Synthesis And Characterization Of Strontium Ferrite Environmental Sciences Essay

Strontium ferrite is a ferromagnetic material and reported as having hexagonal magnetoplumbite type (M-type) structure. It is the most widely used permanent magnets throughout the world, which account for about 90wt% of the annual production of permanent magnets. In this study, the strontium ferrite is synthesized using sol-gel methods and the magnetic properties were analyzed.

Chapter 1 gave introduction about the structure of M-type hexagonal strontium ferrite. Besides, some general magnetic properties will be discussed. Commercial applications of strontium ferrite would be discussed as well.

Chapter 2 is all about the experimental details, including the synthetic techniques used for strontium ferrite, description of instrument used and procedures carried out.

Chapter 3 concentrated on the results on magnetic susceptibility of hexagonal strontium ferrite. Comparison between strontium ferrite and cation-substituted strontium ferrite was made.

Chapter 4 concluded the whole investigation of this study. Suggestions for future studies were also discussed. Better understanding of the properties and practical applications of strontium ferrite can be achieved through this study.

ABSTRACT

The properties of magnetoplumbite type (M-type) hexagonal strontium ferrite has been investigated. The attempt of substitution of cobalt(II) oxide and titanium(IV) oxide in order to produce a quaternary system of the type SrO-Fe2O3-XO where X represents the dopant cation was made. The synthesis is based on sol-gel method where ethylene glycol is the gel precursor. This technique was employed because it was found to be able to produce nanoparticles of cation substituted strontium ferrite. Moreover, sol-gel method can produce high yields of strontium ferrite particles.

Overall, the magnetic properties were observed to be change after the cation substitution. Co(II)-Ti(IV) substitution in SrFe12O19 with different ratios were made in this study to investigate the effect of cation substitution in magnetic properties of strontium ferrite. Co(II)-Ti(IV) substitution in strontium ferrite with mole ratio of 0.4 showed the best magnetic properties that we desired for. The mass susceptibility where X = 0.4 was found to be increase sharply compared to the unsubstituted one. Except the cobalt titanium substitution with mole ratio of 0.4, other cation substitution ratios showed decrease in mass susceptibility which is not desirable. Therefore the cobalt-titanium substitution for SrCoxTixFe12-2xO19 with X = 0.4 is the best to improve magnetic properties of strontium ferrite for various commercial applications.

REVIEW

Strontium ferrite has been a subject of continuous interest and intensive study for several decades due to the fact that this compound has been the the most widely used permanent magnets, which account for about 90wt% of the annual production of permanent magnets since shortly after its discovery in the 1950s. Strontium hexaferrite, SrFe12O19, is a ferrimagnet and is also known as ceramic permanent magnet. When compared with alnico-magnets, strontium ferrite has high coercivity, moderate remenance, corrosion resistance and excellent chemical stability [5]. Iron(III) oxide (Fe2O3) is the principal components in SrFe12O19 which gives rise to its magnetic properties. Within the five different crystallographic sites of strontium ferrite, the iron ions are coupled antiferromagnetically. Due to its high magnetocrystalline anisotropy field in its structure, SrFe12O19 exhibits high saturation magnetization and high coercivity [1]. The high magnetic permeability in strontium ferrite enables it to store strong magnetic fields, which is stronger than iron. Strontium ferrite is often produced as nanoscale size powder, which can be sintered into solid cores.

Strontium ferrite has been used for several important industrial applications, such as permanent magnets, microwave devices and high density perpendicular recording media, with proper doping in order to improve properties of strontium ferrite [1]. SrFe12O19 has also been investigated as a medium for magnetic recording and magneto-optical recording and for long (millimetre)-wave devices [2]. Efforts have made to the development of novel synthetic methods which facilitate the production of fine hexagonal ferrite particles and to possible ways of reducing their high intrinsic magnetocrystalline anisotropy.

The objective in this study was to attempt the synthesis of cation substituted M-type hexagonal ferrite SrCoxTixFe12-2xO19 using the sol-gel method. The sol-gel method has been used widely to produce fine particles of a variety of oxides. The effect of doping strontium ferrite with cobalt (II) and titanium (IV) oxides to produce quaternary systems of SrO-Fe2O3-XO, where X represents the dopant cation would be tested. The fine particles of cation substituted ferrite produced by using sol-gel technique is desirable because the grain size of the materials used in magnetic recording is the main factor determining the level of background noise at low density.

Magnetic properties of strontium ferrite would be focus in this study. Magnetic susceptibility balance would be used to determine the mass susceptibility for both strontium ferrite and cation-substituted strontium ferrite produced using the sol-gel method. The mass susceptibilities of the samples were compared to determine the optimum amount of cation needed to dope to ferrite to give the best magnetic behaviour.

CRYSTAL STRUCTURE OF M-TYPE HEXAGONAL SrFe12O19

According to crystalline structure, hexaferrite can be classified into four types, these include M, W, Y and Z types hexaferrites which correspond to (SrO + MeO):Fe2O3 ratios of 1:6, 3:8, 4:6 and 5:12 respectively. SrFe12O19 is classified as M-type hexaferrite.

The hexagonal SrFe12O19 was first prepared by Adelsk¨old in 1938 [2]. He also confirmed that the crystal structure of this compound to be iso-structural with the naturally occurring ferrite mineral magnetoplumbite, and therefore it has the M-type structure. Later structural refinements for strontium hexaferrite have confirmed his determination [2]. Strontium ferrite is classified as hexagonal ferrite. It is denoted as having the space group P63/mmc. According to the research made by Kimura et al, the lattice parameters measured are found to be: a = 0.588 36nm and c = 2.303 76nm at room temperature [2].

As shown for M-type hexaferrite BaFe12O19 in Fig. 1.1, the crystalline structures of different types of hexaferrites are remarkably complex. The unit cell contains ten oxygen layers. A unit cell is sequentially constructed for four blocks, they are S (spinel), R (hexagonal), S* and R*. The S and R blocks have equivalent atomic arrangements and are rotated around the c-axis at 180° with respect to S* and R* blocks. R or R* block consists of three O2− layers while S or S* block contains two O2− layers; with one oxygen site in the middle layer substituted by a Ba2+ ion [16]. The structure of strontium ferrite is similar to that of barium ferrite, by just substituting the barium ion with strontium ion.

Fig. 1.1: Structure of barium hexaferrite

Occasionally, a unit cell is comprises of two formula units. The unit cell consists of 64 ions per hexagonal unit cell, which are 2 strontium ions, 38 oxygen ions and 24 ferric ions. The structure of magnetoplumbite are made of a layer of hexagonal close packed arrangement of oxygen and strontium ions, which is sandwiched between two spinal blocks containing a cubic close-packed arrangement of oxygen atoms with iron atoms.

The iron atoms are positioned at five interstitial crystallographically different cation sites of the close-packed layers, namely 4f1 (tetrahedral site, A sites), 12k, 4f2, 2a (octahedral sites, B sites) and 2b (trigonal bipyramidal site) [15]. The tetrahedral iron oxide is FeO4, octahedral iron oxide consists of six oxygen ions, which is FeO6, and the formula for trigonal bipyramidal iron oxide is FeO5. A schematic M-type structural representation and the five Fe3+ sites are shown in Fig. 1.2 by Collomb et al. [15].

Figure 1.2: The crystal structure sketch map of the hexagonal M-type phase and the five Fe sites with their surroundings are displayed.

The 2b sites only occur in the same layer with strontium ion. 12k site is the octahedral site of S and R blocks. There are two tetrahedral (4f1) sites and one octahedral (2a) site in centre of S block. The two octahedral (4f2) sites are found in the R block, adjacent to the strontium-containing layer.

The M-type structure of strontium ferrite gives rise to its magnetic properties. Cation substitution to strontium ferrite may give chances whereby altering the structure and thus influence the magnetic properties.

MAGNETIC PROPERTIES OF M-TYPE HEXAGONAL SrFe12O19

Strontium hexaferrite is a ferrimagnetic material. Since the free electrons in SrFe12O19 are in close proximity and remain aligned even the external magnetic field have been removed, it is able to retain a permanent magnetic field and is recognized as ferrimagnetic material.

In 1950s Gorter predicted that the iron ions at the trigonal bipyramidal (2b) and octahedral (2a, 12k) sites have their spin orientation antiparallel to that of the iron ions at the 4f sites [2]. The antiparallel 4f1 and 4f2 and parallel 2a, 12k and 2b sublattices form the ferrimagnetic structure. The magnetic ordering corresponding to the magnetoplumbite structure of hexagonal strontium ferrite is well illustrated in Fig. 1.3.

In S block, the majority α-sublattice consists of four octahedral ions and the minority β-sublattice contains two tetrahedral ions whereas R block contributes three octahedral ions and one trigonal ion to the majority sublattice and two octahedral ions to the minority sublattice.

Figure 1.3: The schematic structure (left) of the SrFe12O19 with Gorter’s magnetic ordering (middle) along the c-axis. The large open circles are oxygen ions, the large broken circles are Sr ions; small circles with a cross inside represent Fe ions at 12k, small circles containing a filled circle inside represent Fe ions at 4f2, small unfilled circles represent Fe ions at 4f1, filled small circles represent Fe ions at 2a and small circles with a unfilled circle inside represent Fe ions at 2b. The magnetic structure suggested by Gorter is shown on the right, where the arrows represent the direction of spin polarization.

From Fig. 1.3, we can summarizes the sites of Fe(III) ions corresponding to the spin direction, as in Table 1.1.

Site

Coordination

Occupancy

Direction of spin polarization

12k

Octahedral

12

Up

2a

Octahedral

2

Up

2b

Trigonal Bypiramidal

2

Up

4f1

Tetrahedral

4

Down

4f2

Octahedral

4

Down

Table 1.1: Fe(III) ion sites in M-type hexagonal ferrite

Hysteresis Loop

The magnetic properties of strontium ferrite can be examined through hysteresis loops. Hysteresis loop can be measured using instruments such as Vibrating Sample Magnetometer (VSM) and SQUID Magnetometry Measurements.

When a magnetic material is placed in a magnetic field, the flux density (B) would lags behind the magnetizing force (H) that causes it, and this form hysteresis loop.

From a hysteresis loop, we can identify the magnetic properties of the material, they are saturation magnetization, remanence or also known as remnant magnetization, and coercivity. A typical hysteresis loop is well illustrated in Fig. 1.4.

Figure 1.4: Typical hysteresis loop (B-H curve)

Initially, there is no applied magnetic field and it is known as unmagnetized state. After magnetic field is applied, it causes alignment. Until maximum magnetizing force applied, maximum flux density achieved at the same time and this phenomenon is known as saturation magnetization. At this point, the maximum number of spin has mobilized. Saturation magnetization is defined as the maximum possible magnetisation of a material. It is also a measure of strongest magnetic field a magnet can produce. The unit of saturation magnetization is in amperes per meter. Strontium ferrite is having high saturation magnetization at which it can store high amount of magnetizing force. As the magnetizing force being slowly removed, the alignment stays at the point where H = 0, this is known as remnant magnetization. Remnant magnetization is the magnetization left in a permanent magnet after an external magnetic field is removed. When a magnet is “magnetized”, it has remanence. It is usually measured in unit Tesla. Strong permanent magnet such as strontium ferrite has high remnant magnetization which means the high amount of magnetic force remains in it even after the magnetizing force is removed. As form Fig. 1.4, negative magnetic field is applied to demagnetize the permanent magnet. When the flux density (B) = 0, there is no magnetizing force remain in the magnet and the negative H needed to demagnetize the magnet is known as coercivity. Negative H is the magnetic field applied in opposite direction. Coercivity is measured in unit amperes per meter. Due to its high uniaxial magnetocrystalline anisotropy with an easy axis of magnetization along the hexagonal c-axis in the structure, SrFe12O19 has high coercivity.

Anisotropy is directional or orientational effects in crystal structure of materials which can provide better magnetic performance along certain preferred axis. Therefore, we need to apply high negative magnetizing force to demagnetize strontium ferrite. Attempts have to be made to lower down the coercivity of strontium ferrite for usage.

Units in Magnetism

The units used in magnetism can be divided mainly into two categories, SI system and c.g.s system. The conversion table shown in Table 1.2 is to clarify the magnetism formulas in both SI and c.g.s systems and the conversion factors between them.

Quantity

Symbol

SI Unit

SI Equation

c.g.s Unit

c.g.s Equation

Conversion Factor

Magnetic Induction

B

tesla (T)

B=µo(H+M)

gauss (G)

B = H+4Ï€M

1 T = 104 G

Magnetic Field Strength

H

ampere/meter 

(A/m)

H = NÃ-I/lc 

( lc – magnetic 

path, m)

oersted (Oe)

H = 0.4Ï€NÃ-I/lc

(lc – magnetic 

path, cm)

1 A/m = 

4 Ï€Ã-10-3 Oe

Magnetic Flux

Φ

weber (Wb)

Φ = BÃ-Ac

(Ac – area, m2 )

maxwell (M)

Φ = BÃ-Ac

(Ac – area, cm2 )

1 Wb = 108 M

Magnetization

M

ampere/meter (A/m)

M=m/V

(m- total magnetic moment, 

V- volume, m3 )

emu/cm3

M=m/V

(m- total magnetic moment, 

V- volume, cm3 )

1 A/m = 10-3 

emu / cm3

Magnetic Permeability of Vaccum

µo

newton/ampere2

µo= 4Ï€Ã-10-7

1

–

4Ï€Ã-10-7

Inductance

L

henry

L=μoμN2Ac/lc

(Ac- area, m2, 

lc – magnetic path, m)

henry

L=0.4πμN2Ac/lcÃ-10-8

(Ac-area, cm2, 

lc – magnetic path, cm)

1

Emf (voltage)

V

volt

V=-NÃ-dΦ/dt

volt

V=-10-8NÃ-dΦ/dt

1

Note: In the above equations, I = current (in amps), N = turns

Table 1.2: Magnetism formulas in SI and c.g.s systems and their conversion factors for the magnetic units.

1.4 PHOTOLUMINESCENCE PROPERTIES OF SrFe12O19

According to the study of G. B. Teh et.al [3] on strontium ferrite, strontium ferrite was found to exhibit photoluminescence behavior. When a sample of strontium ferrite is excited at a certain wavelength, highest intensity of photoluminescence emission peaks was obtained. The ability of strontium ferrite to photoluminesce could be due to the oxygen vacancies in their lattice structure. The oxygen vacancies are assumed to cause the particles to exhibit photoluminescence behavior by acting as traps for mobile excitation. The oxygen vacancies have effective +2 charges, making them powerful electron capture centers. Valence electron would gain sufficient energy to jump from the valence band to the conduction band and leaving a gap known as hole during excitation. F-centers, which is the region where contain high amount of electrons would formed when the excited electrons being trapped in oxygen vacancies. These rich electron centers would lead to emission of luminescence when the holes and electrons recombine.

1.5 SYNTHESIS ROUTE OF SrFe12O19

The processing routes used for synthesis of strontium ferrite affect its properties much. Traditionally, this ferrite powder is synthesized by a mixed oxide ceramic method, which involves the solid-state reaction between SrCO3 and Fe2O3 at a high calcination temperature (about 1300°C). However, uncontrolled particle morphology, larger particle size and agglomerates would be the biggest disadvantages of this technique. Besides, contamination would be introduced to the sample while subsequent milling of the calcined ferrite powder and this would affect the magnetic properties become less desirable. Therefore, the narrowed particle size distribution, refined particle size and minimal particle agglomeration has been the main concern during the synthesis of strontium ferrite.

In order to improve the magnetic properties, numerous nonconventional soft synthetic routes have been carried out, including sol-gel synthesis [3], hydrothermal reaction [6], co-precipitation [7], citric acid method [8] and microemulsion processing [10].

In this study, the synthesis of strontium ferrite employed the sol-gel technique. It is a wet chemical route employing ethylene glycol as gel precursor. Sol-gel technique is the technique of using chemical substances which have high solubility in organic solvents to synthesize precursor compounds. The compounds are easily transformed into hydrated oxides on hydrolysis. The metal alkoxides formed can be removed easily using hydrolysis and thermal treatment and therefore results in hydrated oxides which are highly purify.

Sol-gel method is used in this study because of its many advantages. Sol-gel technique is able to produce homogeneous nanosized crystallites. This method is tend to give shaped materials directly from a solution without passing through the powder processing and the fact that the annealing temperature is very low compared with other conventional technology. The crystalline size and properties of the ferrite produced are largely affected by calcinations temperature [3]. Sol gel method has the advantage that the crystal growth of particles is easier to control by varying the heat treatment [11]. It was reported that at 500˚C it produced only maghemite, γ-Fe2O3. A mixed product of magnetic α-Fe2O3 and M-type SrFe12O19 were obtained at 600˚C. As the calcination temperature increase to 800˚C and above, there are only M-type SrFe12O19 phase was observed. Sol-gel synthesis is able to produce high yields of SrFe12O19 nanoparticles. It is also able to produce nanocrystallite of cation substituted SrFe12O19. Nanoparticle size of strontium ferrite is desirable and aimed to synthesize because nanoparticles tend to give better magnetic properties. Nanoparticles give few magnetic domains, probably single domain. Single domain tends to give higher magnetic induction because there are no oppose magnetic domain. Single domain aligns in one direction only. These properties are ideal for the making of permanent magnet.

1.6 CATION SUBSTITUTION IN SrFe12O19

In order to improve the magnetic properties of strontium ferrite, many studies have been carried out. One of them is cation substitution in strontium ferrite. Rare earth and other metal cations are used for substitution for strontium and iron respectively [5]. The pair doping of SrFe12O19 such as a La-Co pair to replace a Sr-Fe pair has been tested [14]. The doping, or known as cation substitution, is aim to improve the magnetic properties of strontium ferrite. Cation substitution results in structural changes in strontium ferrite. As the physical properties of ferrite change, the magnetic properties would be affected due to the fact that magnetic properties are determined by the arrangement of iron ions in crystal structure. In this study, Co-Ti pair will be doped to the strontium ferrite. Cobalt titanium substitution will produce a quaternary system of the type SrO-Fe2O3-AO where A represents the dopant cation.The cobalt titanium substitution gives rise to the new formula, SrCoxTixFe12-2xO19 where X is the number of mole of cation substituted in.

1.7 Commercial Applications

Strontium ferrite is widely used as permanent magnet because it has direction of easy magnetization and the hexagonal c-axis which are perpendicular to the plane of the plate. The properties that are desirable in using as permanent magnet include high saturation magnetization, high remnant magnetization, high coercivity, high Curie temperature and high magnetocrystalline anisotropy.

Besides, SrFe12O19 is also commonly used in high-density data storage magnetic recording media. Nanoparticles of SrFe12O19 with single domain and low coercivity are crucial in used for magnetic recording media. M-type strontium ferrite nanoparticles have attracted much attention due to their good frequency characteristic, low noise, high output, in particular, excellent high frequency characteristic and wide dynamic frequency range [4]. There are two types of recording medium, namely particulates and thin films. Tape and floppy is categorized in particulate and hard drive is belongs to thin film. Information is stored by magnetizing material. The recording head can apply magnetic field (H) and align domains to magnetize the medium. It can also detect a change in the magnetization of the medium. Magnetic recording media prefers high saturation magnetization; make it to store as much information. High value of remnant magnetization is required in recording media to make sure that all materials stored in the hard disk still remained even the power supply (applied magnetic field) is switched off. Low coercivity is important in magnetic recording media. When the positive magnetic field is applied, this charging manages the medium to store data. On the other hand, negative magnetic field applied to retrieve back the data, this is called discharges. Therefore, less current is needed to retrieve the data in the low coercivity medium. As a result, less heat generated and this saves the electricity.

In general, strontium ferrite has high value of uniaxial anisotropy field, high coercive force and high saturation magnetization. The high coercivity of strontium ferrite has to be lowered down and saturation magnetization has to be simultaneously increased if it is to be useful for magnetic recording purposes. It has been reported that the substitution of cations such as Co(II) for the ion Fe(III) in strontium ferrite has lowered the coercive force. Therefore, many studies were carried out to achieve better magnetic properties of strontium ferrite for commercial applications.

CHAPTER 2: EXPERIMENTAL

Sample Preparation

Synthesis of M-type SrFe12O19

Synthesis of Cation Substituted SrFe12O19

Sample Characterization

Magnetic Susceptibility Balance MK1

2.1 Sample Preparation

2.1.1 Synthesis of M-type SrFe12O19

The sol-gel technique was used to synthesize M-type SrFe12O19 whereby the ethylene glycol acts as gel precursor. The starting materials, strontium nitrate, Sr(NO3)2 and iron(III) nitrate-9-hydrates, Fe(NO3)3·9H2O were used due to their high solubility in ethylene glycol. Calculation below was made to determine the weight of materials needed to be used.

Relative Molecular Mass of materials:

Strontium nitrate, Sr(NO3)2 = 211.63 g/mol

Iron(III) nitrate-9-hydrates, Fe(NO3)3·9H2O = 404 g/mol

(Note: All answers have to be converted into 3 significant figures.)

No. of mol of 1 g Sr(NO3)2 = Mass of Sr(NO3)2

RMM of Sr(NO3)2

= 1g

211.63g/mol

= 4.7252×10-3 mol

Sr : Fe = 1 : 12

No. of mol of Fe(NO3)3·9H2O needed = 4.7252×10-3 mol x 12

= 5.6702×10-2 mol

Mass of Fe(NO3)3·9H2O needed = No. of mol of Fe(NO3)3·9H2O needed x RMM of

Fe(NO3)3·9H2O

= 5.6702×10-2 mol x 404g/mol

= 22.9 g

From the calculation, 1g of strontium nitrate and 22.9g of iron(III) nitrate-9-hydrates were needed in the synthesis and were weighted. Strontium nitrate would provided 1 mol of strontium ions and iron(III) nitrate-9-hydrates would provided 12 mol of iron ions in the synthesis of strontium ferrite, which matched the molecular formula of SrFe12O19. The strontium nitrate and iron(III) nitrate-9-hydrates were readily dissolved in ethylene glycol with slight heat applied due to their high solubility in it. The mixture was heated slightly and stirred with a magnetic bar until the mixture was fully dissolved. The resultant solution is in transparent reddish color. The magnetic stirring bar was removed.

The mixture was heated to 100°C and it would slowly transform into a gel form. The gel was dried with continuous heating at 100°C for 3 hours. The dried gel was then transferred to a crucible to remove traces of organic precursor. A mixture of metal oxides in dispersed nanoclusters form was obtained. The dried gel was then annealed in a furnace at 800°C for 3 days with extensive ground with a pestle in a mortar after annealed at interval of each day.

2.1.2 Synthesis of Cation Substituted SrFe12O19

Cation substituted strontium ferrite was synthesized by using cobalt(II) ions and titanium(IV) ions to substitute the iron ions in M-type hexagonal strontium ferrite. The substitution of Co(II) and Ti(IV) gives the compound a new molecular formula, which is SrCoxTixFe12-2xO19 where the x denoted different ratios. In the synthesis of cation substituted SrFe12O19, the ratios of cations used, x, is in between 0.2 to 6.0 (0.2 ≤ x ≤ 6.0), where x = 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0. The same method described in section 2.1.1 was used for the synthesis, by only adding two new starting materials, which are the cobalt(II) nitrate and titanium(IV) ethoxide to give the Co2+ and Ti4+ cations.

Calculation as described below was made to calculate the weight of materials needed respectively.

Relative Molecular Mass of materials:

Strontium nitrate, Sr(NO3)2 = 211.63 g/mol

Iron(III) nitrate-9-hydrates, Fe(NO3)3·9H2O = 404 g/mol

Cobalt(II) nitrate, Co(NO3)2.6H2O = 291.04 g/mol

Titanium(IV) ethoxide, Ti(CC2H5)4 = 228.11 g/mol

(Note: All answers have to be converted into 3 significant figures.)

Example used for the calculation: SrCo0.2Ti0.2Fe11.6O19, x= 0.2

No. of mol of 1 g Ti(CC2H5)4 = Mass of Ti(CC2H5)4

RMM of Ti(CC2H5)4

= 1g

228.11g/mol

= 4.3838×10-3 mol

0.2 mol of Ti needed 1 mol of Sr.

4.3838×10-3 mol of Ti needed (4.3838×10-3 mol x 1) mol of Sr.

0.2

Therefore, 0.021919 mol of Sr is needed.

Mass of Sr(NO3)2 needed = 0.021919mol x 211.63 g/mol

= 4.64 g

0.2 mol of Ti needed 11.6 mol of Fe.

4.3838×10-3 mol of Ti needed (4.3838×10-3 mol x 11.6) mol of Sr.

0.2

Therefore, 0.25426 mol of Fe is needed.

Mass of Fe(NO3)3·9H2O needed = 0.25426mol x 404g/mol

= 103 g

Mass of Co(NO3)2.6H2O needed = 4.3838×10-3 mol x 291.04g/mol

= 1.28 g

The calculation above were used to calculate the weight of starting materials needed for other cation ratios, x for 0.4, 0.6, 0.8, 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 respectively as well. The weight needed for each material was tabulated in Table 2.1.

x

Weight of materials needed (g)

Sr(NO3)2

Fe(NO3)3·9H2O

Co(NO3)2.6H2O

0.2

4.64

103

1.28

0.4

2.32

51.4

1.28

0.6

1.55

31.9

1.28

0.8

1.11

23.0

1.28

1.0

0.93

17.7

1.28

2.0

0.46

7.08

1.28

3.0

0.31

3.54

1.28

4.0

0.23

1.77

1.28

5.0

0.19

0.71

1.28

6.0

0.15

0.00

1.28

Table 2.1: Weight of materials needed for synthesis of Co(II)-Ti(IV) substituted strontium ferrite

For the series of different substitution ratios (x), the corresponding strontium nitrate, iron(III) nitrate-9-hydrates, cobalt(II) nitrate and titanium(IV) ethoxide were weighed and dissolved in 100ml ethylene glycol. The oxides obtained after ignition were then annealed in a furnace at 800°C for 3 days with extensive ground with a pestle in a mortar after annealed at interval of each day. The preparation for strontium ferrite and cation substituted strontium ferrite is shown in Fig. 2.1 in flow chart array.

Figure 2.1: Schematic diagram of the procedure for synthesis of strontium ferrite and cobalt-titanium substituted SrFe12O19.

Sample Characterization

Magnetic Susceptibility Balance MK1

The magnetic properties of strontium ferrite and cobalt-titanium substituted strontium ferrite produced by the method described above were examined using the Magnetic Susceptibility Balance MARK 1 (MK1) by Sherwood Scientific Ltd, England. The magnetic susceptibility balance apparatus was shown in Fig. 2.2.

Figure 2.2: Magnetic Susceptibility Balance MK1 by Sherwood Scientific Ltd, England.

The basic design principle of Magnetic Susceptibility Balance MK1 was shown in Figure 2.3. Magnetic Susceptibility Balance determines the magnetic properties by placing two couple of moving magnets with the beam in between where the stationary sample is ready to be measured. Basically, the possible deflection in the beam and the movement being made of a particular sample either solid or liquid could be observed in a balanced system which possesses a magnetic field. Meanwhile, the coil within the instrument is conducted with current required in order to make compensation of the magnetic force produced by the sample. Either paramagnetic or diamagnetic could be resolved in a plus or minus relatively on display with the aid of the direction that the beam swifts.

Figure 2.3: Basic design principle of Magnetic Susceptibility Balance MK1 by Sherwood Scientific Ltd, England.

Magnetic susceptibility is defined as when the magnetising field is applied to the sample, how much is the ratio of the intensity of magnetism induced by the sample in response to the magnetising field which it is subject. In this experiment, mass susceptibility was the main concern. Mass susceptibility, xg, is defines by the mathematical formula below:

𝑥g= 𝑥v/d

Where d = density of substance

𝑥v is the volume susceptibility, calculated by using the formula:

𝑥v = I/H

Where I = intensity of magnetism produced in a substance

H = intensity of applied magnetic field

Based on the magnetic properties of magnetic substances, they can be classified into one of the three groups. Among them, there is paramagnetic material which would attract by a strong magnetic field, diamagnetic which repelled by magnetic field and ferromagnetic which is unique to retain their own magnetic field. After the external magnetic field is removed, ferromagnetic materials are still able to retain a permanent magnetic field. This is happened due to their free electrons are in close proximity and remain aligned without the magnetic field. Strontium ferrite and cobalt-titanium substituted strontium ferrite were found to be ferromagnetic due to the overshooting value observed on the display when the samples were introduced. To overcome this problem, a non-magnetic material sodium chloride, was used to ‘dilute’ the large magnetism induced by the samples.

Procedure was carried out. First, the range knob of Magnetic Susceptibility Balance MK1 was turned to the x1 scale and was allowed to warm up for 10 minutes before use. The zero knob is adjusted until the display reads 000. An empty sample tube of known weight was placed into the tube guide and the reading, Ro was taken. 0.0005g sample + 0.2820g NaCl (sample length, l =3cm) was packed into the sample column. The weight of samples and sodium chloride were fixed for all measurements made. The packed sample tube was placed into tube guide and the reading, R was taken. The steps were repeated for all the eleven samples.

The mass susceptibility, 𝑥g is calculated using the formula:

𝑥g= CBal* l * (R-Ro)

109 * m

Where: l = length of sample (cm)

m = mass of sample (gram)

R = balance reading for sample + tube

Ro = balance reading for empty tube

CBal = the balance calibration constant (=1)

CHAPTER 3: RESULTS AND DISCUSSION

3.1 Results of Mass Susceptibility

3.1.1 Mass susceptibility of M-type SrFe12O19

3.1.2 Mass susceptibility of Cation Substituted SrFe12O19

3.1.3 Table of mass susceptibility of SrFe12O19 and Co(II)-Ti(IV)

substituted SrFe12O19

3.1.4 Graph of mass susceptibility, 𝑥g (cgs) against Co(II)-Ti(IV) ratio

3.2 Findings and Discussion

3.1 RESULTS OF MASS SUSCEPTIBILITY

Mass susceptibilities of the samples were calculated using the formula:

𝑥g= CBal* l * (R-Ro)

109 * m

Where: l = sample length (cm)

m = sample mass (gram)

R = balance reading for sample in tube

Ro = balance reading for empty tube

CBal = the balance calibration constant (=1)

* All the answers are adjusted to 4 significant figures.

3.1.1 Mass Susceptibility of M-type SrFe12O19

𝑥g = CBal* l * (R-Ro)

109 * m

= 1 x 3cm x [936-(-036)]

109 x 0.0005

= 1.944×10-4

3.1.2 Mass Susceptibility of Cation Substituted SrFe12O19

SrCo0.2Ti0.2Fe11.6O19

𝑥g = CBal* l * (R-Ro)

109 * m

= 1 x 3cm x [798-(-036)]

109 x 0.0005

= 1.668×10-4

SrCo0.4Ti0.4Fe11.2O19

𝑥g = CBal* l * (R-Ro)

109 * m

= 1 x 3cm x [671-(-036)]

109 x 0.0005

= 1.414×10-4

SrCo0.6Ti0.6Fe10.8O19

𝑥g = CBal* l * (R-Ro)

109 * m

= 1 x 3cm x [654-(-036)]

109 x 0.0005

= 1.380×10-4

SrCo0.8Ti0.8Fe10.4O19

𝑥g = CBal* l * (R-Ro)

109 * m

= 1 x 3cm x [542-(-036)]

109 x 0.0005

= 1.156×10-4

SrCo1.0Ti1.0Fe10O19

𝑥g = CBal* l * (R-Ro)

109 * m

= 1 x 3cm x [441-(-036)]

109 x 0.0005

= 0.954×10-4

SrCo2.0Ti2.0Fe8O19

𝑥g = CBal* l * (R-Ro)

109 * m

= 1 x 3cm x [236-(-036)]

109 x 0.0005

= 0.544×10-4

SrCo3.0Ti3.0Fe6O19

𝑥g = CBal* l * (R-Ro)

109 * m

= 1 x 3cm x [162-(-036)]

109 x 0.0005

= 0.396×10-4

SrCo4.0Ti4.0Fe4O19

𝑥g = CBal* l * (R-Ro)

109 * m

= 1 x 3cm x [145-(-036)]

109 x 0.0005

= 0.362×10-4

SrCo5.0Ti5.0Fe2O19

𝑥g = CBal* l * (R-Ro)

109 * m

= 1 x 3cm x [-006-(-036)]

109 x 0.0005

= 0.060×10-4

SrCo6.0Ti6.0O19

𝑥g = CBal* l * (R-Ro)

109 * m

= 1 x 3cm x [-066-(-036)]

109 x 0.0005

= -0.060×10-4

3.1.3 Table of mass susceptibility of SrFe12O19 and Co(II)-Ti(IV) substituted SrFe12O19

The mass susceptibilities of the samples calculated were summarized in Table 1.

Samples

Sample length, l (cm)

Sample mass, m (gram)

Empty tube reading, R0

Reading for tube + sample, R

Mass susceptibility, xg

SrFe12O19

0.1

0.0005

-036

936

1.944×10-4

SrCo0.2Ti0.2Fe11.6O19

0.1

0.0005

-036

798

1.668×10-4

SrCo0.4Ti0.4Fe11.2O19

0.1

0.0005

-036

671

1.414×10-4

SrCo0.6Ti0.6Fe10.8O19

0.1

0.0005

-036

654

1.380×10-4

SrCo0.8Ti0.8Fe10.4O19

0.1

0.0005

-036

542

1.156×10-4

SrCo1.0Ti1.0Fe10O19

0.1

0.0005

-036

441

0.954×10-4

SrCo2.0Ti2.0Fe8O19

0.1

0.0005

-036

236

0.544×10-4

SrCo3.0Ti3.0Fe6O19

0.1

0.0005

-036

162

0.396×10-4

SrCo4.0Ti4.0Fe4O19

0.1

0.0005

-036

145

0.362×10-4

SrCo5.0Ti5.0Fe2O19

0.1

0.0005

-036

-006

0.060×10-4

SrCo6.0Ti6.0O19

0.1

0.0005

-036

-066

-0.060×10-4

Table 3.1: Mass susceptibility, xg of SrFe12O19 and Co(II)-Ti(IV) substituted SrFe12O19

3.1.4 Graph of mass susceptibility, 𝑥g (cgs) against Co(II)-Ti(IV) ratio

A graph of mass susceptibility of SrFe12O19 and Co(II)-Ti(IV) substituted SrFe12O19 against Co(II)-Ti(IV) ratio was plotted and shown in Graph 3.1.

Figure 3.1: Graph of mass susceptibility, 𝑥g(cgs) against Co(II)-Ti(IV) ratio

3.2 FINDINGS AND DISCUSSION

Magnetic susceptibility is a measure of response of electrons in sample to an applied magnetic field. Electrons produce magnetic moments at where the electrons spin circularly around the nucleus following right-thumb rule. The net magmetic moment is the sum of moments from all electrons. There are three types of magnetism; they are ferromagnetic or ferrimagnetic, paramagnetic and diamagnetic. Strontium ferrite is a ferrimagnetic compound due to its high magnetic induction, B when magnetic field, H is applied. The magnetic moments of ferrimagnetic strontium ferrite is aligned parallel with applied magnetic field. It is a naturally magnet because the magnetic moments are point at one direction even there is no magnetic field is applied. The magnetic moment of M-type hexagonal ferrites strongly prefer the hexagonal axis direction, which is the c-axis.

Within the grain boundaries of ferrimagnetic particles, the domains are aligned in two directions opposing when there is no magnetic field applied. As the magnetic field is applied and strength of applied field (H) increases, the magnetic moment of ferrimagnetic material become aligns with H. The magnetic domains with aligned magnetic moment grow at expense of poorly aligned ones. In the end, the magnetic domains become single domain when the applied magnetic field increases until a point. Single magnetic domain is desirable as it is easily to rotate the particles for usage. Single domain also gives higher magnetic induction because there are no oppose magnetic domain, the single domain align in one direction only. These properties are ideal for making of permanent magnet. To a higher chance of obtaining single domain in particles of a compound, the compound often produced in nanoparticles. Nanoparticles tend to give few magnetic domains, probably single domain. This is due to the very small size of particles tend to give the smallest amount of grain boundary, therefore the chance of getting magnetic domains in opposite direction is small compare to the large size particles which have more grain boundaries. Therefore in this study, sol-gel technique which is able to produce nano-sized particles was employed.

The magnetic properties of the hexagonal strontium ferrites are strongly dependent upon the synthesis conditions and the site preference of the substituted cations among the five different Fe3+ sublattices namely, tetrahedral (4f1), trigonal bipyramidal (2b) and octahedral (12k, 2a and 4f2) of hexagonal structure [13].

Mass susceptibility is the ratio of the intensity of magnetism induced in the sample to the magnetising field applied in response to the density of the substance. In commercial application, the mass susceptibility is desirable as having high value, for the usage of strong permanent magnet and recording media.

For the composition where x = 0.2, the substituted strontium ferrite recorded decrease in mass susceptibility. The magnetic properties were not as desirable as the value of susceptibility needs to be relatively high.

In the specimen with x = 0.4, a dramatically increase in mass susceptibility was measured. The increase in susceptibility indicated that the substitution of Co(II)-Ti(IV) had filled up the minor β-sublattice (spin-down) of the magnetoplumbite structure and thus enhanced the measured magnetisation along the α-sublattice (spin-up) axis.

In the specimen with x = 0.6, the mass susceptibility decreased. The large decrease of susceptibility indicated that at this ratio, the Co(II)-Ti(IV) cations may well have occupied the cation sites which were in the α-sublattice (spin-up).

For x = 0.8 to 5.0, it was found that both specimens recorded similar values of susceptibility. As the substitution of Co(II)-Ti(IV) increased, the susceptibility showed a rapid decrease. It might therefore be expected that further substitution will subsequently produce a superparamagnetic-like strontium ferrite. Superparamagnetism is a phenomenon by which magnetic materials may exhibit a behavior similar to paramagnetism at temperatures below the Curie temperature.

For x = 6.0 shows negative value of mass susceptibility, indicates the formation of cobalt oxide and titanium dioxide which are diamagnetic. There are no iron oxides which gives ferromagnetic properties.

From the previous published study, the partial substitution of Fe3+ ions with a Co2+ + Ti4+ pair was attempted [4]. However, the coercive force reduces and simultaneously saturation magnetization also reduces. Therefore, the ratio of substituted cations is very important in modifying the magnetic properties of ferrite.

Single domain particles of Ti-Co substituted M-type hexaferrite posses attractive properties for the recording media applications [13]. Such substitutions at iron site are effective in reducing the coercivity and magnetocrystalline anisotropy but require higher annealing temperature for the single phase formation. It has also been reported that when synthesizing Ti-Co substituted hexaferrites, it is difficult to avoid the formation of cobalt ferrite. Although the coercivity decrease by the substitution of Co-Ni but at the same time the saturation magnetization of the materials decrease which limit their applications in the high density recording media.

Chapter 4: Conclusions

4.1 Conclusions

4.2 Future Work

4.1 CONCLUSIONS

The effect of cation substitution on magnetic properties of strontium ferrite is discussed in the previous chapter. The magnetic properties are differing corresponding to the different cation substitution ratios. There existed significant trend which corresponded to the changes in substitution ratio in strontium ferrite. The mass susceptibility is the highest at x = 0.4 in SrCoxTixFe12-2xO19. This indicates that this cation substitution ratio gives best magnetic behaviour where the magnetization is the highest. This phenomenon is favourable for commercial application such as making of permanent magnet and magnetic recording media.

4.2 FUTURE WORK

In this study, the magnetic properties of M-type hexagonal SrFe12O19 and cation substituted SrFe12O19 was studied. One of the magnetic properties, mass susceptibility of the ferrites was determined in this study, by using magnetic susceptibility balance. Besides magnetic susceptibility balance, the study on magnetic properties can be improved by using SQUID magnetometer or vibrating sample magnetometer (VSM) [12] to investigate the saturation magnetisation, remnant magnetisation and coercivity. These three magnetic properties are the essential one to determine their magnetic behaviour for various applications. However, these two instruments are not available in our laboratory.

The cobalt-titanium substitution is replacing the iron sites instead of strontium sites. The strontium site could be substituted with other transition elements or rare earth elements with the similar atomic radii, example for bismuth and lanthanide.

In future, further study could be carried out by substituted lanthanide completely to replace strontium ferrite to investigate whether the M-type hexagonal structure remains.

One of the ways to enhance magnetic properties is to produce single domain particles by pairing the divalent-tetravalent substitution in strontium ferrite with appropriate synthetic methods. Besides cation substitution by Co2+-Ti4+ pair, sol gel derived strontium ferrite with iron substituted by Zn2+, Ti4+ and Ir4+ have been carried out [9]. They are Zn2+-Ti4+ pair and Zn2+-Ir4+ pair. If this is true, it will have a great impact on the technology of tomorrow.