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Research
Research Archive
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Prof. Kannan M. Krishnan
(Principal Investigator)
Magnetic Actuation of MEMS Devices
Personnel:
Dr. Alec Pakhomov, Visiting Scientist
Vasilios Tsirimiagos, Graduate Student
Chris Petz, Undergraduate Student
Abstract:
This project was a collaborative effort between Microvision
and the UW. Micro-electro-mechanical system (MEMS)-based
scanners are a natural choice for the scanning mirror
requirements of scanned beam displays. By developing
a MEMS-compatible magnetic film deposition material
and process, actuation forces can be directly applied
to the scanner.
The project team is developed materials and processes
for fabrication of hard micromagnets for magnetic actuation
of MEMS devices. The first phase of the project is focused
on:
- Selection of magnetic materials with parameters consistent
with the device requirements.
- Fabrication of hard magnetic films by appropriate
means.
- Investigation of heat treatments leading to the desired
properties yet compatible with silicon processing.
- Recommendations of optimum film preparation techniques
and patterning methods for Phase II.
Hard magnetic films often suffer from instability over
time and temperature, or incompatibility with certain
MEMS fabrication techniques. The goal of this first
phase of the project was to identify those magnetic
films that overcome these limitations. It is anticipated
that there will be a phase II to this project whereby
the University of Washington and Microvision team will
perform the deposition of the material optimized in
Phase I in thin film form and patterned onto actual
MEMS. Both deposition and patterning will be optimized
to achieve best torque performance.
Coercivity Limits and Mechanism in Nanocomposite
Fe-Nd-B Alloys
Dr. Er. Girt (postdoctoral fellow)
Dr. W. Grogger (postdoctoral fellow)
Dr. D. Crew, U.W. Australia (Collaborator)
Dr. T. Schrefl, U. Vienna(Collaborator)
Prof. Z. Althunian, McGill University(Collaborator)
Prof. G. Thomas, UC Berkeley (collaborator)
Abstract:
We have studied the magnetic behavior of a range of
Fe13.1NdxB (2 < x < 147) alloys as a function
of the Nd content and annealing temperature (400-700ƒC)
and characterized them by a wide range of measurements
including x-ray diffraction, differential scanning calorimetry,
thermogravimetric analysis and electron microscopy.
These annealed samples show a systematic increase in
room-temperature coercivity with Nd content, culminating
in the largest value (~2.8T) ever reported in the Fe-Nd-B
system when x ~ 147. Energy-filtered images, using spatially
resolved measurements of inner-shell ionization edges,
was critical in evaluating the particle shapes (platelets
with the crystallographic c-axis normal to the plate),
size (~100 x 40 x 25 nm) and distribution. For such
randomly oriented, non-interacting particles, the largest
observed coercivity, Hc ~ 2.8 T is ~90% of the theoretical
limit expected for Stoner-Wohlfarth coherent rotation
behavior including demagnetization effects. Initial
magnetization curves and a systematic series of minor
loop measurements show that the coercivity mechanism
changes from the pinning-type at low Nd content, to
the nucleation-type at higher Nd concentrations. In
addition, the samples show a low-temperature spin reorientation
and a peak in coercivity as a function of temperature.
To understand these results we have carried out a systematic
evaluation of the physical microstructure and the distribution
of magnetic spins at the appropriate length-scale.
Publications
D. C. Crew, Er. Girt, M. Guilot, D. Suess, T. Schrefl
and Kannan M. Krishnan, " The effect of magnetic
interactions between grains on reversal behavior in
diluted Nd2Fe14B", PRB (in press)
Er. Girt, Kannan M. Krishnan, G. Thomas and E. Girt,
"Coercivity limits and mechanism in nanocomposite
Fe-Nd-B alloys", J. Mag. Mag. Mat., 231, 219 (2001)
R. C. Woodward, N.T. Gorham, R. Street, D.C. Crew,
E. Girt and Kannan M. Krishnan, "Coevrcivity, time
dependence and reversible magnetisation in Nd-rich Nd-Fe-B
alloys", IEEE Trans. Mag. 37, 2493 (2001)
Er. Girt, Kannan M. Krishnan, M. Guillot, I. P. Swainson,
Z. Altounian, G. Thomas, "Structural and magnetic
properties of Nd2Fe17-xGax (x = 0 - 2)", J. Appl.
Phys. 87, 5323 (2000)
Er. Girt, Kannan M. Krishnan, G. Thomas, Z. Altounian,
"Approaching the theoretical coercivity of Nd2Fe14B
: microstructural evaluation and interparticle interactions",
J. Appl. Phys. (in press)
Er. Girt, Kannan M. Krishnan, G. Thomas, Z. Altounian,
"Nanocomposite Nd-rich Nd-Fe-B alloys: approaching
ideal Stoner-Wohlfarth type behavior", Appl. Phys.
Lett. 76, 1746 (2000)
Y. Zhang, W. Tang, G.C. Hadjipanyis, C. Chen, C. Nelson
and Kannan M. Krishnan, " Evolution of microstructure,
microchemistry and coercivity in 2:17 type sm-Co magnets
with heat treatment ",IEEE Trans Mag. 37, 2525
(2001)
Y. Zhang, W. Tang, G. C. Hadjipanayis, J. Liu, M. S.
Walmer, E. C. Nelson and Kannan M. Krishnan, "Effect
of cell size on magnetic properties and domain structures
of sintered Sm(Co,Cu,Fe,Zr) 2:17 magnets", J. Appl.
Phys. (in press)
. Y. Zhang, M. Corte-Real, G.C. Hadjipanayis, J. Liu,
M. S. Walmer and Kannan Krishnan, "Magnetic hardening
studies in sintered Sm(Co, Cux, Fe, Zr)z 2:17 high temperature
magnets" J. Appl. Phys. 87, 6722 (2000)
M. Benaissa, Kannan M. Krishnan and E.E.Fullerton,
"Magnetic anisotropy and its microstructural origin
in epitaxially grown bicrystal Sm-Co thin films",
IEEE Trans. Mag., 34, 1204 (1998)
Tai Nguyen, Kannan M. Krishnan, L. H. Lewis, Y. Zhu
and D.O. Welch, "Microstructure and composition
in rapidly quenched FeNdB-based hard magnet alloys",
J. Appl. Phys.,79, 4848 (1996)
Patterned Media by Ion-Beam Irradiation G.J.
Kusinski, (Graduate Student)
J. D. Wright (Graduate Student)
D. Weller, IBM/Almaden (Collaborator)
B. Terris, IBM/Almaden (Collaborator)
G. Thomas, UC Berkeley (Collaborator)
R. Ranjan, Seagate (Collaborator)
Abstract
(111) textured (Co0.3nm/Pt1nm)10 multilayers with perpendicular
anisotropy were grown using electron beam evaporation
and were patterned into magnetic arrays by a Ga Focussed
Ion Beam (FIB). The reversal processes of these arrays
were studied by a magnetic soft x-ray transmission microscope
(X-TXM) at a spatial resolution of 25nm. Element specificity
(Co L3 absorption edge) and magnetic contrast due to
magnetic circular dichroism (MCD) were used for the
first time to image such nanometer scale elements. Samples
imaged at remanance, after applying a field of +10kOe,
show significant decrease in intensity for irradiated
regions as a result of the change in magnetization from
out of plane to in plane (Figure 1a). The matrix and
the squares have the same intensity, and the irradiated
lines have lower intensity, demonstrating upward-perpendicular
magnetization and in-plane magnetization, respectively.
Reversing the applied field direction resulted in partial
switching of the 240nm square patterns at fields below
the sample coercivity (HC^ = 6.3kOe). At remanance,
after applying a vertical field of -3.4kOe, the matrix
is still magnetized perpendicular (upward), the FIB
lines are magnetized in-plane but the squares are magnetized
downward at the edges and upward in the centers, as
shown by dark and bright contrast, respectively (Figure
1b). The reversal process appears to originate at the
edges of the patterns and propagate into both the squares
and the surrounding region. The same area imaged at
750eV, (off the Co edge), shows no contrast indicating
no topographical changes due to the patterning. Details
of the magnetizing experiment including the reversal
mechanism for the samples exposed to different irradiation
dose are discussed in the paper. These results are correlated
with the microstructure, which was investigated using
conventional, high resolution and energy filtered transmission
electron microscopy (TEM).
Publications
G.J. Kusinski, K.M. Krishnan, G. Denbeaux, G. Thomas,
B.D. Terris and D. Weller "Magnetic imaging of
ion- irradiation-patterned Co/Pt multilayers using complementary
electron and photon probes', Appl. Phys. Lett. 79, 2211
(2001)
D. Weller. L. Folks, M. Best, E.E. Fullerton, B.D.
Terris, G. J. Kusisnski, Kannan M. Krishnan and G. Thomas,
'Growth, structural and magnetic properties of high-coercivity
Co/Pt multilayers", Jour. Appl. Phys., 89, 7525
(2001)
G.J. Kusinski, K.M. Krishnan, G. Denbeaux, G. Thomas,
B.D. Terris and D. Weller "Magnetic imaging of
ion- irradiation-patterned Co/Pt multilayers using complementary
electron and photon probes', Appl. Phys. Lett. 79, 2211
(2001)
Role of Stoichiometry and Structure in Colossal
Magneto-Resistive Oxide Thin Films
Dr. H. L. Ju (Postdoctoral Fellow)
Dr. A. R. Modak (Postdoctoral Fellow)
Prof. S. Sundar Manoharan (Visiting Scientist)
Dr. H. C. Sohn (Postdoctoral Fellow)
Dr. O.I. Lebedev (Collaborator)
Prof. D. Lederman (Collaborator)
Abstract:
Synthesis by pulsed laser ablation and novel sol-gel
chemical routes. Fundamental investigation of the role
of elemental substitutions, crystallinity, epitaxy,
strain, oxidation state and grain boundaries in determining
their galvanomagnetic properties. Characterization by
electron microscopy and x-ray scattering/dichroism.
Tailored structures to reduce field dependence and submicrometer
magnetoresistive devices are being developed. Current
work is in CMR manganites and new work is being initiated
in half-metallic ferromagnets.
We prepared a range of well characterized samples by
the sol-gel method developed in our laboratory. By careful
doping we controlled the divalent doping (in one set
of samples) and the oxygen content (in the second set).
We measured the temperature dependent magnetotransport
behaviour of these complex oxides. The electronic structure
of these well characterized samples was measured by
electron energy-loss spectroscopy using the newly installed
CM200/FEG microscopes. No discernible change in the
Mn L3,2 edges were observed either as a function of
doping or oxygen content. However, the O-K edge(1s->2p
transitions) intensity, sensitive to the 2p hole density
on oxygen sites, varied systematically with the resistivity
for both sets of samples.
By these measurements, we showed that these manganites
are charge-transfer type insulators with significant
conductivity due to holes on oxygen sites. This provides
new insight into th transport mechanism in these oxide
and suggests that the double exchange model, which implicitly
assumes a Mott-Hubbard type insulator, has to be revised
to include the role of oxygen hole mobility in the transport.
Publications
H. L. Ju, H.C. Sohn and Kannan M. Krishnan, "
Evidence for oxygen 2p hole driven conductivity in La1-xSrxMnO3
and La0.8Sr0.2MnOz CMR thin films", Phys. Rev.
Lett., 79, 3230 (1997)
Kannan M. Krishnan and H. L. Ju, "Role of stoichiometry
and structure in colossal magneto-resistive La1-xSrxMn1-yRuyO3+?
", Phys. Rev (B), 60, 14793 (1999)
H. L. Ju, Kannan M. Krishnan and D. Lederman, "Evolution
of strain dependent transport properties in ultra-thin
LSMO films", Jour. Appl. Phys., 83, 7073 (1998).
S. Sundar Manoharan, H. L. Ju and Kannan M. Krishnan,
" Unusual substitution effect of Ru in bulk and
thin films of La0.7Sr0.3Mn1-xRuxO3" , Jour. Appl.
Phys., 83,7183 (1998)
H. L. Ju and Kannan M. Krishnan, "Effect of vacuum
annealing on oxygen stoichiometry and resistivity in
sol-gel derived films", Sol. Stat. Comm. , 104,419
(1997)
O.I. Lebedev, C. Van Tandeloo, S. Amelinckx, H. L.
Ju and Kannan M. Krishnan, "HREM study of strained
epitaxial La0.7Sr0.3MnO3 thin films, Phil Mag A, 90,
673-691 (2000)
Kannan M. Krishnan, A. R. Modak, C. A. Lucas, H. Baumann
and R. Michel, "Role of epitaxy and polycrystallinity
in the magneto- resistance and magnetization of La0.8Sr0.2MnO3
thin films", J. Appl. Phys., 79, 5169 (1996
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