Key Engineering Materials
ISSN: 1662-9795, Vol. 855, pp 172-176
doi:10.4028/www.scientific.net/KEM.855.172
© 2020 Trans Tech Publications Ltd, Switzerland
Submitted: 2019-11-18
Revised: 2020-03-09
Accepted: 2020-03-30
Online: 2020-07-27
Blocking Temperature of Magnetite Nanoparticles Fe3O4 Encapsulated
Silicon Dioxide SiO2
Togar Saragi1,a*, Hotmas D Sinaga1,b, Feni Rahmi1,c, Gustiani A Pramesti1,d,
Adi Sugiarto1,e, Arnold Therigan1,f, Norman Syakir1,g, Sahrul Hidayat1,h
and Risdiana1,i
Department of Physics, Universitas Padjadjaran, Jl. Raya Bandung-Sumedang Km. 21 Jatinangor
Jawa Barat Indonesia
1
t.saragi@phys.unpad.ac.id, bhotmas15001@mail.unpad.ac.id, cfenirahmi.fr@gmail.com,
gustiani16001@mail.unpad.ac.id, eadi16002@mail.unpad.ac.id, fyonathantherigan@yahoo.co.id,
g
n.sjakir@phys.unpad.ac.id, hsahrul.hidayat@phys.unpad.ac.id, irisdiana@phys.unpad.ac.id
*a
d
Keywords: blocking temperature, co-presipitation, magnetite, nanoparticles, FC, ZFC, SiO2
Abstract. One of the important characteristics of magnetic materials is the measurement of
magnetic characteristics through Superconducting Quantum Interference Device (SQUID)
especially magnetization temperature dependence M(T)ZFC and MTFC measurement. In this work,
we reported temperature dependence of magnetization for magnetite nanoparticles with and without
SiO2 under magnetic fields of 100 Oe. The magnetite nanoparticles were synthesized by coprecipitation method. It was calculated that the blocking temperature of magnetite nanoparticles
Fe3O4 without and with SiO2 encapsulation is 118.38 K and 209.03 K, respectively. The blocking
temperatures of magnetic nanoparticles increase by SiO2 encapsulation.
Introduction
Nano magnetic particles, generally in a single domain state with uniaxial anisotropy [1] have
been studied very intensively due to their fundamental as well as technological relevance [2,3]. The
magnetic stability of nanoparticles will become an important issue in technological applications due
to thermal agitation by decreasing the particle size [4]. For ferromagnetic or ferrimagnetic
materials, with small enough nanoparticles their magnetization becomes thermally fluctuated. As an
influence of magnetic particle size on magnetic properties, the superparmagnetic materials have a
high saturation magnetization, although smaller than that of bulk of ferromagnetic and the
coercivity changes with the particle size, and at small enough size, the coercivity and remanen
become zero. In the single domain structure, superparamagnetic materials can fluctuate randomly
by thermal fluctuation at high enough temperatures just as an atom spin in paramagnetic materials.
At low temperatures, the thermal energy becomes smaller and the magnetic moments become
blocked. This temperature in which the magnetic moment becomes blocked is called the blocking
temperature (TB). From M(T) curve point of view, there are two different curves for zero-fieldcooling (ZFC) and filed-cooling (FC) measurement. In ZFC curve, the moment increased with the
temperature and then decreased, while the moment decreased in FC curve. The temperature at the
peak point of ZFC curve is also called as the blocking temperature. Below blocking temperature,
superparamagnetic material looses its preferred direction of magnetization in zero magnetic fields
[5]. In other word, blocking temperature TB can be defined as the temperature between the blocked
state and the superparamagnetic state.
The blocking temperature is related to applied field and particle size. Nanoparticles with an
uniaxial anisotropy randomly flip the direction of their magnetization. This effect is induced by
thermal energy. The average time to perform such a flip is given by the relaxation time:
∆𝐸𝐸
𝜏𝜏 = 𝜏𝜏0 exp �
�
(1)
𝑘𝑘𝐵𝐵 𝑇𝑇
where τ0 is the length of time characteristics of the probed materials or the attempt frequency of
order magnitude around 10-9 to 10-12 s [6], the anisotropy energy ∆E is the energy barrier the
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Key Engineering Materials Vol. 855
173
magnetization flip has to overcome by thermal energy or the energy barrier between the
degenerated double-well potential, kB is the Boltzman conxtant and T is the temperature in Kelvin,
K is the anisotropy constant and V is the particle volume. Eq. 1 gives a connection between the time
τ and the temperature TB . If the moment is detected by a technique with a measuring time, τm, the
particle behaves superparamagnetically when τ < τm and the particle is in the blocked state when τ >
τm. The blocking temperature, TB of superparamagnetic blocking of noninteracting particles is
determined using the well-known Neel–Brown formula (τm=τ) defined by [7]
𝐾𝐾𝑒𝑒𝑒𝑒𝑒𝑒 𝑉𝑉
𝑇𝑇𝐵𝐵 =
𝜏𝜏
(2)
𝑘𝑘𝐵𝐵 ln � 𝜏𝜏𝑚𝑚 �
0
In the quasi-static magnetic measurements (τ~101–2 s), and Eq. (2) yields
∆𝐸𝐸
𝑇𝑇𝐵𝐵 =
(3)
25𝑘𝑘𝐵𝐵
where Keff is the effective magnetic anisotropy constant consisting of the volume (KV) and surface
(6KS/d, where d is the linear particle size) contributions. From Eq. 3, it can be calculated the the
anisotropy energy ∆E.
In this work, we reported the blocking temperature of iron oxide with and without silicon dioxide
encapsulation. The samples were synthesized by simple co-precipitation method [8-10]. All samples
sintered at temperature of 80°C for 4 hours. The magnetization dependence temperature M(T)ZFC
and M(T)FC investigate by SQUID measurements.
Experiment
Magnetic nanoparticles of Fe3O4 were synthesized by co-precipitation method [8,10]. After
washing, the obtained nanoparticles of Fe3O4 were dried at 80°C. Another sample of Fe3O4 with
SiO2 encapsulation were dispersed in di-water, washed and dried at 80°C. The samples of Fe3O4
without SiO2 encapsulation referred to as Fe3O4 sample and the samples of Fe3O4 with SiO2
encapsulation referred to as Fe3O4.SiO2 sample. The particles size was measured by HR-TEM
measurement. All samples were characterized by superconducting quantum interference device
(SQUID) in order to investigated the magnetic properties. Temperature dependence of magnetic
moment M(T) were measured by zero-field cooling (ZFC) and field cooling (FC) in external field
of 100 Oe from 300 K to 10 K. The curve of M(T)ZFC measurement, then fitted to get the function
of M(T) equation and then derived it to obtain the first derivate function of dM/dT. The value of
blocking temperature, TB revealed from the first derivative zero of magnetization. The information
of magnetic configuration can be analyzed from M(T)FC curve.
Result and Discussion
Fig. 1 shows the M(T)ZFC and M(T)FC dependence for sample Fe3O4 (a) and Fe3O4.SiO2 (b) in
applied magnetic field of 100 Oe. It is found that, the M(T)ZFC dependence for sample of Fe3O4 can
be fitted by a function in Eq. (4).
(4)
𝑀𝑀 = 𝐴𝐴0 + 𝐴𝐴𝐴𝐴 + 𝐵𝐵𝑇𝑇 2 + 𝐶𝐶𝑇𝑇 3 + 𝐷𝐷𝑇𝑇 4 + 𝐸𝐸𝑇𝑇 5 ,
-6
-8
where A0 = -0.13709, A = 0.09422, B = -0.001, C = 5.181157×10 , D = -1.32923×10 , E =
1.33497×10-11. While the The M(T)ZFC dependence for sample of Fe3O4.SiO2 can be fitted by a
function in Eq. (5).
(5)
𝑀𝑀 = 𝐴𝐴0 + 𝐴𝐴𝐴𝐴 + 𝐵𝐵𝑇𝑇 2 + 𝐶𝐶𝑇𝑇 3 + 𝐷𝐷𝑇𝑇 4 + 𝐸𝐸𝑇𝑇 5 ,
-4
-6
-8
where A0 = 2.22735, A = 0.03934, B = 7.06446×10 , C = -7.02875×10 , D = 2.26417×10 , E =
-2.51974×10-11.
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Magnetism and its Application
Fig. 1. M(T) dependence of nanoparticles of Fe3O4 (a), and Fe3O4.SiO2 (b). Solid line in (a) and
(b) is a fitting line using Eq. (4) and (5), respectively.
Fig. 2 shows the first derivative zero (dM/dT=0) of Fe3O4 and Fe3O4.SiO2. The intersection
point of dM/dT curve at zero value of magnetization is interpreted as a blocking temperature of
samples. It is found that the blocking temperature of Fe3O4 is 118.38 K, while the blocking
temperature of Fe3O4.SiO2 is 209.03 K. The blocking temperature increased by SiO2 encapsulation
process. From Fig. 1 and Fig. 2, it was observed that around these temperature of 118.38 K (Fe3O4)
and 209.03 K (Fe3O4.SiO2), both of the M(T)ZFC curve exhibited a maximum, while the first
derivative of M(T)ZFC reached zero. These points represent the temperature blocking of
superparamagnetic nanoparticles. The M(T)ZFC increased by increasing of temperature from 10 K to
118.38 K (Fe3O4) and from 10 K to 209.03 K (Fe3O4.SiO2), indicating that the nanoparticles
magnetic moments increasingly parallel to the magnetic field. At temperature below 118.38 K and
209.03 K, nanoparticles of Fe3O4 and Fe3O4.SiO2 are in a blocked state, respectively, while at above
the temperature of 118.38 K and 209.03 K, nanoparticles of Fe3O4 and Fe3O4.SiO2 are in a single
domain superparamagnetic, respectively [8,11]. The blocking temperature is known to be very
sensitive to particle size distribution [12]. Due to the values of blocking temperature resulted from
the present analysis, it can be interpreted that the particles size of Fe3O4.SiO2 is larger than that of
Fe3O4. From our previous work, it is confirmed that the particle size of Fe3O4 measured by HRTEM is about 11 nm, while the core of Fe3O4.SiO2 without calculating the size of encapsulating
layer is about 10 nm [13]. With including the encapsulating layer, the size of Fe3O4.SiO2 is about
14.2 nm, which is good agreement with the interpretation of the present result. The sample with
higher blocking temperature has the larger particle size.
Fig. 2. First derivative curve of M(T)ZFC of nanoparticles of Fe3O4 and Fe3O4.SiO2.
Key Engineering Materials Vol. 855
175
Fig. 3 shows the temperature dependence of M(T)FC of nanoparticles Fe3O4 and Fe3O4.SiO2. The
magnetization curve of both samples increased with decreasing temperature, however the value of
this magnetization did not continue to increase as explained in the Curie-Weiss theory. The
magnetization value slightly decreased at 66 K and 183 K for Fe3O4 and Fe3O4.SiO2, respectively.
We assume, that it was probably due to the suddenly snaps of the system into a different magnetic
configuration. The encapsulation process will change all properties of magnetic nanoparticles of
Fe3O4 such as blocking temperature, particles size and temperature of magnetic configuration
changed as shown in Table 1.
Table 1. The blocking temperature, critical temperature and particles size of Fe3O4 and Fe3O4.SiO2.
Sample
Blocking
temperature
(K)
118.38
209.03
Fe3O4
Fe3O4.SiO2
Particles
size (nm)
11.00[13]
14.20
Temperature of
magnetic configuration
changed (K)
66.00
183.00
∆E
(×103kB)
3.00
5.20
Fig. 3. M(T)FC dependence of nanoparticles of Fe3O4 and Fe3O4.SiO2.
By using of Eq.(3), it was calculated that the anisotropy energy ∆E of the samples Fe3O4 and
Fe3O4.SiO2 is 3.0×103kB and 5.2×103kB, respectively. The energy barrier of magnetization flip
which is overcome by thermal energy of sample increasing by increased of blocking temperature
and also the size of particles.
Summary
The magnetic properties of Fe3O4 and Fe3O4.SiO2 have successfully investigated by analyzed M(T)
curve from SQUID magnetometer measurement. It is found that the encapsulation process will
change all properties of magnetic nanoparticles of Fe3O4. The blocking temperature, particles size
and critical temperature increased from 118 K to 209 K, 11 nm to 14.2 nm and 66 K to 183 K,
respectively by SiO2 encapsulation.
Acknowledgment
This work was supported by Riset Kompetensi Dosen Unpad (RKDU) 2019 (3nd Year), No.
3331/UN6.D/LT/2019, and partially supported by Academic Leadership Grant of Universitas
Padjadjaran 2019, No. 3339/UN6.D/LT/2019. We would like to thanks to Koike Lab, Dept. of Apl.
Phys. Graduate School of Engineering Tohoku University Japan for XRD and SQUID-VSM
Measurements.
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Magnetism and its Application
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