Defect Induced Ferromagnetism in Undoped ZnO Nanoparticles
Srabantika Ghose1*, R. Ranganathan2 and Debnarayan Jana1
1
Dept. of Physics, University of Calcutta, 92, A.P.C. Road, Kolkata-09, India
Department of Experimental Condensed Matter Physics, Saha Institute of Nuclear Physics, Kolkata-64, India
*
Corresponding author’s e-mail: sgphycu@gmail.com, Tel.: + 91-33-2350 8386 ext. 413.
2
Abstract
We have studied here the changes in magnetic
property of ZnO nanoparticles due to systematic
reduction of size by mechanical milling in N2
atmosphere. From Raman and photoluminescence (PL)
spectra, we have found an increase in defect density due
to size reduction. From room temperature PL spectra
defect related shift of green emission has been observed
for ferromagnetic ZnO nanoparticles.
Keywords: ZnO, nanoparticles, ferromagnetism, point
defects
is observed. But when we increase the milling time
green emission re-originated with high intensity and a
shift of this emission towards lower energy occurs.
Interestingly samples with shifted green luminescence
have the ferromagnetic property.
Introduction
ZnO is an attractive material for diluted
magnetic semiconducting (DMS) systems due to its
wide band gap (3.37eV) and large binding energy
(~60meV) at room temperature. Recently it has been
observed that though bulk ZnO is diamagnetic but
ferromagnetism (FM) can be found in its nano states. It
has been understood that native defects, especially that
are residing at grain surface region play an important
role to trigger this FM [1]. To identify the role of
different defects on this FM, further investigation is
required.
Fig. 1
Fig. 2
Experimental details
High purity ZnO powder (99.99%) has been
ball-milled for different milling time (60, 240, 480 and
960 minutes) in pure N2 atmosphere. XRD, TEM,
magnetic (SQUID), optical (UV-visible absoption, PL,
Raman) and PAL measurements have been carried out.
Results and Discussions
All samples show pure wurtzite structures in
XRD measurements (Fig.1). Due to 16 hours milling
crystallite size of ZnO nanoparticles decreases to 23±1
nm. From SQUID measurement it has been observed
that magnetic property of ZnO nanoparticles changes
from completely diamagnetic to weakly ferromagnetic
when the crystallite size decreases from 39±1 nm to
30±1 nm (Fig.3). Further reduction of the size of the
samples increases the ferromagnetic nature of ZnO
samples.
PL spectra (Fig. 4) show that all samples have
green luminescence originated from defects. For lower
milling time (upto 240min) a quench of green emission
Fig. 3
Fig. 4
Fig.1: XRD data of all samples; Fig. 2: SAED image for
960 min milled sample; Fig. 3: M-H graph from SQUID
measurement; Fig. 4: Room temperature PL spectra of
all samples.
Acknowledgement
S. Ghose acknowledges U.G.C.(India) for providing her
RFSMS fellowship and Prof. S.K.Ray (IIT, Kharagpur)
for PL measurements.
Reference
[1] S. Ghose et al., Surface defects induced
ferromagnetism in mechanically milled nanocrystalline
ZnO, J. Appl. Phys., 114,073516(2013)