Experimental comparison of the structural, magnetic, electronic, and optical properties
of ferromagnetic and paramagnetic polycrystalline Zn
1-x
Co
x
O „x =0,0.05,0.1…
Sasanka Deka, Renu Pasricha, and P. A. Joy*
Physical and Materials Chemistry Division, National Chemical Laboratory, Pune 411008, India
Received 7 March 2006; revised manuscript received 10 May 2006; published 13 July 2006
Ferromagnetism at room temperature is observed in one case whereas paramagnetism down to 12 K is
observed in the second case when polycrystalline Zn
1-x
Co
x
O is synthesized under almost identical conditions.
Identical x-ray diffraction, optical absorption, and x-ray photoelectron spectroscopy results indicate the incor-
poration of Co
2+
ions in the ZnO lattice in both cases. However, electron diffraction studies show the presence
of Co metal impurities in the ferromagnetic samples, indicating that pure Co doped ZnO is not likely to be
ferromagnetic.
DOI: 10.1103/PhysRevB.74.033201 PACS numbers: 75.50.Pp, 75.60.-d, 78.40.Fy
The theoretical and experimental studies on the magnetic
impurity doped semiconductor systems, known as diluted
magnetic semiconductors DMS, raised tremendous hope
for these materials to be used in spintronics.
1
Spintronic de-
vices like spin-valve transistors, spin light emitting diodes,
logic devices, and nonvolatile storage are some possible ap-
plications where DMS materials are proposed to be used.
Ohno showed that the III-V semiconductor GaAs, when
doped with Mn, becomes ferromagnetic with a Curie tem-
perature T
C
of 110 K.
2
In the III-V system, it is generally
accepted that the hole carriers generated by the Mn-doping
acceptor induces ferromagnetic ordering due to the
Ruderman-Kittel-Kasuya-Yosida RKKY interactions. The-
oretical calculations based on the Zener model on II-VI sys-
tems suggested that Mn-doped ZnO would show ferromag-
netic FM behavior with a T
C
300 K, provided the
semiconductor is p-type.
3
Similarly, Sato et al. predicted
from ab initio calculations that hole-doped Zn
1-x
Mn
x
O is
ferromagnetic at room temperature and ZnO doped with V,
Cr, Fe, Co, and Ni will be ferromagnetic without any carrier
doping treatment.
4
Ueda et al. first experimentally observed
that Zn
1-x
Co
x
O is ferromagnetic up to 300 K, but with re-
producibility 10%.
5
ZnO is a transparent wide band gap 3.3 eV semiconduc-
tor and is a promising candidate for UV-laser devices, having
a higher exciton binding energy 60 meV. Motivated by the
theoretical predictions and the first report on ferromag-
netism, much effort has been focused recently on the synthe-
sis and fabrication of room temperature ferromagnetic DMS
materials based on ZnO.
6
There are many reports where
Zn
1-x
Co
x
O is found to be ferromagnetic at room tempera-
ture. But in most of the cases the observed magnetic moment
B
/Co ion is very low, much less than that expected. Con-
troversies started on both the theoretical and experimental
issues regarding the origin of ferromagnetism in the case of
Zn
1-x
Co
x
O. Recently Spaldin showed that additional p-type
carriers are required to obtain ferromagnetic ordering in
Zn
1-x
Co
x
O and the calculated magnetic moment is 3.1
B
per impurity atom which is closer to the value of 3
B
for
Co
2+
ion.
7
Interestingly, some reports stated that ferromag-
netism solely depends on the synthesis conditions.
8,9
Jin et
al. did not observe any ferromagnetism, down to 3 K, for Cr
to Cu-doped ZnO thin films.
10
Antiferromagnetic coupling
has been observed by Yoon et al.
11
at room temperature and
down to 2 K in Zn
1-x
Co
x
O. Presence of Co metal clusters is
shown to be responsible for the origin of ferromagnetism in
the Co-doped ZnO thin films.
12,13
Recent studies on poly-
crystalline samples show that Zn
1-x
Co
x
O is not ferromag-
netic at room temperature.
14–18
All the reports on the studies on magnetic and nonmag-
netic Co doped ZnO are made on samples synthesized by
different methods. In this Brief Report we compare the struc-
tural, magnetic, electronic, and optical properties of ferro-
magnetic and nonmagnetic Zn
1-x
Co
x
O x =0, 0.05, and 0.1
synthesized under almost identical conditions. Though x-ray
diffraction XRD, x-ray photoelectron spectroscopy XPS,
and optical absorption studies showed identical results for
two series of samples, electron diffraction studies show evi-
dence for the presence of Co metal impurities in the ferro-
magnetic samples.
Two batches of polycrystalline Zn
1-x
Co
x
O x =0, 0.05,
and 0.1 powder samples have been synthesized by an auto-
combustion method.
19
The first batch is synthesized using
glycine as the fuel 2 moles of glycine per mole of metal ion
whereas a mixture of glycine and dextrose 2 moles of
glycine+ 1 mole of dextrose was used for the second batch.
For convenience the 0%, 5%, and 10% Co doped samples
are coded as G0, G5, and G10, respectively, for the first
batch and D0, D5, and D10 for the second batch. All the
as-synthesized Co doped samples G5, G10, D5, and D10
were green in color. The powder samples were characterized
by x-ray diffraction Philips 1830, Cu K radiation. Se-
lected area electron diffraction SAED patterns were ob-
tained using a JEOL model 1200 EX transmission electron
microscope operating at 120 kV, camera length of 80 cm,
field limited aperture of 100 m. Electron diffraction ring
patterns were simulated using the computer program
JECP/PCED.
20
Magnetization measurements as a function of
field and temperature were carried out using a vibration
sample magnetometer EG & G PAR 4500. Electronic ab-
sorption spectra were recorded on a JASCO V-570 spectro-
photometer. Room temperature photoelectron spectra were
recorded on a VG Microtech Multilab ESCA 3000 spectrom-
eter using a nonmonochromatized Mg K x-ray source
h = 1253.6 eV.
The powder XRD patterns of the as-synthesized samples
PHYSICAL REVIEW B 74, 033201 2006
1098-0121/2006/743/0332014 ©2006 The American Physical Society 033201-1