4. CONCLUSION
In summary, we studied the role of the magnetic interaction between magnetic clusters on
heating dissipation of magnetic fluid of prepared Mn0.7Zn0.3Fe2O4 nanoparticles in an alternating
field. Our results implied that magnetic interactions between magnetic clusters affect directly to
the large value of the SLP for Mn0.7Zn0.3Fe2O4 fluids. Reducing Mn0.7Zn0.3Fe2O4 nanoparticles
concentration in fluid means decreasing dipolar interaction between colloidal particles and help
25
30
35
40
45
50
55
0 300 600 900 1200 1500
80 Oe
70 Oe
60 Oe
50 Oe
40 Oe
T (oC)
t (s)
(a)
25
30
35
40
45
50
55
60
0 300 600 900 1200 1500
80 Oe
70 Oe
60 Oe
50 Oe
40 Oe
T (oC)
t (s)
(b)
30
40
50
60
70
0 300 600 900 1200 1500
80 Oe
70 Oe
60 Oe
50 Oe
40 Oe
T (oC)
t (s)
(c)
colloidal particles move easier in fluid. Therefore, SAR achieved higher value at lower
concentration. Maximum SAR of 58.7 W/g is achieved in the AC magnetic field of 80 Oe while
the frequency is set to 178 kHz. Thus, these nanoparticles could also be used as effective heat
mediator in AC induction heating.
Acknowledgement. This study was supported by Vietnam National Foundation for Science and
Technology Development (NAFOSTED) under grant number 103.02-2015.74 and Program of
Development in the field of Physics by 2020 under grant number KHCBVL.03/18-19. The authors are
thankful to Institute of Materials Science, Graduate University of Science and Technology and Ton Duc
Thang University.
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Vietnam Journal of Science and Technology 56 (1A) (2018) 50-58
DYNAMIC EFFECTS OF DIPOLAR INTERACTIONS ON THE
SPECIFIC LOSS POWER OF Mn0.7Zn0.3Fe2O4
Pham Hong Nam
1, 2, *
, Luong Le Uyen
3
, Doan Minh Thuy
3
, Do Hung Manh
1
,
Pham Thanh Phong
4, 5
, Nguyen Xuan Phuc
1
1
Graduate University of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay,
Ha Noi, Viet Nam
2
Institute of Materials Science, VAST, 18-Hoang Quoc Viet Road, Cau Giay, Ha noi, Viet Nam
3
Department of Physics, Quy Nhon University; Binh Dinh Province, Viet Nam
4
Theoretical Physics Research Group, Advanced Institute of Materials Science,
Ton Duc Thang University, Ho Chi Minh City, Viet Nam
5
Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Viet Nam
*
Email: namph.ims@gmail.com
Received: 15 August 2017; Accepted for publication: 20 February 2018
ABSTRACT
In this work, isothermal magnetization and initial dc susceptibility of spheroidal, nearly
monodisperse Mn0.7Zn0.3Fe2O4 nanoparticles (typical diameter: 20 nm) prepared by a
hydrothermal route have been measured between 10 and 300 K. The high-temperature inverse
magnetic susceptibility was always found to follow a linearly temperature dependence. The
deviation from the standard superparamagnetic behavior is related to dipolar interaction among
nanoparticles. The results are well explained using interacting superparamagnetic model, which
is basically a mean field theory. As a consequence, the dipolar interaction affected the specific
loss power of Mn0.7Zn0.3Fe2O4
Keywords: magnetic nanoparticles, interacting superparamagnetic model, spinel.
1. INTRODUCTION
In recent years, magnetic fluid hyperthermia (MFH) therapy has been considered as a
promising therapy for cancer treatment [1]. In the MFH therapy, energy dissipated from
magnetic nanoparticles (MNPs) in an alternating magnetic field can be used to locally raise the
temperature more above physiological temperature (37
o
C), in targeted tumor tissues, thereby
destroying them without harm to surrounding healthy tissue [2]. The large specific loss power
(SLP) is the key required characteristic for clinical hyperthermia. The magnetic fluid containing
MNPs with large SLP can minimize the dose of MNPs, which applied to the patient body, while
maintaining enough heat to kill the cancer cell. Zinc ferrite (ZnFe2O4) and manganese ferrite
(MnFe2O4) nanoparticles are among the most biocompatible agents for MFH. These particles are
Dynamic effects of dipolar interactions on the specific loss power of Mn0.7Zn0.3Fe2O4
51
typically coated with a biocompatible polymer to prevent their aggregation and biodegradation
for in vivo applications [3]. Up to now, theoretical descriptions of magnetic fluids are based on
models consisting of non-interacting particles [4]. Therefore, such behavior has typically been
not observed experimentally in both suspension [5] and biological systems [6]. In the absence of
magnetic fields, interparticle interactions can produce clustering and formation of structures in
suspension [4]. In fact, influence of dipolar interactions on the heating capacity is not so clear
and apparently contradictory results have been reported [7]. The experimental studies regarding
an increase [6], a decrease [8] or a non-monotonic [4] variation of SLP with dipolar interactions
have been reported. From the point of view of theory, most theoretical works agree that SLP
tend to decrease in the presence of strong interactions [4] although a limited increase in a
restricted range of MNPs concentration has also been reported [8]. One of the existing
approaches is the interacting superparamagnetic (ISP) model [9], which is particularly suitable to
account for the effect of dipolar interactions on otherwise superparamagnetic nanoparticles. In
this work, we show that the dynamical aspects of dipolar interaction actually play a major role
on the specific loss power of Mn0.7Zn0.3Fe2O4 nanoparticles.
2. EXPERIMENTAL
Mn0.7Zn0.3Fe2O4 nanoparticles (NPs) having mean diameter of about 20 nm were prepared
by a hydrothermal process employing a Teflon lined stainless steel autoclave. More detailed
information on the synthesis of Mn0.7Zn0.3Fe2O4 NPs is available in ref. [10]. FeCl3, MnCl2,
ZnCl2, HCl and NaOH (Merck 99.9 %) were used as received. The FeCl3, MnCl2, and ZnCl2
were dissolved in aqueous hydrochloric acid solution, and then the sodium hydroxide was
slowly added into the solution. The reaction mixture was stirred for about 30 min. Finally, the
solution was transferred into a Teflon lined stainless-steel autoclave with a filling degree of
80 %. After heating at 180
o
C for 12 h, the autoclave was cooled down to room temperature. The
products were washed several times with hot de-ionized water and acetone and finally dried in
an oven at 80
o
C for 5 h. A X-ray diffractometer (XRD) D 5000 with CuK ( = 0.15406 nm)
radiation was used to determine crystal structure and to estimate grain sizes of the samples. The
particle size of sample was determined by using X-ray diffraction and transmission electron
microscopy (TEM) (JEOL, JEM-1010). All magnetic measurements were carried out on
Quantum Design Physical Property Measurement System (PPMS) system. A homemade unit, in
which a RDO generator produced AC magnetic field with the amplitude in the range 50 – 80 Oe
at a fixed frequency of 178 kHz, was utilized to measure the magnetic inductive heating of
Mn0.7Zn0.3Fe2O4. The temperature change of the fluid was directly monitored by dipping an
optical sensor into the fluid. The concentrations of the fluid, NPs dispersed in water, were 3
mg/mL, 5 mg/mL and 7 mg/mL.
3. RESULTS AND DISCUSSION
The XRD pattern of Mn0.7Zn0.3Fe2O4 nanoparticles is shown in Fig. 1 where clear peaks
corresponding to Bragg diffraction from (220), (311), (222) (400), (422), (333), (440), (620) and
(533) planes. It is well concord with standard JCPDS (No. 10-0319). No other oxide (Fe2O3) or
impurity peaks were observed which infers the phase purity of the Mn0.7Zn0.3Fe2O4. In addition,
the calculated lattice constant of 8.430 Å reveals the cubic structure of Mn0.7Zn0.3Fe2O4. Using
Scherrer’s equation, the calculated crystallite size is 20 nm for the high intensity (311) plane.
The size, shape and morphologies of the Mn0.7Zn0.3Fe2O4 nanoparticles were further determined
Pham Hong Nam, Luong Le Uyen, Doan Minh Thuy, Do Hung Manh, Pham Thanh Phong, Nguyen Xuan Phuc
52
by TEM. The TEM image (Fig. 2a) evidenced that the particles are having almost spherical in
shape. The mean particle size was estimated to be 20 nm, which is close to that obtained from
the XRD data, suggesting that each particle here is a single nano-crystallite.
Figure 1. X-ray diffraction patterns of the Mn0.7Zn0.3Fe2O4 sample.
Figure 2. TEM image and particle size histograms of the Mn0.7Zn0.3Fe2O4 sample.
Figure 3a presents the zero-field-cooled (ZFC) magnetization profile of Mn0.7Zn0.3Fe2O4
nanoparticles under an applied field of 100 Oe. The Curie temperature (TC) was estimated to be
450 K. It should be noticed that TC value of our sample are much higher than the reported TC
value of its bulk counterpart (343 K) [11]. However, no finding the appearance of blocking
temperature (TB) in sample, which can be due to the existence of strong interparticle interactions
origin from the multi-domain behavior of the sample. Therefore, to test this hypothesis of our
system, we have estimated critical diameter for single domain by following equation [12],
(1)
in which Dcr is the critical diameter, wp is the energy density of the magnetic domain and Ms is
the spontaneous magnetization. It is clear that the particles can be considered as single domain
when particle size is smaller than Dcr. Because the Mn0.7Zn0.3Fe2O4 ferrite is a crystal with a
20 30 40 50 60 70 80
(2
2
0
)
(3
1
1
)
(2
2
2
)
(4
0
0
)
(4
2
2
) (
3
3
3
)
(4
4
0
)
(6
2
0
)
(5
3
3
)
2 (degrees)
In
te
n
si
ty
(
ar
b
.
u
n
it
s)
100 nm
(C) a) b)
Dynamic effects of dipolar interactions on the specific loss power of Mn0.7Zn0.3Fe2O4
53
cubic symmetry, the energy density of the magnetic domain can be calculated by following
expression [12]
(2)
where kB is the Boltzmann constant, TC is the Curie temperature, K1 is the magnetocrystalline
anisotropy constant, and a is the crystalline lattice constant. By substituting K1 = 3.8 × 10
4
erg/cm
3
[13], TC = 343 K [10], kB = 1.38 × 10
-16
erg/K, and calculated lattice constant a = 8.45 ×
10
-8
cm, we obtained wp = 0.226 erg/cm
2
. Putting in Ms = 418 G [14] into Eq. (2), Dcr =15.5 nm
is obtained. The fact that this value is smaller than the experimental value (20 nm) indicates the
multi-domain nature of the sample.
Figure 3. The plot of dc magnetization vs. temperature (a) and inverse susceptibility plotted as a function
of the quantity T/Ms
2
for Mn0.7Zn0.3Fe2O4 sample (b). Straight lines are fits to high-temperature data.
As known, in paramagnetic regime, where the magnitude of the magnetic moments
associated to magnetic ions does not change with temperature, the physically significant
information can be extracted plotting 1/ as a function of temperature by the Curie-Weiss law
[9]:
(3)
in which, N is the number of MNPs per unit volume and the saturation magnetization is Ms =
N . This was done in Fig. 3b for our sample. The curve shows that the ideal Curie-Weiss law
correspondence for a linear behavior is indeed measured at high temperature and deviations from
linearity at very low temperatures, which can be ascribed to some type of particle blocking. The
straight line constantly intercept the temperature axis estimating the positive value of θ, implies
a predominant ferromagnetic interaction among magnetic moments. The origin of the
ferromagnetic interaction is attributed to dipolar coupling. Therefore, in this case, it could be of
interest to check for the presence of dipolar interactions between the Mn0.7Zn0.3Fe2O4 MNPs to
better predict the magnetic response of this sample. A comprehensive analysis of the possible
presence of dipolar interactions was carried out with the help of a mean-field model, recently
proposed by Allia et al. [9]. The use of this model could allow us to estimate dipolar interactions
at a temperature region, in which the so-called interacting superparamagnetic (ISP) regime
describes the behavior of interacting nanomagnets. It is well-known that in superparamagnetic
nanoparticles, their hysteretic magnetization curves are well described in terms of Langevin
functions and in some cases, the classical ‘‘superparamagnetic’’ scaling law of the reduced
magnetization M/Ms with Ms(H/T) has been approximately observed; at low temperatures,
0
2
4
6
8
10
0 100 200 300 400 500 600
H = 100 Oe
M
(
em
u
/g
)
T (K)
T
C
= 450 K
(a)
0
2
4
6
8
10
12
14
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
-1
-1
(
O
e
g
/e
m
u
)
T/M
s
2
(K g/
2
emu
2
)
(b)
Pham Hong Nam, Luong Le Uyen, Doan Minh Thuy, Do Hung Manh, Pham Thanh Phong, Nguyen Xuan Phuc
54
deviations from the Ms(H/T) law in samples containing chemically homogeneous particles are
usually ascribed to single-particle blocking and of random, collective interactions among
particles [15].
Figure 4. Reduced magnetization for Mn0.7Zn0.3Fe2O4 sample, measured at seven different temperatures,
and plotted as a function of Ms(H/T) (a) and H/Ms (b). Dark line is the fitting of M(H) curves to a Langevin
function.
Figure 4 shows the reduced magnetization as a function either of Ms(H/T) (Fig. 4a) and
H/Ms (Fig. 4b). It is clearly that our sample don’t obey the classical ‘‘superparamagnetic’’
scaling law. This analysis confirms the inner coherence of the ISP model. On the other hand, the
ISP model could be used to describe for the real magnetic interaction behavior of
Mn0.7Zn0.3Fe2O4 nanoparticles. In the case of magnetic nanoparticles with the interacting
superparamagnetic behavior, the magnetization vs. magnetic field can be described by a
modified Langevin function [9]:
(4)
in which Ms = Nμ is the saturation magnetization, kB is Boltzmann constant, T* is related to the
dipolar energy εD through the relation [9],
(5)
The best fits with Eq. (4) to the data are shown by the lines in Fig. 4b, proving the validity
of the ISP approach. However, in order to gain a deeper insight on this problem, we determined
the value of the effective magnetic anisotropy constant (Keff) from the magnetization data at 10 K
using the law of approach to saturation [16]:
(6)
where Ms is the saturation magnetization, f is the high-field susceptibility, and B is function of
Ms and K, and is given by the following expression [15]:
(7)
From the magnetization curves near the saturation region, B may be deduced. Using Eq.
(7), Keff may be calculated from the above expression. The values of Keff is found to be 1.13 x 10
6
erg/cm
3
, which is larger than the estimated value for bulk ferrite (8.5 x 10
5
erg/cm
3
[17]). This
increase in the effective anisotropy can be associated with the enhanced surface anisotropies in
-1.2
-0.8
-0.4
0
0.4
0.8
1.2
-0.4 -0.2 0 0.2 0.4
10 K
50 K
100 K
150 K
200 K
250 K
300 K
M
/M
s
M
s
(H/T) (10
4
x erg. g
-1
K
-1
)
(a)
-1.5
-1
-0.5
0
0.5
1
1.5
-0.06 -0.04 -0.02 0 0.02 0.04 0.06
10 K
50 K
100 K
150 K
200 K
250 K
300 K
fit
M
/M
s
(b)
H/M
s
(T.g/emu)
Dynamic effects of dipolar interactions on the specific loss power of Mn0.7Zn0.3Fe2O4
55
the nanoparticles. A further confirmation of the veracity of the anisotropy constant value was
obtained from the values of Hc at 10 K. For example, for an assembly of noninteracting
randomly oriented single-domain cubic particles the value of coercivity can be determined by
the expression Hc = 0.64Keff/ Ms, while for uniaxial particles Hc = 0.98Keff/Ms. The values of Hc
are 1498 Oe and 2247 Oe by the law of approach to saturation, respectively. Variations with
respect to these theoretical values can be associated, for example, with interparticle interactions
[15].
In order to study the AC magnetic heating characteristic of Mn0.7Zn0.3Fe2O4 nanoparticles,
the dependence of the heat generation on altering the applied magnetic fields of the sample was
measured at fixed frequencies of 178 kHz and under different magnetic field amplitudes from 40
to 80 Oe. The strength and frequency of the applied AC magnetic field is chosen so that the
high values of SAR is achieved maintaining the safety limit for application in hyperthermia
treatment (Hf ≤ 5×109 Am‒ 1s‒ 1 ) [7]. The experiments were performed for 25 min with
nanoparticles at three different conentrations, viz. 3.0, 5.0, and 7.0 mg/mL. The Specific
Absorption Rate for the nanoparticles can be determined using the following expression,
(8)
where Ci is the specific heat capacity of the i
th
component in ferrofluid, mi is the mass of
component (Mn0.7Zn0.3Fe2O4 nanoparticles and water, respectively), m is the mass of the
Mn0.7Zn0.3Fe2O4 nanoparticles in ferrofluid and dT/dt is the initial slope of the time dependent of
temperature curve. In these experiments, we used the linear relations in ranges 0 – 5 minutes
intervals in order to calculate dT/dt. The results for temperature rise are shown in Fig. 5. It can
be seen that in low applied field (40, 50 Oe), after about 20 minutes of heating, the temperature
of the sample comes to saturation, however, a sharp increase in temperature is noticed for
higher strength of the applied field. This shows that power loss due to Brownian relaxation
dominates at smaller applied field (40, 50 Oe), while that due to Neel’s relaxation favors
comparatively at larger applied field (60, 70 and 80 Oe). The initial temperature rising rate and
SLP of samples was listed in Tab. 1.
Table 1. The initial heating rate (dT/dt) and SLP of magnetic fluids at different particles concentration
under applied fields (40 - 80 Oe) at fixed frequency 178 kHz of Mn0.7Zn0.3Fe2O4 ferrofluid samples.
Applied field
(Oe)
dT/dt (
o
C/s) SLP(W/g)
3 mg/mL 5 mg/mL 7 mg/mL 3 mg/mL 5 mg/mL 7 mg/mL
40 0.0033 0.0132 0.0121 4.6 11.1 7.2
50 0.0131 0.0175 0.0216 18.3 14.6 12.8
60 0.0227 0.0319 0.0422 31.6 26.7 25.2
70 0.0338 0.0479 0.0527 47.1 40.2 31.5
80 0.0421 0.0545 0.0625 58.7 45.6 37.3
As can be seen from Table 1, values of dT/dt for Mn0.7Zn0.3Fe2O4 NPs fluids increases as
almost a linear trend with the ferrite concentration. In addition, it is interesting that when
increasing Mn0.7Zn0.3Fe2O4 NPs concentration in fluids, the SLP values light decrease, which
could be related to effect of interparticle interactions. To analyze the effect of dipolar interaction
Pham Hong Nam, Luong Le Uyen, Doan Minh Thuy, Do Hung Manh, Pham Thanh Phong, Nguyen Xuan Phuc
56
between colloidal clusters on the SLP of magnetic fluids, we focus on fluids with low
aggregation. As shown in Table 1, the highest SAR value is 58.7 W/g for Mn0.7Zn0.3Fe2O4 NPs
fluids with concentration of 3 mg/mL and decreasing with the increasing of Mn0.7Zn0.3Fe2O4 NPs
concentration. The increasing of SLP value when decreasing concentration of Mn0.7Zn0.3Fe2O4
NPs fluids was revealed in the recent report of Presa et al. [18]. They suggest that magnetic
interactions take place inside a particle (magnetic cluster) seem to be responsible for the
changing of SLP value. For superparamagnetic fluids, hysterisis is vanished, SLP value was
dominated by Néel and Brown relaxation loss. The particle-particle interactions strongly effect
on the Néel relaxation time of heating dissipation, resluting in decreasing SLP value when
increasing strengh of interactions [19, 20]. In our case, we may imply that the heating capacity
was effected by the interactions between magnetic colloidal clusters. These interactions not only
affect on the relaxation of each moment in Mn0.7Zn0.3Fe2O4 particles (Néel relaxation), but also
impact strongly on rotation of each clusters that mean impact on Brownian relaxation loss. When
decreasing Mn0.7Zn0.3Fe2O4 concentration the distance between clusters increases and reduces
strength of dipole interactions so that the rotation of clusters smoother that make the Brown
relaxation processes more convenient. At this time the contribution of Brown relaxation loss on
heating dissipation is dominated. The increasing of SLP value with the decreasing
Mn0.7Zn0.3Fe2O4 NPs concentration is as consequence of Brown loss contribution. We imply that
magnetic interaction between clusters plays an importance role in heating capacity of magnetic
fluid because it affects directly on Brown loss. The effect of the dipolar interaction on the
specific absorption rate of iron oxide nanoparticles have been described in previous works [21,
22]. Furthermore, there are also interesting reports on the impact of particle interactions on the
collective behavior of multicore nanoparticles ferrofluids for hyperthermia [23, 24]. The
magnetic ordering and exchange interactions within the multicore nanostructures may lead to
increase a tenfold of SLP for multicore nanoparticle systems with respect to that of single core
materials as recent report of Lartigue et al. [23]. However, further studies are necessary to
describe how magnetic interaction between clusters effect on each heating dissipation loss.
Figure 5. (a)–(c) Temperature vs. time curves for Mn0.7Zn0.3Fe2O4 nanoparticles with different
concentrations (3, 5, and 7 mg/ml). The fixed frequency is 178 kHz and the applied fields are 40, 50, 60,
70 and 80 Oe.
4. CONCLUSION
In summary, we studied the role of the magnetic interaction between magnetic clusters on
heating dissipation of magnetic fluid of prepared Mn0.7Zn0.3Fe2O4 nanoparticles in an alternating
field. Our results implied that magnetic interactions between magnetic clusters affect directly to
the large value of the SLP for Mn0.7Zn0.3Fe2O4 fluids. Reducing Mn0.7Zn0.3Fe2O4 nanoparticles
concentration in fluid means decreasing dipolar interaction between colloidal particles and help
25
30
35
40
45
50
55
0 300 600 900 1200 1500
80 Oe
70 Oe
60 Oe
50 Oe
40 Oe
T
(
o
C
)
t (s)
(a)
25
30
35
40
45
50
55
60
0 300 600 900 1200 1500
80 Oe
70 Oe
60 Oe
50 Oe
40 Oe
T
(
o
C
)
t (s)
(b)
30
40
50
60
70
0 300 600 900 1200 1500
80 Oe
70 Oe
60 Oe
50 Oe
40 Oe
T
(
o
C
)
t (s)
(c)
Dynamic effects of dipolar interactions on the specific loss power of Mn0.7Zn0.3Fe2O4
57
colloidal particles move easier in fluid. Therefore, SAR achieved higher value at lower
concentration. Maximum SAR of 58.7 W/g is achieved in the AC magnetic field of 80 Oe while
the frequency is set to 178 kHz. Thus, these nanoparticles could also be used as effective heat
mediator in AC induction heating.
Acknowledgement. This study was supported by Vietnam National Foundation for Science and
Technology Development (NAFOSTED) under grant number 103.02-2015.74 and Program of
Development in the field of Physics by 2020 under grant number KHCBVL.03/18-19. The authors are
thankful to Institute of Materials Science, Graduate University of Science and Technology and Ton Duc
Thang University.
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