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CHAPTER 1
INTRODUCTION
The name "garnet" comes from the Latin granatus, a grain possibly in reference to malum
garanatum (pomegranate) a plant with red seeds similar in shape, size and color to some
garnet crystals. Garnet exists both in amorphous and polycrystalline form. The garnets are
classified as naturally occurring garnet and chemically synthesized garnet [1*].
1.1 Natural garnets
Garnets are neo-silicates of general formula C3A2(SiO4)3, and have 8 formula units in a unit
cell. The space group of garnet is ‘Ia3d’ i.e. a body centered cubic lattice. The C- site is
usually occupied by divalent cations (Ca2+, Mg2+, Fe2+) and the A-site by trivalent cations
(Al3+, Fe3+, Cr3+) in an octahedral/tetrahedral framework with Si occupying the tetrahedral
site.
• c-site (dodecahedral) is the largest cation site. In this site eight oxygen ions in
positions 96 h-sites form the corners of dodecahedral configuration which amounts to
a cube with the faces slightly bent along one diagonal of each. Each unit cell has 24 c-
site with a orthoromhbic point group symmetry 222.
• a-site (octahedral) is the next largest cation site. In this site six oxygen ions in position
96 h-sites forn an octahedron streched along one three fold axis. Each unit cell has 16
a-site in rohmbohedral point group symmetry 3 bar.
• d-site (tetrahedral) is the smallest cation site. In this site four oxygen ions in positions
96 h-sites form the corners of tetrahedral configuration. Each unit cell has 24
(tetrahedral)-site with a tetrahedral point group symmetry 4 bar.
* Note : The referencrces cited from internet, not being peer reviewed are * marked and are given seperately in Bibliography
2
Figure 1.1 (a) Dodecahedron geometry of c-site, (b) octahedron shape of a-site and (c) tetrahedron
shape of d-site. Oxygen occupies the corners and cations occupy the center of polyhedra [3*].
96 h-sites with a point symmetry 1 bar and are occupied by oxygen ions. In this structure
tetrahedron shares two edges with neighoring dodecahdeons, the octanhedron shares six edges
with dodecahedrons, and each dodechedron shares ttwo edges with tetrahedrons, four edges
with other dodechedrons. The octahedron and tetrahedra do not share a common edge. All
edge-sharing involves at least one dodecahdron [1].
Figure 1.2 Garnet unit cell (BCC), as network of tetrahedral (bluish), octahedral (pinkish) and
dodecahedral (circular) sites, contains 160 atoms (64 cations and 96 oxygen ions) [2*].
3
1.1.1 Applications of natural garnets
• Garnet sand is a good abrasive and a common replacement for silica sand in sand
blasting.
• Garnet sand is also used for water filtration media.
• For water jet cutting, garnet extracted from hard rock is suitable since it is more
angular in form, therefore more efficient in cutting.
1.2 Synthetic garnet
The general formula is A3B2(CO4)3. As in case of natural garnet we have silicon at C site but
here we can put large no. of material including Ge, Ga, Al, V and Fe and can have the
variable properties. This is a class of garnet which is prepared in laboratory both in pure and
doped with other ions form. We need to dope the garnet in order to have the variable
properties so that it can be used for various applications like in electronics and magneto-optics
industry. There are two very important synthetic garnets that we will discuss in later sections.
These are yttrium iron garnet (YIG) and yttrium aluminium garnet (YAG) [1*].
1.2.1 Yttrium iron garnet (YIG)
Yttrium iron garnet (YIG) is a kind of synthetic garnet which is ferrimagnetic with chemical
composition Y3Fe2(FeO4)3, or Y3Fe5O12 and Curie temperature 550 K. It has advantageous
properties like Faraday rotation: -3000-(-4000) degrees/cm (Ce-doped YIG films), Saturation
magnetization: 1800-2500 G (bulk YIG at room temperature), High Quality factor in
microwave frequencies, low absorption of infrared wavelengths up to 600 nm and Very small
line-width in electron spin resonance [5*].
1.2.1.1 Crystalline structure of YIG
YIG belongs to space group Oh10-Ia3d. It is a body centered cubic bravis lattice. In YIG, the
five Fe (III) ions occupy two octahedral and three tetrahedral sites, with the Y (III) ions
coordinated by eight oxygen ions in an irregular cube. The iron ions in the two coordination
sites exhibit different spins, resulting in magnetic behavior [1].
4
Figure 1.3 Arrangement of sites in YIG [6*].
It has three different crystallographic sites with 16Fe3+ cations in octahedral [a] sites, 24Fe3+
cations in the tetrahedral (d) sites and 24 Y3+ cations in the dodecahedral{c} sites. Neither of
these polyhedra is regular and oxygen lattice is much distorted. The magnetic contribution
arises from the antiparallel alignment of the Y3+ magnetic moments in the{c} site to the
resultant of the antiferromagnetically coupled magnetic moments in the [a] and (d) sites. The
cation distribution at the[a] and (d) sites of garnet is expected to play the most important role
in controlling its magnetic properties. The strongest magnetic interactions in pure YIG is
related to the inter-sublattice exchange, i.e. super exchange interaction betweenFe+3 iron in
octahedral and tetrahedral through intervening O2- ions [16].
Figure 1.4 (a) distribution of sites in unit cell and (b) distribution of sites in an octant of unit cell with
О tetrahedral cation, ∆ octahedral cation, • dodecahedral cation [1].
5
1.2.1.2. Superexchange interactions
At 0 K, the magnetic moment of YIG is 5µB . The origin of this moment is the consequence of
the super exchange antiferromagnetic interaction between trivalent iron ions, of which there
are 3 in tetrahedral sub-lattice and 2 in octahedral sub-lattice. Each Fe3+ ion is in 3d5
electronic configuration and has a moment of 5µB. so the difference in iron sub-lattice
moments is 5µB since magnetic moment of tetrahedral sublattice is opposite to that of
octahedral sublattice [1].
The Fe3+a –O2- -- Fe3+
d super exchange linkage geometry is clearly exhibited by figure1.5.
Although this (a)-(d) linkage is by far the strongest for super exchange interaction, there are
other linkages too, one (a)-(a) linkage and four (d)-(d) linkages which will lead to
antiferromagnetic interaction in the (a) and (d) sublattices. These intra sublattice interactions
can become important when the tetrahedral or octahedral iron is substantially depleted by
nonmagnetic ion substitution [4].
1.2.1.3. Magnetization
Magnetization refers to alignment of the dipole or the magnetic moment of electrons or atoms
or in a specified direction after application of the external magnetic field in the same
direction.
The temperature dependences of magnetization of the octahedral Mocta iron sublattice and the
tetrahedral sublattice Mtetra are not quite alike with Mocta decreasing less rapidly with
increasing temperature than Mtetra in the region 100 K < T < 500 K so that the net
magnetization M decreases less rapidly than Mtetra or Mocta [4].
6
Figure 1.5 Spontaneous magnetization curve of YIG in bohr magneton per formula unit vs
temperature. The solid curve is experimental and dotted curve is in presence of molecular field
assuming no a-a site and d-d site interactions [5].
Figure 1.6 Magnetization of the tetrahedral sublattice(left) and octahedral sublattice(right) of YIG in
Bohr magnetons per formula unit vs. temperature. The curve shown is a molecular field fit [5].
7
Different magnetization values can be obtained by substitutions in the yttrium garnet as Fe
ions occupy two types of crystallographic sites: for one molecule, there are three Fe 3+ ions in
the tetrahedral sites and two Fe 3+ ions in octahedral sites. As we know magnetization is the
difference between the magnetizations of the magnetic sub lattices (by the theory of
ferrimagnetism).
Ms = ׀Mtetra – Mocta׀
We can reduce the Ms by two methods. In first method, we substitute the Fe ion by the non
magnetic ions like as Al ions which will occupy tetrahedral sites and hence Mtetra decreases.
In second method, we substitute dodecahedral site of non magnetic yttrium ions by magnetic
ions like as gadolinium ion whose magnetization counteracts the resultant of the
magnetization of Fe ions [4].
1.2.1.4. Curie Temperature
The Curie temperature is defined by the vanishing of the spontaneous magnetization of the
material i.e. the vanishing of the magnetization in the absence of any field.
The value of Curie temperature depends upon the Fe3+ ions as the only magnetic ion also as it
depend upon the number n of Fe3+a –O2- – Fe3+
d linkages per formula unit. In YIG there are n
= 24/5 such linkages per formula unit Y3Fe5O12 so that Tc / n = 550/ (24/5) which is nearly
equal to an average value of 115 K obtained for eight different oxides of Fe3+ by Gilleo
(1958) [4].
Also, the Curie temperature changes as we substitute nonmagnetic ion in a-cite or d-cite. In
the same way it is also possible to estimate the composition of a substituted YIG by means of
a measured Curie temperature and the saturation magnetization.
8
1.2.2. Preparation of Polycrystalline sintered YIG and the Substituted
garnets
The preparation of polycrystalline garnet samples of high purity, uniform grain size and
density requires a good amount of experience in ceramic technology and the understanding of
sintering technique.
The best fine grained garnets are obtained normally by hot pressing of garnet powders pre-
fired and ball-milled in a conventional way. Hot pressing is carried out at relatively low
temperatures and for shorter durations compared to normal sintered materials. In this
technique we need greater homogeneity and the fineness of the powder since inhomogeneity
or traces of other phase present in the pre sintered powder compact cannot be smoothened out
in the short duration of the hot pressing [8].
1.2.3. Applications
• Yttrium–iron garnet materials possess the highest quality factor, in microwave regime,
viz. the smallest line-width in magnetic resonance, among the magnetic materials [1].
• These materials also own high saturate magnetization, which can be tailoredly
designed by forming solid solution with Gd3Fe5O12 and Y3Al5O12, etc. Yttrium–iron
garnet materials, in ceramics or single crystal form, are thus widely used for magnetic
microwave devices, such as circulators, oscillators and phase shifter [2].
• YIG is used in microwave components like as YIG-Tuned filters, YIG-Tuned
Oscillator, junction circulator and phase shifter.
• It also used in bubble devices.
• It also used in magneto-optic storage devices for example systems aspects of optical
data storage technology.
• It also used in integrated optics, magneto-optic display and infrared light intensity.
• Yttrium iron garnet is also exceptionally efficient as both a transmitter and transducer
of acoustic energy.
9
In summary, the magnetic garnets have found maximum usage in various non reciprocal
ferrite devices, like circulators, isolators, phase shifters in microwave communication and
radar transmit receive chains. In the recent times, Yttrium Iron Garnet (Y3Fe5O12; YIG) and
the substituted garnets (e.g. Al-YIG, Gd-YIG etc.) with controlled magnetization have found
increasingly more applications in mobile phones. An important characteristic of garnets, with
respect to other ferrites, is their having very low power loss to electrical signals (at microwave
frequencies) during transmission. However, when garnets, being hard and brittle, are made
into appropriate sizes, by careful diamond cutting and other machining operations, to suit
components design, many stress induced defects are generated which lead to unacceptable
level of increased electrical and magnetic losses. Hence, to relieve the residual strains in the
material, it is desirable to subject the material to appropriate thermal treatments in air or other
gaseous ambience. For observing the extent of adverse effects due to residual strains and
charge defects in oxide magnetic material, the most sensitive way is to measure magnetic
resonance linewidth (∆H). This can be done by employing microwave electron spin resonance
(ESR) spectrometer. The ESR linewidth (∆H), to a large extent, manifests the magnetic losses
in a material at resonance.
1.3. Aim of Project
To study the effect of residual strains arising due to attrition/machining etc. in the
polycrystalline sintered magnetic garnets viz. YIG and the substituted Al-YIG and Gd-YIG
via measurement of the ESR linewidth (∆H) and to see how do the different heat treatments
and ambient atmospheres (air, N2 and O2 ) affect ∆H values.
10
CHAPTER 2
Residual Strain, Basics of Magnetic Interaction and
ESR Line-width
2.1. Residual Strain
When apply stresses (external forces or heat gradient) on a body or material it undergo some
deformation and even if we remove the stress applied, the body still have stresses due to
deformation known as residual stress and associated strain is residual strain [8*].
2.2. Super exchange interactions
In this interaction, the spins of magnetic ions are coupled by an interaction via the electron
system of the oxygen anions which lie between. This mechanism corresponds to a so called
configuration interaction, i.e. a ground configuration and an excited state of this configuration
interact and form by superposition a new state with lower energy. The theory predicts
maximum super exchange interaction for a configuration in which the magnetic ions and the
oxygen form an angle of 180 degrees and M-O distance is small. The interaction for the 90
degree configuration is much smaller [2].
2.3. Electrons Spin Resonance (ESR)
It is a branch of absorption spectroscopy in which radiation having frequency in microwave
region is absorbed by paramagnetic substance to induce transition between magnetic energy
level of electron with unpaired spin. Magnetic energy splitting is done by applying a static
magnetic field [1*].
2.3.1. Basic principle of ESR
The unpaired electrons are excited to a high energy state under the magnetic field by the
absorption of microwave energy. The excited electron changes its direction of spin and
relaxes into the ground state by emitting phonons. Microwave absorption is measured as a
function of the magnetic field by ESR spectroscopy.
11
2.3.2. Theory of ESR:
Every electron in an atom travels in an orbit around a nucleus has orbital angular momentum.
Within this orbit, it also spins about its own axis, and has spin momentum and spin quantum
number s = 1/2, with magnetic components ms = +1/2 and ms = -1/2.
In presence of external magnetic field B0 (let us assume), some electrons’ magnetic moments
align themselves parallel to the external magnetic field and some align anti parallel to the
external magnetic field. Since each alignment has specific energy associated with it therefore
the one whose alignment is parallel to the external magnetic field corresponds to low energy
state (E-1/2 = --1/2 gµBB0) and the one whose alignment is anti parallel to the external field
corresponds to high energy state (E+1/2= +1/2 gµBB0). The separation between these two states
is
∆E = gµBB0
Where g is a Lande’ g factor (measure of the contribution of the spin and orbital motion to its
total angular momentum) and µB is Bohr magneton. This equation implies that the splitting of
the energy levels is directly proportional to the magnetic field's strength.
Figure 2.1 splitting of energy level in presence of crystal field [9*].
Now, the technique of ESR is purely to make electronic transitions from the lower level, often
the ground state, to the higher level (shown in fig 2.1). An electron can move between the two
energy levels by either absorbing or emitting electromagnetic radiation of energy E = hν.
12
So the resonance condition, E = ∆E leads to the fundamental equation of ESR:
hν = gµBB0.
This equation works smoothly in frequency range of around 9-10 GHz with fields
corresponding to about 3500 G.
So, if this resonance absorption of radiation is continue, then there must be some phenomena
by which electrons move back to lower energy level from higher energy level. This process is
known as a Relaxation processes and are measured in terms of relaxation time. In absence of
relaxation, saturation occurs in which continuous absorption of energy by electron present in
lower state leads to equal population in both states. Hence there will be No further absorption,
No further resonance, No further signal and Broadening in signal.
Relaxation time should be sufficiently rapid to prevent saturation of upper energetic level at
the same time sufficiently slow to yield narrow spectral peaks. Ratio of number of electrons in
upper energy level to those in lower energy level is given by BOLTZMANN LAW.
n1/n2 = exp (– gµBB/kT)
Where n1 and n2 are no. of electrons in state 1 and state 2, g is gyromagnetic ratio, µB is bohr
magneton, B is applied magnetic field, k is Boltzmann constant and T is temp.
We have seen that the spinning electron has orbital motion around the nucleus. Consider this
system placed in a steady magnetic field Bo, with the axis of spin rotation inclined at some
13
angle to the applied field. The spinning electron acting as a small magnetic dipole will
experience a torque tending to turn it into alignment with the field, but this cannot take place
as the electron spin has orbital momentum about the nucleus. The axis of the spinning
electron will then precess around the magnetic field axis, as shown in figure 2.2.
Figure 2.2 vector diagram for the precession of a magnetic dipole under the influence of a static field and a rotating field [12*].
ωo =γBo is the relation which relate Larmor precession frequency ωo to applied field Bo.
Where γ is magnetogyric ratio of electron and is the ratio of magnetic moment to mechanical
moment of inertia. γ = gµB/(Һ/2π).
By putting the value of γ in the above equation and we lead to the fundamental equation of
ESR as hν = gµBB0. This equation is used to determine g in an ESR experiment by
measuring the field and the frequency at which resonance occurs. If g is the ratio of the
unpaired electron’s spin magnetic moment to its angular momentum then it differs from the
free electron value. Since an electron’s spin magnetic moment is constant, then the electron
must have gained or lost angular momentum through spin-orbit coupling because the
mechanisms of spin orbit coupling are well understood, the magnitude of the change gives
information about the nature of the atomic or molecular orbital containing the unpaired
electron [7].
Because of electron nuclear mass differences, the magnetic moment of an electron is
substantially larger than the corresponding quantity for any nucleus, so that a much higher
electromagnetic frequency is needed to bring about a spin resonance with an electron than
with a nucleus, at identical magnetic field strengths. For example, for the field of 3350 G
shown at the right, spin resonance occurs near 9388.2 MHz for an electron compared to only
about 14.3 MHz for 1H nuclei.
14
2.3.3 Understanding of g – factor
The g-factor is the ratio of spin magnetic moment β to spin angular momentum s, divided by
orbital magnetic moment M to orbital angular momentum p. Each electron has two types of
permanent magnetic moments, an orbital moment and also a single spin moment. The
electrons will also possess orbital and spin angular momentum [2].
The correlation between angular momentum and magnetic moment is based on the principle
that a current i, circling a single loop of area A in vacuum µo, creates a magnetic field
identical to that of a magnetic moment M.
M = µoiA
If an electrons of charge e travels in an orbit of radius r at a frequency of f times per second,
then
M = µoπr2ef
The orbiting electron also creates orbital angular momentum p (moment of momentum) about
the axis, where
p = moωr2 = mo2πfr2
this momentum vector is anti parallel to M, as p is clearly positive and M negative since it
contains as electronic charge e. hence, the magnetic and angular moments are interrelated as
M = (µoe/2mo)p
This signifies that the magnetic and mechanical moments of circling electrons are inter-
dependent on each other.
The spin angular momentum is quantized as s = sħ, Where s is the spin quantum number.
The fundamental unit of magnetic moment, bohr magneton, is the fundamental magnetic
moment of spin. i.e.
β = µoeħ/2mo
15
Hence, g-factor : g = βp/sM = 1/s
As we know spin quantum no. for a single free electron is ½ hence g = 2. The value of g is 1
when the magnetic moment is due to orbital motion alone and is 2 for the spin alone, but if
coupling exists between the spin and orbital moments it has a value larger than 2.
In principle, the ESR spectra can be generated either by varying incident
photon frequency at the same time holding the magnetic field constant or doing the reverse. In
practice, it is usually the frequency which is kept fixed. By increasing an external magnetic
field, the gap between the ms = +1/2 and ms = −1/2 energy states is widened until it matches
the energy of the microwaves, as represented by the double-arrow in the diagram above. At
this point the unpaired electrons can move between their two spin states. Since there typically
are more electrons in the lower state, due to the Maxwell-Boltzmann distribution, there is a
net absorption of energy, and it is this absorption which is monitored and converted into a
spectrum [2].
2.3.4 Experimental aspects of ESR:
In this technique, klystron oscillator of frequency 9 GHz delivers a power of 30-300 mW.
The energy is transmitted by means of a waveguide i.e. a rectangular copper or brass tubing of
dimensions equivalent to wavelength of radiation. The sample is placed inside the resonant
cavity, which has small hole in each end wall to transmit power in and out. The purpose of
cavity is to concentrate energy on to the sample by multiple reflections of the travelling
microwave from the two end walls. After coming out from the cavity, the microwave power is
sensed by semiconducting crystal detector which acts as a rectifier which converts the micro
power into direct current [7].
The absorption of energy by the paramagnetic sample can be seen by direct observation of the
crystal current while slowly varying the field. As the field approaches the value corresponding
to resonance, power is absorbed by the sample so that the power transmitted through the
cavity to the crystal is reduced.
16
Figure 2.3 ESR experimental setup of ESR [11*].
2.3.4.1 Microwave Bridge:
There are different parts of Microwave Bridge as microwave oscillator, attenuator, bridge,
cavity, detector and reference arm.
2.3.4.1.1 Microwave oscillator: The most suitable oscillator for the frequency around 9.1
GHz and of power 30-300 mW is klystron. A klystron can only be tuned over a small region.
In reflex klystron the electrons from the cathode are accelerated towards the rf gap by the
beam voltage. Because of the negative repeller voltage the electrons turn to rf gap. The
frequency of the oscillations is adjusted by mechanically changing the dimension of the
resonant cavity. The klystron frequency is tuned to the resonant frequency of the microwave
cavity with sample in place. For cooling the klystron water is used and for protecting it from
reflected power from the microwave circuitry isolator is used.
2.3.4.1.2 Attenuator: If all the power from the klystron goes to the sample it can saturate the
signal. It is therefore necessary to attenuate the microwave power reaching the cavity. A
special form of attenuator also known as isolator is used just after the klystron to prevent the
power reflected back to the klystron and causing unwanted oscillations. Sometimes a power
leveler is also introduced between the klystron and the attenuator. This keeps power output
constant independent of input.
17
2.3.4.1.3 Bridge: The microwave power enters the cavity by a hole in one walls and leaves by
a hole in the other wall. A small dip in the detector current indicates the resonance absorption.
This bridge sensitivity is poor and in order increase the sensitivity we use reflection cavity
instead of a transmission cavity and bridge can be balanced so that no power from klystron
reaches the detector.
2.3.4.1.4 Cavity: Cavity is generally of the reflection type where the power enters and leaves
the same hole. The cavity is designed to store microwave energy in standing waves of the
wavelength corresponding to the frequency of the microwave oscillator. The magnetic field is
concentrated in the center of the cavity so sample should be kept at the center.
2.3.4.1.5 Detector: A common detector for the microwave signal is a crystal diode which
works as a microwave rectifier. It consists of a semi conducting silicon crystal with a metal
point contact. The detector crystal works best at certain level of detector current. To obtain the
desired bias level some microwave power is fed directly to the detector. The noise of detector
is inversely proportional to the frequency.
2.3.4.1.6 Reference Arm: This branch off some microwave power from the klystron directly
to the detector to give the bias current, which is important since the biasing of the detector
through the reference arm allows operation at very low power levels. The microwave cavity
can then be perfectly matched without any reflection and no adjustments have to be made
while changing the attenuation.
2.3.4.2 Magnet:
Magnet is by far the most expensive single component in ESR spectrometer. Initially we used
a permanent magnet with auxiliary coils to provide the variation of few hundred gauss but
now days we use electromagnet with a field range from 500 to 5000 gauss. In most
instruments a range of scanning rates is available and it varies from a few milli gauss to
several hundred gauss per minute so that spectra of greatly different overall widths maybe
conveniently observed. This can be achieved by mechanical means or electronically. Line
width of the hyperfine spectra of free radicals in solution can be as low as a few milligauss.
Also, the line width for paramagnetic ions and for radicals in solid matrices can be higher
than few milligauss and hence the magnetic field should be homogeneous as well as stable.
18
2.3.4.3 Spectrometer:
Spectrometer should have maximum sensitivity and stability and it should have a good
resolution. A spectrometer to be used for many different experiments should also be highly
versatile.
Figure 2.4 A typical power absorbed (Pabsorb) Vs H curve, length AB is Full Width at Half Maximum (FWHM), denoted as line width (∆H) also frequency is X band [7].
Low line-width ∆H corresponds to low losses in sample in a corresponding frequency range.
If there are impurities or porosities in the sample then the line width will appear broader. In
the figure 2.5 the distance between the two peaks will give the value of ∆H, while the
inflation point is the point at which resonance takes place.
Figure 2.5 Field derivative of power absorbed (dP/dH) Vs H curve [7].
The points A and B in figure 2.4 are same as P1 and P2 in figure 2.5. Hence, the basic
equation of ESR measurements is ∆E = gβHo and also ∆E = hν.
19
The frequency is kept constant and marker sample is used. So, we get the equation as
gm Hm= geff Hsample
Where, gm and Hm are corresponding g-value and field of the marker. The marker sample
generally used is Tetra Cyno Ethylene (TCNE) or Di Phenyl Pienyl Hydroxide (DPPH) with g
values of 2.00277 and 2.0036 respectively. The marker’s position in the ESR spectra is shown
by a hiccup in the curve. Surface finish and stoichiometry also contribute to ∆H.
2.3.5 The parameter required to meet the application of ESR
• Ms: magnetization of the material
• M-H curve of the material
• Tc: Curie temperature
• ∆H: ESR-line width
ESR line width (∆H):
∆H = ∆Hint + ∆Ha + ∆Hp + ∆Hresi
Where, ∆Hint intrinsic line width, ∆Ha is the broadening due to magnetocrystalline anisotropy,
∆Hp is the broadening due to porosity; ∆Hresi is the broadening due to residual stresses.
In order to have low losses in garnet ∆H should be less and which can be obtained by any
reducing one term and keeping other almost constant. Since in this project, the heat treatment
of the garnet will be done in order to reduce the stresses and hence indirectly we are reducing
∆H by reducing ∆Hresi as residual stresses reduces by the heat treatment.
20
CHAPTER 3
Literature Review on Substituted Yttrium Iron Garnet
Yttrium iron garnet (YIG) and substituted YIG are interesting ferromagnetic materials in part
because they have potential application in microwave devices. Ferrimagnetic garnets are
assigned to cubic structure (space group Ia3d); every cell contains 8 formula unit of
Y33+Fe5
3+O12. Y3+ ion cannot occupy the octahedral and tetrahedral sites because of its large
ion radius, so R3+ ion can only occupy dodecahedral sites which have larger space. In the case
of ferrimagnetic garnet Y3Fe5O12, the ion distribution structure can be represented by writing
the garnet formula as {Y3}[Fe2](Fe3)O12, { }, [ ], ( ) representing 24c (dodecahedral), 16a
(octahedral) and 24d (tetrahedral), respectively.
3.1. Aluminium substituted garnets (Al-YIG) Y3 Fe5-YAlYO12
The Al 3+ ions preferably occupy the tetrahedral sites in garnet structures, although a small
fraction of them occupy the octahedral sites. This substitution of Al ion in place of Fe ion
leads to decrease in magnetization as well in Curie temperature Tc because of reduction in
Fe3+ –O– Fe3+ interaction and hence stability of MS with temperature is lower.
Figure 3.1 Saturation magnetization at room temperature for garnet having the composition Y3Fe5YAlYO12 [4].
21
Figure 3.2 line width ∆H at room temperature of garnets having composition Y3Fe5-YAlYO12 [4]
Figure 3.3 (A) line width of Y3Fe5-5YAl5YO12 as function of temperature. (B) Saturation magnetization Vs temperature of comp. Y3(Fe5-YAlY)O12 for 0 < y < 0.15 [4].
22
The amount Al substitution is laying in this interval 0 ≤ y ≤ 0.27. Since higher amount lead to
the instability of characteristics with temperature, we cannot substitute large amount of Al as
the characteristics like effective line width ∆Heff and spin wave line width ∆Hk get affected as
shown in fig. 16 and 17, as y increase the tendency of ∆H to decrease with temperature
decreases.
Table 3.1 ESR parameters for Y3AlxFe5-XO12, at frequency 9.3 GHz [4]
Type
4πMS
(Gauss)
∆H
(Oe)
Tc
(K)
Y 35 1200 40 225
Y34 1000 40 210
Y 39 800 40 195
Y 38 760 40 190
Y 37 680 40 180
Y 33 615 40 175
Y 30 565 35 160
Y 32 420 35 135
Y 31 370 35 125
Y 36 290 30 115
3.2. Yttrium gadolinium garnets (Gd-YIG) Y3-xGdxFe5O12
The Gd ions occupy the dodecahedral sites. This substitution leads to decrease in
magnetization of garnet without practically changing the Curie temperature Tc and hence
stability of Ms with temperature is higher.
23
Figure 3.4 saturation magnetization at room temperature for garnets having the composition Y3-xGdxFe5O12 [4]
For x > 0.3 (approx), the curve of Ms versus temperature exhibits low temperature
compensation points. Fig shows that alpha decreases with x, down to x = 0.4. conversely, the
line width delta H increase with x this is due to the reduction of Ms in terms of delta Ha
which is related to line width broadening by anisotropy equation, with K1 constant with x.
beyond x = 0.55, this type of garnet is not often used, because the comparatively broad line
width and a lower stability with temperature also as shown in fig.
Figure 3.5 linewidth ∆H at room temperature for garnet having the composition Y3-xGdxFe5O12 [4]
24
Table 3.2 ESR parameters for Y3-XGdxFe5O12 , at frequency 9.3 GHz [4]
Type
4πMS
(Gauss)
∆H
(Oe)
geff
Tc
(K)
Y11 1600 60 2.00 553
Y12 1420 65 2.01 553
Y13 1250 75 2.01 553
Y14 1100 95 2.02 553
Y15 900 140 2.03 553
The gadolinium ion is the only magnetic rare earth which can be used in the manner described
above. The gadolinium ion is the S ion, it does not exhibit spin orbit coupling casing rapid
damping of the gyromagnetic movement (rapid relaxation) which increases line widths ∆HK,
∆Heff and ∆H to prohibitive proportions in terms of microwave application. The other rare
earth magnetic ions can be used only at very low doping levels.
Figure 3.6 Saturation magnetization as a function of temperature for Y3-xGdxFe5O12 [4].
25
3.3 Indium substituted Yttrium Iron Garnet (YIG) InxY3 Fe5-xO12
In3+ substitution in the YIG led to increase in the lattice parameter. The increase in
magnitudes of parameter were 12.362 to 12.407 ˚A for the samples with x=0.1 and 0.4,
respectively. The observed increase can be justified by considering the larger ionic size of
In+3 (0.792 ˚ A) compared to that of Fe+3 (0.642 ˚ A) [16].
Also, as the In3+ concentration increases (x) from 0.1 to 0.2 MS decreases and for x>0.2 it’s
rising. The latter rise can be understood as the substitution of In3+ for [a] which is
antiferromagnetically coupled to the iron cations at octahedral sites. Based on the Neel’s
theory of ferrimagnetism in ferrite, the substitution of a non magnetic ion like In3+ for Fe3+ in
[a] site can lead to the rise of total saturation magnetization. But the decline in Ms for x=0.2
as compared to x=0.1 can be understand by the Yafet-Kittle model in which possibility of
canted triangular spin configuration is considered once a non magnetic cation is substituted
for a magnetic sublattice. Spin canting would contribute to the lack of spin alignment (non-
collinear spin arrangement) even at high fields. Ofcourse, spin canting in magnetic ferrites can
also be initiated by other contributing sources such as: unbalanced distribution in occupation
of iron cations at[a] or (d) sites, which could induce both topological and exchange interaction
disorder, structural defects in the surface layer, e.g. vacancies in A-sites which causes a local
spin canting in B-sites and vice versa [16].
3.4 Cerium substituted Yttrium Iron Garnet (YIG) CexY3-x Fe5O12
Cerium-substituted YIG (Ce:YIG), in particular, has been found to exhibit a large magneto-
optic effect and low propagation loss, which will be good candidate materials for the devices
with higher quality [11]. A low coercivity, high-remanence, soft magnetic material, having a
hysteresis loop, is required for microwave operation. For a magnetic material to be applied in
microwave devices, the most important static magnetic properties are the saturation
magnetization (Ms), anisotropy constants, Neel temperature, remanent magnetization,
coercivity (Hc). In general, Ms and Hc are required for applications [10].
26
The saturation magnetization of Y2.9Ce0.1Fe5O12 is 28.0emu/g. Saturation magnetization of the
sample increase 2.0emu/g than that of the pure YIG (saturation magnetization of pure YIG is
26.0 emu/g [12]. In a YIG system, non-magnetic Y3+ ions occupy dodecahedral (c-) sites and
magnetic Fe3+ ions occupy octahedral (a-) and tetrahedral (d-) sites. The magnetic moment
caused by two Fe3+ ions in an a-site is aligned anti-parallel to that caused by three Fe3+ ions
in a d-site, leaving a net moment from Fe3+ in the d-site. Therefore, the saturation
magnetization of YIG is given by the magnetic Fe3+ in the d-sites. The paramagnetic trivalent
Ce3+ ions can be substituted for non-magnetic Y3+ ions in c-sites, but not for Fe3+ ions in a-
or d-sites. The magnetic moment of Ce3+, substituted for Y3+ in c-sites, which can be
parallel to the magnetic moment of Fe3+ in the d-sites, meaning that the saturation
magnetization of Ce:YIG is different from that of pure YIG. At room temperature with
increasing Ce3+, the saturation magnetization of the YIG samples increased slightly from
about 26.0emu/g, reaching a maximum value of 28.0 emu/g at x = 0.1. However, cerium ions
tend to exist in a diamagnetic tetravalent state Ce4+ (that is, no electron in the 4f shell) and to
precipitate as CeO2 [10]. Consequently, an excessive addition to YIG material leads to the
non-magnetic inclusion of CeO2 inside the material, which is of no benefit to enhance the
magnetic and magneto-optical properties of YIG material [13].
27
CHAPTER 4
EXPERIMENTAL
4.1 Preparation of the spheres of Pure-YIG, Gd-YIG, Al-YIG
Already prepared garnet samples (Y3Fe5O12 as Pure-YIG, Y3-XGdXFe5O12 as Gd-YIG and
Y3AlXFe5-XO12 as Al-YIG) are cut down so that we can have a little chunk of the sample and
then with the help of cavity which have a cylindrical shape with a tangential hole at the
bottom from which a tangential compressed air is moved in and a very fine holes in the cover
of the cavity at top to move out. The chunk of sample tumble inside the cavity and since the
inner surface of cavity has an abrasive hard coating of Silicon Carbide so after sometime
finally that chunk achieve almost spherical shape and then polishing of those spherical
samples is done by using similar type of cavity with inner surface covered with emery paper
of different grades. Now to remove the strains developed during grinding and polishing
process different heat treatment of the samples are carried out.
Figure 4.1 (a) cavity used making the sphere of 1mm dia. out of small chunk of garnet with
inner coating of hard silicon carbide (b) cavity used for polishing the sphere of 1mm dia. with
emery paper inner coating.
28
Annealing of the sample in presence of Argon gas: Heat up the sample in the inert gas
environment upto 1000 degree C and then kept the sample at 1000 degree C for 2 hours so
that soaking takes place then switch off the furnace and sample starts cooling in inert gas
(Argon gas) environment. This whole process is take place in the tubular furnace.
Annealing of the sample in presence of Oxygen gas: Heat up the sample in the Oxygen gas
environment upto 1000 degree C and then kept the sample at 1000 degree C for 2 hours so
that soaking takes place then switch off the furnace and sample starts cooling in Oxygen gas
environment. This whole process is take place in the tubular furnace.
Annealing of the sample in presence of Air: Heat up the sample upto 1000 degree C in
presence of air and then kept the sample at 1000 degree C for 2 hours so that soaking takes
place then switch off the furnace and sample starts cooling. This whole process is take place
in the Muffled furnace.
Cooling of the sample in Air: Heat up the sample upto 1000 degree C and then kept the
sample at 1000 degree C for 2 hours so that soaking takes place then takeout the sample out of
the muffled furnace and put it in the air and it eventually cools down within 5-10 minutes.
4.2 Characterization
4.2.1 SEM (Scanning Electron Microscopy)
The scanning electron microscope (SEM) is a type of electron microscope that images the
sample surface by scanning it with a high-energy beam of electrons. The electrons interact
with the atoms that make up the sample producing signals that contain information about the
sample's surface topography, composition and other properties such as electrical conductivity.
The types of signals produced by an SEM include secondary back-scattered electrons (BSE),
characteristic (cathodoluminescence), specimen current and transmitted electrons.
4.2.2 EDX (Energy Dispersive X-ray Spectroscopy)
EDX is an analytical technique used for the elemental analysis or chemical characterization
of a sample. In this spectroscopy, the investigation of a sample is done through the
interactions between electromagnetic radiation and matter, analyzing X-rays emitted by the
29
matter in response to being hit with charged particles. Its characterization capabilities are due
in large part to the fundamental principle that each element has a unique atomic structure
allowing X-rays that are characteristic of an element's atomic structure to be identified
uniquely from one another.
4.2.3 XRD (X-Ray Diffraction)
X-ray diffraction is a versatile analytical technique for identification and quantitative
determination of the various crystalline forms, known as ‘phases’, of compounds present in
powdered and solid samples. Identification is achieved by comparing the X-ray pattern or
‘diffractogram’ – obtained from an unknown sample with an internationally recognized
database containing reference pattern for more than 70,000 phases. Modern computer
controlled diffractometer systems use automatic routines to measure, record and interpret the
unique diffractograms produced by individual constituents in even a highly complex mixture.
4.2.5 ESR (Electron Spin Resonance)
ESR experiments were carried out using a Varian E-112 E-line Century Series X-band ESR
Spectrometer, utilizing 100 kHz field modulation. Tetracyanoethylene (TCNE, g = 2.00277)
used as a standard for g-factor measurements. Typical operating parameters of the
spectrometer were: modulation amplitude ~ 1G (gauss) and microwave power = 5mW.
Samples were centered in the cavity to minimize effects due to any asymmetry of the
magnetic field and to assist normalization.
4.2.6 VSM (Vibrating Sample Magnetometer)
A vibrating sample magnetometer or VSM is a scientific instrument that operates on
Faraday's Law of Induction, which tells us that a changing magnetic field will produce an
electric field. This electric field can be measured and can tell us information about the
changing magnetic field. A VSM is used to measure the magnetic behavior of magnetic
materials. A sample is placed inside a uniform magnetic field to magnetize the sample. The
sample is then physically vibrated sinusoidally, typically through the use of a piezoelectric
material.
30
CHAPTER 5
RESULTS AND DISCUSSION
On the different magnetic garnet samples various characterizations viz M vs H, XRD,
SEM/EDX and ESR were carried out to have learning experience of the various techniques,
but importantly the insight into the relationship of coercivity (Hc) and ESR line-width (∆H)
with thermal heat treatments of garnets (to release stresses and other defects) in different
gaseous environment (air, nitrogen, oxygen). Magnetization value has bearing on both Hc and
∆H, therefore the Ms measurement.
5.1 Room Temperature M Vs H curves for garnet samples
For the 3 different sintered samples of magnetic garnet viz YIG, Al-YIG and Gd-YIG,
wherein the extent (x) of Al or Gd substitution in Y3Al5-XFeXO12 or GdXY3-XFe5O12 is not
known, M-H hysteresis curves were experimentally obtained. These are given in figs 5.1&5.3.
5.1.1 Estimation of Magnetization
In the analysis of M-H curve obtained at room temperature, as expected, the highest
magnetization is shown by Pure-YIG (4πMs = 1700 G) and lower values by Al- YIG (4πMs =
1300 G) and by Gd-YIG (4πMs = 270 G). The reason being, in YIG (Y3Fe5O12) there are
three crystallographically different cations sites exists as 24 d sites (tetrahedral), 16-a sites
(octahedral) and 24-c sites (dodecahdedral). The magnetic interaction between the Fe3+ ions in
‘a’ and ‘d’ sites is strongly antiparallel. The Y3+ (at c-site) in YIG has no magnetice moment,
so the net moment of YIG is solely due to an unequal distribution of Fe3+ ions in ‘a’ (3Fe) and
‘b’ (2Fe) sites. In the case of Gd-YIG garnet, the moment on the Gd3+ (4f7) ion in c-site is anti
parallel to the resultant moment of Fe ions in a and d sites. So the magnetization decreases in
proportion to Gd substitution for Y in case of Gd-YIG.
Now, considering the case of Al-YIG, here Al3+ goes (28%) into the tetrahedral site (d-site)
because its ionic size is smaller than Fe3+ ion. Since Al3+ ion doesn’t have magnetic moment,
31
so its substitution decreases the overall magnetization because in one octant of unit cell there
are 2 a-sites and 3 d-sites and both are anti parallel to each other so it is the dominant Fe
moment on tetrahedral site which is responsible for the net decrease (d-a site) in
magnetization and on replacing some of the tetrahedral Fe ion by substituting Al ion leads to
decrease in magnetization.
Figure 5.1 Magnetization Vs Applied field at room temperature of YIG. Saturation Magnetization is
24.97 emu/g or 1650 Gauss. Here the coercivity is 482 gauss as found by zooming the scale.
Figure 5.2 Magnetization Vs Applied field at room temperature of Al-YIG. Saturation Magnetization
is 18.43 emu/g or 1200 Gauss. Here the coercivity is 484 gauss as found by zooming the scale.
32
Figure 5.3 Magnetization Vs Applied field at room temperature of Gd-YIG. Saturation Magnetization is 0.0038 emu/mg or 270 Gauss here the coercivity is 480 gauss as found by zooming the scale.
Figure 5.4 Magnetization Vs Applied field at R.T. of YIG (annealed in O2) in spherical form of 1mm diameter approx. Here the coercivity is around 48 gauss as found by zooming the scale.
33
Figure 5.5 Magnetization Vs Applied field at R.T. of YIG (in stressed state) in spherical form of 1mm diameter approx. Here the coercivity is around 52 gauss as found by zooming the scale.
5.1.2 Composition estimation of the Al-YIG and Gd-YIG samples from Room
Temperature Magnetization values
Since, the saturation magnetization for each of the 3 garnet samples has been measured. Now,
from the data graph of figure 3.1 and 3.4, the variation of magnetization with composition for
Al-substituted YIG (Al-YIG) and Gd-substituted YIG (Gd-YIG) at room temperature is
shown.
So, as we know Y3-XGdXFe5O12 as Gd-YIG and Y3Fe5-YAlYO12 as Al-YIG and from the
corresponding saturation magnetizations, using the plot in figure 3.1 and 3.4 the value of x is
2.7 and y is 0.33. Hence, the chemical formula for Gd-YIG is closed toY0.3Gd2.7Fe5O12 and
that of Al-YIG as Y3Fe4.67Al0.33O12.
34
5.2 XRD of YIG, Gd-YIG and Al-YIG sintered samples
X—ray diffraction revealed that the YIG showed only garnet phase and no other phases like
YFeO3 and α-Fe2O3. Even substituted garnets like Al-YIG and Gd-YIG had not showed traces
of other phases. The structure is crystalline in the substituted garnets. A little shift of the
peaks of X-ray diffraction of substituted garnets takes place with respect to the peaks of YIG
because the lattice constant of substituted garnets change and hence, by bragg’s law angle θ
changes hence shifting of the peaks take place.
Figure 5.6 XRD pattern of a sintered disk sample of YIG.
35
Figure 5.7 XRD pattern of a sintered disk sample of Al-YIG.
Figure 5.7 XRD pattern of a sintered disk sample of Gd-YIG.
36
Figure 5.9 A collective view of XRD spectra of (A) Pure-YIG, (B) Gd-YIG and (C) Al-YIG.
5.2.1 Lattice Parameter and Density Measurement from XRD data
From the XRD pattern, the lattice parameters for the samples are calculated as
a = d * (h2 + k2 + l2) ½, where a = lattice parameter, d = diffraction plane spacing and h,k,l are
miller indices of plane.
We found out a = 12.362 Å for P-YIG. So the density of the material is
Volume of the unit cell = 12.3623 * 10-30 = 1889.14 * 10-24 cc
Molecular weight of Pure-YIG = 737.95 g (Y3Fe5O12)
Mass of unit cell with 8 Formula Unit = 737.95 * 8 / 6.023 * 1023 g
Therefore, theoretical density = 5.19 g/cc
37
Similarly, we found out a = 12.351 Å for Al-YIG. So the density of the material is
Volume of the unit cell = 1884.11 * 10-24 cc
Molecular weight of Al-YIG = 728.46 g (Y3Fe4.67Al0.33O12)
Mass of unit cell with 8 FU = 728.46 * 8 / 6.023 * 1023 g
Therefore, theoretical density = 5.14 g/cc
Also, we found out a = 12.465 Å for Gd-YIG. So the density for the material is
Volume of the unit cell = 1936.858 * 10-24 cc
Molecular weight of Gd-YIG = 923.2450 g (Y0.3Gd2.7Fe5O12)
Mass of unit cell with 8 FU = 923.2450 * 8 / 6.023 * 1023 g
Therefore, theoretical density = 6.33 g/cc.
Table 5.1 YIG, Gd-YIG and Al-YIG sintered samples
Composition Lattice parameter
(computed) (Å)
Theoretical
density (g/cc)
Bulk density
(g/cc)
% of Theoretical
density
Y3Fe5O12 12.372 5.19 5.11 98.4
Y3Fe4.67Al0.33O12 12.355 5.14 5.02 97.4
Y0.3Gd2.7Fe5O12 12.465 6.33 5.98 94.5
38
5.3 EDX analysis of sintered garnet samples
Figure 5.10 Elemental analysis of Al-YIG with Accelerating Voltage: 15.0 kV at Magnification: 1000
Figure 5.11 Elemental analysis of YIG with Accelerating Voltage: 15.0 kV at Magnification: 1000
Figure 5.12 Elemental analysis of Gd-YIG with Accelerating Voltage: 15.0 kV at Magnification 1000
39
Table 5.2 Atomic % of various constituent of substituted different Sintered garnets
Atom % of Yttrium
Atom % of Gadolinium
Atom % of Iron
Atom % of Aluminium
Atom % of Oxygen
Pure YIG
Experimental Atom % 13.20 -- 26.80 -- 60.00
Theoretical Atom % 15.00 -- 25.00 -- 60.00
Error in Atom % (+/-) 1.80 -- 1.80 -- 0.00
Al-YIG
Experimental Atom % 16.82 -- 21.74 1.44 60.00
Theoretical Atom % 15.0 -- 23.35 1.65 60.00
Error in Atom % (+/-) 1.82 -- 1.61 0.21 0.00
Gd-YIG
Experimental Atom % 2.14 11.16 26.70 -- 60.00
Theoretical Atom % 1.50 13.50 25.00 -- 60.00
Error in Atom % (+/-) 0.64 2.34 1.70 -- 0.00
Compositions of constituent phases in sample were carried out by EDX analysis. The detail of
the measurement location and the results are shown in EDX data above. The data shows the
percentages of Fe atom and Y atom in YIG, also Al and Gd atom in substituted YIG are
approximately in agreement with the theoretical data.
40
5.4 ESR (Electron Spin Resonance) Spectra at Room Temperature
For the ESR studies at low and high power levels, samples are generally taken in the form of
spheres. At X-band, the size of the polished sphere is normally 1 mm which is much smaller
as compared to rf wavelengths inside the sphere material.
Figure 5.13 ESR curve of 1mm sphere of Al-YIG at room temp. at X band frequency (9.1 GHz)
Figure 5.14 ESR curve of 1mm sphere of YIG at room temperature at X band frequency (9.1 GHz).
41
Figure 5.15 ESR curve of 1mm sphere of Gd-YIG at room temp. at X band frequency (9.1 GHz)
Figure 5.16 ESR spectra (dP/dH Vs H) for same material (Al-YIG) with internal stress and other defects subjected to different heat treatments.
42
Figure 5.17 ESR spectra (dP/dH Vs H) for same material (Gd-YIG) with internal stress and other defects subjected to different heat treatments.
Figure 5.18 ESR spectra (dP/dH Vs H) for same material (YIG) with internal stress and other defects subjected to different heat treatments.
43
How the resonance line-width (∆H), manifesting the microwave power loss, is affected by thermal treatment is clearly revealed. ∆H is maximum (= 80 G) for 1mm sphere, as prepared in tumbler while oxygenated sample has shown the lower ∆H = 50 G.
ESR of chunk of garnet (without any heat treatment) has shown highest ∆H due to additional shape anisotropy besides other factors. The secondary peak in the ESR spectra is due to body resonance.
Since, as prepared sphere (of YIG, Al-YIG or Gd-YIG) has residual strains which developed during the making of sphere through tumbler but here shape anisotropy doesn’t is zero because of spherical shape, hence only residual strains contribute to the ∆H. Therefore, sphere shows ∆H less than chunk.
Now, on doing the heat treatment of the stressed sphere of garnet in presence of nitrogen i.e. heating the sample in presence of nitrogen at 1000 oC and then keep the sample at that temperature for 2hrs for soaking and then allowing it to cool down. The stresses get relieved during the process and therefore the value of ∆H decreases.
Slow cooling in presence of nitrogen allows only stresses to get relieved. So, in fast cooling in presence of air shows little decrease or no decrease in ∆H because now oxygen plays an important role in decreasing the value of ∆H. This is because of minor change in stoichiometry or sintering condition (like as reducing atmosphere) leads to oxygen deficient structure that appeared in preparing high temperature sintered garnet YIG. In order to maintain charge neutrality in lattice, this oxygen deficit is compensated for by a corresponding valency change of Fe3+ ions to Fe2+. Hence in this way hopping takes place which leads to increase in ∆H. Hence, the heat treatment in presence of oxygen reduces the deficiency of oxygen and consequently decreases the ∆H.
Since, the fast cooling in presence of air gives a little exposure of oxygen to the sample while slow cooling in the presence of air gives sufficient oxygen for the highly compact garnet sample to move into the interstitials and hence finally slow cooling in presence of oxygen gives maximum exposure of oxygen to suppress the hopping of Fe3+ ions to Fe2+ions. Hence, ESR line-width decreases from fast cooling in presence of air to slow cooling in presence of air and then to slow cooling in presence of oxygen.
44
Table 5.3 ESR parameters for sintered garnet samples of YIG, Al-YIG and Gd-YIG.
Composition
Density (g/cc) X-ray Bulk
4πMs (Gauss)
∆H (X-band) (Gauss)
Ho (Gauss)* Resonance field
geff *
Y3Fe5O12
5.19 5.11 1650
Chunk 80 3220 2.10 As it is ground sphere# 70 3085 2.01 Slow cooling in N2 60 3215 2.02 Fast cooling in air 50 3245 2.00 Slow cooling in air 40 3110 2.08 Slow cooling in O2# 30 3240 2.00 Y3Fe4.67Al0.33O12
6.33 5.6316 1200
Chunk 200 3300 1.98 As it is ground sphere 80 3180 2.04 Slow cooling in N2 60 3260 2.00 Fast cooling in air 60 3230 2.00 Slow cooling in air 55 3150 2.06 Slow cooling in O2 50 3070 2.10 Y0.3Gd2.7Fe5O12
5.14 5.02 270
Chunk 430 3055 2.12 As it is ground sphere 75 3100 2.09 Slow cooling in N2 70 3000 2.16 Fast cooling in air 60 3180 2.04 Slow cooling in air 50 3240 2.00 Slow cooling in O2 40 3380 1.92
* without taking any correction to demagnetizing field due to Sphericity in the sample or porosity and other impurities etc.
# the M-H hysteresis curve of pure YIG spherical sample has shown coercivity (Hc) = 52 G of as ground spherical sample while Hc reduced to 48 G for the oxygen annealed sample. This effect is also seen in resonance field Ho as well as ∆H.
45
5.5 SEM micrographs of sintered garnets
Pure-YIG
Figure 5.19 SEM photographs of a fractured surface of sintered YIG (Y3Fe5O12) at different
magnifications (fig. on left side A,B,C are without treatment, on right side D,E,F are annealed in O2).
46
Al-YIG
Figure 5.20 SEM photographs of a fractured surface of sintered Al-YIG (Y3AlxFe5-xO12) at different magnifications (fig. on left side A,B,C are without treatment, on right side E,F,G are annealed in O2).
47
Gd-YIG
Figure 5.21 SEM photographs of a fractured surface of sintered Gd-YIG (GdxY3-xFe5O12) at different magnifications (fig. on left side A,B,C are without treatment, on right side D,E,F are annealed in O2).
48
The SEM micrographs of fractured surface of garnet and substituted garnets show that single
phase material were formed after sintering. Also, in the diagram the left side micrographs are
of the fractured surface of the sample without undergoing any heat treatment while the
micrographs on the right side are of the fractured surface of the similar sample as in the left
side also at same magnification but undergone an heat treatment of slow cooling after heating
to 1000 oC in presence of oxygen.
Micrographs indicate that as Al-substituted YIG has more porosity as compared to pure YIG
and Gd-substituted has little less porosity than Al-substituted YIG.
Here pure YIG and Gd-YIG shows larger grain growth as compared to Al-YIG. The average
grain size of these samples ranged between 8 µm - 15µm.
Here the sample undergone annealing heat treatment in presence of oxygen shows larger grain
growth and hence less porosity.
49
CHAPTER 6
CONCLUSIONS
In this project, three magnetic garnet sintered disk samples viz. YIG, Al-YIG and Gd-YIG
were provided to do some preliminary study of the adverse effects of residual strains arising
from shape processing (machining, cutting, attrition etc.) of garnet samples for microwave
ferrite component making. From the present ESR line-width (∆H) measurements on attrition
processed spherical (1mm dia.) ferrimagnetic garnet samples, very useful semi-qualitative
findings have come out. In common to all the garnet compositions (YIG, Al-YIG, Gd-YIG), it
is found that at room temperature -
1. The attrition-processed spherical (~1mm dia.) samples having residual strains (and
other possible charge defects) show higher ESR line-width (∆H) from 70 G to 80 G.
2. On simple air annealing the sample at ~1000 oC for 2hrs, whereby, the samples being
significantly relieved of residual stresses, has led to considerable (~25%) reduction in
X-band ∆H from 80 G to 60 G.
3. A comparison of the samples annealed in N2 atmosphere vis-à-vis O2 has shown that
only the oxygen annealed sample has further reduced the ESR line-width ~15 to 18 %.
Giving a clue that if some of the Fe ions present in Fe2+ state and cause hopping
between Fe2+ and Fe3+ and (thereby high magnetic losses or ∆H) could be suppressed
on oxygenation.
Thus, the microwave losses in the ferrite or garnet material can be reduced by proper
annealing treatments.
Future scope of work
Present consistent findings of significant reduction in ESR line-width by appropriate thermal
treatments, in oxygen ambience, leading to low magnetic losses should be further tested/
validated for the actual size shaped ferrite and garnets pieces used in microwave components
and to be evaluated in the real device performance.
50
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