating spin waves in a cross-like Permalloy structure that allows for simultaneous excitation of MSSW in one of the wires and MSBVW in the other. Micro-BLS measurements were conducted as a function of applied field and pumping frequency to probe the efficiency of the generation of the two types of spin waves. Two dimensional spatial profiles were obtained to explore possible interference of the two types of spin waves at the center of the cross-like structure. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W4.00013: Lattice Symmetry Breaking of Spin Wave Propagation in Two-Dimensional Magnonic Crystals Glade Sietsema, Michael E. Flatt\'e We solve the Landau-Lifshitz-Gilbert equation for spin waves in a two-dimensional magnonic crystal using the plane wave expansion method[1]. In doing this we have found that the inclusion of the dipolar field in the LLG equation results in the dispersion relations and linewidths having a lower symmetry than the crystal latice. The magnitude of this symmetry breaking is determined by the strength of the dipolar field relative to the exchange field. Adjusting the crystal parameters can change the relative strength of these fields, thereby allowing this effect to be enhanced or reduced. We have also calculated the Green's functions for this system, which show highly directional propagation of the spin waves depending on the excitation frequency.\\[4pt] [1]arXiv:1111.2506 [Preview Abstract] |
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