Nanostructured magnetic dots grown by electron lithography
have been studied using an atomic force microscope (AFM)
("non-magnetic") in the presence of a magnetic field. Cobalt
dots of thickness t = 40 nm and lateral dimensions in the
200-300 nm range exhibit an unusually large expansion when
subject to low fields (10-20 Oe). This expansion
("magnetostriction") is orders of magnitude larger than the
expected magnetostriction of bulk Co, e.g. values as large
as \deltal/l = 1x10^-2 have been observed for H = 10
Oe. The same technique applied to bulk sample renders the
expected magnetostriction values. Moreover, applying
magnetic field at a frequency higher than the response of
the feedback system and scanning the AFM tip has allowed
simultaneous detection of topography and distribution of
magnetostrictive response at the surface (using lock-in
techniques). With this technique, domains and domain walls
of the Co dots can be imaged with a 1 nm lateral resolution.
In elliptical dots with a 350 nm long axis we have found
evidence for two domains with opposite magnetization along
the long axis of the dot. The domain wall width in these
dots is found to be about 35 nm. High resolution domain
imaging can be applied to other systems, however they need
to display a large enough field induced deformation.
[L22.002] Ferromagnetic Resonance Force Microscopy Imaging
Wei Chen, Melissa Midzor, Michael Cross (Caltech), Philip Wigen (Ohio State University), Chris Hammel (Los Alamos National Laboratory), Michael Roukes (Caltech)
Magnetic resonance force microscopy (MRFM) has been used to
investigate magnetostatic waves on microscopic samples of
YIG. This work elucidates the nature of scanned probe
(local) imaging in ferromagnetically-coupled systems.
Scanning was performed with a specially-designed ultrasharp
tip with Permalloy (NiFe) deposited solely in the tip
region, to yield a spatial sensitivity of <10um. This has
provided the first direct imaging of fundamental and higher
order magnetostatic modes in micromagnetic systems. The
modal dependence upon applied field and sample size was
measured and compares well with theoretical models. However,
unlike traditional ferromagnetic resonance detection
technique, MRFM not only serves as a non-perturbative
detection tool of magnetostatic modes, but also can locally
change their dispersion relations via the strong field
gradients generated from the cantilever tip. As a result,
when the tip is positioned closely to the YIG surface,
certain modes of the magnetostatic waves are either enhanced
or depressed, depending on their respective wavelengths.
This corresponds to the fact when the tip is further away,
the dispersion of the FMR modes is mainly determined by the
sample size. As the tip moves closer to the surface, a new
regime emerges where the FMR dispersion is dominated by the
local magnetic field. A quantitative model based on DE
theory is proposed, and it explains the main features of the
observed tip influence on different magnetostatic modes.
[L22.003] Imaging of the domain wall excitations in YIG film with Scanning ac Hall Microscopy
M. Marchevsky, S. Bhattacharya (NEC Research Institute, Princeton, NJ 08540)
A new technique of Scanning ac Hall Microscopy is applied to
visualize a local dynamic response of the magnetic domains
in yttrium-iron garnet film. By using a small ac excitation
field, the spatial distribution of the domain wall
vibrations was imaged at the different applied dc magnetic
fields. We observe a "localization" of domain wall motion
near the topological defects of the domain structure and an
evolution of the excitation patterns with the external
field. When a small in-plane dc field gradient was applied,
localized "beams" of the domain wall excitation are
observed; they typically span defects of the opposite
topological "sign". We speculate on the origin of the
observed phenomena in terms of a competition between the
magnetostatic "elastic energy" and domain wall pinning by
defects. We also discuss the potential of our imaging
technique for studying local dynamic properties of magnetic
materials.
[L22.004] Theory of a magnetic microscope with nanometer resolution
David Penn (National Institute of Standards and Technology), Peter Apell (Department of Applied Physics, Chalmers University, Sweden), Peter Johansson (Department of Physics, University of Lund, Sweden)
We propose a theory for a type of apertureless scanning near
field microscopy that is intended to allow the measurement
of magnetism on a nanometer length scale. A scanning
tunneling microscope (STM) is used to scan a magnetic
substrate while a laser is focused on the STM tip. The
electric field between the tip and substrate is enhanced in
such a way that the circular polarization of the scattered
light due to the Kerr effect, which is normally of order 0.1
percent, is increased up to two orders of magnitude for the
case of a Ag or W tip and an Fe sample. Apart from this
there is a sustantial background of circular polarization
that is non-magnetic in origin. This circular polarization
is produced by light scattered from the STM tip and
substrate.
[L22.005] Magnetic imaging of a buried SmCo layer in a spring magnet
J.C. Lang, J. Pollmann, G. Srajer, D. Haskel, J. Maser (Advanced Photon Source, Argonne National Lab, Argonne, Illinois 60439), J.S. Jiang, S.D. Bader (Materials Science Division, Argonne National Lab, Argonne, Illinois 60439)
Magnetic domain mapping results obtained using an x-ray microprobe are reported for an Fe/SmCo spring magnet. The magnetic structure as a function of the externally applied magnetic field was observed for the buried SmCo layer of the material using magnetic dichroism contrast at the Sm L-III edge ( 6.716 keV ). Clear boundaries between different magnetic domains could be distinguished. Contrary to expectations, the magnetic domain walls were not completely oriented parallel to stacking disorders in the SmCo layer. Moreover, in one remanent phase, they even seemed to be perpendicular to the stacking disorders. The work at the APS is supported by the U.S. Department of Energy, Office of Science, under Contract No. W-31-109-ENG-38.
[L22.006] Imaging Magnetic Vortex Cores
Teruo Ono (Graduate School of Engineering Science, Osaka University)
In a thin circular magnetic dot of micrometer or submicrometer size, a curling spin structure may be stabilized. Spin directions change gradually along the edges so as not to lose too much exchange energy but to cancel total dipole energy, resulting in a vortex spin configuration. In the vicinity of the dot center, however, the angle between adjacent spins becomes large and the exchange energy increases. As a result, at the center of the vortex structure, the spins within a small spot will turn out-of plane. Although such a magnetic vortex core structure has been theoretically predicted, direct experimental evidence has been lacking. We succeeded to clearly observe the magnetic vortex core in permalloy (Ni80Fe20) circular dots with diameter from 0.1 to 1 micrometer by using magnetic force microscopy (MFM).1 MFM observation for permalloy circular dots was carried out after applying various magnetic fields and the annihilation field of the magnetic vortex core was estimated.
1. T. Shinjo et al., Science 289 (2000) 930.
[L22.007] Stress determination and magnetization reversal detection in FeSiAl(N) films using magnetic force microscopy with in-plane magnetic field capability
J. Snyder, C.C.H. Lo, J. Leib, R. Chen, B. Kriegermeier-Sutton, M.J. Kramer, D. Jiles (Ames Laboratory, USDOE and Department of Materials Science and Engineering, Iowa State University), M.T. Kief (Seagate Technology, Minneapolis, MN 55435)
A system has been developed which has the capability to
image domain structure using magnetic force microscopy (MFM)
while an in-plane magnetic field is applied in situ. The
magnetization reversal in a series of highly stressed
rf-sputtered FeSiAl(N) films has been studied using this
system. All films exhibited a stripe domain structure in
zero applied field indicative of a perpendicular component
of domain magnetization which alternates in sign. The films
all exhibited a similar sequence of magnetization processes:
on reducing the applied field from saturation a fine stripe
domain structure nucleated and then coarsened as the field
was decreased to zero. Local switching of domain contrast
was observed along the steepest part of the hysteresis loop
as the perpendicular component reversed. As the reverse
field was increased toward saturation the stripe domains
disintegrated into smaller regions. The observation is
consistent with an interpretation that the domain
magnetization rotated non-coherently into the sample plane.
The saturation field and the film stress exhibited similar
trends with nitrogen partial pressure. The results suggest
that the perpendicular anisotropy, which caused the
formation of the stripe domain structure, was induced by the
film stress via magnetoelastic coupling.
[L22.008] Observation of Magneto Mechanical Phase Transformation in Gd_5(Si_xGe_1-x)_4 with Magnetic Force Microscopy (MFM)
J.E. Snyder, P. Xi, J. Leib, C. C. Lo, S. J. Lee, D. C. Jiles (Ames Laboratory, Iowa State University)
Using MFM, we have observed the magnetic domains on the
surface of a Gd_5(Si_2.09 Ge_1.91) sample, and
how those domains change with temperature in the vicinity of
magnetic and structural phase transformations. This material
undergoes such transformations close to room temperature,
the exact temperatures of which depend on composition, and
on whether the sample is being heated or cooled. Stripe
domain patterns were observed on one face of the sample,
indicating ferromagnetic order. MFM images for a series of
temperatures around 293K showed systematic dramatic changes
in domain sizes and shapes, and systematic differences
between heating and cooling, reflecting the phase
transformations taking place in the sample. The other face
showed no definitive domain patterns. This indicated that
the sample consisted of two different magnetic phases. VSM
measurements confirmed that a combination of different
magnetic phases could be present in the sample.
[L22.009] Nanoscale Magnetic Domains in Epitaxial Ni (111) Films
R. Naik, S. Hameed, J. Valenzuela, P. Talagala, L.~E. Wenger (Wayne State U.), V.~M. Naik (U. Michigan-Dearborn)
The magnetic properties and domain structures of epitaxial Ni (111) films grown on Ag(111)/Si(111) by molecular beam epitaxy have been studied for thicknesses varying from 15 to 500 nm. Both ferromagnetic resonance and magnetization data clearly showed the presence of an uniaxial perpendicular anisotropy energy, Ku \sim 3 \times 10^-7 ~erg/cm^3, which is essentially independent of film thickness. Although K_u < ?\pi M_s^2, disordered stripe domains were observed in films with t \ge ( 50 nm by magnetic force microscopy with the average domain size increasing with the film thickness. A theoretical model of periodic stripe domain structures with tilted partial flux closure domains has been successfully applied to explain the thickness dependence of these stripe domains. The model predicts that these films have a significant volume fraction of flux closure domains with the tilt angle (made with respect to the film normal) varying from 68^o to 82^o with increasing film thickness. The tilt of the moments in the flux closure domains would correspondingly give rise to an in-plane magnetization, in accordance to the experimental observation in these Ni films.
J.V. is supported under NSF-REU- EEC-9721446
[L22.010] Analysis of disordered stripe magnetic domains in strained epitaxial Ni(100) films.
L.E. WENGER, S. HAMEED, P. TALAGALA, R. NAIK (Wayne State University), V.M. NAIK (U. of Michigan-Dearborn), R. PROKSCH (Asylum Research)
Disordered stripe domain structures have been previously
reported^1,2 on strained epitaxial Ni(001) films over a
wide range of film thickness (10 < t < 220 nm), which
covers both K_u > 2\pi M_s^2 and K_u < 2\pi M_s^2
where K_u is the uniaxial perpendicular
anisotropy energy and 2\pi M_s^2 magnetostatic shape
anisotropy energy. While a periodic domain model including a
\mu*-effect (rotational permeability) was tenable for
films with t < 25 nm, a model with tilted partial flux
closure domains is more successful in explaining the
evolution of domain structures for films with t > 25 nm.
This model predicts no closure domains for t < 25 nm, a
significant deviation of magnetization direction in the
closure domain from the film normal with increasing film
thickness, and a significant volume fraction (\sim 40%)
of closure domains for thicker films (t > 70 nm).
Correspondingly, the closure domains give rise to an
in-plane magnetization in these strained Ni(001) films in
agreement with experiment. ^1 H.J. Hug, et al., J.
Appl. Phys. 79, 5609 (1996). ^2 S. Hameed, Bull. Amer.
Phys. Soc. 45, 450 (2000).
[L22.011] Two-dimensional magnetic domain patterns
M. Kiwi (P. Univ. Católica, Santiago, CHILE), J.R. Iglesias, S. Goncalves (Instituto de F\'\isica, UFRGS, Porto Alegre, BRAZIL), O. Nagel (Departamento de F\'\isica, Univ. Nac. del Sur, Bah\'\ia Blanca, ARGENTINA)
Two-dimensional magnetic garnets exhibit complex and fascinating magnetic domain structures, like stripes, cells, mixed states of stripes and cells and labyrinths, which have challenged theorists for a long time. They change reversibly when the intensity of an externally applied magnetic field is varied. By Monte Carlo simulations we investigate the pattern formation and the thermodynamics of these systems as a function of the intensity of the external magnetic field. Our simulations yield patterns which are similar to the ones observed experimentally. The ordering transition temperature, relaxation energy and average domain size of each configuration are computed. General trends, as functions of the model parameters, are presented and discussed.