Electrochemical and Solid-State Letters, 7 共12兲 G309-G312 共2004兲
G309
1099-0062/2004/7共12兲/G309/4/$7.00 © The Electrochemical Society, Inc.
Effects of High Dose Ni, Fe, Co, and Mn Implantation
into SnO2
Y. W. Heo,a,* J. Kelly,b D. P. Norton,a A. F. Hebard,b S. J. Pearton,a,**,z
J. M. Zavada,c,*** and L. A. Boatnerd
a
Department of Materials Science and Engineering and bDepartment of Physics,
University of Florida, Gainesville, Florida 32611, USA
c
U.S. Army Research Office, Research Triangle Park, North Carolina 27709, USA
d
Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37813, USA
The effects of high dose (3 ⫻ 1016 cm⫺2 ) implantation of Ni, Fe, Co, or Mn ions into bulk, single-crystal SnO2 substrates carried
out at substrate temperature of ⬃350°C to avoid amorphization of the implanted region on the magnetic properties of the material
are reported. X-ray diffraction showed no evidence of secondary phase formation in the SnO2 . The Mn-implanted samples
remained paramagnetic, as also reported for samples doped during thin film growth, but the Fe, Co-, and Ni-implanted SnO2
showed evidence of hysteresis with approximate Curie temperatures of ⬃120 K 共Co and Cr兲 or 300 K 共Fe兲. The carrier density in
the implanted region appears to be too low to support carrier-mediated origin of the ferromagnetism and formation of bound
magnetic polarons may be one explanation for the observed magnetic properties. The much reduced Curie temperature seen in
Co-implanted SnO2 compared to material doped during pulsed laser deposition suggests the residual implant damage degrades the
magnetic properties.
© 2004 The Electrochemical Society. 关DOI: 10.1149/1.1814596兴 All rights reserved.
Manuscript submitted May 25, 2004; revised manuscript received June 21, 2004. Available electronically October 25, 2004.
SnO2 is an n-type electronic oxide that is widely used for liquid
crystal display applications,1 gas sensing,2 ferroelectric transparent
thin-film transistors,3 and windows for solar cells.4 There is also
interest in wide bandgap, transparent oxides for use in spintronic
functions. The use of carrier spin, in addition to charge, appears
promising for a new class of devices such as polarized light emitters,
chips that integrate memory and microprocessor functions, magnetic
devices exhibiting gain, and ultralow power transistors.5-14 The use
of carrier spin in metallic multilayersforms the basis of hard drives
in information storage. The control of spin-dependent phenomena in
electronic oxides or more conventional semiconductors may lead to
devices such as spin light-emitting diodes 共spin-LEDs兲, spin field
effect transistors 共spin-FETs兲, and the spin qubits for quantum
computers.5,6,12 A key requirement in realizing most devices based
on spins in solids is that the host material be ferromagnetic above
room temperature. In addition, it is necessary to have both efficient
spin-polarized carrier injection and transport. For these reasons,
there is interest in developing dilute magnetic oxides and semiconductors which exhibit ferromagnetism upon doping with a few percent of transition metals. In these alloys, a stoichiometric fraction of
the constituent atoms is replaced by transition metal atoms. If
single-phase ferromagnetic material can be achieved, then potentially they can be used to inject spin-polarized carriers.
Theoretical predictions of magnetic ordering temperatures in excess of room temperature for 5 atom% Mn doping in GaN, diamond,
and ZnO have been reported.15 There have been numerous recent
reviews on developments in transition metal-doped GaN and other
wide bandgap semiconductors,13,14,16,17 but far less on ferromagnetism in electronic oxides. Sato and Katayama-Yoshida have covered the first principles design for both semiconductor and oxide
spintronics,18 while reviews have appeared on the experimental status of TiO2 , SnO2 , Cu2 O, and some ZnO.19,20 The initial results on
rutile-phase SnO2 doped with Mn showed paramagnetic behavior,
although the lattice constant changes with Mn concentration suggested it was soluble in the lattice.21 Subsequently, Ogale et al.22
reported a very high Curie temperature 共650 K兲 in Co-doped SnO2
grown by pulsed laser deposition. The magnetic moment was also
very high 共7.5 Bohr magnetron per Co兲.
* Electrochemical Society Student Member.
** Electrochemical Society Fellow.
*** Electrochemical Society Active Member.
z
E-mail: spear@mse.ufl.edu
In this paper we report on the magnetic properties ofNi, Fe, Co,
and Mn implantation into SnO2. In the case of Mn implantation, the
material remained paramagnetic, while the other transition metals
produced ferromagnetic properties up to ⬃120 K 共Co and Cr兲 or 300
K 共Fe兲. Higher Curie temperatures may be possible by doping the
SnO2 during thin film growth, to avoid residual implantation damage that appears to be detrimental to the magnetic properties.
Experimental
Bulk, rutile phase, n-SnO2 crystals with n-type carrier density
⬎1018 cm⫺3 were used in these experiments. The SnO2 crystals
were oriented with the surface normal perpendicular to the SnO2
共110兲 direction. The bandgap of this material is reported to range
from 3.7 to 4.6, depending on the growth conditions and carrier
concentration.4 The room-temperature electron mobility was
25 cm2 /V.s. The samples were implanted at 350°C to avoid amorphization by Fe, Ni, Co, or Mn ions with a dose of 3.1016 cm⫺2 and
energy of 250 keV. In all cases the projected range of ions was
⬃0.15 ␮m. After implantation the samples were annealed for 5 min
at 700°C in an N2 atmosphere to try to remove the ion implantation
damage. The magnetic properties were obtained using a commercially available RF-superconducting quantum interface device
共SQUID兲 共Quantum Design MPMS兲. None of the films showed any
evidence for second phase formation from X-ray diffraction 共XRD兲
measurements.
Results and Discussion
Figure 1 shows the magnetic data from the Co-implanted sample.
Hysteresis was detectable at 100 K 共top兲, but not at room temperature, which is consistent with the temperature dependence of the
difference in field-cooled and zero field-cooled magnetization 共bottom兲. The difference between the two plots eliminates para- and
dia-magnetic contributions and indicates the presence of hysteresis
if the difference is nonzero. Although ferromagnetism is the usual
explanation for hysteresis, spin glass effects, cooperative interactions between superparamagnetic clusters, or superparamagnetism
below a blocking temperature can also be the cause. All of these
effects, however, are magnetic phenomena involving the ordering of
spins, and it is in this sense that we refer to the hysteresis measured
by the ⌬M (T) data as ‘ferromagnetic’. The plot at the bottom of
Fig. 1 has a strong positive curvature and approaches zero at
⬃120 K. Identification of T c from such a plot as the temperature at
which ⌬M (T) equals zero is at best a statement about the disap-
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Electrochemical and Solid-State Letters, 7 共12兲 G309-G312 共2004兲
Figure 1. Evidence of hysteresis loops in the magnetization vs. field curves
for Co-implanted SnO2 at 100 K 共top兲 and temperature dependence of difference in field cooled and zero-field cooled magnetization 共bottom兲.
pearance of hysteresis at the 250 Oe field in which the data are
acquired. Similar data taken at lower fields will have a higher T c .
These rather unconventional shapes in the temperature-dependent
magnetization are common in most DMS materials. This behavior
has been predicted in theories that include the effect of randomness
and disorder on percolating ferromagnetic clusters.24,25 We did not
observe the very high Curie temperatures seen in Co-doped pulsed
laser deposited SnO2 , 22 which suggests that the residual implant
damage is degrading the magnetic properties. This is similar to the
result in Mn-implanted ZnO.23 In addition, we obtained a magnetic
moment of only 0.35 Bohr magneton per Co ion, far below the value
of 7.5 Bohr magneton obtained in the PLD material.22 This also
indicates that we were not getting the degree of Co solubility obtained in the thin film growth.
The Mn-implanted material remained paramagnetic but both the
Fe and Ni-implanted SnO2 showed hysteresis in magnetization vs.
field loops 共Fig. 2兲. Table I summarizes the Curie temperature and
magnetic moment data obtained from these samples. In both cases,
the magnetic moments are quite low and suggest that only a fraction
of the implanted transition metal ions are contributing to the magnetization. The most promising results are clearly from the Feimplantation. It is important to examine the material for evidence of
second phases, as small Fe or iron oxide particles might contribute
to the magnetization. We could not detect any second phases by
Figure 2. Evidence of hysteresis loops in the magnetization vs. field curves
for Fe-implanted SnO2 at 300 K 共top兲 and for Ni-implanted SnO2 at 100 K
共bottom兲.
XRD 共an example is shown for the Fe-implanted SnO2 in Fig. 3兲.
Moreover, we got some preliminary X-ray photoelectron spectroscopy data which indicates the Fe to be in the 2 ⫹ oxidation state,
consistent with the material being single phase. By sharp contrast,
the unimplanted samples showed paramagnetic behavior within the
detection limits of the SQUID system 共Fig. 4兲. A method to confirm
the substitutionality of the implanted ions is extended X-ray absorption spectroscopy 共EXAFS兲. Such measurements and a search for a
polarized carrier population via anomalous hall effect 共AHE兲 or polarized light emission from structures using the implanted SnO2 as a
carrier injection layer are needed to prove the ferromagnetism does
not arise from second phases.
Table I. Curie temperature and magnetic moment data obtained.
Implanted Ion
Curie temperature
共K兲
Fe
Ni
Co
⬃300
⬃120
⬃120
Mn
n/a
Comments
0.2 Bohr magnetons per ion
0.32 Bohr magnetons per ion
0.35 Bohr magnetons per ion;
above room temperature
ferromagnetism in thin films22
paramagnetic
Electrochemical and Solid-State Letters, 7 共12兲 G309-G312 共2004兲
G311
Figure 3. XRD scans for Fe-implanted bulk SnO2 show in both log and
linear versions.
Hall measurements showed that the samples retained n-type conduction, with average carrier density in the low 1018 cm⫺3 range.
This carrier density is generally considered to be too low to produce
carrier-mediated ferromagnetism and a more likely mechanism is
the bound magnetic polaron model.24,25 In this case, many localized
spins due to the transition metal ions interact with a much lower
number of weakly bound carriers, leading to polarons. The extent of
these polarons increases as the temperature is lowered and the transition temperature occurs essentially when the polaron size is the
same as that of the sample. The overlap of the individual polarons
produces long-range interactions and energetically it is favorable for
the spin polarization to develop. This model is inherently attractive
for low carrier density systems such as many of the electronic oxides. The polaron model is applicable to both p-and n-type host
materials and has become the most generally quoted model for magnetism in oxides.
Conclusions
Fe, Co, Ni, and Mn implantation into bulk, single crystal SnO2
produces ferromagnetic behavior in the first three cases. The magnetic moments are low 共0.2-0.3 Bohr magnetons per ion兲 for all of
these transition metal-doped samples, while the Fe-doping looks the
most attractive because it has the highest Curie temperature. This
suggests that Fe-doping during epitaxial growth of SnO2 may be an
attractive method for obtaining higher Curie temperature, due to the
absence of residual implant damage, which most likely degrades the
magnetic properties. Our results are consistent with recent reports
from Coey et al.26,27 who found that room-temperature ferromag-
Figure 4. Magnetization vs. field curves for unimplanted SnO2 at 10 K 共top兲
and temperature dependence of difference in field cooled and zero-field
cooled magnetization 共bottom兲.
netism in both Fe- and Mn-doped SnO2 bulk samples formed by
sintering. In their case, Co-doping also reduced the Curie temperature relative to the other two elements. They proposed a mechanism
of ferromagnetic coupling of transition metal ions via an electron
trapped in a bridging oxygen vacancy to explain the high Curie
temperatures. They also pointed out that use of thin films should
enhance the magnetic properties relative to bulk samples produced
by equilibrium processing.
Acknowledgments
The work at the University of Florida is partially supported by
AFOSR grant under grant no. F49620-03-1-0370, by the Army Research Office under grant no. DAAD19-01-1-0603, the Army Research Laboratory, AFOSR 共F49620-02-1-0366, G. Witt and
F49620-03-1-0370兲, NSF共CTS-0301178, monitored by Dr. M.
Burka and Dr. D. Senich兲, by NASA Kennedy Space Center Grant
NAG 10-316 monitored by Mr. Daniel E. Fitch, and the National
Science Foundation 共DMR 0400416, Dr. L. Hess兲.
The University of Florida assisted in meeting the publication costs of this
article.
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