Signatures of spin-glass freezing in Co/CoO nanospheres and nanodiscs

Article abstract of DOI:10.1016/j.jmmm.2012.03.028

We present a study of the magnetic properties of Co nanoparticles having a combination of both spherical and disk shapes. The hcp Co nanospheres with an average diameter of 11 nm and nanodiscs of dimensions ～2.5×15 nm ² were prepared by thermal decomposition of di-cobalt octacarbonyl in the presence of an amine surfactant. The as-synthesized nanoparticles were oxidized to grow an antiferromagnetic layer. High resolution transmission electron microscopy showed the presence of a ferromagnet/antiferromagnet (Co/CoO) interface with a 2.2-nm thick CoO shell on the spherical nanoparticles and 0.5 nm thick on nanodiscs. We report the temperature and field dependent DC magnetization, frequency, field, and temperature dependent AC susceptibility, and the radio frequency transverse susceptibility. A low temperature paramagnetic behavior was observed in the DC magnetization at high fields and is assigned to defects in the CoO shell that are not coupled to the antiferromagnetic lattice. Our results support the existence of a low temperature frozen, disordered magnetic state, characterized by a strong exchange coupling between the structurally disordered, spin-glass CoO shell and Co core.

Full text of DOI:10.1016/j.jmmm.2012.03.028

Journal of Magnetism and Magnetic Materials 324 (2012) 2512–2518
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Signatures of spin-glass freezing in Co/CoO nanospheres and nanodiscs
a,b,n
c
d
a
D. Srikala
, V.N. Singh , B.R. Mehta , S. Patnaik
a
School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India
b
c
Department of Physics and Electronics, Hans Raj College, Delhi University, Delhi 110007, India
Physics of Energy Harvesting, National Physical Laboratory, Delhi 110012, India
d
Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We present a study of the magnetic properties of Co nanoparticles having a combination of both
Received 25 November 2011
Received in revised form
spherical and disk shapes. The hcp Co nanospheres with an average diameter of 11 nm and nanodiscs of
2
dimensions ꢀ2.5 ꢁ 15 nm were prepared by thermal decomposition of di-cobalt octacarbonyl in the
1
0 March 2012
presence of an amine surfactant. The as-synthesized nanoparticles were oxidized to grow an
antiferromagnetic layer. High resolution transmission electron microscopy showed the presence of a
ferromagnet/antiferromagnet (Co/CoO) interface with a 2.2-nm thick CoO shell on the spherical
nanoparticles and 0.5 nm thick on nanodiscs. We report the temperature and field dependent DC
magnetization, frequency, field, and temperature dependent AC susceptibility, and the radio frequency
transverse susceptibility. A low temperature paramagnetic behavior was observed in the DC magne-
tization at high fields and is assigned to defects in the CoO shell that are not coupled to the
antiferromagnetic lattice. Our results support the existence of a low temperature frozen, disordered
magnetic state, characterized by a strong exchange coupling between the structurally disordered, spin-
glass CoO shell and Co core.
Available online 30 March 2012
Keywords:
Spin-glass
Exchange bias
Magnetism
&
2012 Elsevier B.V. All rights reserved.
1
. Introduction
interface, exchange bias is also observed in systems with ferri-
magnet (FI) [10,11] or spin glass (SG) [12] components. Signifi-
cant advances in fine particle production by various techniques
favoured the studies of EB in transition metal particles like Co, Fe,
Ni or alloys of these metals [13–15]. Generally the partial oxidation
of these metals allows the growth of AFM shell at the expense of
the core and by changing the size of the metal particles, the
amount and phase of oxide interface in the system can be varied by
keeping the shell thickness constant [12]. For example, spherical
Co nanoparticles of 14.5 nm size with 1.9 nm CoO shell thickness
lead to a significant increase in exchange bias and coercivity as
Magnetic nanoparticles have attracted immense interest due
to the large number of current and foreseen applications in many
areas of technology. Since in the nanoscale the elemental magnetic
materials lose their robust room temperature ferromagnetism,
a substantial parts of recent experimental and theoretical work
has focused on techniques to overcome the superparamagnetic
limit, particularly in exchange-coupled nanomaterials [1]. When
a system with a ferromagnetic (FM) and antiferromagnetic (AFM)
interface is cooled in an external magnetic field through the
ordering temperature of the AFM shell (the N e´ el temperature
compared to 2.5 nm Co
on a value of about 480 Oe at 5 K in the case of CoO shell whereas
it is ꢀ130 Oe in Co (cooled in 5 kOe external magnetic field at
3 4
O shell [16]. The exchange bias field takes
(T
N
)) with the Curie temperature (T
C
) of the FM being larger than
T
N
, it is possible to induce additional unidirectional anisotropy due
3 4
O
to the interfacial magnetic coupling. This exchange bias (EB) is
experimentally reflected as an asymmetric shift in the hysteresis
loop in the field cooled protocol and the magnitude of shift in the
loop is an estimate of the exchange bias field HEB [2–6]. Such
materials have potential applications in ultrahigh-density mag-
netic random access memory [7], and in spin valves for magnetic
storage [8,9]. In addition to the different systems in FM/AFM
350 K). In contrast, Co nanocubes of ꢀ50–60 nm edge length with
a CoO layer of ꢀ4 nm thickness give rise to exchange bias field of
380 Oe at 5 K [17]. Similarly it is reported that Co (25 nm)/CoO
(5 nm)/Cu (2 nm) elongated nanoring arrays exhibit exchange
bias of 260 Oe at 90 K [18]. Real magnetic nanostructures have
a distribution of sizes, and random orientations of magnetiza-
tion, making their behavior very complex and challenging to
understand. In fact in nanostructured systems, the surface region
exhibits modified magnetic properties compared to the core.
Broken exchange bonds and competing magnetic interactions
inevitably lead to spin canting and spin-glass like behavior
[19,20]. Spin glass behavior arises because of spin disorder
n
Corresponding author at: Department of Physics and Electronics, Hans Raj College,
Delhi University, Delhi 110007, India. Tel.: þ91 11 27667458; fax: þ91 11 27666338.
E-mail address: srikala.d@gmail.com (D. Srikala).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jmmm.2012.03.028

D. Srikala et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2512–2518
2513
and frustration [21]. Therefore, both the surface and interfacial
effects play an important role in nanostructured materials. In this
paper, we have studied cobalt nanoparticles having spherical and
disk shapes, prepared by thermal decomposition of a carbonyl
material. The magnetic behavior of the system has been found to
be strongly dependent on the external applied fields and exchange
coupling between the core and disordered shell. Spin glass
behavior is observed at low temperatures. We also discuss effective
anisotropy peaks and irreversible behavior of magnetic spins in the
system.
solution on a carbon coated copper grid and dried at room
temperature. The particle size and size distribution were averaged
over ꢀ100 nanoparticles. DC magnetization measurements were
carried out in a Quantum Design superconducting quantum
interference device (SQUID) in the temperature range 2–330 K
and in applied magnetic fields up to 4 T. AC susceptibility
measurements were carried out in the SQUID in the temperature
range 5–50 K as a function of DC magnetic field and frequency. To
study the dynamic magnetic properties, radio frequency trans-
verse susceptibility (RFTS) measurements were performed using a
self-resonant tunnel-diode oscillator in the temperature range
2.5–200 K and in the applied magnetic fields up to 72 T.
2
. Experiments
Cobalt nanoparticles were prepared by thermal decomposition
3. Results and discussion
2 8
of di-cobalt octacarbonyl (Co (CO) ) in the presence of different
surfactants [22]. The synthesis was carried out in a three neck
flask with a condenser. A solution containing the surfactants 0.4 g
octadecylamine and 0.2 g trioctylphosphine oxide in 10 mL of
o-dichlorobenzene (o-DCB) was degassed with high purity argon
Fig. 1(a) shows the TEM image of as-synthesized cobalt nano-
particles having both spherical and disk shapes formed together. The
average diameter of nanospheres is 11 nm and the dimensions of
2
disks are ꢀ2.5 ꢁ 15 nm . The formation of elongated particles i.e.,
for 20 min and then heated to 180 1C. Then 0.54 g of Co
2
(CO)
8
nanodiscs along with spherical nanoparticles were possible because
of the linear amine surfactant used during the synthesis.
Fig. 1(b) shows the high resolution TEM image of as-synthesized
cobalt nanosphere of radius 11 nm without any oxide shell with
dissolved in 8 mL of o-DCB was rapidly injected into the reflux
bath. The solution was heated for 2 min and then cooled to room
temperature. The nanoparticles were extracted from the colloidal
solution by adding equal volumes of ethanol and hexane followed
by centrifugation for 20 min at 1400 rpm.
˚
lattice spacing 2.02 A corresponding to [002] plane of hcp phase.
After the extraction of nanoparticles from the colloidal solution and
oxygen passivation, the core–shell structure is shown in Fig. 1(c) and
(d). Fig. 1(c) shows nanospheres with a Co core of diameter 6.6 nm
and CoO shell of thickness 2.2 nm, and Fig. 1(d) shows a nanodisc
with 0.5 nm CoO shell thickness. The hexagonal close packed crystal
structure of nanoparticles was determined by electron diffraction
spectroscopy with rings assigned to the corresponding crystal planes
as depicted in Fig. 2(a). EDAX confirms the presence of both cobalt
The size and the shape of Co nanoparticles after extracting
from the colloidal solution were studied by transmission electron
2
microscopy (TEM) using a technai G (200 kV) microscope. The
crystal structure was identified by using electron diffraction
spectroscopy (EDS) and elemental composition was ascertained
by energy dispersive absorption X-ray spectroscopy (EDAX). The
samples for TEM studies were prepared by dropping the colloidal
Fig. 1. (a) TEM image of Co nanospheres and nanodiscs, (b) HRTEM image of the as-synthesized Co nanosphere of 11 nm diameter before extracting from the colloidal
solution, and (c) HRTEM image of nanospheres with a core shell structure after extraction. The diameter of the core is 6.6 nm and the thickness of the shell is 2.2 nm,
(d) HRTEM image of nanodisc with 0.5 nm thickness of CoO shell.

2
514
D. Srikala et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2512–2518
2.5
114
2
11
03
2.0
1.5
ZFC
FC
1
004
1
10
02
101
1
.0
.5
0
0
1
5
2
9
6
1
5
1/nm
3
2
2
2
2
C
Cu
1
0
Co
Co
Si
O
0
50 100 150 200 250 300 350
T (K)
Cu
Co
Fig. 3. Zero-field cooled (ZFC) and field-cooled (FC) magnetization as a function of
temperature in different applied magnetic fields using a SQUID magnetometer. A
low temperature rise in magnetization indicates the presence of defects that are
not aligned in the AFM lattice.
0
2
4
6
8
10
Energy (KeV)
Fig. 2. (a) Electron diffraction rings complying with hcp Co with polycrystalline
nature, and (b) EDAX spectrum exhibiting the presence of Co and O. Elements
carbon and copper are from the carbon coated copper grid upon which colloidal
solution was dispersed. Si peak observed is due to the Si detector used in TEM.
temperatures. At 5 kOe the ZFC curve does not show a peak even
after going down to 2 K, but both FC and ZFC curves exhibit a rise
in the magnetization (Fig. 3(c)). This rapid rise in magnetization
was attributed to the defects in the CoO shell [16]. As EDAX
analysis confirmed the absence of any magnetic impurities in the
sample we conclude that at sufficiently high magnetic fields these
defect moments are prevented from coupling into the AFM lattice
and hence these crystal defects appear as paramagnetic impu-
rities [16,24,25]. The standard analysis procedure was followed to
calculate the moments associated with these paramagnetic impu-
rities by performing Curie–Weiss law analysis by a linear fit to the
ZFC 1/M vs T data for Tr50 K. The moment per nanoparticle of
and oxygen as shown in Fig. 2(b), along with carbon, copper and
silicon from the grid and the detector.
Fig. 3 shows the zero-field cooled (ZFC) and field-cooled (FC)
DC magnetization curves at various applied fields in the tem-
perature range 2–330 K. Magnetization as a function of tempera-
ture shows a prominent low temperature peak centered at 17.6 K
in 100 Oe field (Fig. 3(a)). Above 30 K there is a gradual increase in
the total magnetic moment up to the measured temperature of
3
30 K as reported in the case of nanodiscs and nanocubes [23]. As
these paramagnetic impurities is estimated to be 268
m
B
and if
2
þ
the applied field is increased to 1 kOe, this low temperature peak
shifted to 11 K (Fig. 3(b)) and disappeared at large fields of 5 kOe
each defect is a Co
ion, which has a moment of 3.8
m
B
, then the
2
þ
measured moments correspond to about 70 Co in the CoO shell
that are not exchange coupled to the AFM lattice. In the presence
of sufficiently large magnetic fields, these defect moments
arrange themselves in the energetically favorable direction, which
in this case is away from the AFM lattice when compared to the
random orientation due to zero magnetic field cooling. Since the
thermal energy at low temperatures is not enough to overcome
the pinning of these moments, there is a sharp rise in the net
magnetization.
(
1
Fig. 3(c)). For measurements at applied fields of 100 Oe and
kOe, an irreversible magnetic behavior defined as the tempera-
ture where the difference between FC and ZFC magnetizations,
normalized to its maximum value at T¼2 K, becomes smaller
than 4% is observed. At 100 Oe the irreversible magnetic behavior
is observed in the whole temperature range and Tirr (irreversible
temperature) cannot be defined as FC and ZFC magnetizations are
forced to merge at the maximum measuring temperature of
3
30 K. On increasing the field to 1 kOe, Tirr is observed at 260 K,
Field dependence of the magnetization at different tempera-
but the magnetic irreversibility persists at low temperatures. On
further increase in the applied field to 5 kOe, the magnetic
irreversibility vanishes at low temperatures. For lower fields the
FC magnetization shows a very weak variation at low tempera-
tures, whereas the peak in ZFC magnetization shifts to lower
S
tures is depicted in Fig. 4(a). A saturation magnetization (M ) of
18.4 emu/g is observed at 300 K, but magnetization does not
saturate at low temperatures even in the presence of 4 T magnetic
field. The slope of M vs H curves at low temperatures is due to the
susceptibility of AFM CoO shell. These magnetization values are

D. Srikala et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2512–2518
2515
3
2
1
0
0
0
0
2
1
0
0
0
ZFC
3 T FC
-
-
10
20
3
00 K
200 K
1
1
3
1
50 K
00 K
0 K
2
0
0 K
10
0
0
1
2
3
4
H (T)
-
-
10
1
1
0
0
.6
.2
.8
.4
20
20
10
0
-10
-
20
10000
0
50
100 150 200 250 300
T (K)
-
-5000
0
5000
10000
H (Oe)
Fig. 4. (a) Field dependence of the magnetization displaying the nonsaturation
Fig. 5. Zero-field cooled and field-cooled (3 T) hysteresis loops measured at
a) 10 K, (b) 30 K and (c) 100 K exhibiting exchange bias effect. A negligible shift
in hysteresis loop is observed at high temperatures. Inset shows the temperature
behavior of CoO shell at low temperatures and inset shows the variation of
(
r
remanence magnetization (M ) and slope (dM/dH) of the virgin magnetization
curves in the field range 1.75–4 T with respect to temperature, and (b) inflection
dependence of the coercive field H
vanished above 100 K, whereas
temperature.
C
and the exchange bias field HEB. Exchange bias
minimum coercivity is observed at room
n
field (H ) and irreversibility field (Hirr; inset) as a function of temperature.
a
very small compared to the bulk Co (162 emu/g for hcp Co [26])
S
saturation magnetization. Such a large reduction of M cannot be
explained just by the interaction of the surface ligands that are
present on the surface of the Co nanoparticles [27] or by
nanoparticle surface-to-volume ratio [28]. We infer that the
the first quadrant merge together into a single curve; the two
corresponding magnetization values differing by less than 1% [29]
are observed in the measured hysteresis loops as shown in the
inset of Fig. 4(b). Hirr at low temperatures support the frozen spins
of the disordered CoO shell in a spin-glass like state. Hence our
system has exchange coupling between spin-glass like shell and a
ferromagnetic core that is strongly affected by the particle–
particle dipolar interactions and interfacial coupling at low
temperatures.
S
reduction of M must be primarily caused by the CoO shell and
the structural disorders that are present inside the shell or at the
interface of FM/AFM. The inset of Fig. 4(a) shows variation of
remanence magnetization (M
that confirms a strong increase in M
r
) as a function of temperature
(by a factor of ꢀ7) as
r
the temperature is lowered. The nonsaturation tendency is
well noticeable by plotting the values of the slope (dM/dH)
of the virgin magnetization curves in the field range of 1.75–4 T,
as a function of temperature. The presence of intrinsic structure
disorders in the CoO shell with Co embedded in it resulted
in a disordered spin configuration, with a net reduction in aver-
age magnetic moment well below the expected one. At high
temperatures the core magnetic moments dominate the shell
anisotropy and reduce the slope in the high field of the virgin
magnetization curves. On lowering the temperature, the random
freezing of the oxide magnetic moments leads to the high field
slope in the measured hysteresis loops.
Fig. 5 shows the temperature dependence of hysteresis loops
measured at T¼10, 30 and 100 K. In order to study the exchange
bias effect the samples were cooled in 3 T magnetic field from
330 K (above the N e´ el temperature of CoO) to the desired low
temperature, followed by M vs H measurements. The measured
ZFC hysteresis loops were symmetric at all the temperatures from
300 K to 30 K, but the loop at 10 K is asymmetric with respect to
the field axis, where it is shifted towards negative field axis by
93 Oe. In the FC measurement, there is a strong enhancement in
the coercive field along the negative field axis which is estimated
to be HEBꢀ1390 Oe at 10 K, 1000 Oe at 30 K and 54 Oe at 100 K
The field derivative dM/dH of the virgin magnetization curves
C
(Fig. 5). The variation in the coercive field H (half the width of the
n
shows a peak at H which is called as the inflection field that
hysteresis loop along the field axis) and exchange bias field HEB is
increases with the decrease in temperature caused by the pro-
gressive increase of effective anisotropy and consequent freezing
of shell moments (Fig. 4(b)). The high irreversible field (Hirr) is
defined as the field above which the two branches of the loop in
shown in the inset of Fig. 5(c). As the temperature decreased from
C
300 K to 10 K the coercivity H increased by a factor of 34, from
41 Oe to 1416 Oe. Thus, the shape of the loops with coercivity and
non-saturation magnetization is due to the presence of two

2
516
D. Srikala et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2512–2518
3
3
2
2
.5
.0
.5
.0
3
2
2
.0
.5
.0
H
= 3.5 Oe
F = 3 Hz
Hac = 3.5 Oe
3
3
3
9
Hz
100 Oe
00 Oe
400 Oe
3 Hz
32 Hz
97 Hz
2
6
00 Oe
10
20
30
40
50
5
10
15
20
25
30
35
1
1
0
0
.5
.0
.5
.0
1
1
1
1
1
1
9
1
.0
1
7.7 K
8
7
6
5
4
0.8
0
0
.6
.4
2
0
40
60
)
80
H2/3 (Oe2/3
0.2
1
0
20
30
T (K)
40
50
5
10 15 20 25 30 35
T (K)
0
Fig. 6. Temperature and frequency dependence of the (a) in-phase
w
(T) and
0
0
Fig. 7. Temperature variation of the (a) real and (b) imaginary part of AC
(
b) out of phase component
w (T) of the magnetic susceptibility for nanospheres
susceptibility at different DC fields.
and nanodiscs.
different magnetic components that are strongly coupled at the
interface: a ferromagnetic component because of Co and a
nonsaturating one which is responsible for the positive slope
even at high fields and is caused by the CoO shell. The effective
P p
phase is further evident from the field dependence of T . T
follows the de Almeida–Thouless (AT) line. The extrapolation of
the AT line back to H¼0 gives the spin-glass transition tempera-
ture which in this case is 17.7 K and is in perfect agreement with
the observed peak in DC magnetization at low field (Fig. 3(a)).
Dynamic magnetic behavior of Co nanoparticles was investi-
gated using resonant RF transverse susceptibility (TS) measure-
ments. It is well established as a sensitive and direct method to
understand the influence of relaxation and interactions that
govern the dynamical magnetic response, and in particular the
anisotropy fields of the magnetic nanomaterials. These measure-
ments were carried out in the temperature range 2.5–200 K by
measuring the shift in the resonant frequency with varying DC
magnetic field. The powder sample was encapsulated in a plastic
capsule that fits into the inductive coil and is placed perpendi-
cular to the external DC magnetic field. In this technique, the
change in the resonant frequency with respect to temperature
and field arising because of change in the coil inductance is
proportional to real part of complex magnetic susceptibility [31].
The TS ratio is calculated as a percentage change from the
measured resonant frequency using the following formula:
anisotropy constant calculated using K¼30K
B B B
T /V, where K is the
Boltzmann constant and T is the blocking temperature taken at
B
6
3
3
30 K, yields 1.963 ꢁ 10 erg/cm . This value is larger than the
5 3
anisotropy of pure spherical particles (K¼3.89 ꢁ 10 erg/cm ).
This reflects the increased contribution from various sources like
shape, surface, stress and unidirectional anisotropies. Tempera-
ture dependence of AC susceptibility is measured at several
frequencies ranging from 3 Hz to 1 kHz. The sample is first cooled
from room temperature to 5 K in a zero magnetic field. Then a
probing AC magnetic field (Hac) of 3.5 Oe amplitude is applied to
measure the susceptibility as the temperature is slowly raised in
0
2
part
of
K steps up to 50 K. Fig. 6 shows the real part
w
and imaginary
00
w
of AC susceptibility. As the frequency is raised, the value
(real part) decreases and the peak shifts towards high
temperatures. The behavior observed is characteristic of both
0
w
0
0
superparamagnets and spin glasses, whereas
w
(imaginary part)
does not follow any sequence with the variation in frequency.
A quantitative measure of the peak temperature shift with
T T
Dw /w (%)¼([w(H)ꢂwsat] ꢁ 100)/wsat, so that the data taken can
frequency is the relative shift in peak temperature,
DT
P
/T
P
, per
be compared at different measurement conditions [32]. Fig. 8
presents the TS ratio taken in the field scan from positive (þ 2 T)
to negative (ꢂ2 T) fields at different fixed temperatures. Two well
defined symmetrically located peaks were observed even at 200 K
as the applied field is swept from positive to negative maximum.
This indicates the presence of interactions responsible for effec-
tive anisotropy. These two peaks do not merge into a single peak
which is consistent with the ZFC–FC curves of DC magnetization
that show a broad plateau (Fig. 3(a)) implying dominance of
effective anisotropy over associated thermal energy at room
decade of frequency [30] and it is calculated to be 0.024 which
falls in the canonical spin-glass that lies between 0.0045 and 0.06.
In canonical spin-glass systems the spin-glass phase is very
sensitive to the external fields and the application of few hundred
Oesterds makes it disappear as seen from the DC magnetization
ZFC and FC loops (Fig.3). AC susceptibility at various bias fields
H
dc with an AC field of 3.5 Oe amplitude and 3 Hz frequency is
shown in Fig.7. As H increases, the peak temperature (T ) and
peak values decreased significantly. The existence of spin-glass
P

D. Srikala et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2512–2518
2517
These curves show evolution of anisotropy fields with the change
in the direction of field.
4. Conclusion
In conclusion, cobalt nanoparticles with spherical and disk
shapes were prepared by the thermal pyrolysis method and are
allowed to grow an antiferromagnetic CoO layer. HRTEM and
magnetization measurements confirm the presence of the AFM
shell. Large coercive fields are observed at low temperatures with
strong exchange bias and nonsaturated magnetization. Magneti-
zation measurements confirmed substantial exchange bias and
the disordered spin-glass oxide shell. Radio frequency transverse
susceptibility showed the presence of effective anisotropy at
characteristics peaks with hysteresis in the peak structures in
field scans, indicating the presence of strong magnetocrystalline
anisotropy at elevated temperatures.
Fig. 8. Field dependent RF transverse susceptibility ratio of Co nanoparticles at
various temperatures for a unipolar field scan from þ2 to ꢂ2 T. The TS data show
distinct peaks associated with the effective anisotropy fields even at high
temperatures.
Acknowledgment
0
0
0
0
0
.12
.09
.06
.03
.00
1
1
2
3 K
01 K
00 K
D. Srikala acknowledges UGC for financial support. DST,
Government of India, is acknowledged for funding the SQUID at
IIT Delhi, and High field magnet facility at JNU, New Delhi.
References
[
1] J. Nogu e´ s, J. Sort, V. Langlais, V. Skumryev, S. Suri n˜ ach, J.S. Mu n˜ oz, M.D. Bar o´ ,
Physics Reports 422 (2005) 65.
[
[
[
2] M. Gruyters, Europhysics Letters 77 (2007) 57006.
3] W.H. Meiklejohn, C.P. Bean, Physical Review 105 (1957) 904.
4] J. Nogu e´ s, I.K. Schuller, Journal of Magnetism and Magnetic Materials 192
(1999) 203.
-
2
-1
0
1
2
[5] V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, J. Nogu e´ s,
Nature 423 (2003) 850.
H (T)
[
[
6] M. Gruyters, D. Schmitz, Physical Review Letters 100 (2008) 077205.
7] S.S.P. Parkin, K.P. Roche, M.G. Samant, P.M. Rice, R.B. Beyers, R.E. Scheuerlein,
E.J.O. Sullivan, S.L. Brown, J. Bucchigano, D.W. Abraham, Y. Lu, M. Rooks,
P.L. Trouilloud, R.A. Wanner, W.J. Gallagher, Journal of Applied Physics 85
Fig. 9. Irreversible nature of the spins contributing to susceptibility in both cycles
i.e., from 0 to þ2 T and from þ2 to ꢂ2 T.
(1999) 5828.
[
[
8] J.C.S. Kools, IEEE Transactions on Magnetics 32 (1996) 3165.
9] S. Araki, M. Sano, S. Li, Y. Tsuchiya, O. Redon, T. Sasaki, N. Ito, K. Terunuma,
H. Morita, M. Matsuzaki, Journal of Applied Physics 87 (2000) 5377.
temperature. The unipolar field scans show that the TS ratio at
zero DC field increased with the increase in temperature. A
feature that is to be noted is the nonsaturation of susceptibility
even at 2 T magnetic field which is again consistent with our DC
magnetization. The susceptibility at 2 T has been considered as
[
[
10] A. Lawska-Waniewska, A. Roig, M. Gich, LI. Casa, K. Racka, N. Nedelko,
E. Molins, Physical Review B 70 (2004) 054412.
11] F.X. Redl, C.T. Black, G.C. Papaefthymiou, R.L. Sandstrom, M. Yin, H. Zeng,
C.B. Murray, S.P.O. Brien, Journal of the American Chemical Society 126
(2004) 14583.
[
12] L. Del Bianco, D. Fiorani, A.M. Testa, E. Bonetti, L. Savini, S. Signoretti, Physical
w
sat for all temperatures to calculate transverse susceptibility
Review B 66 (2002) 174418.
ratio. At low temperatures, the percentage change in the TS ratio
at zero field to the saturation field is small and this change
increased with temperature and is maximum at 200 K. These two
[13] G.H. Wen, R.K. Zheng, K.K. Fung, X.X. Zhang, Journal of Magnetism and
Magnetic Materials 270 (2004) 407.
[
14] J.F. L o¨ ffler, J.P. Meier, B. Doudin, J.P. Ansermet, W. Wagner, Physical Review B
7 (1998) 2915.
5
peaks correspond to the effective anisotropic fields 7H
have contribution from higher-order magnetocrystalline aniso-
tropy constants, dipolar interactions, surface spin disorder, etc.
K
which
[15] J. Nogu e´ s, J. Sort, V. Langlais, S. Doppiu, B. Dieny, J.S. Munoz, S. Surinach,
M.D. Baro, S. Stoyanov, Y. Zhang, International Journal of Nanotechnology 2
(2005) 23.
[16] D. Srikala, V.N. Singh, A. Banerjee, B.R. Mehta, S. Patnaik, Journal of Physical
[
33]. The broadening of the peak structure is due to the distribu-
Chemistry C 112 (2008) 13882.
17] C.P. Gr a¨ f, R. Birringer, A. Michels, Physical Review B 73 (2006) 212401.
18] D. Tripathy, A.O. Adeyeye, N. Singh, R.L. Stamps, Nanotechnology 20 (2009)
[
[
tion of anisotropy in nanoparticles and is prominent at low
temperatures because of frozen magnetic moments. In the uni-
polar field scans from positive to negative field sweep, there is a
015304.
[19] R. L o´ pez-Ruiz, F. Luis, J. Ses e´ , J. Bartolom e´ , C. Deranlot, F. Petroff, Europhysics
Letters 89 (2010) 67011.
noticeable difference between the positive peak at þH
K
and at
[
20] R.H. Kodama, A.E. Berkowitz, E.J. McNiff Jr., S. Foner, Physical Review Letters
7 (1996) 394.
21] M. Bandyopadhyay, S. Dattagupta, Physical Review B 74 (2006) 214410.
negative ꢂH
K
field. The larger positive anisotropy peak is due to
7
the orientation of the spins towards the overall susceptibility
which when passing through zero field could not rearrange in the
direction of the negative fields leading to asymmetry in peak
[
[22] V.F. Puntes, D. Zanchet, C.K. Erdonmez, A.P. Alivisatos, Journal of the
American Chemical Society 124 (2002) 12874.
23] J. Waddell, S. Inderhees, M.C. Aronson, S.B. Dierker, Journal of Magnetism and
[
K
heights. The exact values of 7H fields could not be calculated as
Magnetic Materials 297 (2006) 54.
the magnetization was not saturated even at 4 T magnetic fields.
Fig. 9 shows the 2D plot of the TS data in two scans at different
temperatures. This plot represents the field sweep from zero to
positive (þ2 T) and from positive (þ 2 T) to negative field (ꢂ2 T).
[24] J.B. Tracy, D.N. Weiss, D.P. Dinega, M.G. Bawendi, Physical Review B 72 (2005)
64404.
25] J. Keller, P. Miltenyi, B. Beschoten, G. G u¨ ntherodt, Physical Review B 66
2002) 014431.
[26] J.R. Childress, C.L. Chien, Physical Review B 43 (1991) (1991) 8089.
0
[
(

2
518
D. Srikala et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2512–2518
[
27] D. Srikala, V.N. Singh, A. Banerjee, B.R. Mehta, S. Patnaik, Journal of
Nanoscience and Nanotechnology 9 (2009) 5627.
28] C.B. Murray, S. Sun, H. Doyle, T. Betley, MRS Bulletin 985 (2001).
29] L.D. Bianco, D. Fiorani, A.M. Testa, E. Bonetti, L. Savini, S. Signoretti, Physical
Review B 66 (2002) 174418.
[31] R.B. Clover, W.P. Wolf, Review of Scientific Instruments 41 (1970) 617.
[32] L. Spinu, H. Srikanth, E.E. Carpenter, C.J.O. Connor, Journal of Applied Physics
87 (2000) 5490.
[33] P. Poddar, J.L. Wilson, H. Srikanth, D.F. Farrell, S.A. Majetich, Physical Review
B 68 (2003) 214409.
[
[
[
30] S.D. Tiwari, K.P. Rajeev, Physical Review B 72 (2005) 104433.