Below are links to PDF files of slides
from some talks I have given. Most contain bonus material that
was not presented at the event due to time or space constraints.
Additional material is available on my publications page.
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Event: MMM 2022, Minneapolis, MN
Date: 7-Oct-2022
Title: HOB-02: Energetics of spin-flop and spin-flip
transitions in homogeneous antiferromagnets
Authors: M. Hu, M.A. Hoefer, and M.J. Donahue
PDF
(709 523 bytes).
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Event: MMM 2020, Virtual Conference
Date: 6-Nov-2020
Title: P3-02: High order methods for computing the
demagnetization tensor for periodic boundaries
Authors: M.J. Donahue and D.G. Porter
PDF
(2 097 381 bytes).
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Event:
Online Spintronics Seminar
Date: 21-May-2020 through 9-Jun-2020
Title: Micromagnetics and OOMMF (tutorial series)
Author: M.J. Donahue
OOMMF
Tutorial Series web page.
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Event: MMM 2019, Las Vegas, NV
Date: 7-Nov-2019
Title: ET-01: Quantitative evaluation and reduction of error in
computation of the demagnetization tensor
Authors: M.J. Donahue and D.G. Porter
PDF
(328 995 bytes).
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Event: OOMMF Workshop, University of York, York, UK
Date: 5-Apr-2017
Title: Introduction to OOMMF micromagnetic modeling
Authors: M.J. Donahue
PDF
(6 033 918 bytes).
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Event: Characterization Techniques for Magnetic Materials
Workshop, TMS2015, Orlando, FL
Date: 15-Mar-2015
Title: Introduction to micromagnetic modeling
Authors: M.J. Donahue
PDF
(7 097 791 bytes).
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Event: International Workshop on Advanced Micromagnetics
(IWAM), San Diego, CA
Date: 21-May-2012
Title: Fast, accurate computation of the demagnetization
tensor for periodic boundaries
Authors: M.J. Donahue
PDF
(332 777 bytes).
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Event: HMM 2009, Gaithersburg, MD
Date: 13-May-2009
Title: OD-4: Quasi-stable vortex magnetization structures in
nanowires with perpendicular anisotropy
Authors: K.M. Lebecki and M.J. Donahue
PDF
(1 760 452 bytes).
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Event: Intermag 2009, Sacramento, CA
Date: 5-May-2009
Title: AS-09: Parallelizing a micromagnetic program for use on
multi-processor shared memory computers
Authors: M.J. Donahue
Abstract,
Publication,
PDF
(437 075 bytes).
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Event: MMM 2008, Austin, TX
Date: 14-Nov-2008
Title: GS-02: Exploiting effective field time derivative
information to improve accuracy of a norm-preserving Landau-Lifshitz
solver
Authors: D.G. Porter and M.J. Donahue
PDF
(100 248 bytes).
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Event: MMM 2008, Austin, TX
Date: 12-Nov-2008
Title: Session XB: Open Forum on Micromagnetic Modeling:
Finite Difference Micromagnetics
Author: M.J. Donahue
PDF
(1 208 223 bytes).
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Event: MMM 2007, Tampa, FL
Date: 6-Nov-2007
Title: AP-21: Precession axis modification to a semianalytical
Landau-Lifshitz solution technique
Authors: D.G. Porter and M.J. Donahue
Abstract,
Publication,
PDF
(188 255 bytes).
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Event: 6th International Symposium on Hysteresis Modeling and Micromagnetics
HMM-2007, Naples, Italy
Date: 4-Jun-2007
Title: Accurate computation of the demagnetization tensor
Author: M.J. Donahue
Abstract:
Analytic formulae for computing the demagnetizing (self-magnetostatic)
field between uniformly magnetized rectangular prisms are commonly
used in the stray field computation of finite-difference based
micromagnetic simulations. These formulae are amenable to Fast
Fourier Transform methods, allowing efficient evaluation of this
long-range interaction. Unfortunately, the numerical behavior of
these formulae is poor, with significant and increasing loss of
accuracy due to catastrophic cancellation as the distance between the
prisms grows.
PDF
(538 894 bytes).
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Event: 10th Joint MMM/Intermag Conference, Baltimore, MD
Date: 11-Jan-2007
Title: Micromagnetics on curved geometries using rectangular
cells: error correction and analysis
Authors: M.J. Donahue and R.D. McMichael
Abstract,
Publication,
PDF
(380 475 bytes),
Gzip-compressed PostScript
(271 820 bytes).
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Event: Micromagnetics: Experiments, Modeling, and Mathematical
Theory Workshop, Bonn, Germany
Date: 5-Sep-2006
Title: Micromagnetics on curved geometries using rectangular
cells: error correction and analysis
Authors: M.J. Donahue and R.D. McMichael
Abstract:
Rectangular meshes are convenient in micromagnetics because they allow
the self-magnetostatic field to be efficiently computed using Fast
Fourier Transform (FFT) techniques. However, geometric edges that are
curved or rotated off the coordinate axes result in partially filled
cells or stair-step edges that produce errors in the
self-magnetostatic field computation.
This talk will discuss two local edge correction methods that are
compatible with FFT-based far field self-magnetostatic computations.
The first correction method is based on a simple empirical formula in
the form of an edge cell anisotropy. The second is a flexible, fully
three-dimensional adjustment computed using the standard
self-magnetostatic algorithms but applied to a local, refined mesh.
To evaluate these correction methods, we use a quantitative measure
that is based on calculating an edge mode resonance for different
orientations of an edge with respect to the rectangular mesh. Applied
to a 350 nm Permalloy square, we find up to a 50% frequency shift
for the uncorrected approach, but less than a 4% shift with either of
the discussed correction methods.
PDF
(1 114 589 bytes), Gzip-compressed PostScript
(783 7 bytes).
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Event: MMM 2005, San Jose California
Date: 1-Nov-2005
Title: CV-11: Magnetization normalization methods for
Landau-Lifshitz-Gilbert
Authors: D.G. Porter and M.J. Donahue
Abstract:
Exact solutions of the Landau-Lifshitz-Gilbert (LLG) equation have
constant magnetization magnitude. Numerical solvers working in
Cartesian coordinates fail to preserve this quantity. It is common
practice to re-normalize magnetization between time steps, but this
can produce errors. For example, consider a single spin precessing
with no damping about a fixed applied field. A first-order Euler
integration step always ends outside the unit sphere, increasing the
angle between the spin and the applied field. Renormalization along
the new magnetization direction results in an increase to the system
energy, resulting not only in errors to energy dissipation rates but
also potentially leading to numerical instabilities. Similar problems
arise with higher order integration schemes, although to a lesser
degree. However, an energy-neutral renormalization---one that
adjusts the spin perpendicular to the precession axis---does not
suffer from this problem. This poster describes such a
renormalization method, and illustrates the results in simple one spin
and complex multi-spin examples. A supplemental approach is also
presented, where an additional term is introduced to the LLG equation
that explicitly directs the magnetization magnitude toward its proper
value. This additional term comes into effect only if the spin
magnitude deviates from 1, so exact solutions remain unchanged.
PDF
(116 311 bytes), Gzip-compressed PostScript
(69 561 bytes).
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Event: HMM 2005, Budapest Hungary
Date: 31-May-2005
Title: TO1-4: Vortex head-to-head domain walls and their formation
in onion-state ring elements
Authors: M.H. Park, Y.K. Hong, S.H. Gee, B.C. Choi, and
M.J. Donahue
Abstract:
Vortex head-to-head (HTH) domain walls in
Ni80Fe20 ring elements were studied using MFM
and micromagnetic simulations. Two types of vortex HTH domain walls,
single- and double-vortex, were observed to be stable in the onion
configuration at remanent state, depending on the geometry.
Micromagnetic simulations of the wall formation process showed that
once the vortex core nucleated the exchange energy began to decrease,
accompanied by an increase in the width of the wall. Vortex nucleation
in the 65 nm thick ring was found to be much faster than in the
40 nm thick ring element due to its higher initial magnetostatic
energy density.
PDF
(2 654 901 bytes), Gzip-compressed PostScript
(5 792 377 bytes).
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Event: HMM 2005, Budapest Hungary
Date: 30-May-2005
Title: MP2-2: Micromagnetic Calculations of Eddy Currents with
Time-Varying Fields
Authors: L. Yanik, E. Della Torre, and M.J. Donahue
Abstract:
This paper extends a recently presented program for solving the eddy
current problem in a limited geometry, by investigating the effect of
time-varying fields. When the applied field was turned off, the wall
motion slowed by several orders of magnitude, but since the wall
energy could be reduced by reducing the amount of wall, it continued
to move, albeit much more slowly . Reversing the applied field had the
effect of nucleating the opposite kind of wall which propagated inward
and eventually annihilated the previous wall.
PDF
(322 653 bytes),
Gzip-compressed PostScript
(197 730 bytes).
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Event: MMM 2004, Jacksonville Florida
Date: 8-Nov-2004
Title: AR-14: Micromagnetic and Analytic Study of Small Zigzag
Sensors
Authors: M.J. Donahue, F. da Silva, and D.P. Pappas
Abstract:
Zigzag shaped elements are studied as prototypes for low field
magnetic sensors. The elements are composed of rectangular blocks
of size L x L/2 x 20 nm, where L varies from 500 nm to 50 nm. The
shape anisotropy of the blocks causes the magnetization to scissor
back and forth between blocks with tilt angle θ relative to the
long axis of the full element.
Results from micromagnetic simulations indicate that the sensitivity
to applied fields decreases with the element size, and effectively
vanishes when the block size L is below 100 nm. As the part size is
reduced, the exchange field becomes proportionally stronger, resulting
in a decreased tilt angle. A simple analytic model shows the
sensitivity to be ΔR sinθ sin²θ/Hk,
where ΔR/R0 is the AMR ratio and Hk is the
effective anisotropy, i.e., the sum of the exchange and shape
anisotropy in each subblock. This response function exaggerates the
effect of the reduced tilt angle: at θ = 45°, the
sensitivity is 92% of the maximum obtained at 55°, but the
sensitivity drops to 40% of the maximum by 25°. The rejection
ratio (i.e., the insensitivity to fields applied in the perpendicular
plane) is studied in a similar fashion.
PDF
(1 633 043 bytes), Gzip-compressed PostScript
(937 139 bytes).
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Event: MMM 2004, Jacksonville Florida
Date: 8-Nov-2004
Title: BF-02: Micromagnetic Eddy Currents in Conducting
Cylinders
Authors: E. Della Torre, L. Yanik, M.J. Donahue, and
E. Cardelli
Abstract:
Recently interest has been expressed in introducing eddy currents into
micromagnetic problems. We have developed a micromagnetic program to
solve for the dynamic magnetization in conducting cylinders as a test
bed for determining errors in these programs. This involves solving
the coupled problems of eddy current and magnetization
calculation. Applying a magnetic field along the wire but opposite to
the magnetization creates a Bloch wall that moves inwards and
generates eddy currents that impedes its progress. This model permits
one to determine the effect of wall bending on its characteristics,
since its curvature increases as it approaches the center of the wire.
The results of this model properly reduce to earlier published
results for a zero thickness wall.
PDF
(84 588 bytes), Gzip-compressed PostScript
(151 278 bytes).
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Event: IFIP Workshop on the Changing Face of Mathematical
Software, Washington, DC
Date: 3-Jun-2004
Title: Standard Problems and Public Code for Micromagnetics
Author: M.J. Donahue
Abstract:
Advances in magnetic devices such as recording heads, field sensors
and magnetic non-volatile memory are dependent on microstructural
details for high performance. Magnetization processes can be modeled
by micromagnetics, which is a continuum theory applicable at the
nanometer scale. In 1995, the micromagnetic modeling activity group
(muMAG) was formed at NIST to address fundamental issues in
micromagnetic modeling through promotion of community standard
problems and development of a micromagnetic public code. The
standard problem suite currently consists of four problems, covering
both equilibrium and dynamic aspects of micromagnetics. A fifth
problem, dealing with thermal effects, is under development. The
public code (OOMMF) is an extensible collection of applications
written in C++ and Tcl/Tk. Use of OOMMF has been acknowledged in
over 100 refereed journal publications. This talk will discuss
results and lessons from the standard problem suite, and provide an
overview of the OOMMF architecture and extension mechanisms.
PDF
(1 393 474 bytes),
Gzip-compressed PostScript
(1 241 150 bytes).
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Event: SIAM Conference on Mathematical Aspects of Materials
Science, Los Angeles, CA
Date: 23-May-2004
Title: Motion of Magnetic Domain Walls in Thin, Narrow Strips
Authors: M.J. Donahue and D.G. Porter
Abstract:
In thin, narrow strips of ferromagnetic material, magnetic domains
orient parallel to the long axis of the strip. In-plane transverse
walls form at the junction between oppositely directed domains.
Micromagnetic simulations show that a magnetic field applied along the
strip axis induces domain wall motion primarily through precession
about the strip demagnetizing fields. A simple model is presented
that quantitatively agrees with results from the full micromagnetic
model, including domain wall momentum and retrograde motion.
PDF
(374 398 bytes),
Gzip-compressed PostScript
(206 247 bytes).
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Event: µMAG meeting, Anaheim, CA
Date: 7-Jan-2004
Title: OOMMF: Where is it, where is it going?
Authors: M.J. Donahue
PDF (1 025 088 bytes),
Gzip-compressed PostScript
(433 307 bytes).
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Event: 9th Joint MMM/Intermag Conference, Anaheim, CA
Date: 6-Jan-2004
Title: BR-12: Velocity of Transverse Domain Wall Motion Along
Thin, Narrow Strips
Authors: D.G. Porter and M.J. Donahue
An extended version of this presentation can be found above.
Abstract,
PDF
(272 652 bytes),
Gzip-compressed PostScript
(101 007 bytes).
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Event: MCSD Seminar, NIST, Gaithersburg MD
Date: 16-Sept-2003
Title: Exchange Energy Formulations for Micromagnetics
Authors: M.J. Donahue and D.G. Porter
Abstract:
Micromagnetics is a continuum theory that models behavior of magnetic
materials at the nanometer scale. Typically, four component energies
are considered: Zeeman (applied magnetostatic), demagnetization
(self-magnetostatic), crystalline anisotropy, and exchange. The
exchange energy represents the adjacent spin-spin interaction that
gives rise to ferromagnetism. Defined in terms of the gradient of the
magnetization components, it is especially sensitive to discretization
details in a numerical implementation. This talk examines several
discretization methods for the exchange energy term, and presents
results on convergence rates, boundary conditions, and iterative
stability. Effects of the discretization method on vortex mobility and
Néel wall collapse are also presented.
PDF
(606 455 bytes),
Gzip-compressed PostScript
(706 452 bytes).
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Event: HMM 2003, Salamanca Spain
Date: 28-May-2003
Title: WA-03: Exchange Energy Formulations for 3D
Micromagnetics
Authors: M.J. Donahue and D.G. Porter
An extended version of this talk is above.
Abstract,
Publication,
PDF
(171 143 bytes),
Gzip-compressed PostScript
(198 242 bytes).
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Event: HMM 2003, Salamanca Spain
Date: 28-May-2003
Title: WD-08: A Test Bed for a FDTD Micromagnetic Program with
Eddy Currents
Authors: L. Yanik, E. Della Torre, and M.J. Donahue
Abstract,
Publication,
PDF
(409 994 bytes),
Gzip-compressed PostScript
(677 294 bytes).
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Event: Applied Math Lab Seminar, New York University
Date: 10-Apr-2003
Title: Exchange Energy Formulations for 3D Micromagnetics
Authors: M.J. Donahue and D.G. Porter
A revised and updated version of this talk can be found above.
PDF
(983 072 bytes),
Gzip-compressed PostScript
(1 259 770 bytes).
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Event: MMM 2002, Tampa Florida
Date: 13-Nov-2002
Title: CC-03: Defect Related Switching Field Reduction in Small
Magnetic Particle Arrays
Authors: M.J. Donahue, G. Vértesy and M. Pardavi-Horvath
Abstract,
Publication,
PDF
(386 375 bytes),
Gzip-compressed PostScript
(1 587 906 bytes).
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Event: MMM 2002, Tampa Florida
Date: 12-Nov-2002
Title: BC-02: Comparison of Exchange Energy Formulations for 3D
Numerical Micromagnetics
Authors: M.J. Donahue and D.G. Porter
Abstract:
Exchange energy is one of the canonical terms in micromagnetic
theories. Because it is derived from the derivative of the
magnetization vector, it is especially sensitive to the numerical
representation selected. In this work we compare three discretized
exchange formulations for 3D numerical micromagnetics on rectangular
grids: the first is derived from the magnetization at a cell and at
each of its 6 nearest neighbors; the second uses a trilinear
interpolation on the 26 nearest neighbors; the third uses 12
neighboring points, 2 forward and 2 backward along lines parallel to
the grid axes. Convergence studies on analytic 1D domain walls show
that the 12 neighbor method is of higher order than the other two, so
this method can attain a specified accuracy with a coarser
grid. Similar results are observed on studies of 3D vortex structures
in small cubes of soft magnetic material, as in muMAG Standard Problem
3. The effect of the exchange representation on the motion of magnetic
structures is also important. Vortex cores tend to move preferentially
towards locations between grid nodes, requiring extra applied field to
overcome this discretization induced artifact. We find that
the 12-neighbor method exhibits somewhat greater vortex pinning on
coarse grids than the others, but at finer discretizations, where the
vortex core is resolved, this formulation exhibits considerably less
pinning.
PDF
(117 684 bytes),
Gzip-compressed PostScript
(127 532 bytes).
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Event: NIST Nanotechnology Open House, Gaithersburg Maryland
Date: 20-Jun-2002
Title: Micromagnetic Modeling
Authors: M.J. Donahue
Abstract:
Micromagnetics is the study, modeling and simulation of magnetic
materials and their behavior at the nanometer scale. The micromagnetic
modeling activity group, muMAG, was formed in 1995 as part of the NIST
Center for Theoretical and Computational Materials Science (CTCMS) to
address fundamental issues in micromagnetic modeling. This talk will
provide an overview of the two main facets of muMAG, the Standard
Problem suite and the OOMMF micromagnetic public code.
PDF
(739 061 bytes),
Gzip-compressed PostScript
(959 541 bytes).
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Event: Intermag 2002, Amsterdam, The Netherlands
Date: 2-May-2002
Title: GR-02, Analysis of Switching in Uniformly Magnetized Bodies
Authors: M.J. Donahue and D.G. Porter
Abstract,
Publication,
PDF
(147 766 bytes),
Gzip-compressed PostScript
(154 440 bytes).
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Event: MMM 2001, Seattle Washington
Date: 13-Nov-2001
Title: AF-04, High Resolution Study of Discretization Effects
in µMAG Standard Problem No. 1
Authors: M.J. Donahue and D.G. Porter
Abstract:
A fine detail, 3D micromagnetic study has been made of magnetization
reversal in muMAG Standard Problem No. 1, a 2 µm x
1 µm x 20 nm rectangle of
Ni80Fe20. In the remanent state, closure
domains form 90° walls with the single domain in the central
portion of the part. As the applied field is reversed, these walls
seed the creation of 180° Néel walls running perpendicular
to the long axis of the sample, starting on one edge and terminating
in a half vortex situated approximately halfway to the opposite edge.
If the magnetization discretization is too coarse, magnetostatic
forces overpower the exchange field and temporarily collapse the wall,
which reforms as a cross-tie and now full vortex. Magnetization
reversal proceeds by growth of the domain formed between the cross-tie
and vortex. Conversely, if the discretization is fine enough, then in
sufficiently strong reversing fields the equilibrium state with
180° Néel walls is found to be unstable---if the
symmetry about the center point is broken, then reversal occurs by
growth of one of the edge domains. Included in this material is a
multiscale approach to self-magnetostatic field calculation.
PDF
(1 361 659 bytes),
Gzip-compressed PostScript
(1 195 881 bytes).
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Event: HMM 2001, Ashburn Virginia
Date: 21-May-2001
Title: Poster III-5, OXS: An Extensible Public Domain Solver for
Micromagnetics
Authors: M.J. Donahue and D.G. Porter
Digest,
PDF (2 374 334 bytes),
Gzip-compressed PostScript
(2 604 874 bytes).
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Event: Scientific Object Oriented Programming Users Group
(Scoop), NIST, Gaithersburg Maryland
Date: 6-Feb-2001
Title: An Introduction to the OOMMF eXtensible Solver Class
Architecture
Author: M.J. Donahue
Abstract: Micromagnetics is the modeling of magnetic behavior
of materials at the nanometer scale. OOMMF (Object Oriented
MicroMagnetic Framework) is a project in MCSD/ITL developing
micromagnetic programs and tools in Tcl/Tk and C++. This talk
provides a brief overview of the
OOMMF
project, and then shifts focus to the class structure of the OXS 3D
solver currently in development. I discuss in particular detail
the Oxs_Ext class, which is designed to provide a uniform framework
for program modification and extension. I also give an explicit
example showing what is involved in adding a new energy term using
this mechanism, and discuss the architectural issues involved in
designing this framework.
PDF
(626 780 bytes),
Gzip-compressed PostScript
(505 299 bytes).
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Event: The 8th Joint MMM-Intermag Conference, San Antonio,
Texas
Date: 10-Jan-2001
Title: EW-11: Non-uniform Thickness in Two-dimensional
Micromagnetic Simulation
Authors: D.G. Porter and M.J. Donahue
Abstract,
Publication,
PDF (1 340 700 bytes),
Gzip-compressed
PostScript (1 526 904 bytes).
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Event: The 8th Joint MMM-Intermag Conference, San Antonio,
Texas
Date: 9-Jan-2001
Title: DF-13: Micromagnetic Calculation of the High Frequency
Dynamics of Nano-Size Rectangular Ferromagnetic Stripes
Authors: O. Gérardin, H. Le Gall, M.J. Donahue
and N. Vukadinovic
Abstract,
Publication,
PDF (417 223 bytes),
Gzip-compressed
PostScript (613 828 bytes).
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Event: AMRI seminar, University of New Orleans
Date: 30-Oct-2000
Title: Micromagnetic Dynamics in Thin Films
Author: M.J. Donahue
Abstract:
Dynamic micromagnetic computations are increasingly important for
rate-sensitive applications such as magnetic memory. In this talk I
give a brief overview of micromagnetics, and then discuss new
solutions to muMAG Standard Problem No. 4, which details magnetization
switching behavior in a 500 x 125 x 3 nm NiFe particle under
Landau-Lifshitz-Gilbert dynamics. We find two reversal modes,
depending on the relative orientation of the applied field with the
initial magnetization state. One mode proceeds by simple propagation
of end domains toward the sample center, but the other involves
counter-rotating domains, 360 degree walls and complex dynamics on
fine length scales. I also present computational and theoretical
results describing the functional dependence of the resonance
frequency and susceptibility to the applied field in the neighborhood
of a switching event.
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Event: OOMMF Workshop 2000, Gaithersburg Maryland
Date: 18-August-2000
Authors: M.J. Donahue, D.G. Porter, R.D. McMichael and
J. Eicke
Titles:
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Event: µMAG Workshop, San Francisco California
Date: 6-Jan-1998
Title: OOMMF Micromagnetics Public Code Project
Authors: M.J. Donahue, D.G. Porter, R.D. McMichael,
J. Eicke and J. Blue
PDF (251 810 bytes),
Gzip-compressed PostScript
(398 506 bytes).
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Event: MMM 1995, Philadelphia Pennsylvania
Date: Nov-1995
Title: Artifacts in Magnetic Resonance Imaging from Metals
Authors: L.H. Bennett, P.S. Wang, and M.J. Donahue
Abstract,
Publication,
PDF (188 103 bytes),
Gzip-compressed PostScript
(343 590 bytes).
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Event: NDE seminar, The Ohio State University, Columbus Ohio
Date: 7-May-1991
Title: Simulation of Radiographs
Author: M.J. Donahue
Abstract:
This talk provides an overview with examples of a radiograph
simulation package developed by the author as part of his
Ph.D. dissertation work in the Welding Engineering Department of The
Ohio State University. This package requires as input a text file
defining a collection of simple objects (ellipsoids, cylinders,
prisms, and intersections of these with half-spaces) having arbitrary
orientations and X-ray absorption coefficients. The user also selects
either parallel beam or cone beam X-ray source geometry. The output
is a simulated radiograph. The talk also includes examples of
tomographic reconstructions using the simulated radiographs, and a
comparison with a reconstruction from actual radiographs.
PDF (292 267 bytes),
Gzip-compressed
PostScript (889 979 bytes).
Date created: September 8, 2017 | Last updated: Oct 10, 2023
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