1. *Corresponding author. Tel.: #1-404-894-6368 fax: #1-
404-894-6369; e-mail: john.zhang@chemistry.gatech.edu.
Journal of Magnetism and Magnetic Materials 194 (1999) 1—7
Synthesis of superparamagnetic MgFe
O
nanoparticles by
coprecipitation
Qi Chen , Adam J. Rondinone , Bryan C. Chakoumakos , Z. John Zhang *
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400, USA
Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6393, USA
Abstract
MgFe
O
nanoparticles have been synthesized by coprecipitation method. Magnetic measurements in combination
with neutron diffraction have determined the existence of a superparamagnetic state in this metal oxide system. The
superparamagnetic relaxation of magnetization in these nanoparticles has been studied by using Mo¨ ssbauer spectro-
scopy. The relaxation time has been correlated with the particle size and temperature and is consistent with Ne´ el
theory. 1999 Elsevier Science B.V. All rights reserved.
Keywords: Spinel ferrites; Magnetic nanoparticles; Superparamagnetic; Neutron diffraction; Mo¨ ssbauer spectroscopy
1. Introduction
Superparamagnetism is a unique feature of mag-
netic nanoparticles and has great relevance to mod-
ern technologies including magnetic resonance
imaging contrast agents, data lifetime in high den-
sity information storage, ferrofluid technology, and
magnetocaloric refrigeration [1—4]. The funda-
mental studies on this subject have mostly focused
on pure metals such as Co, Ni and Fe [5—8]. In
metal nanoparticles, size is the only control to their
superparamagnetic properties, and the super-
paramagnetic state is usually observed in metal
particles within a few nanometer size ranges. The
applications of superparamagnetic metal nanopar-
ticles also are limited due to the chemical instability
of pure metals. Superparamagnetic properties of
materials are determined by magnetic anisotropy,
which comes from electron spin-orbital angular
momentum coupling at lattice sites in the crystal
structure. The major factors that control the
strength of magnetic couplings are the magnitude
of magnetic moment on each coupling component,
the distance between them, and the symmetry of the
lattice site. These factors correspond to the crystal
chemistry issues of chemical composition, lattice
constant, and coordination environment at the lat-
tice sites, respectively. In pure metal systems, these
crystal chemistry issues are basically fixed. Aside
from size, little variation can be chemically applied
to change the crystal chemistry of pure metal
nanoparticles to vary and control their super-
paramagnetic properties.
However, these crystal chemistry issues can be
substantially varied in a controlled fashion in metal
0304-8853/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 5 8 5 - X

2. oxides [9]. Spinel ferrite, MFe
O
(MCo, Fe,
Cd, Zn, Mn, or Mg) is a particularly important
magnetic material system [10]. Its nanoparticles
may possess novel magnetic properties, particularly
superparamagnetic behavior. At superparamag-
netic state, the collective behavior of the magnetic
nanoparticles is the same as that of para-
magnetic atoms. Each particle behaves like a
paramagnetic atom but with a giant magnetic
moment. There is a well-defined magnetic order
within each nanoparticle. Furthermore, spinel
ferrite nanoparticles provide great experimental
systems in which one can study the correlation
between the crystal chemistry and superparamag-
netic properties of magnetic nanoparticles. When
M in the chemical formula of spinel ferrite is
a metal carrying no magnetic moment such as
Mg, the magnetic couplings purely originate
from the magnetic moment of Fe cations and may
be relatively weaker. Magnetic anisotropy in
MgFe
O
could be lower than that of other spinel
ferrites in which all the metal cations have large
magnetic moments. Therefore, it is logical to anti-
cipate that the nanoparticles of MgFe
O
may
possess superparamagnetic properties even at rela-
tively large sizes.
Superparamagnetic properties of MgFe
O
have
been observed by Wirtz et al. in the particles which
were dispersed in MgO single crystals as solid solu-
tions [11,12]. Such MgFe
O
particles were pre-
pared by thermally diffusing Fe into MgO wafers.
After aging the samples at 700 and 800°C,
MgFe
O
precipitated out in particle form in the
MgO matrix. These MgFe
O
particles in solid
MgO matrix certainly expanded the super-
paramagnetic properties beyond pure metal sys-
tems and into much more diverse metal oxides.
However, the applications of these MgFe
O
particles are limited since they are confined
within MgO crystals in a random solid solution
form.
We herein report the preparation of MgFe
O
nanoparticles by coprecipitation and the physical
characterization of MgFe
O
nanoparticles. The
MgFe
O
nanoparticle powders are formed
through chemical reaction of MgCl
and FeCl
in
basic solutions. At room temperature, the nanopar-
ticles display a paramagnetic behavior. However,
the neutron diffraction studies show a ferrimagnetic
state in these MgFe
O
nanoparticles. These stud-
ies demonstrate that these MgFe
O
nanoparticles
are truly superparamagnetic. Our studies have
clearly shown the superparamagnetic properties of
MgFe
O
nanoparticles even with a size close to
20 nm. The studies on the fluctuation of nano-
particle magnetization directions by Mo¨ ssbauer
spectroscopy show that the superparamagnetic
relaxation time of nanoparticles correlates with the
particle size and temperature as Ne´ el theory has
predicted. Since our MgFe
O
nanoparticles are in
a free standing powder form, the potential applica-
tions of these superparamagnetic nanoparticles can
be broadly explored.
2. Experimental
2.1. Nanoparticle preparation
Mg spinel ferrite nanoparticles were prepared
using the coprecipitation method. The starting ma-
terials, FeCl
) 6H
O and MgCl
) 6H
O solids were
dissolved and mixed in water with concentrations
of 0.8 and 0.4 M, respectively. The aqueous mixture
was added into a 6 M NaOH solution to form
a precipitate. The slurry was then placed in
a boiling water bath and digested for two hours.
After the digestion, the slurry was filtered and
washed until the pH in the solution became neutral.
The resultant powder was heated in air in a tube
furnace at various temperatures.
2.2. Magnetic measurement
Magnetic properties of the spinel ferrite
nanoparticles were studied by using a Quantum
Design MPMS-5S SQUID magnetometer with
a magnetic field up to 5 T. A sample space oven was
used to study magnetic properties at temperatures
up to 800 K.
2.3. Neutron diffraction
Neutron diffraction studies were conducted by
using the HB4 powder diffractometer at the High-
Flux Isotope Reactor at Oak Ridge National
2 Q. Chen et al. / Journal of Magnetism and Magnetic Materials 194 (1999) 1—7

3. Fig. 1. X-ray diffraction patterns (Cu K -radiation) of
MgFe
O
nanoparticles.
Laboratory. The sample was placed in a vanadium
can for data collection at room temperature over
the two-theta range of 11 to 135° in steps of 0.05°.
The wavelength was precisely determined to be
1.4991(1) As based on the refinements of Si standard.
An array of 32 equally spaced (2.7°) He detectors
can be step-scanned over a range of up to 40°. The
data were corrected for the variation in detector
efficiencies, which were determined by using a
vanadium standard. The General Structure Analy-
sis System (GSAS) program was used to refine the
nuclear and magnetic structures of the sample and
to determine the cation distribution and lattice
magnetic moment in spinel structure [13].
2.4. Mo¨ ssbauer spectroscopy
The superparamagnetic relaxation behavior of
MgFe
O
nanoparticles was studied with Fe-57
Mo¨ ssbauer spectroscopy by using an Austin S-600
Mo¨ ssbauer Spectrometer (Austin Science Associ-
ates Inc.). A triangular waveform was employed to
drive the linear motor in a constant acceleration
mode. Experiment temperatures were controlled by
Janis SVT Research Cryostat System. MOSMOD
Mo¨ ssbauer Analysis Software was used to treat
the data. For Mo¨ ssbauer spectroscopy studies,
nanoparticles were mixed with wax to form pellets
that contained about 2—3% nanoparticles by
weight.
3. Results and discussion
X-ray diffraction characterization showed broad
diffraction peaks from the powder sample that pre-
cipitated out of the solution. The sample was iden-
tified as a mixture of MgO, Fe
O
and MgFe
O
in a poor crystalline state. After heating the sample
in air at temperatures above 400°C, the mixture
was transformed into crystalline MgFe
O
nano-
particles that contained Fe
O
impurity less than
2.5% by weight. By controlling the temperature of
heat treatment, the size of these nanoparticles can
be varied from a few nanometers to about 20 nm.
Fig. 1 shows the typical X-ray diffraction patterns
of these MgFe
O
nanoparticles. As the size of
the nanoparticles decreases, the width of diffraction
peaks increases. The size of the nanoparticles was
determined from the diffraction peak broadening
with use of the Scherrer equation. The nanoparticle
size determined by X-ray diffraction is consistent
with the size observed directly under high-resolu-
tion transmission electron microscope. This con-
sistency on the particle size implies that the
MgFe
O
samples are nanocrystallites. The chem-
ical composition was determined with ICP analy-
sis, and a ratio of two was given for Fe to Mg.
The temperature dependence measurements on
magnetic susceptibility of MgFe
O
nanoparticles
showed that the variation of magnetic susceptibility
depended upon the sample cooling conditions in
SQUID magnetic field. A typical magnetic behav-
ior of these MgFe
O
nanoparticles is represented
by a sample with 11 nm size (Fig. 2). When the
sample is cooled from room temperature to 1.7 K
without an external magnetic field (zero field
cooled; ZFC), its magnetic susceptibility initially
increases as temperature is increased from 1.7 K,
and then starts to decrease at a temperature that is
considered as a blocking temperature (plot a in
Fig. 2). When the same sample is cooled under
a magnetic field (field cooled; FC), its susceptibility
is highest at 1.7 K. It decreases as temperature
increases, and overlaps with the results obtained
from the zero field cooling measurement when tem-
perature rises above the blocking temperature (plot
b in Fig. 2). The blocking temperature changes as
the magnitude of applied magnetic field changes.
Q. Chen et al. / Journal of Magnetism and Magnetic Materials 194 (1999) 1—7 3

4. Fig. 2. Temperature dependence of the magnetic susceptibility
for field cooled and zero field cooled 11 nm MgFe
O
nanopar-
ticles at H50 G and 100 G. Fig. 3. Field dependence of the magnetization for 11 nm
MgFe
O
nanoparticles at 50 and 300 K.
Fig. 2 displays the different temperature-dependent
changes of magnetic susceptibility at 50 Gauss field
(plot a and b) and 100 Gauss field (plot c and d).
Below the blocking temperature, the magneti-
zation of MgFe
O
nanoparticles displays a hyster-
esis loop as the applied magnetic field changes
strength and direction (plot a in Fig. 3). The
MgFe
O
nanoparticles with 11 nm size have a co-
ercivity of 165 Gauss. When the temperature of
magnetization measurement is raised above the
blocking temperature, the hysteresis behavior dis-
appears, and the nanoparticles show a paramag-
netic magnetization (plot b in Fig. 3). Neutron
diffraction studies were performed to determine the
magnetic structure of MgFe
O
nanoparticles at
temperatures above the blocking temperature. Fig. 4
shows the neutron diffraction pattern of MgFe
O
nanoparticles at 300 K. A clearly-defined magnetic
structure is displayed, and the MgFe
O
nanopar-
ticles have an antiferromagnetic order, which is
consistent with well-documented bulk MgFe
O
as
a ferrimagnetic material. Mg cations occupy 40%
tetrahedral and 30% octahedral lattice sites in
a unit cell. Fe cations occupy the remaining oc-
tahedral sites. The unit cell is cubic and has a lattice
constant of 8.3874(5) As . The tetrahedral lattice site
has a magnetic moment of 3.25 , and the magnetic
moment is !1.25 at the octahedral lattice site.
Our high temperature magnetic measurements
have shown the MgFe
O
nanoparticles have
a magnetic transition at 620 K that fits with re-
ported Curie transition temperatures between 605
and 710 K in bulk MgFe
O
materials [10].
The results from magnetic measurements and
neutron diffraction studies unambiguously demon-
strate that the MgFe
O
nanoparticles prepared
from our coprecipitation method are super-
paramagnetic at room temperature. The magnetic
orders in the nanoparticles are well preserved
above the blocking temperature. However, the
magnetic anisotropy is overcome by the thermal
activation and an external magnetic field, and the
magnetization direction follows the direction of
applied magnetic field as a paramagnetic material
does. As the temperature decreases, the thermal
activation energy is reduced. When the temperature
drops below the blocking temperature, the mag-
netic anisotropy is no longer overcome and the
magnetization direction within each nanoparticle
aligns along the easy axis of the nanoparticle. Since
the easy axes of the nanoparticles are randomly
arranged, the magnetic susceptibility of the
4 Q. Chen et al. / Journal of Magnetism and Magnetic Materials 194 (1999) 1—7

5. Fig. 4. Neutron diffraction patterns ( 1.4991 As ) of MgFe
O
nanoparticles at 300 K. The “goodness of fit”, is 1.23. Below the
pattern, the first row of the sticks marks the peaks from the magnetic scattering of Fe
O
impurity. The second row of the sticks marks
the peaks from the magnetic scattering of MgFe
O
nanoparticles. And the third row of the sticks corresponds to the peaks from nuclear
scattering.
nanoparticles starts to steadily decrease below the
blocking temperature, as shown in Fig. 2. When
a stronger external magnetic field is applied, the
magnetic anisotropy of the nanoparticles is over-
come at a relatively lower temperature, and hence,
the blocking temperature shifts to a lower value.
In the superparamagnetic state, the magneti-
zation direction of MgFe
O
nanoparticle is not
fixed along any of the easy axes. It fluctuates among
the easy axes of magnetization when there is no
external magnetic field [14]. The superparamag-
netic relaxation time, is the average time that it
takes the particle magnetization to jump from one
direction to another. The relaxation time depends
on the size of the particles and the temperature,
which is approximated by Ne´ el as
exp(K»/k ¹), (1)
where K is the anisotropy constant of the particle,
» the particle volume, k the Boltzmann constant,
and ¹ the temperature.
is a constant and is
usually taken as
10 s.
The superparamagnetic relaxation of MgFe
O
nanoparticles and its correlation with the particle
volume and temperature has been studied by using
Mo¨ ssbauer spectroscopy. Fig. 5 shows the Mo¨ s-
sbauer spectra of MgFe
O
nanoparticles with
a size of 6 nm at various temperatures. At 300 K,
there is only a doublet pattern due to the nuclear
quadruple splitting of Fe-57, and there are no mag-
netic hyperfine interactions. When the temperature
is reduced to 95 K, a sextet pattern emerges in
addition to the reduction of the intensity of the
doublet pattern. The sextet component increases as
the temperature decreases to 70 K. At 55 K, the
doublet component disappears and only the sextet
pattern remains. The Mo¨ ssbauer spectra of 12 nm
MgFe
O
nanoparticles are displayed in Fig. 6.
There is also no magnetic hyperfine interaction in
these nanoparticles at 300 K, and hence, there is
Q. Chen et al. / Journal of Magnetism and Magnetic Materials 194 (1999) 1—7 5

6. Fig. 5. Mo¨ ssbauer spectra of 6 nm MgFe
O
nanoparticles at
various temperatures.
Fig. 6. Mo¨ ssbauer spectra of 12 nm MgFe
O
nanoparticles at
various temperatures.
only a quadruple interaction-induced doublet pat-
tern. When the temperature decreases to 95 K, the
spectrum shows a significant sextet component in
comparison to the one in the spectrum obtained
from 6 nm MgFe
O
nanoparticles at the same
temperature. For 12 nm nanoparticles even at
70 K, the Mo¨ ssbauer spectrum shows a single sex-
tet pattern. The mathematical fit of Mo¨ ssbauer
spectra has given a magnetic hyperfine field of 44 T
for these nanoparticles at 55 K.
The changes of Mo¨ ssbauer spectra in Figs. 5 and
6 clearly show the correlation between the relax-
ation time and the particle volume and temper-
ature as Eq. (1) has expressed. Fe-57 Mo¨ ssbauer
spectroscopy has an experimental measurement
time
*
about 10 s, which is the Larmor pre-
cession time of the Fe-57 isotope [15—17]. When
the relaxation time is smaller than
*
, the fluctu-
ation of the particle magnetization direction is so
rapid that the average of internal magnetic hyper-
fine field is zero to the Fe-57 nucleus. Conse-
quently, there is only a quadruple doublet splitting
in the spectrum. increases as temperature de-
creases. Eventually, approaches
*
, and a sextet
pattern starts to appear. However, some Fe-57
nuclei still experience a rapid superparamagnetic
relaxation and the doublet pattern remains visible.
When the temperature is low enough, the relax-
ation in the nanoparticles are much slower and
the spectrum becomes a pure sextet pattern.
From Eq. (1), it is clear that the increase of is
faster in the nanoparticles with a larger size as
temperature decreases. Therefore, the sextet com-
ponent in the Mo¨ ssbauer spectrum of 12 nm
MgFe
O
nanoparticles at 95 K is stronger than
the same component in the spectra of 6 nm
nanoparticles at the same temperatures as shown in
Figs. 5 and 6. At 70 K, the spectrum of 12 nm
nanoparticles has already shown a pure sextet pat-
tern. However, there still is a substantial doublet
6 Q. Chen et al. / Journal of Magnetism and Magnetic Materials 194 (1999) 1—7

7. component in the spectrum of 6 nm nanoparticles
at 70 K. The large temperature range for the super-
paramagnetic transition is considered as the result
of the size distribution in MgFe
O
nanoparticles.
4. Conclusions
MgFe
O
nanoparticles have been prepared by
coprecipitation method. The magnetic susceptibil-
ity measurements show that the magnetization goes
through the blocking temperature as the temper-
ature is varied. The nanoparticles display hysteresis
behavior below the blocking temperature, and they
show paramagnetic magnetization when the tem-
perature rises above the blocking temperature.
Neutron diffraction studies demonstrate that mag-
netic orders still exist in the nanoparticles above the
blocking temperature. These results unambigu-
ously show that MgFe
O
nanoparticles possess
typical superparamagnetic characteristics. The
superparamagnetic relaxation of the nanoparticles
has been studied by temperature-dependent Mo¨ ss-
bauer spectroscopy. The relaxation time clearly
correlates with the particle size and temperature.
The successful synthesis of superparamagnetic
MgFe
O
nanoparticles certainly expands the
superparamagnetic system into metal oxides, and
consequently, potential superparamagnetic mater-
ial systems would be greatly increased. Further-
more, the flexibility in the crystal chemistry of
metal oxides will elucidate the mechanism of super-
paramagnetic state and facilitate the fine tuning of
superparamagnetic properties in nanoparticles for
specific applications.
Acknowledgements
We thank Professor Angus Wilkinson of Geor-
gia Tech for his help during the data analysis of
neutron diffraction. The financial support from
Georgia Tech to this research is acknowledged.
AJR is supported in part by a William Henry
Emerson Chemistry Fellowship. The neutron dif-
fraction studies were carried out at Oak Ridge
National Laboratory, which is managed by Lock-
heed Martin Energy Research Corp. for the US
Department of Energy under contract number
DE-AC0596OR22464.
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