Enhancement of ferromagnetism upon thermal annealing in pure ZnO
S. Banerjee, M. Mandal, N. Gayathri, and M. Sardar
Citation: Appl. Phys. Lett. 91, 182501 (2007); doi: 10.1063/1.2804081
View online: http://dx.doi.org/10.1063/1.2804081
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APPLIED PHYSICS LETTERS 91, 182501 共2007兲
Enhancement of ferromagnetism upon thermal annealing in pure ZnO
S. Banerjeea兲
Surface Physics Division, Saha Insitute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India
M. Mandal
Chemical Sciences Division, Saha Insitute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India
N. Gayathrib兲 and M. Sardar
Material Science Division, Indira Gandhi Center for Atomic Research, Kalpakkam 603 102, India
共Received 10 August 2007; accepted 10 October 2007; published online 29 October 2007兲
We report here the enhancement of ferromagnetism in pure ZnO upon thermal annealing with the
ferromagnetic transition temperature Tc above room temperature. We observe a finite coercive field
up to 300 K and a finite thermoremanent magnetization up to 340 K for the annealed sample. We
propose that magnetic moments can be formed at anionic vacancy clusters. Ferromagnetism can
occur due to either superexchange between vacancy clusters via isolated F+ centers or through a
limited electron delocalization between vacancy clusters. Isolated vacancy clusters or isolated F+
centers give rise to a strong paramagneticlike behavior below 10 K. © 2007 American Institute of
Physics. 关DOI: 10.1063/1.2804081兴
Recently, ferromagnetism 共FM兲 has been observed in un- After thorough stirring and boiling, 1 mmol NaOH was
doped wide band gap semiconductor such as TiO2, ZnO, added slowly in 1 ml aqueous solution. Ethyl alcohol was
HfO2, SnO2, In2O3, Al2O3, CeO2, etc., 1–10 in reduced di- added after refluxing the mixture for 1 h and allowed to
mensional form such as nanoparticle and thin films. Obser- stand overnight. The precipitated ZnO white powder ob-
vation of ferromagnetism in these undoped systems created tained after centrifuging was heated at 50 ° C for 24 h to dry.
more excitement and threw open a wider debate as to the This as-prepared sample was annealed at 900 ° C for 2 h and
origin of magnetism in these wide band gap semiconductors. the resulting powder was faint yellow in color. The structural
The ferromagnetism in these low dimensional systems has characterization and chemical purity of the sample were car-
been attributed to oxygen defects and has been suggested to ried out using x-ray diffraction 共XRD兲, scanning electron
arise due to low dimensionality.9 The saturation moments in microscopy 共SEM兲, energy dispersive x-ray analysis
all these materials are typically 10−4 – 10−3 emu/ g. However, 共EDAX兲, and photoluminescence.14 The particle size was be-
until now, there has not been any explanation of how exactly tween 200 and 500 nm. The magnetic property was mea-
ferromagnetism arises due to the oxygen defects. The nature sured using MPMS-7 共Quantum Design兲. For the zero field
and specific organization of the defects which can give rise cooled 共ZFC兲 data, the sample was cooled down to 2 K in
to ferromagnetism is an important issue. There exists evi- the absence of magnetic field and the data were taken while
dences that, perfect crystallinity and having more oxygen can warming up up to 340 K in the presence of 100 Oe field. The
degrade FM.11 Recently, it has been shown that oxygen va- field cooled 共FC兲 data were taken while warming the sample
cancy occuring in the O共3兲 sites in bulk Ga2O3 can give rise after cooling it to 2 K in the presence of the 100 Oe field.
to ferromagnetism.12 Another observation recently reported The hysteresis data were taken at various temperatures up to
is that ZnO grown by sol-gel technique also shows ferromag- 300 K and up to a field of 1 T.
netic behavior.13 The most interesting observation reported in Figure 1 shows the ZFC and FC magnetization curves
the same paper was that the ferromagnetism of the ZnO was
destroyed upon intentionally doping with magnetic impurity
such as Mn.13 In this letter, we report on the observation of
ferromagnetism in pure ZnO grown by micellar method.
Transition metal doped ZnO is being commonly used for
magneto-optic applications but our experimental results
opens up a different avenue of using pure ZnO itself 共i.e.,
without transition metal doping兲 for magneto-optic applica-
tion. We also give a possible mechanism that we conjecture
for the appearence of ferromagnetism in ZnO in this letter.
ZnO has a simple wurtzite structure where all oxygen
and Zn sites are equivalent. We selected pure ZnO for our
investigation for magnetization measurement. The samples
were prepared by miceller method using ZnSO4 共1 mmol兲
and sodium dodecyl sulphate micelles 共100 ml of 10−2M兲.
FIG. 1. ZFC and FC magnetization curves of the as-prepared sample taken
a兲
Electronic mail: sangambanerjee@saha.ac.in at 100 Oe field showing the distinct bifurcation around 340 K. Insets: com-
b兲
Present address: Material Science Section, Variable Energy Cyclotron Cen- parison of M-H curves of the as-prepared and annealed 共900 ° C兲 samples at
ter, 1/AF Bidhannagar, Kolkata 700 064, India. 300 and 2 K.
0003-6951/2007/91共18兲/182501/3/$23.00 91, 182501-1 © 2007 American Institute of Physics
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182501-2 Banerjee et al. Appl. Phys. Lett. 91, 182501 共2007兲
FIG. 2. ZFC and FC magnetization curves of the 900 ° C annealed sample
taken at 100 Oe field showing the distinct bifurcation above 340 K. Inset:
the thermoremanent magnetization of as-prepared and the annealed samples FIG. 3. M-H loops of the 900 ° C annealed sample at T = 2, 15, 70, and
showing the finite value up to 340 K. 300 K up to 2000 Oe showing the nonsaturation. Inset共a兲: expanded low
field region of the M-H loops showing the finite coersive field. Inset共b兲:
M-H loops at the same temperatures up to 1 T field showing saturation at
least at 300 K.
taken at 100 Oe for the as-prepared sample. The distinct bi-
furcation of the FC and ZFC starts at around 340 K, which is
also seen from the finite thermoremanent magnetization in crease in the ferromagnetic contribution on annealing. This
the inset of Fig. 2. Both the ZFC and the FC data show comparison between the as-prepared and the annealed
paramagneticlike behavior. The insets of Fig. 1 show the samples clearly indicates that the observed enhancement of
M-H data of the as-prepared sample taken at 2 K up to ferromagnet is not due to any magnetic impurity in the
2000 Oe and at 300 K up to 1 T after the necessary back- sample rather it is an annealing effect. These phenomena of
ground diamagnetic subtraction. There is no clear hysteresis enhancement of ferromagnetism upon thermal annealing is
at both temperatures. The 300 K data show an S-shaped be- explained now.
havior with saturation similar to a superparamagnetic behav- One way in which magnetic moments could arise from
ior. There is no saturation of magnetization at 2 K up to atomic defects in ZnO is as follows. In the case of single
2000 Oe. The M-H data in the insets have been compared neutral oxygen vacancy the electronic states at the O vacancy
with those of the annealed sample which will be further dis- can be obtained using molecular orbital theory. At each of
cussed below. the four Zn ions next to an oxygen vacancy, one sp3 hybrid
Figure 2 shows the ZFC and FC magnetization curves of 4s and 4p orbitals can be formed which points towards the
taken at 100 Oe for the annealed sample. The distinct bifur- vacancy. From these four orbitals, one a1 and three t2 共de-
cation of the FC and ZFC starts well above 340 K. It is generate兲 orbitals centered at the O vacancy are formed.
obvious from the inset that the thermoremanent magnetiza- Careful local cluster calculations by many groups gives a
tion has been enhanced significantly upon thermal annealing. splitting of 2.7 eV or more, between a1 and t2 levels.15 We
The ZFC magnetization increases as a function of tempera- do a simple tight binding modeling with four orbitals, with
ture for T ⬎ 10 K, indicating that there are blocked moments an interorbital hopping matrix element of t = −0.7 eV and get
which start to contribute to the magnetization when the tem- the same energy level structure as that obtained from other
perature is increased. The FC magnetization decreases as the more refined calculations.15 So a neutral oxygen vacancy
temperature increases, since the frozen moments start ran- will have two electrons in a1 state, i.e., a singlet with zero
domizing due to thermal energy. This behaviour is seen in spin because this splitting is greater than the Hund’s coupling
most dilute magnetic semiconductor systems. It is interesting energy of ⬃1 eV. Hence, a single neutral oxygen vacancy
to note that there is a sharp increase in the magnetization in will not have any net magnetic moment. Extending this cal-
both the FC and the ZFC data below 10 K. The origin of this culation to vacancy clusters, we have found that for a cluster
strong paramagnetic behavior will be discussed below. with more than three oxygen vacancies, the seperation be-
In Fig. 3 we show the 共M vs H兲 hysteresis curves of the tween the energy levels are lower than the Hund’s coupling
sample at four different temperatures up to 2000 Oe after the energy thus giving rise to a net moment in the cluster. On
necessary background diamagnetic subtraction. We observe annealing, oxygen vacancies can migrate and form clusters
clear hysteresis at all temperatures up to 300 K, as seen in because it reduces the bond energies favoring decrease in the
inset 共a兲 of Fig. 3. The inset 共b兲 shows the hysteresis up to strain field energy.16 Such oxygen vacancy clusters have also
1 T, showing nonsaturation of the magnetization at 2 K. been observed in high temperature superconducting oxides17
Magnetization saturation can be observed for 300 K. upon annealing which shows up as the peak effect in the
The comparison of the M-H data of the annealed and the current density measurements.
as-prepared sample, as shown in the inset of Fig. 1, indicates Here, we will consider a cluster with three near neighbor
that there is an enhancement in the ferromagnetic contribu- oxygen vacancy configuration, with 12 basis states around
tion on annealing. There is a higher coercivity for the an- ten Zn atoms 共of which one Zn atom is shared between three
nealed sample compared to the as-prepared sample at 2 K. oxygen vacancies, which are in the basal plane ⬜ to the z
The saturation magnetization at 300 K is also higher for the axis兲 as shown in Fig. 4共a兲. We assume that the hopping
annealed sample, indicating that there is definitely an in- matrix element between the orbitals 共three in number兲 cen-
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182501-3 Banerjee et al. Appl. Phys. Lett. 91, 182501 共2007兲
magnitude of moments that can arise from oxygen vacancies,
we assume the oxygen nonstoichiometry to be 0.1% 共i.e., one
in 1000 oxygen atom is absent兲. If only one tenth of these
vacancies generate a moment of 1B, then we get a moment
of 4 ⫻ 10−3 emu/ g which is close to the saturation moment at
T = 15 K and 2000 Oe seen by us.
To summarize, we have reported here observation of en-
hancement of ferromagnetic contribution in pure ZnO
sample upon thermal annealing. The as-prepared sample
shows very weak ferromagnetic contribution compared to the
annealed sample. The enhancement of ferromagnetism has
been attributed to formation of oxygen vacancy clusters upon
thermal annealing. The finite thermoremanent magnetization
obtained from the difference between the ZFC and FC data
FIG. 4. 共a兲 Tetrahedral structure of ZnO showing three oxygen vacancies up to 340 K indicates that the ferromagnetic transition tem-
labeled as VO and the orbitals 共labeled 1–12兲 used for calculating the energy perature Tc of the pure ZnO sample investigated to be above
levels for the three oxygen vacancy clusters. 共b兲 The calculated energy lev- 340 K. We argue that large moments can be generated, if one
els showing the occupancy of six electrons 共each neutral oxygen vacancy
has an occupancy of two electrons兲 and hence a net moment arising due to
has vacancy clusters of three or more oxygen vacancies. This
degeneracy of the third and the fourth energy levels 共−0.725 eV兲. observation indicates that if the already existing wide band
gap metal oxides such as ZnO were used, optoelectronic de-
tered about the Zn atom, which is shared by three vacancies vices can be made magnetic 共without transition metal dop-
larger by a factor of 2, i.e., 2t, which is reasonable since ing兲 due to small oxygen vacancy cluster then a different
these orbitals are centered about the same Zn atom, and as- avenue of magneto-optic applications will be opened up.
suming a small matrix element of −0.2 eV between Zn or- The authors would like to thank Professor S. Chaudhury,
bitals surrounding one O vacancy and the Zn orbitals sur- IACS for the PL measurements, Professor A. De for discus-
rounding the neighboring O vacancy, we get energy levels at sion, Mr. Souvik Banerjee for SEM and EDAX measure-
−5.26, −0.739, −0.725 共twofold degenerate兲, 0.7 共sixfold de- ments, and Mr. Sushanta Banerjee for XRD measurements.
generate兲, and 1.625 共twofold degenerate兲, and these are
shown schematically in Fig. 4共b兲. We see that with a small 1
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