Work supported by:
National Science Foundation NSF/DMR 0203069
Campbell Endowment at UW
-cobalt
An anamolous peak has been observed in zero field cooled
measurements on systems composed of ε-phase cobalt nanoparticles.
The samples were synthesized via a thermal decomposition process
resulting in spherical nanoparticles surrounded by an organic
surfactant [1,2]. In addition to the blocking temperature peak, which is
dependant on particle volume [3] and displays interesting interaction
effects, an additional peak appears at 8 K regardless of particle
diameter. Zero field cooled and field cooled magnetization versus
temperature measurements on spherical particles with mean
diameters in the range of 5-20 nm were performed with a SQUID
magnetometer, including careful background signal checks.
Measurements after heat treatment, to promote oxidation, show that
the blocking temperature peak can be effectively removed in fully
oxidized powders. The 8 K peak, however, remains after heat
treatment, but with a reduced magnitude, and M(H) dependance is
hysteretic below this temperature. Additional measurements were
performed on commercial micrometer-sized cobalt oxide powder,
where this feature is also present, but gives a much weaker
contribution to susceptibility. Unreacted synthesis precursors as well
as samples with different surfactant species all showed at least some
indication of a peak at 8 K. This feature, universal for the system
under investigation, has not been observed in Co nanoparticle
systems prepared by co-sputtering or co-evaporation of Co with nonmagnetic materials. Furthermore, hysteresis measurements, both
before and after heat treatment, show a bias when cooled in field from
30 K to below the peak temperature. Analyses of the data leads us to
conclude that freezing of frustrated uncompensated spins on the
surface of cobalt oxide is the most reasonable explanation of this
anomalous low temperature behavior.
D
 K AV 

 m ~  0 exp 
 k BTB 
1.2E-02
1.0E-02
Moment (emu)
The particles are precipitated from
solution onto a thin carbon film for
transmission electron microscopy (TEM).
This TEM image shows a self assembled
monolayer of 10 nm spheres.
(Phillips 400 T at 100 keV)
FC
8.0E-03
6.0E-03
4.0E-03
ZFC
2.0E-03
TB
0.0E+00
0
100
200
300
Temperature (K)
For more information concerning particle synthesis and self assembly please see
“Controlled Self-assembly of Colloidal Cobalt Nanocrystals Mediated by Magnetic Interactions”
Poster presentation 2-ypm52 Location H
ZFC curve before heating. 6
nm spheres with TOPO and
OA surfactants.
3.0E-03
2.0E-03
1.0E-03
0.0E+00
The 8 K peak may be associated with a layer of cobalt oxide at the
surface as suggested by the figure below.
1
10
Applied External Field
1000
5.0E-04
Co3O4
Co3O4 shell
Before heating
After heating
After heat treatment, the -cobalt core is completely replaced with the
spinel phase of cobalt oxide, Co3O4. The blocking peak, originally at 60
K, is absent in the ZFC plot to the middle right. If the 8 K peak is due to a
surface effect, then one would expect that the magnitude of the peak to
remain the same before and after heat treatment. The magnitude of the
peak susceptibility, however, has dropped by nearly a factor of ten, but
this is accounted for by the shoulder of the blocking peak.
The interesting observation is the second
peak at low temperature on the ZFC curve
and its associated sharp uptake in
magnetization as seen in the FC curve. The
focus of this poster presentation is to further
characterize and describe this feature.
100
Tem perature (K)
-cobalt core
where m is the characteristic measurement
time ~20s, 0 is the microscopic fluctuation
attempt time ~10-10s, KA is the anisotropy
constant ~0.21 J/m3 for -Co, and V is the
magnetic particle volume.
The barrier
height may be approximated by 27 kBTB
where kB is Boltzmann’s constant and TB is
the
blocking
temperature,
which
corresponds to the peak temperature in
moment versus temperature measurements
(as identified to the left).
Two curves are plotted here: zero field
cooled (ZFC) and field cooled (FC). ZFC
measurements start at low temperature
after cooling the sample in zero external
field. Magnetization is then measured as
the temperature slowly increases.
FC
measurements start at high temperature
where a field is applied and the
magnetization is measured as the
temperature slowly decreases. The rate of
cooling is constant in both cases.
4.0E-03
M/H (emu/Gg)
Surfactants
A uniaxial magnetic particle has two stable
27kBTB magnetization orientations as seen in the
figure to the left. If an external field is
applied along the easy axis then one of the
two orientations has a lower energy as seen
to the right. As the temperature increases,
thermal fluctuations provide the activation
energy for the system to relax over the
energy barrier into the field direction. This
No External Field
behavior is well known and follows
Epsilon cobalt nanospheres surrounded by a
mixture of organic surfactants (TOPO and
OA)
are
synthesized
via
thermal
decomposition of cobalt carbonyl in dichlorobenzene solvent. The diameter D may
be controlled between 5 and 20 nm, and the
surfactant length is approximately 1.7 nm.
5.0E-03
The low temperature peak consistently appears at 8 K, regardless of
particle diameter. Peak susceptibility, however, decreases as particle
diameter increases; suggesting that the peak is due to a surface effect.
One possible explanation is surface oxidation. In order to explore this
possibility, ZFC measurements were performed before and after heating
the sample in air at 120ºC for 24 hours. The temperature has been
plotted on a log scale to reveal the peak at 8 K in the ZFC plot to the right.
M/H (emu/Gg)
M. Beerman, A. Pakhomov, Y. Bao, and K. Krishnan
Department of Materials Science and Engineering
University of Washington, Seattle, WA 98195, USA
4.0E-04
3.0E-04
ZFC curve of the same
sample after heating.
2.0E-04
1.0E-04
0.0E+00
1
10
100
1000
Temperature (K)
The phases were identified by x-ray diffraction using a Rigaku 12.5 kW
rotating anode x-ray source and goniometer. The diffraction peaks in scan
a (lower right figure) correspond to –cobalt, and the peaks in scan b
correspond to Co3O4.
XRD a. before and b. after
heating.
Magnetic measurements were performed on a
Quantum Design MPMS 5 superconducting
quantum interference device.
[1] V. Puntes et al. Science 291 (2001) 2115
[2] Y. Bao, et. al., JMMM, in press.
[3] L. Néel, Annales de Géophysique 5 (1949) 99
The cobalt particle reaction precursor is Co2(CO)8. A sample was dried and measured on the SQUID
magnetometer. The ZFC – FC curve below does show some indication of the peak at 8 K. One
possible explanation of the peak is that some clustering of the precursor results in small particle
aggregates with an associated blocking peak at 8 K. Another explanation is direct oxidation of the
precursor to form the uncompensated surface spin state, which is consistent with the heat treatment
measurements.
A powder sample was prepared with 6 nm spherical particles. The sample was cooled in
an applied field of 1000 G to the measurement temperature. Hysteresis plots between
+/- 2T at various temperatures appear in the figure below. The applied field axis is
reduced so that the exchange bias shift in the hysteresis loop is noticeable.
Spin-glass-like freezing is one possible physical mechanism that can lead to a particle sizeindependent maximum in the ZFC curves. Bulk spin glasses have been well studied* and show
a shift in the glass transition to lower temperatures as the applied field increases. We observed
similar shifts in our measurements, which might be an indication of spin-glass freezing. The
variation in saturation magnetization at low temperatures observed in γ–Fe2O3 and Pd-Fe
nanoparticle systems has been attributed to surface spin-glass like freezing.** The anisotropy is
not bound to the crystal lattice due to the irregularities at the region of broken bonds near the
surface. The spin glass transition temperature Tg is approximately 8K in this case. At
temperatures above Tg, the surface spins are strongly paramagnetic and do not couple even in
high fields as suggested by the figure (below left). In the presence of an applied field below Tg
the spins will freeze into the field direction (below right).
1.0
Co2(CO)8 Precursor
ZFC - FC Low Temperature
0.8
6.0E-05
ZFC
5.0E-05
4.0E-05
3.0E-05
2.0E-05
1.0E-05
0.0E+00
0
10
20
30
40
50
60
Temperature (K)
Moment (emu)
Moment (emu)
FC
Co3O4
T = 200 K
T = 50 K
0.4
T = 25 K
0.025
5G
10 G
0.020
20 G
40 G
0.015
100 G
200 G
0.010
500 G
0.005
0.000
-cobalt
T=5K
0.2
1G
0
T = 100 K
0.6
0.030
M/H (emu/Oe)
The evidence as outlined in the Heat Treatment section of this presentation is compelling for the
surface oxide explanation of the 8 K susceptibility peak. However, a thorough investigation requires
background measurements of the synthesis precursors as well as a commercially obtained Co3O4
powder.
Exchange bias was originally observed in samples consisting of micron sized cobalt
particles surrounded by a cobalt oxide (CoO) shell.* Bias is due to exchange interaction
between the ferromagnetic core and the anti-ferromagnetic shell. The nanoparticle
samples that we are discussing here have an –cobalt core and a Co3O4 shell. The Neel
temperature for Co3O4 is 40 K (as opposed to 270 K for CoO).
T > Tg
5
10
15
20
Tem perature (K)
T < Tg
0.0
The figure to the left is the family of field cooled susceptibility curves from which the linear susceptibilty 0, non linear
susceptibility 2 and the non linear exponent  were obtained and plotted in the figure to the right.
-0.2
-0.4
The ZFC plot below was obtained from the commercial Co3O4 powder. At first look, the 8 K peak is
absent, however, a closer inspection reveals some indication of a peak. Again this is consistent with
the surface oxide interpretation. We have previously observed that the magnitude of the 8 K peak is
inversely proportional to the particle diameter, and this sample consists of a wide size distribution of
particles ranging from hundreds of nm to tens of microns. The surfaces of the smallest particles in this
distribution are likely contributing to the 8 K peak.
-0.6
Applied field
-0.8
-1.0
-10000
-5000
0
5000
10000
Field (Oe)
The exchange field H-ex is the extent of the shift of the hysteresis loop from the
origin. The figure below is a plot of H-ex as a function of temperature. The blocking
temperature of 6 nm particles is approximately 80 K, and the greatest exchange field
occurs just below this temperature at about 50 K.
Co Oxide Commercial
Micron Sized Powder
9.E-05
8.E-05
6.E-05
400
5.E-05
4.E-05
3.E-05
2.E-05
1.E-05
0.E+00
0
50
100
150
200
250
300
Temperature (K)
350
H-ex (Oe)
M/H (emu/Gg)
7.E-05
300
200
100
0
0
50
100
150
200
In order to test the applicability of this model, we estimated the linear 0 and non-linear 2
contributions to magnetic susceptibility, and the critical exponent , as a function of
temperature in the range 2.5-10K. These data were extracted from a family of field cooled
(FC) curves measured at different fields with a low ramp rate on a sample containing 6-nm
particles, then compared to the following expansion for the susceptibility:
M
 0  2 H 
H
Isothermal magnetization curves were extracted from the set of FC measurements. Then
linear susceptibility 0 was found from the initial slope of the isothermal M(H) curve. The
inflection point of this curve gives the transition temperature, which is about 7.5 K. A power
law fit was employed to obtain 2 and . The non-linear susceptibility is well behaved below
the transition temperature and is two orders of magnitude smaller than 0. Above the
transition temperature, however, 2 and  are not well determined due to the limitations of the
measurements. The nonlinear exponent  is constant below Tg, with a value of 3/2 for low
fields. These trends do not compare well to bulk spin glasses, where the nonlinear
susceptibility has a maximum at the freezing temperature, and the exponent  has a
minimum.
Temperature (K)
8K
10
* W.H. Meiklejohn, C.P. Bean, Phys. Rev., 102, 1413 (1956).
20
30
40
50
Magnetic property measurements of the surfactant-coated core-shell Co/Co3O4 nanoparticles reveal
contributions from three interacting and correlated entities: single domain metallic cobalt, which is
primarily responsible for the blocking phenomena; antiferromagnetic Co3O4 which is revealed in
exchange bias; and the surface layer of the oxide where spins are coupled below a critical temperature,
which may be defined as either spin glass freezing or ferromagnetic Curie transition. Since the blocking
effect can be controlled by annealing, the low temperature behavior can be isolated and studied
separately from blocking, while the interaction between surface and core may be examined by studying
the system in the pre-oxidized state.
Contact Information
Please visit the Integrated Nanosciences & Mesoscale Engineering Group website at:
http://depts.washington.edu/kkgroup
* K. H. Fischer, J. A. Hertz, Spin Glasses, Camridge University Press (1991).
** B. Martinez, et. al., Phys. Rev. Lett., 80, 181 (1997).
Or you may email the authors at:
Kannan M. Krishnan: kannanmk@u.washington.edu
Temperature (K)
Michael Beerman: mbeerman@u.washington.edu