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Research
Research Archive
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Prof. Kannan M. Krishnan
(Principal Investigator)
Advanced Magnetic and Magneto-Optic Recording Media
The continued increase in recording densities in magnetic
and magneto-optic recording is dependent on the development
of new ultrathin heterostructures and novel materials.
In addition it is simportant to establish appropriate
correlation between growth, magnetic properties, microstructure
and recording performance. Examples of our work in this
area includes the devlopment of new Mn3Ga2 thin films
with perpendicular anisotropy and good Kerr rotation,
epitaxial growth of c-axis oriented Co-Cr perpendicular
recording media and studies of spin-reorientation transitions
in such materials . Current work is on the development
of ordered intermetallics thin films and lithographically
patterned media to overcome the superparamagnetic limit
to magnetic recording.
Publications
Werner Grogger and Kannan M. Krishnan "Scatter
diagram analysis of Cr segregation in Co-Cr based recording
media", IEEE Trans Mag. 37, 1465 (2001)
Werner Grogger, Roger A. Ristau, Thomas T. Thomson,
Samuel D. Harkness, Rajiv Ranjan and Kannan M. Krishnan,
"Quantitative measurements of Cr-segregation in
Co-Cr based magnetic recording media by EFTEM and scatter
diagram analysis" , Appl. Phys. Lett. 80, 1165
(2002)
Kannan M. Krishnan , "Magnetism and Microstructure:
Imaging Techniques and Structure-Property Correlations
in Information Storage Materials", NATO/ASI on
Magnetic recording beyond 2000, Edited by G. Hadjipanayis,
(in press)
G. Kusinski, Kannan M. Krishnan, G. Thomas and E. C.
Nelson, Domain structures and temperature-dependent
spin-reorientation transitions in c-axis oriented Co-Cr
thin films", J. Appl. Phys., 87, 6376 (2000)
. D. J. Rogers, Y. Maeda and Kannan M. Krishnan, "Investigations
of the compositional distribution in epitaxially grown
Co-Cr thin films with enhanced saturation magnetisation",
J. Mag. Mag. Mat. 163, 393 (1997)
. D. Donnet, Kannan M. Krishnan and Y. Yajima, "
Domain structures in epitaxially grown cobalt thin films",
Jour.Phys. D., 28, 1942-50 (1995)
Kannan M. Krishnan, "Enhanced saturation magnetization,
electronic structure and compositional segregation in
epitaxially grown Co-Cr thin films", Appl. Phys.
Lett., 67, 2238 (1995)
LBL-37029
Kannan M. Krishnan, T. Takeuchi, Y. Hirayama and M.
Futamoto, "Epitaxial growth and enhanced saturation
magnetization of Co1-x Crx media suitable for perpendicular
magnetic recording." IEEE Trans. Mag. 30, 5115
(1994).
Kannan M. Krishnan, T. Takeuchi, Y. Hirayama, D. M.
Donnet, Y. Honda and M. Futamoto, "Micromagnetics
and microstructure of epitaxially grown Co and Co-Cr
films suitable for perpendicular magnetic recording",
Jour. Mag. Soc. Jap., 18, S1, 15 (1994)
Kannan M. Krishnan , T. Takeuchi, D. M. Donnet and T.
Tanahashi, "Magnetism and magnetization structure
of c-axis oriented single crystal cobalt films grown
epitaxially on rigid underlayers", Jour. Appl.
Phys., 75, 7579 (1994)
Kannan M. Krishnan , Y. Honda, Y. Hirayama and M. Futamoto,
"Microstructure and epitaxy of c-axis oriented
single crystal cobalt films grown on rigid underlayers",
Appl. Phys. Lett., 64, 1499 (1994)
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)
Magnetically Actuated Shape Memory Alloy Thin
Films
Heusler Ni2MnGa alloys are known to exhibit a martensitic
transition (TM ~202 K), a magnetic Curie temperature
(Tc ~ 376 K) and the shape-memory effect. Both Tm and
Tc are a function of the stoichiometry. Control of the
composition can make the two transitions to coincide
offering the possibility of magnetically driving the
shape memory transition. The associated strains have
vast potential for magnetically actuating microelectromechanical
(MEMS) systems provided the challenge of growing this
material in thin film form with sufficient control of
the stoichiometry / crystallography can be met. In this
context, highly [100] oriented Ni60Mn20Ga20 films, 1000
‰ thick, were grown on NaCl, MgO and Si by ion-beam
sputtering under various growth temperatures. The stoichiometry
and crystal structure of the polycrystalline films were
confirmed to be Ni60Mn20Ga20 and tetragonal ( ao ~ ,
co ~ ). The magnetic properties of Ni60Mn20Ga20 films
depend on the growth temperature and the nature of the
substrate. Ferromagnetic properties at room temperature
were found only in the films grown above 300oC for all
the substrates, whist the films grown below 300oC do
not show ferromagnetism due to its poor crystallity.
The films on NaCl and MgO substrates show an increasing
magnetization (Ms) with increasing growth temperature
arising from an improved degree of crystallinity but
does not depend on specific preferred orientation. For
the case of Si substrates, Ms dramatically decreases
for films grown at 400oC after continuously increased
up to 350oC. This is due to the formation of a silicide
between Si substrate and the metal. The maximum specific
magnetization (sm) of the Ni60Mn20Ga20 films was 44.0,
33.0 and 14.66 emu/g for Si ,MgO and NaCl substrates,
respectively. Magnetic domain structures and the crystal
structure of the film (including the shape memory transition)
as a function of temperature observed by in situ electron
microscopy will be presented and compared with similar
measurements of bulk, single crystal Ni2MnGa samples.
Publications
JaePyoung Ahn, Ning Cheng, T. Lograsso and Kannan M.
Krishnan, "Magnetic properties, structure and shape-memory
transitions in Ni60Mn20Ga20 thin films grown by ion
beam sputtering", IEEE Trans Mag. 37, 2141 (2001)
Growth Modes in Complex Oxide Thin Films
The understanding of the microscopic processes involved
in epitaxial growth and its impact on fundamental properties
of ultra-thin films has long been a central problem
in surface and materials physics. For example, many
fundamental studies of high-Tc superconductors are based
on measurements of ultrathin layers, where a one unit-cell
thick layer constitutes an attractive system to investigate
dimensionality effects in vortex matter or the nature
itself of high-Tc superconductivity. However, even though
the possibility of growing a single unit cell, atomic
layer-by-layer, coherently over large lateral distances
has been debated in recent years, it still remains an
open question. In this context, since the physical properties
of ultrathin layers might be seriously influenced by
the specific growth mode, a major point of interest
is to find the growth mechanism of complex oxides and
the basic structural growth unit. To resolve this long-standing
but fundamental issue of growth modes, we present a
consistent approach based on "ex-situ" quantitative
structural and chemical characterization of epitaxially-grown
ultrathin layers in YBa2Cu3O7-x/PrBa2Cu3O7 superlattices.
From the analysis of superlattices with a non-integer
number of YBa2Cu3O7-x layers, contrary to a layer-by-layer
growth, we provide concrete evidence for a two dimensional
block-by-block growth mechanism in which the growth
units are single perovskite unit cells.
Publications
M. Varela, W. Grogger, D. Arias, Z. Sefrioui, C. Leon,
C. Ballesteros, Kannan M. Krishnan, and J. Santamaria,
"Direct evidence for block-by-block growth in high-temperature
superconductor ultra-thin films, Phys. Rev. Lett., 86,
5156 (2001)
.
M. Varela, C. Ballesteros, W. Grogger, Kannan M. Krishnan,
D. Arias, Z. Sefrioui, C. Leon and J. Santamaria, "High-Resolution
and Energy-Filtered Transmission Electron Microscopy
of YBa2Cu3O7-x/ PrBa2Cu3O7 superlattices" , J.
Alloys and Compounds 323-4, 558 (2001)
Interface Roughness and Giant Magneto Resistance
J. Santamaria, Me. Gomez, M.C. Cyrille, C. Leighton,
Kannan M. Krishnan and I. K. Schuller, " Interfacially
dominated giant magnetoresistance in Fe/Cr superlattices",
Phys. Rev. B. 65, 12412 (2001)
J. Santamaria, Me. Gomez, M.C. Cyrille, C. Leighton,
Kannan M. Krishnan and I. K. Schuller, " Interfacially
dominated giant magnetoresistance in Fe/Cr superlattices",
Phys. Rev. B. 65, 12412 (2001)
M. C. Cyrille, S. Kim, M. E. Gomez, J. Santamaria,
C. Leighton, Kannan M. Krishnan and I. K. Schuller,
"Effect of sputtering pressure induced roughness
on the microstructure and perpendicular GMR of Fe/Cr
superlattices", Phys. Rev. B. 62, 15079-15083 (2000)
M. C. Cyrille, S. Kim, M.E. Gomez, J. Santamaria, K.
M. Krishnan and I. K. Schuller, "Enhancement of
perpendicular and parallel giant magnetoresistance with
the number of bilayers in Fe/Cr superlattices",
Phys. Rev. B. 62, 3361-3367 (2000)
Giant Moment in Fe-N Thin Films
M. A. Brewer, C. J. Echer, Kannan M. Krishnan, T. Kobayashi
and A. Nakanishi, " Magnetic and physical microstructure
of Fe16N2 films grown epitaxially on Si(001)",
Jour. Appl. Phys., 81, 4128 (1997)
M. A. Brewer, Kannan M. Krishnan and C. Ortiz, "Epitaxial
Fe16N2 Films Grown on Si(100) by Reactive Sputtering",
J. Appl. Phys., 79, 5321 (1996).
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