1

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].

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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].

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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.

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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.

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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

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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.

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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

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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.

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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.

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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.

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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.

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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].

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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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