APPLIED PHYSICS LETTERS 99, 232507 (2011)
a)
Suman Mandal, Krishnakumar S. R. Menon, S. K. Mahatha, and S. Banerjee
Surface Physics Division, Saha Institute of Nuclear Physics, Kolkata 700 064, India
(Received 26 September 2011; accepted 20 November 2011; published online 9 December 2011)
The observation of finite magnetic moment in antiferromagnetic materials is quite unusual and has
been immensely investigated in nanoparticle systems. Here, the structural and magnetic properties
of NiO particles are explored by x-ray diffraction, extended x-ray absorption fine structure, and
magnetization measurements. Using similar-sized particles with different surface defect structure,
we show that the observed magnetic enhancement, which is present even beyond finite-size limit,
is due to the surface effects. However, the well known spin glass freezing is found to occur only in
In spite of their compensated nature, the origin of
observed finite magnetic moment in antiferromagnetic
experiments were performed at Powder diffraction beamline
(MCX) of Elettra synchrotron centre, Italy with a fixed x-ray
energy of 8 keV. Crystallite size and morphology of the par-
ticles were characterized by transmission electron micro-
scope (TEM). Magnetization data were obtained with a
quantum design Superconducting Quantum Interference De-
vice (SQUID) magnetometer. Local structure was character-
ized by extended x-ray absorption fine structure (EXAFS)
measurements at the Ni K edge, in both conversion electron
(AFM) materials and various unusual phenomena like loop-
shift upon field cooling, enhanced coercivity in nano-regime
1–13
have been a debated issue in literature.
A clear under-
standing is awaiting for their technological relevance as well
as for basic science. In 1961, N e´ el proposed that observed
magnetic enhancement can be understood by realizing two
sublattice uncompensated moment at the surface of the parti-
cle. However, observed moment is found to be too large to
be expected from the two-sublattice model and that has been
extended to multi-sublattice model as the new finite size
12,21
yield mode (CEY) (which is near surface sensitive)
and
transmission mode (TRANS) (which is bulk sensitive) at
XAFS beamline of Elettra synchrotron centre, Italy.
The cell parameters from XRD data are extracted by
3
effect in AFM nanoparticles. The multi-sublattice ordering
has been demonstrated to be true nano-scale phenomenon
13
22
Rietveld method using the FullProf program suites. The
observed and calculated diffraction patterns for 800Q (pre-
and is absent in particles with larger size especially beyond
nano-range (>100 nm). Recent observations of spin glass
behavior in different AFM nanoparticle systems are not
properly understood and have been mostly attributed to sur-
ꢀ
ꢀ
pared at 800 C and quenched) and 800R (prepared at 800 C
and relaxed) samples are shown in Fig. 1(a), as an example.
Determined lattice constant largely depends on preparation
temperature [Fig. 1(b)], expanded in nano-regime. In Fig.
1(b), we also label the average crystallite size as determined
from TEM images, which increases with synthesis tempera-
7
face spin disorder. The observations of vacancy induced
1
4–20
magnetic moment in non-magnetic systems
us to approach the problem from structural studies.
motivated
12
ꢀ
In this context, particle size dependence of magnetic
properties is mostly studied; however, magnetic properties of
same-sized particles with different surface conditions must
be investigated to properly understand the surface effects on
the observed magnetic enhancement. For this purpose, we
have prepared NiO particles from thermal decomposition of
ture and nearly saturates (ꢁ0.2 lm) beyond 800 C. Particle
sizes of Q and R samples are found to be nearly same for a
particular synthesis temperature, as explicitly shown in the
TEM images [Figs. 1(c) and 1(d)] for the samples prepared
ꢀ
at 800 C. In Fig. 1(e), we also present the TEM image of
ꢀ
NiO nanoparticles (NP22) prepared at 400 C.
In Figs. 2(a) and 2(b), we show the phase-corrected mag-
ꢀ
highly pure Ni-nitrate salt at various temperatures (550 C,
ꢀ
ꢀ
2
8
00 C and 1000 C). Different synthesis temperatures yield
nitude of Fourier transform (FT) of EXAFS signal (k v(k)) for
the first two shells (the Ni-O and Ni-Ni shell) measured in
different particle sizes; however, the surface of a fixed size
particle (prepared at particular temperature) was modified by
varying the cooling rate during synthesis. Some set of sam-
ples (Q samples) were quenched to room temperature (RT)
from the synthesis temperatures while others (R samples)
were allowed to relax for several hours to RT inside the
furnace. NiO nanoparticles were prepared by thermally
CEY mode, as indicated. Background correction and normal-
fol-
23,24
ization of raw EXAFS data are performed in ATHENA
lowing conventional procedure. Normalized v(k) data are then
used to model in ARTEMIS with the phase shift and back-
25
scattering amplitude calculated from FEFF6 code. The first
˚
peak around 2.085 A is due to the scattering from the nearest
neighbour shell (Ni-O shell) containing 6 oxygen atoms. The
˚
second peak around 2.95A is due to the scattering from next
ꢀ
decomposing Ni-hydroxide at 400 C as described else-
12
where. Single phase NiO powders were confirmed by high
resolution powder diffraction (XRD) experiments at RT and
typical diffraction patterns are shown in Fig. 1(a). XRD
shell (Ni-Ni shell) containing 12 Ni atoms. The different FT
magnitudes of Q and R samples [Figs. 2(a) and 2(b)] already
signify different local environments around Ni sites. Hardly
any changes are observed between Q and R samples when
a)
Electronic mail: krishna.menon@saha.ac.in.
0003-6951/2011/99(23)/232507/4/$30.00
99, 232507-1
VC 2011 American Institute of Physics
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