This paper presents the magnetic behavior of CeO2-δ films doped with two non-magnetic transition metal elements: copper and zinc. High quality films were grown on LaAlO3 (001) substrate using a pulsed laser deposition technique. Detailed structural characterization and magnetic property measurements were performed. Our results showed that Cu-doped CeO2-δ films exhibit room temperature ferromagnetism while Zn-doped CeO2-δ films are non-magnetic.
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…describe the overall significance
of this paper?
This paper reports a study of the
magnetic behavior of Ce1xCuxO2δ
xO2δ films grown on
001) substrates. The films
were prepared by pulsed laser
deposition and were thoroughly
characterized using state-of-the-art
characterization techniques. Though
both the dopants are nonmagnetic
in nature, the doped films showed
very different characteristics. Cu
doped films exhibited ferromagnetic
characteristics, while the Zn doped
films showed diamagnetic behavior.
…describe this work to a
materials science and engineering
professional with no experience in
your technical specialty?
The operation of a spintronic
device depends on the injection
of spin-polarized carriers into the
semiconductors. Presently, much
research is going on to develop
efficient spin injection materials.
However, despite extensive
efforts, efficient injection of spin
polarized carriers into nonmagnetic
semiconductors continues to be a
major hurdle in this field. In this
study, we have investigated cerium
oxide based materials for application
…describe this work to a
There is a pressing need in the field of
spintronics for efficient spin injection
materials. The volumes of research
on diluted magnetic semiconductors
(DMS) reflect this need. To date
extensive research on DMS has been
done, however, there has been far
less research on diluted magnetic
dielectric materials (DMD). In this
paper we are reporting our work on
CeO2 based DMDs.
In the field of semiconductor electronics the progress over the past halfcentury cannot be overstated. Since the invention of the transistor in 1947, there has been unprecedented scientific and industrial progress in this field. Miniaturization is one of the most amazing aspects of this progress because it yields better performance at reduced cost and reduced power consumption. However, the miniaturization of electronic and magnetic devices is becoming increasingly challenging because the physical properties of the materials have become the limiting factors in device fabrication. These difficulties and challenges stem from the operational physics of these devices. When the gate length of transistors decreases, the probability of electron tunneling increases. Thus, there are very real physical barriers to the scaling of complementary metal oxide semiconductor (CMOS) technology to the atomic level. The reality is that simply making the devices smaller can not be used as the primary method of performance improvement, and new methods will need to be developed. One of the possible new approaches to device design is based on the utilization of both the charge and spin degrees of freedom of electrons in device design. This area of research is broadly defined as spintronics.1–6
The operation of a spintronic device depends on the injection of spin-polarized
carriers into the semiconductors. Presently, much research is under way to develop efficient spin injection materials. However, despite extensive efforts, efficient injection of spin polarized carriers into nonmagnetic semiconductors continues to be a major hurdle. Initial efforts to inject spin in semiconductors by using ferromagnetic metal electrodes in direct contact with semiconductors resulted in a very small (~1%) degree of spin polarization of injected carriers.7 For efficient spintronic devices, it is essential to achieve much higher spin injection efficiency.
Some earlier studies showed that spin polarized carrier injection can be achieved by using a thin tunneling barrier layer (spin-filter) of a ferromagnetic dielectric between a nonmagnetic metallic electrode and the semiconductor.8,9 Figure 1 shows a schematic of a tunnel barrier. The barrier height Φavg at
temperatures above the transition temperature(TC) of the magnetic barrier
is shown by the dotted line. Below TC there is an exchange splitting (2Δex) of the conduction band leading to different barrier heights for the spin directions. The difference in barrier heights leads to a different transmission factor for each spin. Given the exponential dependence of the tunnel current on the
barrier height, a high degree of polarization can be achieved in this manner.
Equation 1 gives the tunneling current for a junction with an average barrier height of Φ following the method of Simmons.10 The model described in Equation 1 has been used by Moodera et al. to describe spin dependent tunneling in EuS.11 'Φ' is the average barrier height, 'S' is the barrier thickness, and 'm' is the effective mass of an electron in the conduction band of the contact material.
Equation 1 shows that for even
slightly different barrier heights there
is an exponentially large change in the
current tunneled through the barrier.
Thus even a small exchange splitting
of the conduction band edge due to an
internal magnetic field in the tunnel
barrier can give rise to a highly spin
The extensive studies on the europium
chalcogenides (EuX, X: O, S, Se)
have demonstrated the feasibility of
spin dependent tunneling through magnetically
polarized barriers.12,13 However,
europium chalcogenides exhibit
Tc much lower than room temperature,
rendering them useless for practical device
applications. For real applications,
spin filter material needs to be ferromagnetic
at room temperature.
In 2006, Tiwari et al.4 reported room
temperature ferromagnetism in cobalt
doped CeO2-δ. This work inspired
dozens of other research groups to explore
CeO2 system, and later on ferromagnetism
was reported in Ni and Fe
doped CeO2 also.14,15 Though the above
inventions lead to much excitement
in the materials science community,
a significant debate continues about
whether the observed properties are the
intrinsic property of the material or an
extrinsic property due to some kind of
precipitates etc. The above controversy
becomes still more severe because all
the dopants tried with CeO2 thus far are
magnetic in nature.4,14,15 If even a tiny
fraction of these elements precipitates
out, it can make the whole material appear
to be magnetic.
This paper reports a study of the
magnetic behavior of Ce1-xCuxO2-δ and
xO2-δ films grown on LaAlO3(001) substrates. The films were prepared
by pulsed laser deposition and
thoroughly characterized using state-of-
the-art characterization techniques.
of Ce1-xCuxO2 Thin
X-ray diffraction patterns of the Ce1-
xCuxO2-δ films grown on LaAlO3 (001)substrates are shown in Figure 2a. Diffraction
patterns of the substrate are
also shown. Major peaks belonging to
the (00l) family of cubic cerium oxide
were observed indicating the film is
highly oriented in relation to the substrate.
No secondary phases were observed
in the films. Because the detection
limit for x-ray diffraction (XRD)
is around 1% this does not completely
rule out secondary phases. θ-2θ XRD
patterns for the films were collected on
the Philips X'Pert system and Φ-angle
diffraction patterns for the films were
collected on the Bruker AXS system.
Previous studies of cerium oxide grown
on LaAIO3 using transmission electron
microscopy showed that the films grew
by domain matching epitaxy (DME).4
In this growth mechanism the 
plane of CeO2 aligns parallel to the
 plane of the LaAlO3 substrate.
The c-axis of the substrate and the film
are parallel in this growth mechanism,
and there is a 45° rotation of the film
relative to the substrate. Figure 2b
shows the four-circle high-resolution
x-ray Φ-scan for the Ce0.97Cu0.03O2 film for the (111) plane showing the cubic
symmetry of the fluorite structure. The
peaks spaced at 90° intervals reveal the
cubic symmetry and a highly oriented
Magnetic characterization of the
films showed two different types of behavior.
In the low concentration x=0.03
sample a ferromagnetic response was
observed with a saturated magnetic
moment of ~1 μB/Cu atom as shown
in Figure 3a and b. As can be seen in
the insets, the values of the saturated
moments at 10 K and 300 K are almost
similar indicating a Tc much higher
than room temperature. The coercivity
of the sample at 10 K was found to be
~70 Gauss and the remnant polarization
was approximately 0.2 μB/Cu. A
very different magnetic behavior was
observed in the x=0.15 film as can be
seen in Figure 4a and b. The saturated
magnetic moment is approximately
0.2 μB/Cu atom. There is no observed
coercivity for the x=0.15 film at any
temperature, and the saturated moment
is approximately 5 times lower than in
the x=0.03 film. This type of behavior
is symptomatic of a superparamagnetic
To understand the observed magnetic
behavior of the films, all possible
electronic configurations of Cu ions
in the material must be considered. Cu atoms in unionized state have an
outer shell electronic configuration of
3d104s1, thus, Cu+ and Cu2+ ions are expected
to have 3d10 and 3d9 configurations,
respectively. In the 3d10 configuration,
all of the d electrons are paired;
therefore, a Cu+ ion does not have a
magnetic moment. In the case of Cu2+
ions with a 3d9 configuration, one unpaired
electron is available. This gives
a spin angular momentum of ½ which
can result in a net magnetic moment of
M ~1.73 μB [M =gμB√(S(S + 1)); g =
2, S = 1/2].16 So an observed magnetic
moment of 1 μB/Cu atom in the case of
x=0.03 indicates that 68% of the copper
ions in the film are in the 2+ state.
On the other hand in the case of x=0.03
the observed magnetic moment of 0.2
μB/Cu atom indicates that 10% of the
copper ions in the film are in the 2+
of Ce1xZnxO2δ Thin Films
Figure 5a shows x-ray diffraction
patterns of the Ce1xZnxO2δ samples.
In the x=0 and x=0.03 scans, only reflections
for the (001) family of Cubic
CeO2δ and the LaAlO3 substrate were
observed indicating highly oriented
films. The x=0.15 film showed a minor
peak belonging to a hexagonal ZnO
(101) peak indicating that some degree
of phase separation occurred in this
film. A high-resolution four-circle phiscan
XRD pattern is shown in Figure
5b for x=0 sample. The peaks spaced at
90° intervals show the cubic symmetry
of the system.
The results of the magnetic characterization
for the zinc-doped films are
shown in Figure 6. As can be seen in
this figure a strictly diamagnetic response
was observed in these films.
This can be understood by the fact that
the zinc ions were found to be in the 2+
state using x-ray photoelectron spectroscopy
(XPS) and Zn2+ ions have no
unpaired electrons and thus has no net
Ce1xCuxO2δ :XPS Analysis
Investigating the valence state of the
copper ions in the cerium oxide matrix
is expected to give further insight to
the origin of the films' magnetic properties.
For this XPS experiments were performed. Figure 7a shows XPS data
for the Ce0.97Cu0.03O2δ film for the Cu
2p lines. XPS peaks for the Cu 2p3/2 and
2p1/2 emissions were centered at about
933.0 eV and 953.4 eV, respectively,
with an energy separation of 20.4 eV.
In the case of elemental copper, peaks
would be positioned at 932.6 eV and
951.0 eV with an energy spacing of
18.4.17 Since these energy values are
very different from the experimentally
observed values, the possibility of the
presence of metallic precipitates in the
films is ruled out. If the Cu exists in
Cu1+ state, 2p3/2 and 2p1/2 peaks should
be centered at 932.4 eV and 952.5 eV,
with an energy separation of 20.1 eV.18
On the other hand in the case of Cu2+,
photoelectron 2p3/2 and 2p1/2 peaks
should occur at 933.5 eV and 953.7 eV,
respectively, with an energy separation
of 20.2 eV.18 The experimentally determined
values of the binding energy and
energy separation of Cu 2p photoelectron
peaks in the x=0.03 film show that
Cu ions are present in mixed valence
state (Cu2+ and Cu+) with the majority
of ions being in the 2+ state. The above
finding is in very good agreement with
the analysis of magnetic data. Figure
7b shows XPS data for the Ce0.85Cu0.15
film. In this case the Cu 2p peaks are positioned at 951.5 eV and 931.5
eV with an energy separation of 20.0
eV. Similar analysis, as for the x=0.03
sample, shows that in x=0.15 films also
Cu ions are again present in mixed valence
state but this time the majority of
them are in Cu+ state in good agreement
with the analysis of magnetic data.18,19
An investigation of the magnetic
properties of Cu and Zn doped CeO2
films found that both dopants are nonmagnetic
in nature, but the doped films
showed very different characteristics.
Cu doped films exhibited ferromagnetic
characteristics, while the Zn doped
films showed diamagnetic behavior.
- S. Das Sarma, American Scientist, 89 (2001), pp.
- S.A. Chambers, Materials Today, 4 (2002), pp. 3439.
- I. Malajovich, J.J. Berry, N. Samarth, and D.D.
Awschalom, Nature, 411 (2001), pp. 770772.
- A. Tiwari, V. M. Bhosle, S. Ramachandran, N. Sudhakar,
J. Narayan, S. Budak, and A. Gupta, Applied Physics
Letters, 88 (2006), pp. 142511 13.
- S. Ramachandran, Ashutosh Tiwari, and J. Narayan,
J. Electron. Mater., 33 (2004), p. 1298.
- M. Snure, D. Kumar, and A. Tiwari, JOM, 61 (6)
(2009), pp. 7275.
- G. Schmidt, D. Ferrand, L.W. Molenkamp, A.T. Filip,
and B.J. Van Wees, Physical Review B, 62 (2000),
- X. Hao, J.S. Moodera, and R. Meservey, Physical
Review B, 42 (1990), pp. 82358243.
- R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G.
Schmidt, A. Waag, and L.W. Molenkamp, Nature, 402
(1999), pp. 787790.
- J.G. Simmons, J. App. Phys., 34 (9) (1963), p. 2581.
- J.S. Moodera, X. Hao, G.A. Gibson, and R. Meservey,
Phys. Rev. Lett., 61 (5) (1988), p. 637.
- Guo-Xing Miao, M. Mόller, and J.S. Moodera, Phys.
Rev. Lett., 102 (2009), p. 076601.
- A. Schmehl, V. Vaithyanhathan, and A. Herrnberger,
Nature Mater., 6 (2007), pp. 882888.
- A. Thurber, K.M. Reddy, and A. Punnoose, J. Appl.
Phys., 101 (2007), p. 09N506.
- S.K. Sharma, M. Knobel, C.T. Meneses, S. Kumar,
Y.J. Kim, B.H. Koo, C.G. Lee, D.K. Shukla, and R. Kumar,
J. Korean Phys. Soc., 55 (2009), p. 1018.
- N.W. Ashcroft and N.D. Mermin, Solid State Physics
(Fort Worth, TX: Harcourt College Publishers,
- J.G. Jolley, G.G. Geesey, M.R. Haukins, R.B. Write,
and P.L. Wichlacz, Appl. Surf. Sci., 37 (1989), p. 469.
- R.J. Bird and P.J. Swift, Electron Spectrosc. Relat.
Phenom., 21 (1980), p. 227.
19. J.F. Moulder, W.F. Stickle, P.E. Sobol, and K.D.
Bomben, Handbook of X-ray Photoelectron Spectroscopy
(Eden Prairie, MN: Perkin-Elmer, 1992).
Paul Slusser and Ashutosh Tiwari are with the Department
of Materials Science and Engineering,
University of Utah, SLC, Utah 84112, USA; Dhananjay
Kumar is with the Department of Mechanical
Engineering, North Carolina A & T Greensboro,
North Carolina. Dr. Tiwari can be reached at email@example.com.