Development of Advanced Magnetic Force Microscopy
Tips for Magnetic Characterization
Romel D. Gomez, Jon Orloff and Klaus Edinger
Department of Electrical Engineering,
University of Maryland, College Park, MD 20742
Sy-Hwang Liou
Department of Physics and Astronomy
University of Nebraska, Lincoln, NE 68588-0111
PROJECT SUMMARY:
This collaborative research effort between the University of Maryland and the University of
Nebraska aims to develop improved magnetic tips to enhance the resolution and sensitivity
magnetic force microscopes (MFM). The goal is to produce MFM tips that improve the
resolution to 1 nm or roughly a factor of 100 better than the state of art. The new capability will
offer researchers unprecedented insight into surface magnetism at the nanometric regime, thus
extending the range of scientific probing and resulting technological innovations. From a
scientific standpoint, the capability to examine magnetism at near molecular dimensions is a
crucial first step in relating classical and quantum mechanical manifestation of surface
magnetsm, while form technological standpoint, understanding magnetization processes at these
length scales is a precursor to the development of novel magnetic devices at reduced dimensions.
This is is multifaceted program and takes full advantage of the strengths of the respective
research groups and our inductrial counterpart. Prof. Sy-Hwang Liou and his associates at the
University of Nebraska will explore the film composition and single domain particle fabrication
on the tips of MFM probes. Prof. Jon Orloff and his group will use focused ion beam technology
to micromachine extremely sharper tips for increased resolution as well as alter the mechanical
oscillatory properties of the cantilever to improve detection sensitivity. Prof. Romel Gomez and
his team will focus on the development, implementation and modeling of magnetic force
microscopy using these advanced probes. And Dr. Roger Proksch will expore the viability of
new probes in magnetic dissipation imaging. The main feature that distinguishes this project is
the nature of the collaboration. It spans a very broad development base and comprises the
interrelationship between film composition, micromachining, MFM implementation and theory.
This project will involve a several students from both universities, including graduate
students and undergraduates. It is anticipated that some parts of this development can lead to
doctoral dissertations, as well as provide valuable research exposure to deserving juniors and
seniors. At all levels, the students are expected to have gained the knowledge and skills
appropriate for a wide range of career choices. In addition to the respective PI's, postdoctoral
associates will be hired to assist in the implementation of various aspects of the project. The
postdocs will improve the overall efficiency, and at the same time expose them to sophisticated
ion beam apparatus and imaging techniques. Creative ideas consistent with the goals of the
research will be encouraged and extensively supported.
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PROJECT DESCIPTION
Introduction: Background and Necessity of Probe Development
The study of the magnetic properties of matter at the nanometer scale is of interest both from
the scientific and the technological points of view. From a scientific standpoint, our present
understanding of magnetism at these length scales is in its infancy at best. It is only in recent
years that experimental tools and sufficient computing power have become available to tackle the
complicated magnetic behavior at these microscopic length scales. In comparison with bulk
magnetic studies where the effects of interfaces, defects, morphological grains and so on are
treated as perturbations and lumped into global parameters, micromagnetic models attempt to
incorporate these parameters locally and combine them with the contributions of the Zeeman,
magnetostatic, magnetoelastic and local anisotropies. Predicting the behavior of magnetic
domain structures is an inherently complex problem, and we are just beginning to formulate
models, starting from well-known magnetic alloys with the simplest geometrical configurations.
The so called “standard problem #1”, initiated by NIST, consists of a rectangular 1um x 2um
strip of a 50 nm Permalloy thin film with no crystalline anisotropy and perfect edges. From a
technological point of view, the ability to measure and control the properties of magnetic
domains is of great interest in the areas of data storage, micro electromechanical systems
(MEMS), magnetic random access memories (MRAMS), magneto-electronics (spin transistors),
magnetic sensors for the industrial and biomedical applications and even as embodiments of
quantum computing. Often, the novel features of these devices, such as the giant magnetoresistance in field sensing applications or the ultimate areal density for recording media, have its
origins upon the nascent micromagnetic domain configurations.
Magnetic force microscopy (MFM) is arguably the most important imaging tool for studying a
wide variety of local magnetic phenomena. Since its development in 1987 [1], it has emerged as
a powerful tool which reveals magnetic processes with unprecedented clarity, resolution and
ease. It allows the direct visualization of magnetic domains and provides the experimental basis
for theoretical modeling. The technique measures change of the interaction force between a
magnetized probe and the local stray magnetic field from the sample, point by point, as the probe
is scanned across the surface. The probe is typically a cantilever made from silicon or silicon
nitride, with a ferromagnetic tip on the free end. The inherent resolution depends upon the
confinement of the interaction at the end of the probe and sensitivity depends upon the ratio of
the cantilever spring constant and the magnetic moment. At present, commercial MFM probes
resolves about 50-100 nm features at force constant of about 0.01 N/m --- roughly equivalent to
resolving the field gradients from a 10-12 emu source at a distance of 50 nm.
Despite the impressive performance and widespread use of the MFM, there are important
probe-related limitations that need to be overcome to realize its full potential . First of these is
the enhancement of resolution and sensitivity. As is well known from microscopy, in order to
measure something at a given scale it is necessary to have a probe whose fundamental size is
well below the size of the object to be measured. In the case of atomic force microscopy, for
example, in order to precisely and accurately characterize a trench with a width of 0.1
micrometers and a depth of 0.2 micrometers, one needs a probe in the shape of a rod with a
length of at least 0.2 micrometers and a diameter of less than 0.1 micrometers. In the case of
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magnetic force microscopy (MFM) the force between the probe and the sample is carried by the
magnetic field. Obviously, the smaller the magnetically active area of the probe, the less it will
be affected by areas from far away since the dipole nature of the field causes it to diminish
rapidly with distance. Therefore, in order to make a high resolution MFM it would be necessary
to create an extremely small magnetic probe. The smaller volume of the magnetic probe, on the
other hand, will result in a lower magnetic moment and a smaller interaction volume and thus a
weaker force. Hence, the lateral resolution of the MFM probe will also be limited by its
sensitivity (e.g. the spring constant of the cantilever). The improvement in resolution would have
to be complemented with an enhancement of the probe sensitivity.
Second is the development of specialized probes whose properties are optimized for a given
specimen and free from instrument-induced distortions. Since the MFM relies on a mutual
interaction, it is inherently invasive. Thus, the measurement process itself could cause
irreversible changes to the system and the measured image may not reflect the intrinsic state of
the sample. Conversely, the probe’s moment itself may change as it moves in varying fields,
which would cause nonlinearities in the instrument response. This would render the image
interpretation to become complicated and equivocal. To overcome these problems, it is necessary
to use probes whose properties are compatible with the sample at hand. For instance, in one
extreme, samples with very low coercivity such as garnets or Permalloy, would require the use of
high sensitivity cantilevers with low moment tips. The weak moment ensures that fringing field
is much lower than the sample coercivity, and increased sensitivity compensates the reduction in
the interaction forces. On the other extreme, the case of permanent magnet samples requires a
probe to be relatively stiff and with the coercivity be higher than the stray field generated at the
sample surface. In highly unusual cases, say where no known materials exist that could
withstand the strength of the specimen field, it may be judicious to use superparamagnetic probes
instead and interpret the images accordingly. Most interesting problems are likely to have
requirements that are midway between these extreme cases, while certain applications, such as
studying the magnetic evolution of soft magnetic materials at high fields would demand other
properties. These might require low moment probes with very high coercivity.
Finally, the MFM requires the fundamental understanding of the magnetic characteristics of
the probes themselves, which can be incorporated into theoretical models of image
interpretation[2,18]. At present the generally accepted model for MFM assumes a point dipole at
the tip apex. This picture is adequate in qualitative descriptions which treat the images as
representations of the distribution of magnetic charges from the divergence of the volume
magnetization or the normal component of the surface magnetization. Several sophisticated
theoretical descriptions for image representation have been proposed in the literature which take
into consideration the finite volume of the tip. See for example ref. [3]. Unfortunately, because
of the absence of direct experimental evidence of the probe’s magnetization distribution, the
models simply provide possible explanations of observed contrast formation rather than offer
precise magnetization reconstruction.
The goal of this work is to address the aforementioned limitations of conventional MFM
probes, by improving the resolution and sensitivity, by developing processes to tailor probes with
predefined moment and coercivity, and by developing characterization and calibration methods
for incorporation into theoretical models of image reconstruction. In this work, we will use our
combined resources in thin film preparation, micromachining using focused ion beam (FIB)
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facility and expertise in magnetic force microscopy. By using this collaborative approach, the
composition, probe size, cantilever mechanical sensitivity will be systematically studied and
tailored over a wide range of parameters. The main feature that differentiates this project from
other probe development efforts is the nature of the collaboration. This effort spans a very broad
based development --- exploring the interrelationships of film chemistry, micromachining, MFM
implementation and theory.
State of the Art
The efficacy of the MFM as a micromagnetic tool as well as the need for advanced tips is
best illustrated in a case study of a micron-sized NiFe (Permalloy) islands. Owing to its unique
characteristics that make it ideally suited for field generation and detection, Permalloy is one of
the most important and well known magnetic alloys. In fact nearly all magnetic recording
systems use Permalloy in one or more components; and it is likely to be extensively used in the
next generation of magnetic devices. Despite the enormous volume of work devoted to this
material, it is only now that we are beginning to understand the dependence of the magnetic
properties with the material shape, edge irregularities, defects and pinning centers, and
magnetization history.
The magnetic force image in Figure 1 shows the micromagnetics of an e-beam lithographed
array of 26 nm thick NiFe elements, with lateral dimension ranging from 4 um to 0.125 um. The
MFM is sensitive to the change in local magnetization rather than magnetization itself, which
consequently produces the strongest contrast at the domain walls. The image exhibits a variety
of domain structures, ranging from the familiar closure structures occurring at the lower aspect
ratios to the least understood “near“ single domain with complex edge structures. The formation
of irregularly shaped domains of the largest islands in the bottom row is enigmatic, but it is
remarkable that a certain degree of similarity exits between the different structures.
It is clear that the image contains a wealth of information relating the energetics and the
history of magnetization to the formation of domain structures. This is discussed in the literature
[4,5] However, it also reveals some glaring limitations of the MFM which could be overcome by
improved probes. While smallest square element --- 0.5 um on the side--- still shows the domain
structure akin to its larger counterparts, it is easy to surmise that the internal domain structures of
a slightly smaller island, say 0.25 um would be very difficult to see. One can use the same
reasoning to suppose that edge structures of the smallest islands with high aspect ratios, e.g., the
bottom left domain, also contains complex closure patterns which can not be resolved under the
present conditions. Understanding these structures is crucial because they play important roles in
switching processes. In most systems, the magnetization reversal commences by the nucleation
of reversed domains at the edges. Technologically, electron scattering at the domain walls are
regarded as noise sources in magnetoresistive sensing applications, and there is great interest
controlling the formation of complex domains, particularly in smaller devices.
The second limitation concerns the image interpretation. The notion that MFM detects the
localized magnetic charges is an approximation to the generalized equations,
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Figure 1. Array of micron to submicron Permalloy islands showing domain configurations
as a function of the aspect ratio.
  K p (mass, Q,o )
Fz
z
2

  Hi (r  r ')
F '(r )      n j nk Mi (r ')
dV '
rjrk
tip i j k
where  is the phase, measured point by point and comprising the image array. The force, F(r)
is the gradient of the interaction energy -grad(M•H). The probe’s magnetization is arbitrarily
oriented, so that the dot product would contain three non-vanishing terms. Furthermore, in the
case of an extended probe tip the interaction energy involves an integration over the effective
volume of the probe. Since the image is proportional to the force gradient, the images are thus
formed by the relative contributions of various second derivatives and weighted by the
component of the probe’s magnetization in specific directions.
In order to make full use of the above equations and quantify the images, it is important to
determine the magnetization distribution of the probe M(r), as well as the prefactor K related to
the cantilever’s mechanical property. Apart from simplified calibration schemes which assumes
the point dipole model for the probe, the best method for mapping the stray field distribution of
the probe tip in three dimensions is electron holography[6]. Unfortunately, the technique is
rather involved and inappropriate for routine calibration. Thus, with the present state of the
technology, it would be nearly impossible to fully reconstruct the magnetization distribution of
the patterns in Figure 1. To add to the problem, the equation above tacitly assumes that the
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mutual interaction between the probe and specimen is small enough that the magnetic properties
are unchanged during the measurement process. This is, of course, highly dependent upon the
strength of the moment of one of the elements with respect to the coercive force of the
other.[18,19] Thus, to satisfy this criterion, it is important to develop custom probes whose
properties are optimized for specific applications. An illustration of this need is shown in figure
2. At zero field, the remanence configuration is a 7 domain pattern with crosstie structure. We
observe the evolution with increasing field, which shows the increase in size of the left and right
domains at the expense of the middle domain. The pinning effects of the crosstie structure is also
quite evident.
An interesting configuration is shown at H= 70 Oe which falsely suggests that the system
exhibits a hybrid configuration, i.e., it is comprised of a multidomain upper structure and a single
domain lower structure. We believe that this is not the case here, but rather, the observed
anomaly is a manifestation of probe-induced switching. During the first half of the MFM scan,
the system was probably in the multidomain configuration akin to that at 62 Oe, but midway into
the scan the slight magnetic field from the probe has induced the switching into the near
saturated configuration. The very next image acquired without changing the external field was
very similar to the image at 92 Oe, which shows no multidomain structures on top. The near
saturation configuration as inferred in the image is shown in the schematic diagram on the right.
Thus, we infer from this phenomenon that the smallest increment in magnetic field by which
domain dynamics can be studied depends upon the size of the field from the probe itself.
Another application of MFM which uses the interaction between the tip and sample to
measure energy dissipation in domain walls has recently been introduced and called "Magnetic
Dissipation Microscopy"[7]. In essence, it measures the amount of damping on the tip, which
can be very sensitive relative to force gradients. Small scale magnetic structures have been
observed using the technique which is otherwise invisible
using conventional force gradient detection. Clearly this
technique will benefit immersely from this project as the
degree of quantification is directly related on how wellcontrolled and well-characterized the probe tips are.
Figure 2. Domain wall motion and magnetic
evolution of a 3 um square Permalloy element as
a function of applied field.
Research Plans
The specific questions that will be addressed in this
proposed project are: “What are the factors that determine
the experimental resolution and sensitivity limit of
magnetic force microscopy and how much can it be
improved through the use of advanced MFM tips?” and
“What are the intrinsic magnetization distributions of a
given probe, and can probes be developed that
approximate point dipoles?” We propose to address these
questions by systematically using a variety of coating
chemistry, thin film fabrication techniques, and focused
ion beam (FIB) technology combined with theoretical
modeling to:
(i) understand the micromagnetics of coated thins on Si
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and SiN3 AFM probes, both as function of chemistry and geometry,
(ii) elucidate the relationship between probe size and MFM resolution,
(iii) understand and control the mechanical properties of micro-machined AFM probes
(iv). develop new models and experimental methods to characterize the MFM tips, and to
quantify their interaction (and perturbation) with the sample,
We will demonstrate the enhanced performance of the MFM by showing the evolution of
magnetic domain structures in nanostructured thin films with a spatial resolution previously
unobtainable. We expect that the proposed studies will play a central role in our continuing
efforts to study magnetization reversal mechanisms and to develop predictive models of the
reversal process. The proposed studies will also guide our efforts to develop new thin film media
for high-density information storage and for magnetic sensor applications.
Proposed research topics
A. Fabrication and Characterization of Nanostructured Magnetic Clusters
“The goals of this research are to synthesize and understand magnetic single domain
particles and thin films with nanoscale magnetic particles that suitable as MFM
probes. The desired properties include a very large magnetic anisotropy or the
superparamagnetic instability inherent in extremely small magnetic particles.”
Since we are ultimately interested in producing probes with highly confined magnetic
structures at the apex of a very sharp point, it is natural to first fabricate and characterize
magnetic clusters in the nanoscale regime on flat surfaces. We wish to seek a set of
fabrication parameters which yield tiny islands having specific properties such as size, single
domain configuration, coercivity, thermal stability and of course, the efficacy of transferring
these clusters at the ends of MFM probes. We envisage several alternatives in producing
these clusters, and these are outlines outlined below.
Granular Magnetic Films
Granular Magnetic Films have the form of small crystallites dispersed in a matrix. The
magnetic properties of these films can be controlled by either changing the size of crystallites
and/or the separation distance between the crystallites[13,14]. The average size of the Fe
crystallites is 4-5 nm and the Fe crystallites are mostly isolated in an amorphous SiO2 matrix.
Nanocrystalline Films
Nanocrystalline films such as, CoxPr1-x and other alloys that have very high magnetic
anisotropy energy has been systematically studied. These films have high coercivities, about 2.5
to 4 kOe, and other favorable properties. The nanostructure and their magnetic properties can be
also tailored by annealing, for example the coercivity at room temperature as high as 45 kOe
were achieved. The nanocrystallite in the film is about 5 nm for a 28 nm thick PrCo// Cr film
annealed at 500 oC. [10]
Nanoparticulate Films
It is known that an assembly of very fine non-interacting high anisotropy magnetic particles is
magnetically hard; i.e., it has a large coercivity. This effect is due to the fact that the particles are
single domains and that magnetization reversal takes place only by rotation of the magnetization
vectors against strong anisotropy forces. In an earlier report, we showed that the CoPt film
containing nanometer-size particles has the coercivity as high as 30 kOe [11]. Figs. 3a and 3b are
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the AFM and MFM images from a CoPt sample. As shown in Fig. 3(a), the 5nm-thick film
contains well-separated nanometer-size crystallites as indicated in the AFM image. The sizes of
the crystallites are in the range of 100nm to 400nm. The height of crystallites is in the range of
20nm to 80nm. The MFM image, Fig. 3(b), was obtained using a CoPt MFM tip magnetized
parallel to the sample surface. The light and dark contrast corresponds to the strength of the
stray-field gradient on the sample surface. The lighter color represents a frequency shift in the
MFM tip when the magnetization of the sample and that of the MFM tip are repulsive. As shown
in Fig. 3(b), crystallites with one light and dark area are single-domain (as indicated by "S"); the
grain that may contain a few crystallites with two or more light and dark areas are multi-domain
(as indicated by "M"). The size of a single-domain crystallite is between 100-200nm.
In this project, we will initially identify and fabricate nanostructured magnetic films suitable
for MFM applications, with the main focus being materials with very high anisotropy energy and
stable magnetic properties over a wide range of temperature. Some promising candidates are
CoPt, Nd2(Co, Fe)14B, and AlNiCo. We will attempt to alter the microstructure and
microchemistry of these and other phases to optimize the structural and magnetic parameters.
Novel Tip Coatings
“If we are to improve currently-used magnetic coating
materials, we will need to improve the crystalline orientations
of the magnetic material; to control the anisotropy of the
magnetic material at the tip. If this can be done, the
magnetization direction of the tip will be well defined, which
is important in the interpretation of the magnetic images. The
crystalline orientations of the magnetic material will be
controlled by using different growth conditions and different
underlayers Such techniques have been extremely successful
for fabrication of nanostructured magnetic films.”
We have initially developed two types of tip coatings
which our preliminary results show marked improvement of
the quality of the data and the ease of interpretation of the
resulting images. The first tip coating developed (“soft” MFM
Figure 3. The topographic (a) and
magnetic (b) images of a 5 nm thick
tips) is based on superparamagnetic Fe particles embedded in
CoPt film on a quartz substrate,
a SiO2 matrix. The Fe particles can rotate freely in the
annealed at 750 oC for 3 hours. "M"
indicates a multi-domain grain. "S"
presence of the stray magnetic field from the sample, and thus
indicates a single domain grain.
cannot cause the sample magnetization to reverse (which has
been a severe problem with conventional “soft” MFM tips).
The force between the tip and the sample is always attractive, so that interpretation of the images
is relatively straightforward (similar to Bitter techniques). These tips have been used to
demonstrate high resolution imaging in magnetic recording heads and in nanostructured magnetic
films [16]. The second tip coating developed (high-coercivity MFM tips) is based on permanent
magnetic materials, such as CoPt alloys[17]. We have developed a very high coercivity (larger
than 10 kOe) tip coating which is useful for investigating high stray field materials and devices.
It is most useful however for obtaining MFM images with a magnetic field applied to the sample.
As long as the applied field is smaller than the coercivity of the tip, the interpretation of the
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images is relatively straightforward. It may ultimately make possible more complete studies of
the dynamics of the reversal process.
So far, these preliminary results are very encouraging in so far as their manufacturability.
However, there are many materials-processing parameters which need to be fine tuned to further
improve the magnetic properties of the tips. For example, a systematic study of how the thickness
of the magnetic coating on the tip affects the tip-sample interactions merits considerable
attention.
Point-Dipole MFM Tips
We have also begun the development a new
tip in which the active magnetic material (in
this case, the amorphous 2605Co alloy ) is
deposited only on the very end of the tip. The
resultant smaller active region of the tip (500
nm in dimensions) results in improved spatial
resolution. Figure 4 shows a scanning electron
microscope image of this tip. The amorphous
2605Co alloy is a soft magnetic material that is
used as the core of high power transformers.
Fig. 4. MFM probe tip with a magnet of size 500nm
on the tip.
The amorphous alloy is structurally and
magnetically isotropic. A coating of an
amorphous magnetic material is expected to be less sensitive to the shape of the tip than a
crystalline magnetic material. The magnetization of the 2605Co amorphous alloy is about
twice that of Permalloy. The coercivity field of the tips is about 3 Oe which is smaller than
the typical stray magnetic field near written bits in hard disk recording media.
In our initial tests, we compared the images obtained using a standard “soft” thin film tip
and this new “point” tip. Both tips were coated with same magnetic material and thus have
soft magnetic behavior (i.e. the tip magnetic moment follows the stray field of the sample).
MFM images from the same area of a reference sample (a recorded tri-bit on a standard hard
disk, provided by Paul Rice of NIST [14] were obtained to compare the “soft” point dipole
tips and the standard “hard” tips. For a “soft” MFM tip, the interaction of the stray field from
the recording disk results in a magnetic force on the tip which is always attractive. The main
distinction is that while the conventional tips correctly show the bright/dark contrast
characteristic of the magnetic charges, the tri-bit transitions appear to be smeared and
difficult to isolated. This is a manifestation of the finite extent of the interaction volume
which degrades resolution. On the other hand, the moment of the “soft” island tip is almost
always directed in the direction of the local field. Hence, the attraction is always attractive
and the images show shades of dark contrast, which indicate the relative strength of the local
field. However, in this case, the tri-bit pattern was clearly resolved because of the extreme
localization of the probe’s magnetic volume. This comparison showed clearly that the point
tip provides a better resolved MFM image. This observation is consistent with a theoretical
analysis which considered the point tip response as a function of the tip geometry and the tipsample spacing. [15,2] This analysis shows that the point dipole tip has a resolution of 0.6r (r
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is the radius of curvature of the apex of the tip), while a conventional tip only has a resolution
of 1.25r.
Ultrafine Tips Prepared Using Focused Ion Beam Lithography
“A probe with a well defined magnetic size can be made by uniform coating of the probe
and post selective micromachining to control the tip taper, mechanical spring constant and
effective magnetic volume.”
Figure 5 shows a scanning electron microscopy image of a very sharp MFM tip that produced
by focused ion beam milling. The tip was first coated with a magnetic film then machined by a
focused ion beam source, so that there is a nanometer-size magnetic particle on the very end of
the tip. The smaller size of the magnetic particle result in much improved lateral resolution. A
magnetic probe can be fabricated by coating an AFM tip made of Si3N4 or Si with an appropriate
material of high permeability. Such a probe can be successfully used for MFM. But since an
AFM probe consists of a structure with dimensions of many micrometers having a sharp point,
the inherent resolution for MFM is not high - the magnetic material covers too large an area. If
such a tip were fabricated and then machined using a high resolution (< 0.05 micrometer) FIB, it
is possible to remove the magnetic material everywhere except at the apex of the probe, as shown
in Fig.4. This has to be done with great care to avoid damaging the magnetic material at the tip
apex, and we believe it can be done much better by developing a coating the Si 3N4 or Si probe
with a "stop-layer" of material having a thickness of only a few tens of nm, so that the tip can be
imaged at high resolution with the FIB prior to the micromachining step. The stop-layer can
subsequently be removed by chemical means after the micromachining step.
The figure below shows an example a sharp probe we created using FIB. The original probe
was pyramidal with facets (having half angles of 25o front, 17o side and 10o front) which tapered
to a point roughly 30 nm. The probes are batch-fabricated using selective etching techniques, so
that the resulting facets are more or less imposed by the crystallographic property of the material.
It is quite difficult to prepare probes that have arbitrary geometry, e.g., very high aspect ratios.
However, as shown in the figure, a very sharp protrusion can be prepared by ultra high precision
FIB milling of the commercial probe. In this example, the nominal radius of the tip is about 300
nm with a length of about 2 microns and a diameter of about 100 nm.
One would suspect that if a probe were coated with a magnetic material, then it is
conceivable that a small amount of magnetic material would be left at the protrusion, and thereby
satisfy our requirement of small magnetic volume, i.e., an ultra high resolution probe. However,
things are not so simple. In some cases, as our initial results suggest, the MFM contrast are very
different from what one would expect by merely reducing the effective volume size while
keeping the magnetization unchanged. A number of important questions emerged from our initial
work. First, we suspect that the process of material removal changes the magnetostatic energy of
the magnetic film, which causes it to remagnetize in some, thus far, unknown manner. Secondly,
we suppose that a reasonable alternative is to micromachine first and later deposit the magnetic
layer. We have successfully implemented this approach and the results are very promising. In
both cases, the nascent issue is how to control the films under those conditions. The exact nature
of the remagnetization process of micromachined magnetic films and the micromagnetics of
magnetic thin films deposited on protrusions can be understood and potentially controlled, only
through a careful systematic study. Finally, there is the issue of ion implantation. The process of
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FIB causes a certain dose of gallium (or other atoms) to be implanted on the probe. While the use
of FIB micromachining on a coarser (1 micrometer) scale is already being used commercially for
the finishing step in the production of read-write heads for magnetic disk drives, the effects on
the magnetic properties on probes at the submicron length scales are still unexplored. Therefore,
while a major portion of this study will be devoted to determining the relationship between probe
tip morphology and the field distribution associated with the tip, substantial emphasis will be
placed on extending the limits of FIB techniques on magnetic thin films.
As shown in the Fig. 6, the domain configuration of a 150nm-thick epitaxial (110) Fe film
obtained by a MFM tip that was machined by a focused ion beam source. The image was
performed with a vertically magnetized tip in a zero applied magnetic filed to the sample. The
full width at half maximum of the Bloch domain wall width was measured to be 60-70nm that
agrees well with the calculated value for that of bulk Fe. This domain image is clearly has higher
resolution than the recently published results [12]. The above MFM tip innovations undoubtedly
lead to an improved imaging capability. We propose a systematic investigation of these advanced
tip technology that has been demonstrated for high-resolution MFM images to continue to
improve the magnetic properties of the MFM tips by means of a thorough and systematic
investigation of tip-sample interactions.
An essential element of this work is our ability to
fabricate magnetically sensitive probes of very small size.
We will do this using two approaches based on high
resolution focused ion beam technology: (1) direct
micromachining; (2) highly localized (50 nm) implantation.
The first makes it possible to shape a probe tip by controlled
sputtering, using one of our high resolution ion beam
systems focused to between 5 and 50 nm to selectively
remove magnetic material from an underlying Si structure.
The second would make it possible to control the chemical
makeup of a probe by implanting material such as Co or Fe
with an ion beam. Our high resolution implantation system
has a mass selecting filter to allow use of an ion source Fig. 5. An MFM probe machined by a focused ion
containing multiple elements and can focus an ion beam to beam source.
50 nm spot size.
Material Removal Using Focused Ion Beam
Removal of material from a surface by
sputtering with an ion beam is a well known
phenomenon. With the development of the liquid
metal ion source (LMIS) in the 1970's and early
1980's it became possible to perform sputtering in a
controlled manner at high resolution. The LMIS is
a electrohydrodynamic ion source of very simple
construction in which a tiny cone of liquid metal is
created from a liquid metal coating a needle-shaped
Fig. 6. The MFM domain image of a 150nm-thick epitaxial (110)
Fe film obtained by a MFM tip that was machined by a focused
substrate. The conical end shape of the liquid
ion beam source. The arrows indicate the magnetization
direction. 180o Bloch domain wall is clear observed. The full
residing on the needle is a result of a global balance
width at half maximum of the Bloch domain wall width was
measured to be 60-70nm that agrees well with the calculated
value for that of bulk Fe.
11
of electrostatic stress and surface tension forces. Ion emission takes place from the end of the
liquid cone, where the electric field is very high (~ 108 V/cm), by means of field evaporation
followed by ionization. The effect is an almost point ion source that has a brightness > 106
A/cm2 sr and is highly space charge dominated.
The apparent source size - the size one “sees” looking back through an ion focusing system - of
an LMIS is ~ 50 nm, which is about an order of magnitude larger than the physical size of the
cone apex. In addition, the energy spread in the beam is  5 eV. These effects are due to
stochastic space charge (mutual repulsion of the ions). Nonetheless, with a simple optical system
consisting of only two electrostatic lenses it is possible, in fact fairly routine, to produce a
focused ion beam with dimension of 10 nm or even less.
In order for a LMIS to work the metal must have certain properties, including low vapor
pressure at the melting point, high surface tension and chemical compatibility with the substrate
needle that supports it. The most commonly used metal is Ga, which has the additional
advantage of carrying enough mass to be an efficient sputtering ion. It is also possible to create a
LMIS from a relatively low melting point eutectic alloy, for example FeGe or NdCo. Then, by
using a mass filter such as crossed electric-magnetic fields, it is possible to select one component
of the ion beam and so produce a pure beam of ions of the desired component, Fe+, e.g.
A typical focusing column used with a LMIS is limited by space-charge effects in the ion
beam, spherical aberration and chromatic aberration. Consequently there is a trade-off between
beam current and beam size. Typical performance would be a spot size d ~ 10 nm corresponding
to a Ga+ beam current of 5 pA ranging up to a spot size of 0.5 micrometers corresponding to a
beam current of 10 nA.
A focused ion beam (FIB) system operated in the energy range 30 - 50 keV with Ga+ ions can
be used to perform precision sputtering that can create structures on a sub micrometer scale with
10 nm precision, a process called micromachining. The sputter yield of Si, for example, is about
4 atoms/ion, and the corresponding removal rate is ~ 1 cubic micrometer per second per
nanoampere of beam current. The process is very analogous to macroscopic machining with a
milling machine in a machine shop, except on a scale reduced by a factor of ~104. When the FIB
is used at higher energies (~ 100 keV) the sputter rate decreases and ions are implanted with little
sputtering. This process was originally used to make novel semiconductor devices and is called
direct implantation, because it does not require the use of resists and lithography to define the
area to be implanted. Thus, by choosing the appropriate energy range and ion species, it is
possible to modify the properties of materials by either changing the shape or changing the
chemical composition.
We have found that if a magnetic probe is fabricated by coating a silicon nitride AFM tip
with a thin (< 100 nm) layer of magnetically hard material, followed by a thin (~100 nm) coating
of Al, than the tip can be imaged with the ion beam, using the FIB as a scanning ion microscope,
and areas selected for micromachining without damage to the magnetic layer. The selected areas
can then be rastered with the ion beam to effect the milling. In this way probes at least as small
as 50 nm can be made. To our knowledge no one has yet tried to fabricate a probe for MFM by
means of direct implantation. We plan to study the practicality of fabricating probes that have
various doping profiles of magnetic materials in order to determine whether this approach will
allow them to be made more simply than by micromachining.
12
C. Quantification and Characterization of Advanced MFM tips
“We propose to study tip-sample interactions by systematically varying the thickness of
magnetic layer on the tip and the aspect ratio. We will fully characterize the tips and their
coatings in relation to canonical field generating structures to develop a more complete
quantitative understanding of the magnetic images.”
As mentioned above, one of the difficulties in interpreting MFM images is the lack of
fundamental understanding of the magnetic moment of the probe. Under this project, we will
develop methods for tip calibration by imaging well defined current loops, whose magnetic
field distribution can be calculated. An example of this approach is shown in figure 6, where
the same probe is used in electrostatic and magneto static imaging modes[17]. The bright
dark patterns of the left image corresponds to the magnetic field emanating from the 10
micron size wire while the contrast on the left is proportional to the electrostatic forces
between the wire and the probe itself. The electric and magnetic field distributions from
simple structures such as these can be calculated with a very high degree of accuracy. Thus,
one can calculate the “ideal” images from these structures and use the experimental data to
deconvolve the raw images and derive the instrument response function. Under this project,
we envisage developing a series of patterned structures with carefully chosen geometries
which would render straightforward deconvolution of the images. Similarly, emphasis would
also be placed on developing algorithms that can efficiently calculate the response function
and provide a convenient way to produce “magnetization” images derived from arbitrary
charge distributions.
Expected Outcome and Potential Impact
The goal of the proposed research is to develop improved methods for understanding and
characterizing the magnetic properties of nanostructured materials. This research should
have significant implications for a variety of advanced technologies, including hightemperature permanent magnets, extremely high-density information storage applications and
magnetic sensor applications. In each of these cases, the atomic-scale structure of the
materials plays a dominant role in the macroscopic magnetic behavior, especially as regards
the thermal and magnetic stability. The magnetic properties of nanostructured materials are
usually controlled by the behavior of the fundamental “magnetic building blocks” - magnetic
grains or clusters - and their interactions. The typical magnetic grain-size of these technology
important materials is in the range of 5-10 nm. To have a better understanding these
magnetic materials, it will be necessary to control and characterize the structure of the
materials on the nanoscale level. We expect that the proposed activities will also have
impacts as follows: The advanced MFM tip can serve as a small and sensitive magnetic
sensor or a local magnetic field source for a variety of applications. It may be possible to use
a similar approach for the improvement of other long-range scanning-probe force microscopy
applications, e. g., to obtain images of ferroelectric domains
13
Fig. 6. Magnetostatic and electrostatic images of a 10 micron wire strip.
Project Mechanics, Manpower and Educational Component
This is a joint collaboration between the University of Maryland and the
University of Nebraska. The thin film fabrication related to tip coating and preparation of
single domain particles will be performed at the University of Nebraska under the
guidance of Prof. Sy-Hwang Liou. The micromachining and mechanical shaping of the
probes will be done at the University of Maryland under the leadership of Prof. Jon
Orloff, and the development of test beds to analyze sensitivity and resolution, the
modeling of instrument response and analysis of MFM images will be conducted at the
University of Maryland under the direction of Prof. Romel Gomez.
The manpower will involve a research associate at the University of Nebraska, a
research associate at the University of Maryland and two graduate students and two
undergraduate students. The post doc at the University of Nebraska will principally
prepare the thin films under various conditions. The post doc at the University of
Maryland will supervise the day to day operation of the project, particularly with regards
to running the focussed ion beam facility and the MFM implementation. The graduate
students will learn and operate the FIB, various MFM set-ups along with peripheral
instruments such as the vibrating sample magnetometer (VSM) for bulk magnetization
studies and electron microscope for structural analysis. They will have very specific
projects that will eventually lead to a doctoral dissertations in electrical engineering. One
will have theses on “the enhancement of MFM resolution using ion milled MFM tips”.
This will involve the understanding of the micromagnetics of magnetic coatings on
asperities and on this basis formulate the necessary conditions for imaging enhancements.
Theoretical models of image reconstruction will be a component of the thesis, along with
design of canonical sample geometries to test the ultimate capabilities of the advanced
probe tips. The other student will concentrate on the studying the magnetic phenomenon
at the nanometric length scales. This will include a first look at the processes occurring at
the interior of domain walls which will be made possible by the enhanced resolution of
14
the imaging technique. It will augment upon the results on the submicron regime that our
group had already acquired on samples such as spin valves, giant magnetoresistive
multilayers and metal alloys. The influence of nanometer size features such as walls and
magnetization ripples on the performance of sensors will be established. These
experiments will become possible with the anticipated improvements in MFM capability.
The main benefit of this work to the undergraduate students is to expose them to
both the excitement and difficulty of doing research, while teaching them practical skill
such as data acquisition, elements of electronic and mechanical design, data classification
and organization. Of course they will also experience hands on training on some of the
most sophisticated imaging tools.
Overall, the students involved in this project, graduate and undergraduates alike
will develop varying proficiency in electronics, machining, computer interfacing and
mathematical methods --- general skills that improve their success in being
experimentalists of the future. Naturally, by the time the Ph.D. students graduate, they
will have become intimately familiar with the arcane details of surface magnetism, which
have been derived from their own research and the courses they have taken. These
individuals would have attained considerable expertise to fill the grossly underhanded
magnetics industry in the United States.
Support is requested for 4 years, and the time line towards its completion is shown
below. The first and a half will be devoted to exploration of thin film coating on tips and
solid surfaces. A practical parameter space which relates the coercivity and magnetic
moment with chemistry, substrate temperature and deposition parameters (such as
magnetron sputtering pressure) and thickness will be made under this phase. The starting
point of development are those films that show great promise on the basis of our
preliminary studies outlined in Section A. In the thin coating on silicon tips, the main
emphasis will be on establishing the relationship between the film thickness (which
determines the interaction force) and coating morphologies and the imaging signal to
noise ratio. These strategies will be used on both conventional MFM probe tips as well as
micromachined tips which will be prepared concurrently under this phase. The second
phase, which commences roughly in year 2, we will now develop the mathematical
models that describe the performance and image formation using the special probes. At
the same time, some sample structures such as nanometer sized current loops and other
test beds will be developed to relate the models with experiments. By this time, we
anticipate that several advanced tip prototypes would be available, and we would use
these tips as the starting point for our models. The last phase of this effort will be
devoted to extensive testing as well further refinement of our probe fabrication procedure
to obtain reliable probes and increased fabrication yield. Finally, we will study domain
dynamics of technologically important films and hopefully show new features of
micromagnetism made possible by using advanced high sensitivity and high resolution
tips.
Table 1. Time-line for project performance.
15
Industrial Collaboration
This project will be conducted with - Digital Instruments/Veeco - which is the
leading MFM manufacturer in the world. In our agreement, the company will provide
support in by supplying conventional probe tips which will be the starting point of our
developments. They will also provide expertise in MFM implementation, in particular,
Dr. Roger Proksch of DI/Veeco, will closely interact with the team in matters repated to
magnetic dissipation microscopy. In exchange, they will have the right of first refusal
towards any potential commercialization or licensing of the technology of fabricating the
improved tips. Since DI instruments are the most widely used instruments, a close
collaboration with them is in line with our desire to enhance MFM capability for the
largest set of scanned probe microscope scientific researchers.
16
V. REFERENCES:
1.Y. Martin and H. Wickramasinghe, “Magnetic Imaging with 1000 A Resolution”,
Appl. Phys. Lett. 50, 1455 (1987).
2. P.Grutter, H.J. Mamin and D. Rugar, “Magnetic Force Microscopy” in Scanning
Tunneling Microscopy II, R. Wiesendanger and H.-J. Guntherodt, eds. Springer
Verlag: Berlin, 1992
3. Sloncjewski, or Hubert
4. R.D. Gomez, T.V. Luu, A.O. Pak, I.D. Mayergoyz, K.J. Kirk and J.N. Chapman
“Domain wall motion in micron-sized Permalloy elements”, Journal of Applied
Physics 85 (1990), in press.
5. R.D. Gomez, T.V. Luu, A.O. Pak, I.D. Mayergoyz, K.J. Kirk* and J.N. Chapman,
Domain wall motion in micron-sized Permalloy elements Journal of Applied
Physics 85 (1990), in press.
6. Scheinfien
7. Proksch and Grutter, Dissipation
8. S. H. Liou, C. L. Chien, “Granular Meter Films as Recording Media”, Appl.
Phys. Lett., 52, 512(1988)
9. S.S. Malhotra, Y. Liu, J. X. Shen, S. H. Liou and D. J. Sellmyer; ``Thickness
Dependence of the Magnetic and Electrical Properties of Fe:SiO2 Nanocomposite
Films”, J. Appl. Phys., 76, 6304 (1994)
10. S. S. Malhotra, Y. Liu, Z. S. Shan, S. H. Liou, D. C. Stafford, and D. J. Sellmyer;
“High Coercivity PrCoCr Thin Film for Longitudinal Magnetic Recording Media”,
J. of Magnetism and Magnetic Materials, 161, 316(1996).
11. S. H. Liou, Y. Liu S. S. Malhotra, M. Yu and D. J. Sellmyer; ``Magnetic
Properties of Nanometer-Size CoPt Particles'', J. Appl. Phys. 79, 5060(1996).
12. S. H. Liou, S. S. Malhotra, John Moreland and P. F. Hopkins,“High Resolution
Imaging of Thin-Film Recording Heads by Superparamagnetic Magnetic Force
Microscopy Tips”, Appl. Phys. Lett., 70, 135(1997).
13. S. H. Liou, and Y. D. Yao, "Development of High Coercivity Magnetic Force
Microscopy Tips" to appear at J. Magn. and Magn. Mater. 1998
14. Paul Rice and Stephen E. Russek, and Bill Haines, “Magnetic Imaging
ReferenceSample”, IEEE Trans on Magn. Vol. 32, 4133(1996).
15. D. Rugar, H. J. Mamin, P. Guethner, S.E. Lambert, J.E. Stern, I. McFadyen, T.
Yogi; J. Appl. Phys. 68, 1169(1990).
16. A.D. Kent , U. Ruediger, J. Yu, S. Zhang, P. M. Levy, Y. Zhong, S. S. P. Parkin;
"Magnetoresistance due to Domain Wall in Micron Scale Fe Wires with Stripe
Domains", IEEE Trans. on Magn. 34, 900(1998).
17. R. D. Gomez, A.J. Anderson, A.O. Pak, E.R. Burke, A.J. Leyendecker,
and I.D. Mayergoyz” Quantification of MFM images using combined electrostatic and
magnetostatic imaging”, Journal of Applied Physics 83, 6226-6228 (1998)
18. K. L. Babcock, V. B. Elings, J. Shi, D. D. Awschalom, and M. Dugas, “Fielddependence of Microscopic Probes in Magnetic Force Microscopy”, Appl. Phys. Lett.,
69, 705(1996).
17
19. Roger Proksch, George D. Skidmore, E. Dan Dahlberg, Sheryl Foss, J. J. Schmidt,
Chris Merton, Brian Walsh, and Matt Dugas, “Quantitative Magnetic Field
Measurements with the Magnetic Force Microscope”, Appl. Phys. Lett., 69,
2599(1996).
18
Item
Budget for the project at the University of Nebraska
Funds
Requested
From NSF
1. Personnel
Research Associate (1)
2.
Total Salaries
Fringe Benefits (FB) 23%
3.
Total Salaries and FB
3.
Undergraduate Assistant (1)
34,000
34,000
7,820
41,820
6,000
Total Personnel Cost
4.
5.
Travel
Other Direct Costs
a. Materials and Supplies
Liquid He
Materials (Substrates etc.)
Operational Expense
b. Publication Costs
c. Other (Machine Shop, etc)
47,820
4,000
2,000
6,180
500
500
1,500
Total Other Direct Costs
10,680
6.
7.
Total Direct Costs
Indirect Cost 44%
62,500
27,500
8.
Total Request
90,000
19
Nebraska
Mel:PostDoc
Gomez(4wk)
1 grad stud
Tuition
Matls
Shipping
Publication
Orloff (3wks)
1 grad stud
Tuition
FIB supplies
Year 1
Year 2
Year 3
Year 4
Totals
90000
90000
90000
90000
90000
360000
30000
56322 58574.9 60917.9 63354.6 239169.3
6500
12203.1 12691.2 13198.9 13726.8 51820.02
12000
22528.8
23430 24367.2 25341.8 95667.74
6000
6000
6240
6489.6 6749.18 25478.78
1900
1500
1500
0
0
3000
1000
500
500
0
0
1000
1200
0
0
1200
1200
2400
6346.15 11914.27 12390.8 12886.5 13401.9 50593.52
12000
22528.8
23430 24367.2 25341.8 95667.74
6000
6000
6240
6489.6 6749.18 25478.78
10000
10000
10000
10000
10000
40000
239497
244997
249917
255865 990275.9
IX. Budget Descriptions
University of Nebraska
1. Personnel :
The personnel who will be responsible for this program include the principal
investigator, a research associate, and an undergraduate research assistant. The principal
investigator, S.H. Liou, will spend 10% of his time on this project. The research
associate will work on the magnetic coating the characterization of magnetic properties of
films, evaluation of magnetic force microscopy tips. He will also coordinate between
groups at the University of Maryland and at the University of Nebraska. The
undergraduate research will help with the magnetic domain images and prepare samples
for scanning probe microscopy studies.
2. Fringe Benefits and Indirect Costs:
Fringe benefits are 23% and indirect costs are 44%.
3. Travel
Travel funds will cover partial expenses for faculty and students to communicate between
groups at the University of Maryland and at the University of Nebraska and attend
professional meetings and regional meetings.
4. Other Direct Costs
These funds will be used for liquid helium, liquid nitrogen, shop materials, sputtering
materials, publication costs, etc.
5. Requested Budget
The requested budget for the project at the University of Nebraska is $90,000 per year.
VI. FACILITIES:
20
1.
2.
3.
4.
5.
6.
Nebraska:
The PI’s of this proposal have a wide variety of deposition systems and
characterization tool for making nanostructured materials and for characterizing
them structurally, magnetically, and electronically. A brief list of existing
capabilities follows.
Two multiple gun sputter systems
Four magnetometers for measuring magnetization, (Quantum Design SQUID,
Lake
Shore AC/DC susceptometer, Lake Shore High temperature virbration sample
magnetometer, Micro-Mag alternating gradient force magnetometer). Samples
can be studied at temperatures from 5 K to 400 K in fields up to 9 T.
Zeiss polarizing microscope with CCD camera for magnetic domain observations.
The ultimate resolution of this system is 0.5 microns.
Atomic force microscopy (Nanoscope IIIa, with frequency detection module for
MFM)
CMRA Central Service Facilities: (a.) X-Ray Materials Characterization Facility,
Electron Microscopy Facility; a EOL2010 TEM, a JEOL JSM840A SEM. The
Department of Physics maintains a well-staffed machine shop, electronic shop and
there is a glass-blower in the Department of Chemistry as well as another machine
shop to which we have access.
University of Maryland
Prof. Gomez' Research Lab:
1. One thermal high vacuum evaporation system, one two target high vacuum
sputtering system with precision gas and thickness monitor.
2. One Digital Instruments Nanoscope II scanning tunneling and contact atomic
force microscope.
3. One Digital Instruments Nanoscope III Multimode Phase Detection
Atomic/Magnetic Force Microscope.
4. One Digital Instruments Nanoscope Dimension 3000 Phase Detection Large
Sample Atomic/Magnetic Force Microscope
5. One Omicron XP ultra-high vacuum surface science system equipped with a
gate valve connected preparation and analysis chambers. Preparation chamber
consists of sample heating stage to 1200 C, ion bombardment for sample
cleaning, two e-beam evaporation guns. Diagnosis chamber includes a
dedicated UHV scanning tunneling microscope and non-contact force
microscope for MFM studies in ultra high vacuum.
6. Two vibrating sample magnetometers, one capable of variable temperature
magnetization measurements.
7. One Guzik spin stand with MR head recording studies.
8. Two high resolution optical microscope with polarization capabilites for
domain observations.
21
9. Various programmable voltage and current sources with asynchronous
communication interfaces.
10. Several Pentium class workstations for data acquisition and analysis.
Prof. Orloff's Research Lab: (Jon writes)
VII. ORGANIZATION/EDUCATION/COLLABORATIONS
Nebraska:
The proposed project will benefit the research group in magnetism at the
University of Nebraska. This group has 9 faculty and about 25 postdoctoral, graduate
and undergraduate researchers. Many of our graduate and undergraduate students and
postdoctoral are already employed in the magnetic storage industry. For example, F.
Foong (Liou’s student) are with Applied Magnetic Inc.; S.S. Malhotra and D. C.
Stafford (Liou’s students) are with HMT Technology Corporation; A. Tsoukatos
(Liou’s postdoctoral) is with the Materials Research Corporation. It is worthwhile to
note that we are also collaborating with researchers at IBM, Digital Instruments Inc.,
Park Scientific Inc., Quantum Peripherals of Colorado, the National Institute of
Standards and Technology and the University of Maryland. The research proposed
here will also have great impact on the development of advanced MFM tips which
will be used by collaborators at national laboratories , companies and other
universities.
University of Maryland:
The PI's are members of several worldclass centers at the University of
Maryland which are directly working in similar problems in magnetics. Prof.
Gomez is a member of the Maryland MRSEC program on Thin films and
Surfaces directed by Prof. Ellen Williams of the Department of Physics. Various
joint collaborative projects are currently underway, including microscopy studies
on metal oxides and silicon surfaces. In addition to the educational goals on
training of graduate students outlined above, we will also piggyback on the highly
successful educational outreach program of the MRSEC. Specifically,
mentorships at the precollege levels, REU's, "Physics is Phun" and pedagogical
tools such as "research nuggets: on the web" will supported by students and PI's in
this research project. Prof. Orloff is the director of the …..
VIII. CREDENTIALS OF PARTICIPANTS
Liou
22
Liou’s research interests include nano-engineering of magnetic films and
studies of interfaces and particle size effects, applications of scanning probe
microscopy. He has more than 140 articles in refereed journals and books, 34
invited presentations, and 1 patent. Liou has worked as a guest research scientist
in the group of nanoprobe imaging for magnetic technology at the National
Institute of Standards and Technology for one year. He is currently collaborating
with groups at Digital Instruments Inc., Park Scientific Inc., IBM, Quantum
Peripherals of Colorado, the National Institute of Standards and Technology and
the University of Maryland in the developing advanced MFM tips
23
Sy-Hwang Liou
Department of Physics and Astronomy and Center for Materials Research and Analysis
University of Nebraska, Lincoln, Nebraska, 68588-0111
Tel. 402-472-2405, Fax: 402-472-2879, E-mail: sliou@unlinfo.unl.edu
Educational Background
1974 B.Sc. in Physics, Soochow University, Taiwan
1979 M.Sc. in Physics, Florida Institute of Technology, Melbourne, Florida
1981 M.Sc. in Physics, Johns Hopkins University
1985 Ph.D. in Physics, Johns Hopkins University
Honors and Awards
Outstanding paper of the Center for Electronics and Electrical Engineering (NIST) 1988
Professional Experience
1995-1996: Guest Research Scientist, NIST, Colorado
1993-present: Associate Prof. of Physics, University of Nebraska
1990-1994: Co-Editor, Applied Physics Communications
1988-1993: Assistant Prof. of Physics, University of Nebraska
1986-1988: Post-doctoral Staff, AT&T Bell Laboratory
1985-1986: Post-doctoral Fellow, Johns Hopkins University
Research:
 Nano-Engineering films - fundamental studies of interfaces and particle size effects.
 Magnetic Force Microscopy - magnetic domain images and advanced MFM tips for
improving magnetic images and local probes.
 High Tc oxide superconductors and magnetic oxides – microstructural
characterization, static and dynamic properties of flux motion, and growth of complex
oxide compounds.
Author of more than 140 articles in refereed journals, 30 invited presentations, and 1
patent.
Selected Publications Related to This Proposal:
1. “High Resolution Imaging of Thin-Film Recording Heads by Superparamagnetic
Magnetic Force Microscopy Tips”, S. H. Liou, S. S. Malhotra, John Moreland and P.
F. Hopkins, Appl. Phys. Lett., 70, 135(1997).
2. “Magnetization Reversal Behavior in Cobalt Rare-Earth Thin Films”, S. S. Malhotra,
Z. S. Shan, D. C. Stafford, S. H. Liou and D. J. Sellmyer; IEEE Trans. on Magn., 32,
4019(1996).
3. “Magnetic Properties of Nanometer-Size CoPt Particles’’, S. H. Liou, Y. Liu S. S.
Malhotra, M. Yu and D. J. Sellmyer; J. Appl. Phys. 79, 5060(1996).
4. “Magnetic Properties and Magnetization Reversal of CoSm//Cr Thin Films”, Z. S.
Shan, S. S. Malhotra, S. H. Liou, Y. Liu, M. J. Yu and D. J. Sellmyer, J. of
Magnetism and Magnetic Materials, 161, 323(1996).
5. “High Coercivity PrCoCr Thin Film for Longitudinal Magnetic Recording Media”, S.
S. Malhotra, Y. Liu, Z. S. Shan, S. H. Liou, D. C. Stafford, and D. J. Sellmyer, J. of
Magnetism and Magnetic Materials, 161, 316(1996).
24
6. “Metal-Insulator Composites Having Improved Properties and Method for Their
Preparation”, C.L. Chien, Gang Xiao, S.H. Liou; U.S. Patent 4,973,525 Nov. 27,
1990.
25