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MICROSTRUCTURAL AND
MAGNETIC PROPERTIES OF
M-TYPE STRONTIUM
HEXAGONAL FERRITES
BY
ANTERPREET BHATIA
PREPARED FOR:
DR. R. RAVINDRAN
DEPARTMENT OF MECHANICAL &
INDUSTRIAL ENGINEERING
RYERSON UNIVERSITY,
TORONTO, CANADA
OUTLINE
1. AIM OF THE WORK
2. INTRODUCTION
3. EXPERIMENTAL TECHNIQUES
4. RESULTS AND DISCUSSIONS
5. CONCLUSIONS
AIM OF THE WORK
The aim of this study is to understand
the relationship among the fabrication
technique, the microstructure and the
related magnetic properties of the Mtype strontium hexaferrites.
INTRODUCTION
• Ferrites are ceramic magnetic materials and are
used in both soft (cubic ferrites) and hard
(hexagonal ferrites) magnetic applications.
• Strontium ferrites SrFe12O19 with hexagonal
structure are attractive candidate materials used
for permanent magnets, magnetic recording media
and microwave absorbers, due to their low cost,
low density, high stability, large electrical
resistivity, and high microwave magnetic loss.
•
Efforts have been made to improve the
magnetic properties of these ferrites by
adopting the different approaches.
For example, in one of the approaches,
researchers are trying to improve the magnetic
properties of these ferrites
by adopting
different kinds of methods of preparation (e.g
(a)
(b)
(c)
(d)
(e)
chemical co-precipitation method,
glass crystallization,
salt melt method,
sol-gel method and
ceramic processing technique etc.
• Similarly, in an alternate approach, with the
addition of elements, such as Aluminium (Al),
cobalt (Co), titanium (Ti), and chromium (Cr),
etc. an enhancement in the magnetic properties
of these ferrites can be found.
•
In both the approaches main significant
effect is on the microstructral behavior of Sr
hexagonal which further impacts the magnetic
properties of these ferrites.
TYPES OF FERRITES
(1) Spinel ferrites
Chemical formula for spinel ferrites is MFe2O4
where M is any divalent metal.
(2) Garnets
Chemical formula for magnetic garnets is
M3Fe5O12
(3) Orthoferrites
Chemical formula for orthoferrites is MFe2O3
where M is yttrium or rare earth.
(4) Hexagonal Ferrites
Hexagonal ferrites are further divided into
following types.
HEXAGONAL FERRITES
CLASSIFICATION
Hexagonal-Type
M
X
Y
Z
W
Chemical Composition
R Fe12 O19
R Me Fe28 O46
R2 Me2 Fe12 O22
R3 Me2 Fe24 O41
R Me2 Fe12 O2
Where R = Sr, Ba or Pb and Me = Fe2+, Ni2+, Mn2+.
METHODS OF PREPARATION
• During the past few decades, ceramic
processing technique has become an alternative
processing route because it is a favourable
process for synthesizing ultrafine grained
samples.
• The ceramic processing technique generally
consists of five basic steps: (1) powder
manufacture, (2) powder blending, (3) presintering or calcination (4) compacting and (5)
sintering.
Figure 1. Block diagram of ceramic method
MICROSTRUCTURAL PROPERTIES
• X-ray diffraction technique (XRD) was
used to determine the structure and phase
of these ferrites.
• The scanning electron microscopy
(SEM) was used to study the grain size,
grain size distribution and surface
morphology of the samples.
Lattice Constants
The lattice constants ‘a’ and ‘c’ from the
diffractogram were calculated by the following
equation
1
−
2 2
 4 h +hk+k l 

d(hkl) =
+
2
2
a
c
3
2
2
Density and Porosity
The X-ray density Dx was calculated by using the
known formula
2nM
Dx =
2
3N a a c
The experimental density of all the samples was
determined by using the Archimedes principle.
The porosity of the samples was calculated using
the relation
P=
(
Dx − D
Dx
) x 100 %
where Dx is X-ray density, D is observed density.
Chemical Reaction
(1-x) SrCO3 + 6 Fe2O3 + (x/2) RE2O3
Sr1-xRExFe12O19 + (1-x) CO2 + (x/4) O2
where RE = La3+, Nd3+ and Sm3+ and x= 0.00 to 0.30
Sr-La
c = SrFe12O19
Sr-Nd
a = Fe2O3
c = SrFe12O19
Sr-Sm
a = Fe2O3
b = Sr3Fe2O7
c = SrFe12O19
Figure 2. XRD pattern for the Sr-La, Sr-Nd and
Sr-Sm samples
5.92
23.08
La
La
Nd
Nd
Sm
5.88
Sm
c (Å )
a (Å )
23.04
23.00
5.84
22.96
0.00
0.10
0.20
Composition (x)
0.30
0.00
0.10
0.20
0.30
Composition (x)
Ionic radii is (1.27 Å) for Sr2+ and (1.22 Å, 1.16 Å and
1.13 Å) for La3+,Nd3+ and Sm3+ respectively. The
replacement of Sr2+ ions by RE3+ ions results in the
decrease of unit cell dimensions of hexagonal
lattice.
Figure 3. Variation of lattice constant ‘a’ and ‘c’
with composition (x) for three series
Table 1 X-ray density Dx, Observed density D and Porosity P (%) of
Sr1-xRExFe12O19 where RE = La3+, Nd3+ and Sm3+ with (x = 0 to 0.30)
RE3+
Compos
ition (x)
D
(g/cm3)
Dx
(g/cm3)
P (%)
La
0
4.20
5.07
17.03
0.10
4.27
5.10
16.20
0.20
4.31
5.13
16.02
0.30
4.37
5.20
15.80
0.10
4.28
5.11
16.26
0.20
4.32
5.15
16.03
0.30
4.39
5.21
15.81
0.10
4.28
5.12
16.31
0.20
4.33
5.16
16.07
0.30
4.39
5.22
15.97
Nd
Sm
Scanning Electron Micrographs
La x = 0
(a)
La x = 0.20
La x = 0.10
(c)
La x = 0.30
(b)
(d)
Figure 4. SEM images for the Sr-La samples
Nd x = 0.10
(a)
Nd x = 0.20
Nd x = 0.30
(b)
(c)
Figure 5. SEM images for the Sr-Nd samples
Sm x = 0.10
(a)
Sm x = 0.20
Sm x = 0.30
(b)
(c)
Figure 6. SEM images for the Sr-Sm samples
4.5
Grain size ( µ m )
La
Nd
4.0
Sm
3.5
3.0
2.5
0.00
0.10
0.20
0.30
Composition (x)
Where S is the area of the section of a
micrograph, x is the linear magnification
and n is the number of grains in this
section.
Figure 7. Variation of grain size with composition
(x) for three series
MAGNETIC PROPERTIES
Saturation Magnetization and Remanence
40
Ms (J/T.kg)
70
Ms
60
50
La
Nd
Sm
La
35
30
40
Mr
25
30
20
0.00
20
0.10
0.20
Composition (x)
0.30
0.40
Mr (J/T.kg)
80
The substitution
of divalent Sr2+
ion by trivalent
RE3+
ion
will
change Fe3+ ion
to Fe2+ ion per
formula unit,
Fe3+ - O2- - Fe3+ exchange interaction
Figure 8. Variation of magnetization (Ms) and
remanence (Mr) of Sr1-xRExFe12O19 with composition x
Coercive Field
275
Hc (kA/m)
La
Nd
Sm
250
225
200
0.00
0.10
0.20
0.30
0.40
Composition (x)
Hc increases due to the enhancement of
magnetocrystalline anisotropy, because Fe2+ is
anisotropic in nature.
Figure 9. Variation of coercive field (Hc) of
Sr1- xRExFe12O19 with composition x.
Curie Temperature
700
La
Nd
Sm
Tc (K)
680
660
640
620
0.00
0.10
0.20
0.30
Composition (x)
Figure 10. Variation of Curie temperature (Tc) with
composition (x)
CONCLUSIONS
Some of the main conclusions drawn out
of the present work are described as:
1.
The replacement of Sr2+ ions by RE = La3+,
Nd3+ and Sm3+ ions in Sr1-xRExFe12O19
hexaferrite yields hexagonal ferrites to a
certain extent of unreacted oxides with an
additional phases in case of Nd3+ and Sm3+
ions substitution.
2. The values of lattice constants ‘a’ and ‘c’
decrease continuously with increasing
substituted amount of rare earth ions for
the three series studied.
Since RE3+ ions have ionic radii less than
that of the ionic radii of Sr2+ ions, the
replacement of Sr2+ ions by RE3+ ions
results in the decrease of unit cell
dimensions of hexagonal lattice.
3. The increase in bulk density with rare earth
substitution may be attributed to
the
atomic
weight and density of these ions, which are
higher than those of strontium ions. The
replacement of Sr2+ by RE3+ ions in the
hexagonal structure leads to a variation in the
bonding and consequently
inter
atomic
distance and density.
4. The average grain size decreased for
all substituted samples.
However, this
decrease is more in case of Sm3+ ion
substitution due to the smaller ionic size and
decrease in unit cell dimension of Sm3+ ions as
compared to other doped ions.
5. The decrease in magnetization (Ms) and
remanence (Mr) values of all the samples may
be due to magnetic dilution with changing of
the Fe3+ (high spin) valence state to Fe2+ (low
spin) state on a site by substitution of the Sr2+
site with RE3+ ions and existence of spin
canting promoting reduction of
super
exchange fields.
6.
The increase in coercive field (Hc) for all the
series can be attributed to the enhancement
of magneto crystalline anisotropy with
anisotropic Fe2+ ions locating on a 2a site
and the grain size reduction.
7.
Curie temperature decreases as rare earth
ions substitution increases. This behavior
can be attributed to the decrease in strength
of super exchange field interactions by the
Fe2+ (low spin) ions.
Thank You