ARTICLE IN PRESS
M. Zeisberger et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 224–227
225
LP4. These surfactants allow for a stable suspension of the
particles.
Three samples were used for the measurements. The first
3
0
3
2
1
0
5
was a powder sample. Another one was a suspension of the
particles in kerosene and the third type of sample was
prepared by drying about 20 ml of the fluid on a small piece
of paper. The paper was rolled and inserted into the
container used for the VSM measurements. In this kind of
sample the particles are immobilized but in contrast to the
powder the concentration is much lower resulting in much
lower demagnetization effects which simplifies the analysis
of the data.
2
1
0
9
10
5
0
-
-
5
-3
0
3
-
-
-
10
15
20
1
8
200 300 400
3
. VSM and XRD investigations
-
1500
-1000
-500
0
500
1000
1500
In order to obtain the specific magnetization of the
particles the powder sample was investigated by VSM. The
data are shown in Fig. 1. The specific saturation
2
magnetization was 77.5 A m /kg (46% of the bulk value
field [kA/m]
Fig. 2. Magnetization data of the immobilized fluid sample and fits of the
low-and high-field range.
of Co) which is slightly higer than the value reported in
Ref. [7] for Co particles with an oxide shell. This difference
to the bulk properties is due to the oxide shell which is
created during the preparation process. Another VSM
measurement was performed with the immobilized sample.
The data which are presented in Fig. 2 correspond to a
modified Langevin function which can be analyzed
according to the method of Chantrell [8] in order to obtain
the parameters of the particle size distribution which is
assumed as lognormal:
X-ray diffraction. The Yꢀ2Y-scan shows a peak at 44.341
corresponding to the cubic (FCC) phase of Co. From the
full-width at half-maximum of this peak which is 2.271 we
obtained a mean particle size of 6 nm.
4. Characterization of the AC behavior
The characterization of the behavior in AC magnetic
fields was performed by two methods: AC susceptometry
(ACS) and calorimetric measurement of the SHP. The
arrangement used for ACS is shown in Fig. 3. The field coil
(+14 mm ꢁ 75 mm, 75 turns) which is connected to a
function generator provides an AC field with an amplitude
up to 60 A/m and a frequency in the range from 20 Hz to
ꢀ
ꢁ
2
1
ln ðd=DÞ
f ðdÞ ¼ pffiffiffiffiffi exp ꢀ
.
(1)
2
2
psd
2s
From separate fits in the low (m ¼ cVH for |m|5m ) and
s
high-field range (m ¼ m (1ꢀH /H) for |m|Em ) we
s
0
s
1
MHz. The sample is in a cylindric container (inner size
obtained D ¼ 7.0 nm and s ¼ 0:34.
+
3 mm ꢁ 7 mm) which is placed inside the pick-up coil (25
In order to obtain information about the phase and
the particle diameter we investigated the powder sample by
turns). A compensation coil is used to cancel out the
background signal. The output signal is detected by a lock-
in amplifier providing the in-phase and out-of-phase
8
6
4
2
0
0
0
0
0
0
signals that correspond to the real (w ) and imaginary part
0
0
(w ) of the susceptibility. Fig. 4 shows the result of the
measurements which were performed for the fluid as well as
for the immobilized sample. The data show that the fluid
0
00
sample shows much higher values of w and w and a more
pronounced frequency dependence than the immobilized
sample which is due to the Brownian rotation of the
particles in the liquid. In the fluid sample w shows a
distinct maximum at 25 kHz which corresponds to a
0
0
-
-
-
-
20
40
60
80
ꢀ
6
Brownian relaxation time of t ¼ 6.4 ꢁ 10 s. From the
B
relation t ¼ pZDh3/k T and a dynamic viscosity of
B
B
Z ¼ 1 mPas for kerosene we estimated a hydrodynamic
diameter D of the particles of 20 nm.
h
The calorimetric measurement of the SHP was per-
formed using an arrangement as shown in Fig. 5. A water
cooled copper coil which is connected to a 5 kW RF
generator provides an AC field of 400 kHz and amplitudes
in the range from 12 to 24 kA/m. The fluid sample is placed
-1000
-500
0
500
1000
field [kA/m]
Fig. 1. Specific magnetization of the Co powder.
Zeisberger, Matthias
