J Supercond Nov Magn
DOI 10.1007/s10948-012-1559-4
ORIGINAL PAPER
Magnetic Field Dependence of Blocking Temperature
in Oleic Acid Functionalized Iron Oxide Nanoparticles
Sanju Tanwar · V.P.S. Awana · Surinder P. Singh ·
Renu Pasricha
Received: 23 March 2012 / Accepted: 27 March 2012
© Springer Science+Business Media, LLC 2012
Abstract We report the synthesis of phase pure, mono-
dispersed Fe
3
O
4
nanoparticles of size ∼10 nm via chem-
ical co-precipitation of ferrous and ferric ions, under con-
trolled pH and temperature. The nanoparticles are oleic acid
functionalized and hence dispersible in organic medium.
The structure and morphology of nanoparticles are deter-
mined by analyzing XRD pattern and TEM micrographs,
confirming the formation of phase pure Fe
3
O
4
nanoparti-
cles. The magnetization studies reveal the superparamag-
netic behavior of the nanoparticles at room temperature. The
changes in blocking temperatures (T
B
) of magnetic nanopar-
ticles with applied magnetic fields (H
ap
), noted from the
cusp of the zero-field-cooled magnetization, the indicate ef-
fects of dipole interactions. A decrease in blocking temper-
ature from 95 K to 15 K has been observed on varying the
magnetic field from 50 Oe to 5000 Oe. T
B
versus H rela-
tion follows the equation T
B
(H ) = T
o
(1 − (H/H
o
))
m
, i.e.
the Néel–Brown model of magnetic relaxation in nanoparti-
cles.
Keywords Superparamagnetic · ZFC-FC · Blocking
temperature · Néel–Brown model
S. Tanwar
Centre for Converging Technologies, University of Rajasthan,
JLN Marg, Jaipur, Rajasthan 302004, India
S. Tanwar · V.P.S. Awana ( ) · S.P. Singh · R. Pasricha
National Physical Laboratory, Council of Scientific and Industrial
Research, Dr. K.S. Krishnan Marg, New Delhi 110012, India
e-mail: awana@mail.nplindia.ernet.in
url: www.freewebs.com/vpsawana
R. Pasricha
e-mail: pasrichar@mail.nplindia.ernet.in
1 Introduction
Unique physical properties of nanoparticles (NPs) are a
topic of intensive research [1]. A special place belongs to
the magnetic properties in which the difference between
the bulk material and the nanophase is especially signifi-
cant. In particular, it has been shown that magnetization (per
atom) and the magnetic anisotropy of nanoparticles are sig-
nificantly larger than those of the bulk specimen [1]. The
change in magnetic properties at nanoscale has been widely
explored to develop superparamagnetic biocompatible iron
oxide nanoparticles. Such magnetic nanoparticles with neg-
ligible coercivity find immense applications in the field of
diagnostic imaging (MRI) [2, 3], drug delivery [4, 5], mag-
netic storage media [6], ferrofluids [7] and electronics [8].
Numerous routes have been employed for the synthesis of
iron oxide nanoparticles; thermal decomposition [9–12], mi-
croemulsion [13, 14], hydrothermal synthesis [15–17] and
co-precipitation [18–20] are few to be named. But still the
synthesis of monodisperse nanoparticles with uniform size
and homogeneous distribution is a great challenge to over-
come. Co-precipitation with addition of a base under inert
atmosphere is a facile and convenient way to synthesize iron
oxides (either Fe
3
O
4
or γ -Fe
2
O
3
) from aqueous Fe
2+
/Fe
3+
salt at room temperature or slightly elevated temperature.
The size, shape, and composition of the magnetic nanopar-
ticles synthesized through this method depends on the types
of salt used (e.g. chlorides, sulfates, nitrates), the Fe
2+
/Fe
3+
ratio, the reaction temperature, the pH value and the ionic
strength of the media. Once the synthesis conditions are
fixed, the quality of the magnetite nanoparticles is mostly re-
producible. The functionalization of nanoparticles prevents
their agglomeration.
A magnetic nanoparticle generally is in a single domain
state with uniaxial anisotropy [21]. For a sample composed