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Varied Synthetic Approaches to
the Development of Room-Temperature Ferromagnetic
and Semiconducting Oxide Nanostructures for Silicon-Based
Spintronics
Funding: NSF/ECS #0224138 (09/15/02-09/14/05)
Project summary:
The transparent semiconductor ZnO
is predicted to be ferromagnetic with a Curie temperature
greater than room temperature when doped with Mn,
resulting in a diluted magnetic semiconductor (DMS).
Experimental work doping ZnO with various transition
metals has resulted in many viable DMS candidates.
We have succeeded in producing DMS superlattice
materials via alternating the deposition of Zn and
Co metals through reactive ion beam sputtering.
Furthermore, we have deposited ZnO and Co from metal
organic (MO) precursors via MO chemical vapor deposition
and produced epitaxial DMS films.
Most recently, we have co-deposited
from bulk ZnO and Cr targets via RF magnetron sputtering
onto sapphire substrates. Experimental and theoretical
studies of Cr as a DMS dopant have, until recently,
given negative results. Our films show ferromagnetic
properties with approximately 1 Bohr magneton per
Cr atom and a Curie temperature above 365 K. Structural
characterization has not yet revealed any secondary
oxide phases or metallic clustering of Cr atoms
that could result in this magnetic response, and
XRD has indicated that the ZnO is oriented. The
ferromagnetic mechanism has not yet determined,
but it is unlikely to be carrier mediated due to
the dielectric behavior of the thin film. Transport
measurements have as-yet been impossible, but co-doping
with Al or other transition metals is next to be
explored and may yield conducting samples and improved
magnetic properties.
Growth and characterization
of Co doped anatase TiO2
diluted magnetic semiconductors by RF sputtering:
Co doped anatase TiO2 is of interest
for a DMS material as recent work has shown ferromagnetic
behavior up to 700K. Research at this stage has
primarily focused on growth of Co:TiO2 by sputter
deposition, followed by characterization of structural
and magnetic properties. Most recent results have
investigated epitaxial Co:TiO2 films with about
2 at% Co, grown by RF magnetron sputtering from
composite oxide targets on matching LaAlO3(001)
substrates. X-ray diffraction (XRD) has shown films
to be highly oriented (001) anatase, while magnetic
hysteresis and thermoremance measurements show room
temperature ferromagnetism in all samples. TEM and
x-ray absorption spectroscopy (XAS) have shown no
evidence of Co metal clusters in the films, with
uniform 2-7 at% Co substitutionally incorporated
into the anatase lattice. Annealing and room temperature
aging lead to an increase of spontaneous moment
an order of magnitude up to ~ 1 µB/Co
atom at 300 K. AFM/MFM studies demonstrate changes
both in surface morphology and distribution of spontaneous
magnetization in the annealed films. Both as-grown
and annealed films were found to be highly resistive.
The origin of ferromagnetism of
transition metal doped oxides remains controversial,
whereas other DMS materials such as Mn doped III-V
and II-VI compounds are understood to exhibit carrier
mediated ferromagnetism. Possible mechanisms of
the ferromagnetic exchange interactions of such
uniform dielectric transition metal-doped oxides
are under investigation. Our final goal is the integration
of such oxide DMS films in silicon based spintronic
devices.
Recent publications:
K. A. Griffin, M. Varela, S. J.
Pennycook, A. B. Pakhomov, Kannan M. Krishnan, “Atomic-scale
studies of cobalt distribution in Co-TiO2 anatase
thin films: Processing, microstructure and the origin
of ferromagnetism,” J. App. Phys., in press
(2006).
K.A. Griffin, A.B. Pakhomov, C.M.
Wang, S.M. Heald, Kannan M. Krishnan, "Intrinsic
Ferromagnetism in Insulating Cobalt Doped Anatase
TiO[sub 2]", Phys. Rev. Lett. 94, 157204 (2005)
Bradley K. Roberts, Alexandre B.
Pakhomov, Vaithiyalingam S. Shutthanandan, and Kannan
M. Krishnan, "Ferromagnetic Cr-doped ZnO for
spin electronics via magnetron sputtering",
J. Appl. Phys. 97, 10D310 (2005)
K. A. Griffin, A. B. Pakhomov,
C. M. Wang, S. M. Heald, Kannan M. Krishnan, “Cobalt-Doped
Anatase TiO2 – a room temperature dilute magnetic
dielectric material,” J. App. Phys., 97, 10D320
(2005).
A. B. Pakhomov, B. K. Roberts and
Kannan M. Krishnan, A. Tuan, V. Shutthanandan, D.
McCready, S. Thevuthasan, and S. A. Chambers,
"Studies of two- and three- dimensional ZnO:Co
structures through different synthetic routes",
Jour. Appl. Phys 95 (11): 7393-7395 (2004).
A.B. Pakhomov, B.K. Roberts and
K.M. Krishnan,
"Transition from granular to dilute magnetic
semiconducting multilayers in ion beam deposited
ZnO/Co", Appl. Phys. Lett. 83 (21): 4357-4359
(2003).
Tuan, A.; Bryan, J.D.;Pakhomov,
A.B.; Shutthanandan, V.; Thevuthasan, S.; McCready,
D.; Gaspar, D.; Engelhard, M.; Rogers, Jr., J.W.;
Krishnan, K.; Gamelin, D.R.; Chambers S.A.; "Epitaxial
Growth and Properties of Cobalt-doped ZnO on ß-Al2O3
Single Crystal Substrates", Physical Rev.
B. 70, 054424 (2004).
Recent oral presentations:
K. A. Griffin, M. Varela, S. J.
Pennycook, A. B. Pakhomov, Kannan M. Krishnan, “Atomic-scale
studies of cobalt distribution in Co-TiO2 anatase
thin films: Processing, microstructure and the origin
of ferromagnetism,” November 2, 2005, 50th
Conference on Magnetism and Magnetic Materials,
San Jose, CA.
K. A. Griffin, M. Varela, S. J.
Pennycook, A. B. Pakhomov, Kannan M.
Krishnan, "Nanoscale studies of cobalt distribution
in Co-TiO2 anatase thin films: Processing, microstructure,
and ferromagnetism," Nanoscale Science and
Technology Workshop, presented September 2005, Seattle,
WA.
K. A. Griffin, A. B. Pakhomov,
C. M. Wang, S. M. Heald, V.
Shutthanandan, S. A. Chambers, Kannan M. Krishnan,
“Cobalt Doped Anatase
TiO2 – a room temperature dilute magnetic dielectric
material for spin-electronics,” Nanoscale Science
and Technology Workshop, presented September 2004,
Seattle, WA.
K. A. Griffin, A. B. Pakhomov,
C. M. Wang, S. M. Heald, Kannan M. Krishnan, “Cobalt-Doped
Anatase TiO2 – a room temperature dilute magnetic
dielectric material,” November 10, 2004, 49th
Conference on Magnetism and Magnetic Materials,
Jacksonville, FA.
Kannan M. Krishnan, Plenary Lecture,
8th Asia-Pacific Electron Microscopy Conference,
Kanazawa, Japan, Nanomagnetism and Spinelectronics:
Properties & Characterization at Relevant Length
Scales, June, 2004
Recent poster presentations:
Kelli Griffin, A.B. Pakhomov, Chongmin
Wang, Kannan M. Krishnan, "Ferromagnetism in
Co doped TiO2 grown by magnetron sputtering,"
presented at APS March 2004 meeting, Montreal, Quebec.
Bradley K. Roberts, A.B. Pakhomov,
Kannan M. Krishnan, ZnO-based
diluted magnetic semiconductors through ion beam
and magnetron sputtering, Workshop on Nanotechnology,
UW (2003).
A.B. Pakhomov, B.K. Roberts, Kannan
M. Krishnan, V. Shutthanandan, D. McCready, S. Thevuthasan,
A. Tuan, and S.A. Chambers, "Phase and dimension
crossovers in magnetic Co/ZnO multilayers,"
presented at MMM-Intermag Joint Meeting 2004, Anaheim,
CA.
June 2003 Spintronics
Mini-symposium Presentations
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