Synthesis of various magnetic nanostructures and the microwave characterizations 1
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Chapter 1 Introduction
1
Chapter 1
Introduction
This work is developed based on the synthesis of various magnetic nanostructures for
the application in microwave absorption. This chapter gives an overview on the
fundamental knowledge on the microwave absorption, the development on magnetic
materials for microwave application and large scale synthesis technologies for the
magnetic materials. After introducing the background information, the motivations of
my research work are listed.
1.1 Fundamentals for microwave absorption
Microwave is the electromagnetic waves with wavelengths ranging from 1 cm to 1 m.
The corresponding frequency range is between 300MHz (0.3GHz) to 300GHz.[1]
Microwave technology has been mainly the exclusive domain of the defense industry.
With the rapid development of communication systems for applications, such as
cellular mobile telephony, broadband wireless access, wireless local area networks
and satellite base cellular communications, microwave technology is closely bound up
with our daily life because these communication systems are employed everywhere,
including corporate offices, private homes and public recreation places. Although we
have benefited from these modern communication systems, the microwave radiation
emitted by these systems have severe consequences on our body.[2] Another problem
caused by the unprecedented growth of communication systems is the severe
Chapter 1 Introduction
2
electromagnetic interference among the electronic devices.[3] Therefore, microwave
absorbing materials are required to alleviate these problems by absorbing the
unwanted microwave.
1.1.1 Description of microwave absorption ability
A schematic diagram for the measurement of microwave absorption ability is shown
in Fig. 1.1. Ideally, the perpendicular incident microwave is converted into two parts:
the reflected and the absorbed microwaves. If P
in
is incident power density at a
measuring point, and P
r
is reflected power density at the same measuring point, P
ab
is
absorbed power by the composite. Their relation can be expressed as
(1.1)
(1.2)
(1.3)
where R and A are reflection loss (R
L
) and absorption loss in decibels (dB),
Incident wave
Reflected wave
Absorber
Metal
Fig. 1.1 A Schematic diagram for the definition of microwave absorption ability.
Chapter 1 Introduction
3
respectively. Thus, the microwave efficiency can be evaluated from R using Eq. (1.3).
Larger absolute value of R means more effective wave-absorbing ability of the
absorbers.[4]
1.1.2 Calculation of microwave absorption ability
The specular method was commonly used as a theoretical approach in explaining the
propagation characteristics of a transverse electromagnetic (TM) wave in a
single-layer absorber backed by a perfect conductor. For incident wave perpendicular
to the surface of a single-layer absorber backed by a perfect conductor, the input
impedance (Z
in
) at the air-material interface is given by: [5,6]
(1.4)
where
is intrinsic impedance of free space,
is propagation factor in the material, is angular frequency,
is the speed of light and t is thickness of the sample. The complex permittivity
and
magnetic permeability
can be measured experimentally. They could be given as
(1.5)
(1.6)
The reflection coefficient (Г) is defined as
(1.7)
Substitute Eq. (1.4) into Eq. (1.7), and then the value ofcan be obtained. Finally,
Chapter 1 Introduction
4
in decibels (dB) can be written as
(1.8)
When the reflection coefficient (Г) reaches its minimal value zero, which means
, so called impedance match, the lowest reflection loss can be obtained.
Usually, the microwave absorbing characteristics is evaluated by the resonance
frequency
and the reflection loss
as well as the thickness of the absorber.
Effective microwave absorption refers to a low
value at a high
position, and a
small for the design of lightweight absorber.
The initial expression of
is defined as
(1.9)
Hence, materials with the property of
are ideal for microwave absorption.
Unfortunately, for the existing materials, the relative permeability generally does not
approach the magnitude of the relative permittivity at microwave frequency band. The
difference between these two parameters can be reduced when the permeability of the
magnetic materials is high.
1.1.3 Snoek’s law
For soft magnetic bulk materials, the product of the intrinsic permeability
and the
resonance frequency
is constrained by a constant related to the saturation
Chapter 1 Introduction
5
magnetization
, as shown by the Snoek’s law[7]:
(1.10)
where
is resonance frequency and gyromagnetic factor. The
expression in Eq. (1.10) is valid for the polycrystalline bulk materials with uniaxial or
cubic magnetocrystalline anisotropy field. It shows that there exist trade-offs between
high permeability levels and operation at high frequency band. Hence an effective
way to attain is using magnetic materials with high
can be used to obtain high
permeability
at microwave frequency band.
There is another expression of Snoek’s law for planar materials, which is given as[8]
(1.11)
where
and
are the out-of-plane and in-plane anisotropy fields, respectively.
Other parameters in Eq. (1.11) have the same meaning as those in Eq. (1.10). When
compared these two equations, the difference lies in the right side could be found. A
smaller
with larger
is good combination for the materials with higher
permeability and higher working frequency, which can be induced by an artificial or
anintrinsic bianisotropy system.[9] Hence, the shape construction of magnetic
particles is another effective way to attain high permeability
at elevated frequency
range.
In a way, the Snoek’ law could be seen as a criterion for magnetic materials used as
microwave absorber. The calculated value from the right side of equations for the
Chapter 1 Introduction
6
Snoek’s law is used to evaluate whether a magnetic material could be an effective
microwave absorber. Researchers usually seek for a large calculated value when
design an absorber. For this part, we could know that both of the enhancement in the
saturation magnetization and the induce shape anisotropic field into the particles can
extend the Snoek’s limitation.
1.1.4 Skin effect
In the view of the Snoek’s law, high saturation magnetization is required to obtain
effective microwave absorption. As such, metallic magnetic materials are commonly
employed as microwave absorbing materials. While for metallic materials, skin effect
is nontrivial due to their relative low resistivity. When a microwave wave penetrates
into a conductive material, mobile charges on the surface are made to oscillate back
and forth in the same frequency as the impinging fields. An alternating electric current
will be brought by the movement of these charges. The current density is greatest at
Fig. 1.2 A schematic diagram of the skin effect.
Field Strength
X
Z
d
Chapter 1 Introduction
7
the conductor’s surface. The decrease in current density with depth is so called skin
effect,[10] as displayed by the scheme in Fig. 1.2. When the depth is d, the
corresponding electric field reduces from initial
to E, as below:
(1.12)
The skin depth is the distance over which the current falls to 1/e of its original
value, which is dependent on the electrical conductivity of the materials as well as the
incident magnetic field. The relationship is described as
(1.13)
where is the angular frequency of current, , is the
absolute magnetic permeability,
.
Hence it can be written as
(1.14)
Where is frequency of the wave in Hz, is resistivity of the medium in Ω·m,
H/m,
is relative permeability of the material.
The skin effect can dissipate the incident microwave by inducing the eddy current on
the surface of the material but hinder the microwave to penetrate into the inner part of
the materials. Thus, the particle size should not significantly exceed the skin depth for
the sake of high microwave absorbing efficiency. For example, the resistivity of iron
Chapter 1 Introduction
8
is
Ω·m, when the frequency is GHz, its relative permeability is
around 10.[11] As a result, the skin depth is around 1 μm. Wu et al.[12] have proven
that the 1μm carbonyl iron particles show better microwave absorbing performance
than 10 μm carbonyl iron particles through experimental results and theoretical
calculations. The difference is well explained by size-dependent skin effect using the
calculation model derived from Landau-Lifshitz-Gilbert (LLG) equation. It is
necessary to reduce the skin effect of metal-based materials to improve their
microwave absorbing ability.
1.2 Magnetic materials for microwave absorption
Based on the Snoek’s law and skin effect, the option of magnetic materials used for
microwave absorbers should have high saturation magnetization, high anisotropy field
and high resistivity.
1.2.1 Metallic magnetic materials
Metallic materials (Fe, Co, Ni and their alloys) are commonly used for microwave
absorbers due to their high saturation magnetizations.[13-15] The problem of the
metallic materials is the skin effect due to the high conductivity, resulting in a very
small skin depth. Hence the metallic particles used for magnetic filler of microwave
absorbers should be pulverized into small particles with a size comparable to the skin
depth. Non-metal elements, such as boron and silicone,[16-18] are doped into these
Chapter 1 Introduction
9
alloys to enhance the resistivity. Besides, the skin effect could be reduced by
structured metallic magnetic particles, such as the Fe nanowires, Co and Fe/Co
nanoplatelets, FeSiB flakes and Ni fibers, as well as hollow structures.[19-23] For
these structured particles, either the radius size or the thickness of magnetic particles
is less than or comparable with the skin depth. Hence the skin effect could be
effectively suppressed.
1.2.2 Ferrites
Ferrites are usually non-conductive ferromagnetic ceramic compounds derived from
iron oxides such as hematite (α-Fe
2
O
3
) or magnetite (Fe
3
O
4
). In the big family of
ferrites, two groups of ferrites, hexagonal ferrites and spinel ferrites, are commonly
used for microwave absorbers. They are named according to their crystal structures.
1.2.2.1 Hexagonal ferrites
There are several types of hexagonal ferrites named as M, W, Y and Z phases.[24-26]
The hexagonal ferrites have attracted intensively attentions due to their very strong
magnetic anisotropy. At room temperature, Co
2
Z barium ferrite (Ba
3
Co
2
Fe
24
O
41
)
shows a c-plane anisotropy with a large out-of-plane anisotropy field of 12 kOe and a
small in-plane anisotropy field of about 0.120 kOe.[27] M-type barium ferrite
(BaFe
12
O
19
) exhibits ferromagnetic resonance around 50 GHz due to its very high
magnetocrystalline anisotropy induced by the anisotropic structure.[28] The
disadvantage of hexagonal ferrites is the relative low saturation magnetization when
Chapter 1 Introduction
10
compared with metallic magnetic materials or spinel ferrites, resulting in an
insufficient high permeability. To overcome this problem, some divalent metal cations
(Zn
2+
, Co
2+
, Ni
2+
etc.) and trivalent cations (Al
3+
, Cr
3+
etc.) are doped to substitute
part of Ba
2+
or Fe
3+
cations.[29-31] The function of the doped element is to change the
quantity of spin down or spin up moments, resulting in an enhancement of the
saturation magnetization.[32] In some cases, a metallic magnetic layer is coated on
the surface of hexagonal ferrites nanoparticles to increase the magnetization.[33]
1.2.2.2 Spinel ferrites
The crystal structure of spinel ferrite is much simpler than that of hexagonal ferrites.
Normal spinel structures are usually cubic closed-packed oxides with one octahedral
and two tetrahedral sites per oxide. The tetrahedral points are smaller than the
Fig. 1.3 Schematic illustration of normal spinel structure, i.e. A
2+
B
3+
2
O
4
. A
2+
is
located at tetrahedral sites (bubbles in green tetrahedron); B
3+
is located at
octahedral sites (yellow bubbles).
Chapter 1 Introduction
11
octahedral points. B
3+
ions occupy the octahedral holes because of a charge factor, but
can only occupy half of the octahedral holes. A
2+
ions occupy 1/8th of the tetrahedral
holes. Fig. 1.3 shows the schematic illustration of spinel structure with A
2+
at
tetrahedral sites (bubbles in the green tetrahedron) and B
3+
at octahedral sites (yellow
bubbles).
Inverse spinel structures however are slightly different in that one must take into
account the crystal field stabilization energies (CFSE) of the transition metals present.
Some ions may have a distinct preference on the octahedral site which is dependent
on the d-electron count. If the A
2+
ions have a strong preference for the octahedral site,
they will force their way into it and displace half of the B
3+
ions from the octahedral
sites. If the B
3+
ions have a low or zero octahedral site stability energy (OSSE), then
they have no preference and will adopt the tetrahedral site. A typical example of an
inverse spine structure is Fe
3
O
4
. In which, Fe
2+
ions and half of the Fe
3+
ions occupy
octahedral sites, while the other half of the Fe
3+
ions occupy tetrahedral sites. So the
electron transfer between Fe
3+
and Fe
2+
gives rise to ion jumps and relaxation in the
Fe
3
O
4
particles and contributes a particular dielectric loss.
It is well known that, MnZn-ferrite (Mn
a
Zn
1-a
Fe
2
O
4
) and NiZn-ferrite (Ni
a
Zn
1-a
Fe
2
O
4
)
are two typical spinel ferrites suitable for microwave application due to their high
permeability. Many researches have doped some magnetic or nonmagnetic
elements,[34-37] such as Cr, Cu, Co and rare earth elements into the MnZn- and
Chapter 1 Introduction
12
NiZn-ferrites to enhance the resistivity and further improve the microwave absorbing
property. As seen from the overviews, no obvious improvement has been made on
increasing the saturation magnetization. The Snoek’s law limitation still exists even
for the doped spinel ferrites, resulting in the resonance frequency positioning at
megahertz. Inspired by hexagonal ferrites, we have known that the Snoek’s law could
be extended by magnetically anisotropic materials. For spinel ferrites, the magnetic
anisotropy can be induced by shaping the materials into anisotropic structures. Fe
3
O
4
is selected for our works due to its relatively high saturation magnetization (~ 90
emu/g), as well as the well-developed synthesis technologies for Fe
3
O
4
.
1.3 Brief review of size-controlled synthesis technology
In the design of microwave absorber, magnetic materials have to be used as fine
particles dispersed in an insulating matrix. Besides the intrinsic properties of magnetic
materials, the microstructure and morphology are also important to the microwave
absorption performance of the composite. The uniformity of magnetic particles in size
and shape is very necessary to form homogeneous composite. For materials with
morphological features on the nanoscale, size-dependent properties become more
important. For magnetic materials, the particle size is of great importance to the
physical properties, such as magnetization, coercivity, domain structure and some
critical temperature points (the Curie temperature, the Néel temperature and the
blocking temperature). The magnetic recording density is remarkably enhanced while
Chapter 1 Introduction
13
the saturation magnetization is reduced as the particle size decreases.[38] The
superparamagnetism of small ferro- or ferri-magnetic nanoparticles opens up the road
to biological applications, such as magnetic resonance imaging, magnetic field guided
drug delivery carriers, bioseparation and hyperthermia agent in cancer treatment.[39]
referring to bioapplications, the dispersiblity and the toxicity of nanoparticles are also
relevant to the particle size.[40,41] Hence the control over the particles size is
significant to various applications. The synthetic route is a key factor that determined
the particle sizes and the development on the synthetic routes regarding to size control
is briefly introduced as following:
1.3.1 Ceramic sintering method
Traditional ceramic sintering is a kind of solid state reaction method for preparation of
polycrystalline solids using a mixture of solid starting materials. The solid materials
do not react with each other at room temperature and high temperatures (such as 1000℃
[42]) are required to form a desired phase. This method has been used to prepare
various solid oxides including transitional metal oxides [43, 44] and ferrites.[45,46]
The morphology of resultant product is dependent on several factors, such as the
reaction temperature, the heating rate, the surface area of the solids materials and their
reactivity and thermodynamic free energy change along with the reaction, and so on.
Experimentally, the temperature and heating rate are more controllable factors
compared with others. The simultaneous control over the particle size, the
Chapter 1 Introduction
14
composition as well as the microstructure of resultant powders are realized by using
different reaction temperatures.[47-49] In the work done by Gharagozlou,[50] the size
of Co-ferrite particles showed an increase trend with calcination temperatures.
However, the agglomeration led by calcination is unavoidable in the solid reaction
process, which makes it difficult to distinguish each particle.
1.3.2 Ball milling method
Ball milling is also a kind of solid state reaction which happens in a sealed vial. This
technique is well established for the preparation of non-equilibrium phases, alloys
with extended solubility limits and nanocrystalline materials based on a mechanical
alloying process.[51] The starting materials as well as a number of balls are
transferred into the vial before the operation, and then ground at a provided speed.
The resulting particles are products of the repeated and cold welding caused by the
ball impacts.[52] The controllable parameters involved in the reaction process are the
operation speed, the ball-to-powder ratio and the grinding period, as investigated by
Serra et al. on the ball-milled Fe
77
Nb
7
B
15
Cu
1
.[53] The product of ball milling is
usually irregular powders in micron scale, and prolonged milling leads to narrower
distributed powders due to the balance is reached between the fracture and welding
process.[54] Gheisari et al.[55] reveal that the higher grinding speed is helpful to
reach such balance, but resulting in relative larger particle size. In general, the
morphology is not controllable because it varies easily with the materials and the
Chapter 1 Introduction
15
experimental conditions. To make an improvement on the ball milling method,
medium-assistant milling process (so called wet-milling, relative to dry-milling
without using medium) has been introduced. The medium could be organic chemicals,
such as ethyl alcohol, methyl alcohol, heptane and stearic acid and so on.[56-59] The
medium is added into the vial together with the starting powders. Unlike the irregular
particles obtained after dry milling process, it is easy to obtain micron-size flaky
particles by wet-milling method. It has been intensively used to produce iron-based
flaky alloys, such as FeAl, FeSiB, FeCo, FeNdB and FeNi.[60-64] So far, the milled
flaky products are all in micron scale and with a broad size distribution.
1.3.3 Wet Chemical methods
Wet chemical methods are commonly used to synthesize a wide variety of
nanomaterials. There are several solution strategies to obtain size-controllable
nanoparticles.
(1) One is the template-direct synthesis, in which various kinds of templates are used
to guide the formation of nanocrystals with specific shape and size. Ion-track etched
membranes and alumina membranes are developed for nanowires, nanotubes and
nanorods.[65,66] These templates are commercially available with various pore sizes
which afford direct control over the diameter of one dimensional products, while the
length increased with the processing time. Nucleotides/nucleic acids are attractive
biomineralization templates for the growth of inorganic nanoparticles.[67] The ability
Chapter 1 Introduction
16
to tune the size of nanoparticles depends on the control over the nucleic acid structure,
composition and sequence.[68] the After template-directed synthesis procedure, the
after-treatment process is needed to remove the template, such as calcination or
solvent extraction.[69] Therefore, in most cases, template-free methods are preferred
due to their simpler operation process.
(2) A second one is the seed-mediate growth. The strategy based on the temporal
separation of nucleation and growth processes is considered to be very efficient to
control the nanoparticles size and shape precisely. Bastús et al.[70] succeeded to
synthesize uniform quasi-spherical gold nanoparticles up to ~200 nm by using a
kinetically controlled seeded growth strategy via the reduction of gold precursor by
sodium citrate. Jana et al.[71] used 4 nm silver nanoparticles as seeds for the
preparation of 42 nm silver rods and 4 μm silver wires. Sun et al.[72] applied this
method to the synthesis of Fe
3
O
4
nanoparticles. 4 nm Fe
3
O
4
nanoparticles were
prepared in advance and used as seeds to prepare larger particles in a following step.
The resultant particles size could be precisely tuned by adjusting the quantity of used
seeds. Li et al.[73] also used Fe
3
O
4
seeds to prepare size-controlled Fe
3
O
4
/Ag
heterodimer nanocrystals. As seen from the previous works,[74-76] the seed-mediate
growth method was intensively applied to the synthesis of ferrite oxides magnetic
nanoparticles with size less than 20 nm. This method usually leads to very narrow size
distribution without any size-selection procedure.
Chapter 1 Introduction
17
(3) A third strategy is the so called one-step synthesis route, which means that the
nucleation and growth processes take place in a continuous process. Typical methods
based on this strategy like coprecipation, hydrothermal and thermal decomposition
have been investigated for a long period.
Coprecipitation process involves two stages:[77] a short burst of nucleation when the
concentration of the species reaches critical supersaturation, and a slow growth of the
nuclei by diffusion of the solutes to the surface of the crystal. The size and shape of
the coprecipitated nanoparticles can be tailored by pH adjustment, ionic strength,
nature of the salts (chlorides, sulfates or nitrates). For the coprecipitation of Fe
3
O
4
nanoparticles, the Fe(Ⅱ)/Fe(Ⅲ) concentration ratio is a relative important factor
which affects the particle size.[78] Until now, the coprecipitation method is not
suitable for precise control over the particle size. A size sorting process is needed to
narrow down the size distribution. The poor crystallinity is another problem,[79]
especially for magnetic particles, because it reduces the magnetism in some extent.
This problem is usually overcome by a further calcination process.
Like coprecipitation process, hydrothermal treatment is also performed in aqueous
media but under high pressure and high temperature. It is commonly used to prepare
metal oxides by employing different precursors.[80,81] During the hydrothermal
synthesis, the grain growth mainly origins from a faster deposition rate than the
dissolution rate of the precursor on the crystal surface. The rates are sensitive to the
Chapter 1 Introduction
18
reaction temperature, the pH value of the solution. For a better control over the crystal
growth, surfactants such as poly(vinylpyrrolidone),[82] cetyltrimethyam-monium
bromide [83] and polyisobutylene bis-succinimide [84] are used. Even though, it is
difficult to control the particle size of magnetic materials synthesized by hydrothermal
route.[85,86] This may be induced by the aggregation of magnetic particles. While the
size control over nonmagnetic nanoparticles, such as hematite,[87,88] is much easier.
Thermal decomposition method refers to the decomposition of organometallic
precursors in high boiling point organic solvent. It is a very promising way to
synthesize the monodisperse semiconductor,[89] bimetallic[90] and oxides[91]
nanocrystals with uniform and controllable size and shape. The study on the reaction
kinetics by Kwon et al.,[92] the formation of high-quality nanoparticles attributes to
the burst of nucleation followed by high growth rate. Two-dimensional and
three-dimensional assemblies[93,94] have proven the uniformity of nanoparticles
synthesized by thermal decomposition method. During the synthesis process, the
concentration of surfactant as well the molar ratio of surfactant to precursors is found
important to the size and shape of final products.[95, 96] The choice of surfactant,
therefore, is a key factor. For the synthesis of Fe
3
O
4
nanoparticles, the commonly used
surfactants are oleic acid (OA), oleylamine (OL), 1,2-hexadecanediol and
oleates.[97,98] Besides, the reaction temperature profile should be properly set for the
synthesis of nanoparticles with desired shape and size.[99] So far, this method has
Chapter 1 Introduction
19
been mainly explored to control the size of magnetite nanoparticles within the range
of 4-20 nm, due to the superparamagnetic limit for magnetite is around 20 nm.[100] It
is expected to exploit the method for the synthesis of larger size nanocrystals (above
100 nm).
1.4 Motivations and objectives
Based on the above introduction, it is necessary to improve the electric and magnetic
properties by structuring the magnetic materials for microwave applications. In this
work, we focus on the synthesis of various magnetic nanostructures, including
core/shell structure, flaky structure, tube- and rod-like structures, as well as
nanocrystals with large sizes. Although these magnetic nanostructures have
morphological features on nanoscale, they actually possess as high magnetizations as
bulk materials. This superiority renders them suitable for the application in
microwave absorbers. The effect of various magnetic nanostructures on the
microwave absorption ability is investigated in this work. To achieve various
magnetic nanostructures, some novel methods are exploited in this work. The high
energy ball milling is a traditional way to prepare alloy materials; however, it is the
first time that jet milling is employed to narrow down the broad size distribution of
ball-milling product. Up to now, most of the chemical syntheses of magnetic particles
have focused on small particles (several ten nanometers or smaller). The synthesis of
uniform large size magnetic nanocrystals with large size (above 100 nm) is still a
Chapter 1 Introduction
20
challenge. The chemical synthesis of Fe
3
O
4
with controllable size has been developed
not only for its multifold applications, but also for its similarity with other spinel
ferrites. In other words, the successful control over Fe
3
O
4
particles may open up a
way to the synthesis of other kind of spinel ferrite. The thermal decomposition
method shows its advantages in size control of Fe
3
O
4
nanoparticles with cubic,
octahedral, polyhedral or spherical shapes, but it is unsuited to synthesize the
magnetic particles with anisotropic shapes. So far it is still a challenge to control the
size of magnetic particles with some anisotropic structures like disk, tube as well as
rod and so on.
Unlike Fe
3
O
4
, great progress has been made on the size controllable synthesis of
α-Fe
2
O
3
nanoparticles with various novel shapes. Hydrothermal method has been used
to synthesize α-Fe
2
O
3
nanoparticles with controllable sizes and shapes. Fe
3
O
4
phase
could be further obtained by annealing α-Fe
2
O
3
in reductive atmosphere. However,
the annealing conditions are critical for the formation of Fe
3
O
4
. Precisely control on
the temperature is required to avoid the impurity of FeO phase or iron phase during
the annealing process. Hence more suitable methods should be developed to convert
α-Fe
2
O
3
to Fe
3
O
4
, while preserving the initial morphology.
The major objectives of this work are to employ different structures to overcome the
limitations of Fe-based alloy and spinel ferrites when used as microwave absorbing
materials. The major objectives are shown as following:
Chapter 1 Introduction
21
1) Fe/SiO
2
particles with core/shell structure were synthesized. The insulating SiO
2
shell layer was used to reduce the skin effect of Fe particles.
2) Fe/Al flakes in micron and submicron scale were fabricated. The flake-like
structures were used to reduce the skin effect and to extend the Snoek’s limitation
of Fe-based alloys.
3) Fe
3
O
4
nanoparticles with different structures were synthesized. The effect of
different structures on the permeability and the resonance frequency were
investigated.
4) Zn-ferrite particles with unusual high saturation magnetization were synthesized.
The effect of enhanced saturation magnetization on the microwave absorption was
explored.
1.5 References
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