1

CHAPTER 1

INTRODUCTION

This chapter deals with a brief introduction of perovskite manganite

materials and M250 grade maraging steel for different applications along with

significance of ultrasonic studies on materials characterisation. The

importance of the nanostructured perovskite manganites as well as a review of

literature on the lanthanum based perovskite manganite materials and

maraging steel is given here in detail. At the end of this chapter, the specific

objectives of the present investigation are highlighted.

1.1 OVERVIEW

In recent years, there is a constant quest for newer materials with

improved properties for technological developments to meet new demands

(Cooper 2001; Hille et al. 2012; Sincerbox 1995). The basic understanding

about the operating/manufacturing process and functional/cost requirements

of materials is essentially required to develop and to enhance the properties of

the existing materials. Therefore, an absolute characterisation of the newer

materials is required before, during and after the processing (Sawas et al.

1998). A complete knowledge of the structure and properties of materials, not

only at the ambient temperature, but also as a function of temperature, is

required (Barsoum and El-Raghy 1996) to select the appropriate materials for

the newer applications (Shanian and Savadogo 2006).

1.2 PEROVSKITE MANGANITE MATERIALS

The advanced materials like perovskite manganite materials with

improved properties are required in many areas such as computer memories,

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switching devices and sensors. The perovskite manganite materials have been

extensively studied in view of the existence of large variety of physical

phenomena such as superconductivity, ferroelectricity and colossal

magnetoresistance (CMR). These materials offer a degree of chemical

flexibility that permits the relation between the structural, electronic and

magnetic properties of these oxides to be examined in a systematic way.

Research on the manganites has revealed new phenomena such as

CMR and dense granular magnetoresistance (Alonso et al. 2003; Levy et al.

1995), that led to the understanding of various physical concepts such as

double exchange (DE; Burzo et al. 2008; Von Helmolt et al. 1993) and the

Jahn–Teller (JT) effect (Lu et al. 2006; Tang and Zhang 2006). Their rich

electronic phase diagrams reflect the fine balance of interactions that

determine the electronic ground state. These compounds represent the

interplay of experiment, theory and application which is an important aspect

of condensed matter physics research. The structure, basic parameters and

properties of perovskite manganite materials are discussed in the first few

sections.

1.2.1 Structure

Gustav Rose, a German geologist, first discovered the mineral

perovskite in the Ural Mountain of Russia in 1839 and named it after the

famous Russian mineralogist Count Lev Aleksevich von Perovski to honour

him. The general chemical formula for perovskite manganite material is

ABO3, where A and B are cations with different atomic sizes and charges that

bond with oxygen. The oxide materials with general formula ABO3 having

structure similar to that of perovskite mineral CaTiO3 are named as

perovskites. These mixed-valent perovskite structures have gained attention

because of their multiferroic, photocatalytic and magnetic properties that are

useful for applications in thin film capacitor (Karan et al. 2009), non-volatile

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memory (Nishi et al. 2011), non-linear optics (Kien et al. 1993) and

photoelectrochemical cell (Hansen 2010). The ideal cubic perovskite structure

belongs to the Pm3m space group symmetry (Yoshii 2010). The unit cell

contains one molecular formula unit of ABO3 wherein the ions occupy special

Wyckoff positions as given next.

A in 1(a) at 0, 0, 0

B in 1(b) at ½, ½, ½

O in 3(d) at 0, ½, ½

The structure of a cubic perovskite is shown in Figure 1.1. The

atoms are generally arranged such that tetrahedral A ions are located at the

centre of the cube, octahedral O ions are featured on the faces of the cube and

12 coordinated B atoms take the corners of a cube.

Figure 1.1 Crystal structure: Perovskite manganites

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The general formula for the lanthanum based perovskite manganites

is La1─xAxMnO3, where A is a divalent alkaline earth ion such as Sr, Ca and

Pb. Lanthanum based perovskite manganites have attracted the attention of

both scientific and engineering communities because of their potential in

technological applications (Martin et al. 2010; Pollak et al. 2002; Zi et al.

2012). More attention has been paid on lanthanum based perovskite

manganite materials not only because of their special physical properties but

also due to their potential applications in magnetic storage (Swagten 2007),

sensor technology (Lekshmi et al. 2012), frequency switching devices

(Ezhilvalavan et al. 2002) and electronic industries (Martin et al. 2010).

The parent compound LaMnO3 is a Mott-type insulator with

antiferromagnetic (AFM) properties (Von Helmolt et al. 1993). With the

doping of A ion in place of La, there is a gradual change from AFM insulating

(ground state) phase to ferromagnetic metallic (FMM) phase due to

progressive increase in the number of holes (Mn3+–Mn4+). These materials are

called mixed-valent manganites as the Mn exists in both Mn3+ and Mn4+

forms. The ionic size of Mn3+ is significantly larger (0.064 nm) than that of

Mn4+ (0.053 nm) ion (Shimada et al. 1984). These compounds show

spectacular magnetic and electronic properties across the composition and

temperature range. They have complex phase diagrams that consist of canted

antiferromagnetic insulator, ferromagnetic insulator (FMI), FMM, charge

ordering insulator, paramagnetic insulator (PMI) and antiferromagnetic

insulator (A-FMI) phases with interesting physical properties such as CMR,

charge ordering (CO), orbital ordering (OO), phase separation (PS) and JT

effect (Chau et al. 2003; Min et al. 2002; Nadgorny 2001; Nadgorny et al.

2007). These are strongly correlated electronic systems, characterised by

complex interplay of lattice, spin, orbital and strain degrees of freedom that

are strongly coupled to each other. In strongly correlated electronic systems,

electron–electron Coulomb repulsive interaction is strong and the electrons

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are localised with small electron mobility (Lekshmi et al. 2012). One of the

eye-catching properties of manganites is the influence of magnetic field on

the electronic conduction known as CMR; there is a reduction by orders of

magnitude in resistance in the vicinity of the Curie temperature (TC) on the

application of magnetic field (Min et al. 2002).

Doped manganites show two types of transitions (Fath et al. 1999),

namely electrical transition from the insulator to metal transition (TMI)

accompanied by magnetic transition from paramagnetic to ferromagnetic

transition (TC). Generally, the CMR is regarded as the most important

parameter to be considered as a function of temperature; it takes the

maximum value near the Curie temperature TC (Cong et al. 2003; Demin et

al. 1998). However, a large magnetoresistance (MR) value is often obtained

in a large magnetic field (up to several Tesla) and at low temperature. The

above conditions limit its applications in many fields, the field sensitivity and

working temperature of perovskite manganites (Wang et al. 2011).

The DE mechanism can be considered as the basis for the

explanation of the paramagnetic transition (PM) to ferromagnetic transition

(FM) and the associated CMR effect (Von Helmolt et al. 1993). Similarly, in

the JT effect, strong electron–phonon interactions are used to explore the

transition in perovskites (Lu et al. 2006; Tang and Zhang 2006). Therefore,

knowledge of the DE and JT effects and CO (Cong et al. 2003; Tomioka et al.

2001) and OO (Terakura 2007; Tomioka et al. 2001), interactions, and their

possible influence on magnetism is an essential requirement while studying

the use of manganite oxides for a wide range of applications.

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1.2.2 Double Exchange

Double exchange interaction is the interaction between pairs of

Mn3+ and Mn4+ ions in mixed-valent perovskites through an oxygen atom. It

is essential for the FM and metallic properties of these manganese oxides

(Von Helmolt et al. 1993). Anderson and Hasegawa (1995) have explained

the DE mechanism through a formalised semi-classical model which is shown

in Figure 1.2. The spins of Mn3+ and Mn4+ ions are represented by S1 and S2

while the resultant spin by S.

Mn4+

O2-

Mn3+

Figure 1.2 Semi-classical models - Double exchange

Zener (1951) has proved the DE mechanism and the states of

manganites are assumed to be uniform. According to his theory of the indirect

magnetic exchange between 3d atoms, FM interactions are favoured when the

magnetic atoms are fairly well separated, leading to active conduction

electrons. The theory was applied to the manganese perovskites with the aim

of explaining the strong correlation between conductivity and ferromagnetism

(Zener 1951). Starting from the insulating AFM LaMnO3 to end member

where electrons are localised on the atomic orbitals, Zener showed how the

system should gradually become more FM upon hole doping (introduction of

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Mn4+ by doping of divalent atom). Further, the problem of the exchange

between Mn3+ and Mn4+ ions by an oxygen ion was considered and then

introduced the concept of simultaneous transfer of an electron from the Mn3+

to the oxygen and from the oxygen to the neighbouring Mn4+, as shown in

Figure 1.3.

a) Double Exchange mechanism b) Mobility of eg electrons

c) Spin canted state d) Hopping mechanism

Figure 1.3 Zener ferromagnetism

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Doping determines the carrier concentration and sign of the charge

carriers (i.e., positively charged holes or negatively charged electrons) in the

La1─xAxMnO3 mixed-valent manganites. Carrier concentration decides the

fluctuated valence of the transition metal cations, that is Mn3+/Mn4+ in

manganites which is responsible for the ferromagnetic DE (Gennes 1996). At

a fixed amount of doping, perovskite compounds show very rich phase

diagram as a function of temperature, magnetic field, external and internal

chemical pressures, etc., which in turn governs their physical properties.

Each individual ion or atom is enforced by a ferromagnetic Hund

(Hund’s rule) coupling JH, as shown in Figure 1.3(b). The conduction

electrons do not change their spin as they progress from ion to ion. These

electrons are able to move in the crystal in a hopping manner when the net

spins of the incomplete d shells are parallel, that is the conduction electrons

lower their kinetic energy if the background of d shell spins or the t2g spins of

manganite is fully polarised.

Zener considered that a direct coupling between d shells which is

not mediated by conduction electrons but by the direct virtual hopping of d

electrons is of opposite sign leading to AFM rather than FM. The coupling

involved in this process is called JAF. It is inferred that the superexchange

interaction leads to AFM alignment of spins. The original idea of DE was to

explain FM state not as a procedure but as a mechanism for electron transfer.

The DE mechanism (Zener 1951) has been used to explain the

existence of metal–insulator (MI) transition behaviour which arises as a result

of a strong exchange interaction between the itinerant eg electrons (or holes)

and localised t2g spins through the Mn–O–Mn path, as shown in Figure 1.3(c).

The effective hopping (teff) of an electron between two nearest neighbour Mn

ions has been studied extensively by Rodriguez et al. (2000). In fact, the

calculation shows that teff = t cos θ/2, where θ is the angle between t2g spins

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located at the two Mn sites involved in the electron transfer, as shown in

Figure 1.3(d). The most important characteristics of the perovskite manganite

materials are the coexistence of metallic conductivity and ferromagnetism.

The tilting and twisting of the oxygen octahedral have been observed due to

the ionic mismatch between La and A ions. Tilting of the octahedral can be

measured with the distortion of the Mn–O–Mn bond angle and bond length

(Rodriguez et al. 2000).

The spins S1 and S2 of the two ions are coupled by exchange

interactions and Hamiltonian (H) as,

H = J S1S2 (1.1)

where J is the exchange coupling constant and has a ferromagnetic and an

AFM contribution. When J is less than zero, it reveals the FM coupling

stabilising an S = 9/2 ground state. On the other hand, J is greater than zero

for AFM with S = 1/2 ground state. In most of the cases, AFM coupling

dominates, leading to antiparallel alignment of the spins, as shown in Figure

1.4.

Figure 1.4 Antiferromagnetic coupling with S = 1/2 ground state

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According to this proposed mechanism, the translocation of an

electron takes place from one Mn to another through an overriding O2─ ion.

The translocation takes place in two steps due to doubly occupation of oxygen

in p orbitals. The movement of an electron takes place from oxygen to the left

Mn followed by a transfer of a second electron from the right Mn into the

vacated oxygen orbital and hence, known as DE.

In addition, another H is developed due to cluster spin

(Papaefthymiou et al. 1987) and is reported as,

H = JS(S+1)/2 ± B(S+1/2) (1.2)

where B is the strength of the DE interaction.

1.2.3 Jahn–Teller Effect

The discovery of high temperature superconductivity and CMR in

the manganese oxides with perovskite structure has reawakened interest in

dynamic, co-operative JT deformations in solids, particularly where they

occur at a crossover from localised to itinerant electronic behaviour. Whereas

electrons in a partially filled cation shell are localised in a transition metal

compound, the cation with high symmetry may leave the localised electron

for manifold orbitally degenerate orbitals. In this case, the cation becomes a

JT ion. With localised dn configuration, a cubic crystalline field quenches the

orbital angular momentum of a twofold orbital degeneracy, leaving behind

orbital angular momentum at a threefold orbital degeneracy. An appropriate

local JT site deformation to lower symmetry removes the orbital degeneracy

at the JT cation and results in either a static or a dynamic deformation. In

mixed-valent manganites, the electronic properties are closely related to the

lattice.

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In La1─xAxMnO3, perovskite manganites, the Mn3+ ion has a d4

configuration. In octahedral symmetry, the d level splits into three t2g and two

eg orbitals. The Mn3+ ion has high-spin configuration, with three electrons

occupying the three t2g orbitals and one electron occupying the doubly

degenerate eg orbitals as t2g3eg

1. According to the JT theorem, the structure

will distort thereby removing the degeneracy of the eg orbitals. In solids, the

orbital degree of freedom of the Mn3+ ion often shows long-range ordering

associated with the co-operative JT effect. It is observed for the most

extensively studied compounds throughout the La1─xAxMnO3 series, that is

LaMnO3, that below a transition temperature TJT, the 3d3x2

−r2 and 3d3y

2−r

2

orbitals are ordered in the ab plane in an alternating manner (Figure 1.5).

Figure 1.6 shows the energy band structure affected by the JT effect.

There are two types of distortions (Q2 and Q3) associated with the

JT effect (Tomioka et al. 2001). The Q3 is a tetragonal distortion that results

in an elongation or contraction of the MnO6 octahedron corresponding to the

filled 3z2 ─ r2 or x2 ─ y

2 orbital. The Q2 is an orthorhombic distortion

obtained by a certain superposition of the 3z2 ─ r2 and x2 ─ y

2 orbitals

(Kanamori 1960). The oxygen framework is described in Pbnm space group

symmetry by two oxygen (O1 and O2) positions, as shown in Figure 1.7(a).

The O1 position is situated on the mirror plane and is attributed to the out-of-

plane oxygens; the O2 position is attributed to the in-plane oxygens.

The rotations of the octahedra are reflected in the deviation from

180° of the Mn–O–Mn tilting angle. The JT effect in LaMnO3 is dominated

by the Q2 distortion with alternating long (l) and short (s) Mn–O2 bond

lengths in the ab plane and a medium out-of-plane Mn–O1 (m) bond length, as

shown in Figure 1.7(b). The medium bond length (m) deviates from the

average bond length, such that m < (l + s)/2 indicates that the JT distortion is

not of a pure Q2 type. A contribution of the Q3 distortion is also present.

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eg orbitals

a) 3z2 –r2 b) x2 –y2 c) 3D axis representation

t2g orbitals

d) xy e) yz f) zx

Figure 1.5 Representation of eg and t2g orbitals of Mn ion

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Figure 1.6 Representation of Jahn - Teller effect

a) Cubic and orthorhombic unit cells b) ab plane

Figure 1.7 JT-distorted perovskite: sketch of alternation of the short and

long Mn–O distances

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1.2.4 Charge Ordering

Charge ordering refers to the ordering of the metal ions in different

oxidation states in specific lattice sites of a mixed-valent material (Cong et al.

2003; Tomioka et al. 2001). The ordering generally localises the electrons in

the material, making it insulating or semiconducting due to the charge

localisation which in turn restricts the electron hopping from one cation site to

another (Rao et al. 1998). CO is not a new phenomenon in metal oxides that

already exists in magnetite (Fe3O4). The study of CO phenomena in doped

rare earth manganites with the general formula La1─xAxMnO3 has attracted

much attention due to the discovery of CMR and other interesting properties

shown by these materials (Ramirez 1977; Rao et al. 1998). The existence of

CO was reported by Wollan and Koehler (1955) and later confirmed through

neutron diffraction by Jirak et al. (1985) and is associated with novel

properties that are useful in understanding the electronic behaviour of

compounds.

In doped manganites, the CO phases are unique manifestations

arising from the interaction between the charge carriers and the phonons

wherein the JT distortions play a significant role. This may be due to the

localisation of charge carriers into specific sites below a certain temperature

known as charge ordering temperature (TCO), giving rise to long range order

throughout the crystal structure.

Although CO would be expected to be favoured when doping level

x = 0.5, due to the presence of equal proportions of the Mn3+ and Mn4+ states,

it is found in various compositions in the doping range 0.3 < x < 0.75,

depending on the La and A ions. In the CO state, the Mn3+ and Mn4+ ions are

regularly arranged in the ab plane with the associated ordering of the dx2

─r2

and dy2

−r2 orbitals (Takei et al. 1986), as shown in Figure 1.8. It also competes

with DE, and promotes insulating behaviour and AFM. Furthermore, the Mn3+

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(eg) orbitals (3dz2) and the associated lattice distortions (long Mn–O bonds)

also develop long-range order, giving rise to OO responsible for the

anisotropy of the electron transfer interactions (Holder 2003). This gives rise

to complex spin orbital–coupled state. Thus, the OO is coupled with JT

distortion. The CO competes with DE, giving rise to an unusual range of

properties sensitive to factors such as the size of the A-site cations, internal

and external pressures, chemical melting of CO state by doping and melting

of CO state by application of magnetic field.

Figure 1.8 CE-type AFM charge ordering

At low temperatures, rare earth manganites are in AFM ordered with

CE- or A-type ordering, but only the former occurs in the charge-ordered

materials where the eg electrons are localised. The CE-type spin ordering is

characterised by the ordering of Mn3+ and Mn4+ ions alternately. The spin

ordering in the ab plane is quite complex and it stacks as AFM along the c-

axis. In A-type spin ordering, the spins order as FM in the ab plane (with the

moments pointing towards the a-axis) and these planes stacked AFM along

the c-axis. A- and CE-type AFM ordering in half-doped manganites (x = 0.5)

is shown in Figure 1.9.

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Figure 1.9 A- and CE- type Charge and Orbital ordering

OO can occur in both A- and CE-type AFM ordering, but they differ

in detail. The CE-type AFM state is attained on cooling a ferromagnetic state

or a charge-ordered paramagnetic state. Figure 1.10 depicts three dimensional

views of A-, C- and F-type charge and OO. Two distinct types of COs can be

delineated. In one, an FMM state transforms to the charge ordered state on

cooling the material. In the other, the CO state is found in the paramagnetic

ground state and hence, there is no ferromagnetism down to the lowest

temperatures.

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C F A

3z2-r

2 3z

2-r

2 and x

2-y

2 x

2-y

2

Figure 1.10 Charge and Orbital ordering - 3D representation

Magnetic field transforms the CO state into the FMM state, when

the average radius of the A-site cations is sufficiently large (<rA> > 1.17 Å).

Figure 1.11 shows schematic diagram of different CO behaviours of

La0.5A0.5MnO3 depending on <rA>. Accompanying with all these factors, co-

operative JT effect induces additional effects such as lattice distortion and

electron localisation in the charge-ordered state. The FMM and CO transitions

are accompanied by spin and OO, whereas the CO insulator is AFM (CE

type).

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Figure 1.11 Charge ordered state of La0.5A0.5MnO3 as a function of <rA>

1.2.5 Colossal Magnetoresistance

The change in the electrical resistance under the application of

magnetic field is known as magnetoresistance (MR) and is a unique intrinsic

property of manganese based oxide materials. MR can be written as (Cong et

al. 2003),

100%MR0

×−

=ρ

ρρ Ho (1.3)

where ρ0 and ρH are the resistivities in the absence and presence of magnetic

field respectively. The value of MR may be negative or positive, depending

on the decrease or increase in the resistivity with respect to the applied

magnetic field (Wang et al. 2011). During the past decades, the following

forms of MR have been studied, having their origin in different physical

aspects:

• Anisotropic magnetoresistance

• Granular and tunnelling magnetoresistance

• Giant magnetoresistance (GMR)

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The large value (20–50%) of negative MR is reported in metallic

multilayers of perovskites (Baibich et al. 1988; Fert et al. 1992), hence

termed as GMR. Owing to such a large negative MR, these materials are used

in applications such as sensors for a quite some time. Though the search for

better magnetoresistive materials never ended and gradually, the perovskite

structured oxide materials of type La1─xAxMnO3 became the centre of

attention for research, after the work of Von Helmolt et al. (1993) and

Charara et al. (1993) reporting a huge MR. Simultaneously, Jin et al. (1994)

reported MR nearly equal to 99% in similar compounds having large value of

MR termed as colossal magnetoresistance (CMR).

The origin of MR in manganites is quite different than that observed

in other forms of MR. CMR effect is an intrinsic property of crystal structure

and has its origin in the spin disorder of conduction electron that can be

suppressed by an application of the magnetic field, resulting in large MR

(Charara et al. 1993; Von Helmolt et al. 1993). The discovery of CMR effect

in manganites and its relation to various electronic and magnetic properties

are resulted in research on lanthanum based perovskites compounds (Jin et al.

1994).

1.2.6 Magnetic Properties of Nano perovskites

Nanoscience and technology is a broad and interdisciplinary area of

research and development activities that has been increasing worldwide in the

recent decade (Shinde et al. 2012; Wang et al. 2007; Whitesides 2005).

Perovskites with features on the scale of nanometer often have properties

dramatically different from their bulk scale counterparts (Bedekar et al. 2008;

Jena et al. 2007; Vidya et al. 2011). The study of nanostructured materials

requires a multidisciplinary approach with inspiring technological promise,

involving novel synthesis and an understanding of physics and surface science

(Siegel and Fougere 1995). Recently, efforts have been made to synthesise

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and characterise the properties of nanostructured perovskites in different

forms with at least one dimension between 1 and 100 nm such as

nanoparticles (Chocha et al. 2011; Rao et al. 2002), nanocubes (Chen et al.

2002), nanorods (Urban et al. 2002), nanobelts (Yan et al. 2009), nanosheets

(Ebina et al. 2002), nanowires (Ma et al. 2002) and nanotubes (Mao et al.

2003), to explore the distinctive physical properties for potential applications

in nanodevices (Newns et al. 2000).

The effect of reducing the physical size of materials is of great

importance from both fundamental considerations and modern practice

(Bedekar et al. 2008). Magnetic nanoparticles show specific properties such

as coercivity (Srinivas et al. 2009) and superparamagnetism (Shull 2007)

which are attributed due to reduced dimensions. CMR effect in these

perovskite manganite materials has created an interest to know about the

electronic, structural and magnetic properties of these materials completely.

These perovskite manganite materials show FM or nearly metallic behaviour

at low temperature, while at high temperatures they show PMI behaviour

(Wang et al. 2011).

The development of nanostructured magnetic materials has been the

source of discovery of spectacular new phenomena, with possible applications

in the fields of information technology, telecommunication or medicine (Coey

1999; Shull and Bennett 1992). Magnetic nanocomposite materials are

generally composed of FM particles (grain size in nanometer scale)

distributed in either a non-magnetic or a magnetic matrix (Gupta et al. 2001;

Huang et al. 2002). The shape (Finger 1977), size (Vedmedenko et al. 2003)

and distribution (Enokizono et al. 1999) of the magnetic particles help in

determining the properties of such materials. The matrix phase separates the

magnetic particles and changes the magnetic exchange interactions. Thus, the

change in exchange interactions affects the transport and magnetic properties.

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Therefore, the understanding and controlling of the structure of

nanostructured materials is essential to obtain desired physical properties.

1.3 MARAGING STEEL

Maraging steels are high strength and high fracture toughness steels

characterised by intermetallic precipitations in iron–nickel martensite

(Vasudevan et al. 1990). The excellent mechanical properties i.e., ultrahigh

strength combined with good fracture toughness, hardness, ductility and

corrosion resistance of maraging steel, make these steels as most preferred

materials for structural applications in strategic sectors such as aerospace,

military and atomic power plants (Leitner et al. 2011; Saul et al. 1970;

Vasudevan et al. 1990). Furthermore, the dimensional stability and easy heat

treatment make these steels an attractive material for use in machinery and

tools applications (Nedjad et al. 2008).

Similarly, because of their properties such as excellent wettability,

good polishability and high resistance to crack propagation, maraging steels

are used as true efficient materials for various applications such as

construction of missile and rocket motors, landing and takeoff gears, wind

tunnel models, recoil springs, flexures and ac motor shafts (Leitner et al.

2011; Rajkumar et al. 2007; Vasudevan et al. 1990).

1.4 BRIEF REVIEW

The research works carried out on perovskite and maraging steel are

reviewed briefly in this section to gain knowledge on the materials selected

for present investigation.

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1.4.1 Perovskite Manganite Materials

Asamitsu et al. (1997) have studied Lal─xSrxMnO3 with x = 0.17 and

observed that the structural transition temperature (TS) is close to the TC. A

notable decrease in TS has been observed from 280 to 220 K at zero fields.

The temperature dependent ultrasonic velocity and the electrical resistivity of

Lal─xSrxMnO3 (0.11 ≤ x ≤ 0.17) perovskite manganite material have been

measured by Fujishiro et al. (1997). The observed anomalies in velocities are

correlated to the polaron ordered phase at x = 0.125. The phase diagram has

been drawn between the polaron ordering temperature TP and composition

from the observed velocity anomalies. The observed results have been used to

explore the structural phase transitions using the diagram of temperature and

magnetic field.

Metal–nonmetal transition of mixed-valent manganese oxide

perovskites has been explained by Kwon et al. (1997). The tolerance factor (t)

and the standard deviation (σ) of the A-site cation sizes in mixed-valent

perovskites with a composition in the range from 0.2 to 0.5 have been

analysed. The report highlighted that the transition temperature is determined

by the interplay of tolerance factor and standard deviation. Furthermore, it is

used to measure the overall and atomic scale distortions. The doping of

manganite with various elements such as Al, Ga, In, Ti, Sn, Fe, Cr, Co, Ni

and Mg has been analysed by Raveau et al. (1998). The role of the

interpolated cation (size, mismatch effect) has been well established along

with their changes in CMR.

The electronic structures of perovskite manganese oxides

La0.84Sr0.16MnO3 and La0.7Sr0.3MnO3 have been investigated by Kuwata et al.

(1998). The electrical resistivity, magnetisation, dilation and sound velocity of

Lal─xSrxMnO3 (0.48≤ x ≤ 0.90) polycrystals have been measured by Fujishiro

et al. (1998). The observed anomaly in sound velocity is strongly due to CO

23

within the Sr concentration which ranges from 0.48 to 0.82. A phase diagram

of the charge ordered state as a function of temperature and the Sr

concentration has been plotted. The low energy optical conductivity in the

PMI state of these materials of the hole doped La0.7Ca0.3MnO3 and

La0.7Sr0.3MnO3 perovskite manganite has been measured by Quijada et al.

(1998). Synchrotron X-ray scattering study of La0.875Sr0.125MnO3 manganite

material has been carried out by Kiryukhin et al. (1998). The transition is

persistent but the CO state can be restored by heating to the CO transition

temperature and then by subsequent cooling.

This phase transition has been ascribed due to the CO in Mn3+ and

Mn4+ ions. The surface boundary magnetisation for a La0.7Sr0.3MnO3

manganese perovskite has been investigated by Park et al. (1999), using spin

resolved photoemission spectroscopy. The existence of a full moment on

boundary magnetisation at a very low temperature has been observed.

However, the same decayed much faster to the bulk magnetisation up on

heating. These results provide a direct insight into the various novel properties

of grain and surface boundaries both in the polycrystalline and manganese

perovskite samples. The specific heat, magnetisation and electrical transport

properties of La0.875Sr0.125MnO3 have been measured by Uhlenbruck et al.

(1999). Two structural phase transitions in addition to an FM transition have

been observed. The low temperature CO state has been found to be stabilised

by an applied magnetic field.

The importance of competing interactions between the orbit, charge

and spin of perovskite manganite in the light of the mechanism of the CMR

has been studied by Tokura and Tomioka (1999). The essential ingredient of

the CMR is not only the DE interaction but also other interactions such as

FM/AFM superexchange interactions and CO/OO instabilities as well as their

strong coupling with the lattice deformation. Mott insulating and metallic

24

phases of a model with eg orbital degeneracy with regard to understanding the

physics of Mn perovskite compounds have been discussed by Motome et al.

(1999).

Majumdar et al. (1999) have computed the zero temperature phase

diagram and Hall response of doped perovskite manganite. Within the model

of DE and JT coupling, hole-doped manganites have been obtained only when

lattice distortions persist in the metallic state. The phase diagram of half-

doped perovskite manganite within the extended DE model has been

enlightened by Jackeli et al. (2000). A rich phase diagram has been obtained

in the mean field theory at zero temperature as a function of AFM

superexchange interactions and internal Coulomb repulsion. The CE-type

AFM state has disappeared and A-type AFM state has been observed only at

small values of Coulomb repulsion as a charge disordered state. Pollert (2000)

has analysed examples of three structural types, namely spinal,

magnetoplumbite and perovskite structures, derived from the close-packed

arrangement of oxygen anions.

Resistivity measurements in La0.67Ca0.33MnO3 and Nd0.7Sr0.3MnO3

samples have been revealed in terms of quantum phase transition ideas to

study the nature of the MI transition in manganese oxides (Smolyaninova et

al. 2000). Ganguly et al. (2000) have carried out X-ray diffraction (XRD), ac,

dc magnetisation and dc electrical resistivity studies on the

La0.67Ca0.33Mn1─xCrxO3 perovskites for different compositions of x (x = 0.00,

0.05, 0.1, 0.15, 0.2, 0.35, 0.5 and 0.6). FM state has been observed in the

above system up to 60% of Cr substitution. The ultrasonic longitudinal and

transverse sound velocities in Lal─xCaxMnO3 (x = 0.33, 0.63, 0.73, 0.83) have

been measured in the temperature range from 50 to 300 K (Zhu et al. 2000).

An anomalous behaviour in sound velocity for both longitudinal and

transverse modes is observed at the FM, CO and AFM transition

25

temperatures. The spin-phonon coupling has been the main reason for the

observed anomaly in the sound velocity.

The important collective electronic phenomena arising from long-

range columbic interactions and magnetic effects in perovskite manganite

systems have been studied by Yoo et al. (2000). Sun and Li (2001) have

studied the conductance network of the magnetotransport properties of half-

metallic nano perovskite manganite. The effects of superparamagnetism and

high order tunnelling on the temperature dependence of MR in such systems

have been examined (Gontchar et al. 2001). The orbital and magnetic

structures of pure and CO manganite, caused by crystal, charge structures and

the JT effect, have been discussed. The model has been formulated based on

the orbital dependency of the magnetic interactions in JT effects. The

composition dependence of the structure and magnetic and electrical

properties of La0.7Sr0.3Mnl─xCrxO3 manganites have been studied (Kallel et al.

2001).

A single magnetic transition from FM to PM phase with a decrease

in TC has been studied with an increase in composition up to x ≤ 0.5. The

ultrasonic sound velocity and attenuation in La0.5Ca0.5MnO3 perovskites at

zero magnetic fields and an external magnetic field over a temperature range

of 77 and 270 K have been measured by Zheng et al. (2001). The occurrence

of dramatic stiffening in sound velocities with attenuation peaks near the CO

transition has shown the existence of an extremely strong electron–phonon

coupling originating from the JT effect. Under an external magnetic field, it is

interesting to note that the stiffening of the sound velocity shifts towards low

temperature (Zheng et al. 2001). X-ray scattering studies of broad peaks in

PMI phases of La0.7Ca0.3MnO3 and Pr0.7Ca0.3MnO3 have been carried out by

Nelson et al. (2001). Wang et al. (2001) have measured the magnetic spectra

of the La0.7Sr0.3MnO3 at various temperatures over wide range of frequencies

26

from 10 to 100 Hz. The relaxation frequency has been reduced from 1.47

MHz to 772 kHz with decrease in temperature from 296 to 77 K.

Rajendran et al. (2002) have exploring the CO and phase transition

temperatures of La0.67Sr0.33MnO3 perovskites through on-line ultrasonic

measurements. Gunnarsson et al. (2002) have studied the temperature

dependence of low field MR and current-voltage characteristics of a low

angle π-crystal grain boundary junction in La0.67Sr0.33MnO3 perovskite

manganite. Criterion of tolerance factor used for the determination of first and

second order magnetic phase transitions in La2/3(Ca1─xSrx)1/3MnO3 magnetic

materials has reported by Mira et al. 2002. A crossover from first to second

order character at a tolerance factor t = 0.92 has been found which also

brought about several variations in other physical properties.

The relationship between the average ionic radius of A-site cation

<rA> and the Mn3+/Mn4+ magnetic coupling on doped La0.5Ca0.55─xSrxMnO3

perovskite material has been studied by Wakai et al. (2003). The

La0.5Ca0.55─xSrxMnO3 perovskite material undergoes the transition from FM to

CO–AFM phase at a low temperature. The FM metallic phase has been

observed even at a low temperature when the composition x = 0 and 0.1. The

colossal magnetoresistive material Lal─xCaxMnO3 with x = 0.25, 0.33 and 0.45

has been investigated by ultrasonic sound velocity and attenuation

measurements (Jaafer et al. 2003). A lattice hardening effect has been

observed below TC for x = 0.25 and 0.33 which indicates the prominent role of

lattice vibrations in the physical properties. In addition, a lattice hardening

together with velocity hysteresis has been observed at x = 0.45. The above

studies suggest the simultaneous occurrence of both FM and CO regions.

Structural, magnetic, magnetocaloric and magnetoresistive

properties of La1─xPbxMnO3 (x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) perovskites

have been studied by Ngugen et al. (2003). The above studies reveal that the

27

conductivity of the perovskites is metallic at low temperature while it is

semiconducting at high temperature. Magnetic phase diagrams of the ground

state of layered perovskite have been studied by Ohsawa and Inoue (2003) in

a mean field approximation for a two-orbital DE model with the

superexchange interaction between localised spins and JT effect. The crystal

structure of perovskite has been explored through high resolution neutron and

synchrotron radiation diffraction between 10 and 400 K (Przeniosło et al.

2004). The resistivity of La0.5Ca0.5MnO3 sample showing a semiconducting

behaviour in the temperature range of 20–300 K has been investigated by

Walha et al. (2004). An increase in the resistivity value at very low

temperature has been explained by CO effect. It has been noted from the

electrical measurements that 5% of calcium deficiency induced a

semiconducting metallic transition with decrease in temperature.

The microstructure, magnetic and electrical transport properties of

La0.67Sr0.33MnO3 have been studied as a function of sintering temperature

(Wallin et al. 2004). It is noteworthy that raising the sintering temperature

induces the formation of an interfacial phase near the grain boundaries. At

1273 K, the formation of interfacial phases near the grain boundaries

enhances the conductivity in the composite. Broad MR across room

temperature has been observed in a composite sintered at 1573 K. The

sintering temperature has a prominent effect on the properties of the grain

boundary. The grain boundary has an important role in determining the

electrical transport behaviour of the composites.

The dramatic reduction of electrical resistivity in a magnetic field

and CO in rare earth manganite of mixed-valent perovskites have been

studied (Arulraj et al. 2005) in the form of manifestations of the complicated

relation between orbital, spin, charge and lattice degrees of freedom. The

lattice strain due to the JT distortions of MnO6 octahedral and their tilt

28

rotations have not been sufficient to provide a unique structure–property

relation. The relaxation has occurred due to the displacement of domain walls.

Chen et al. (2005) have studied the temperature dependence of the elastic

properties of polycrystalline manganite Lal─xCaxMnO3 (0.5 ≤ x ≤ 0.85)

through ultrasonic measurements. It has been concluded that the longitudinal

modulus started to soften with decrease in temperature from higher

temperature to TCO, and stiffened dramatically just below TCO.

Wang et al. (2005) have investigated the permeability of

La0.7Sr0.3MnO3 perovskite with mean grain size ranging from 45 to 1200 nm

at 293 K. It has been noted that the quasi-static relative permeability

decreases from 87 to 11 with mean grain size, while relaxation frequency

increases from 205 kHz to 231 MHz. The grain size dependence of

permeability and relaxation frequency has been discussed and related to the

demagnetising field induced by distributed air gaps between the grains. The

magnetotransport properties at low temperature in La0.67Sr0.33MnO3

nanoparticles, La0.67Sr0.33MnO3/Al2O3 nanocomposites and ferromagnetic

manganite thick films have been explained by Mukhopadhyay et al. (2006).

The results have shown low temperature upturn in resistivity and enhanced

low field MR of varying magnitude below the resistivity minima.

Neutron powder diffraction and magnetisation measurements have

been carried out by Ono et al. (2006) as a function of temperature in

La1─xCaxMnO3 with x = 0.18. Schlottmann (2006) have studied the phase of

La1─xCaxMnO3, for small x using a mean field slave boson formulation for eg

electrons in two orbitals with excluded multiple occupancy. Electronic

structure calculations on the compound La0.5Ca0.5MnO3 have been performed

to study the relationship between the magnetic ordering, the CO and the

geometry of the compound (Baldomir et al. 2007). An AFM ordering induces

charge disproportionate through JT distortion. A full disproportionate in Mn3+

29

and Mn4+ occurs for the experimental geometry and allows predicting the

experimentally found A-FMI state. Quite a few experimental and theoretical

studies focusing on the size effect of perovskite manganites have been

reported (Dong et al. 2007; Lu et al. 2007; Wang et al. 2007) in which some

interesting effects associated with the downsizing of the materials to tens of

nanometers are revealed.

Rao et al. (2006) synthesised perovskite nanoparticles with an

average diameter of 20–40 nm by the polymeric precursor sol–gel method. It

is inferred that in the 20 nm particles, the CO and the AFM phases are

observed respectively in the bulk below 250 and 160 K are completely absent.

Instead, an FM transition is observed at 95 K followed by an MI transition at

75 K. The 40 nm particles show a residual CO phase, but a transition to the

FM state also occurs at a slightly higher temperature of 110 K. The ac and dc

electrical conductivities of the nanocrystalline La1 ─ xMxGa1 ─ yNyO3 ± δ (where

M = Sr; □ (vacancy), x = ─0.10 to 0.15; N = Mn, Mg; y = ─0.10 to 0.15)

compositions are measured by Rambabu (2007). The above observed results

show that the conductivity of nanocrystalline perovskites increases when the

grain size decreases.

The photoluminescence properties of bulk and nano

La0.90Eu0.05Nb2O7 sheets have been studied (Ozawa et al. 2007) and it is found

that the photoluminescence properties of nanosheets increase by 20 times than

those of the bulk perovskites. The structural, magnetic and transport

properties of nanosized La0.7Sr0.3MnO3 and La0.67Ca0.33MnO3 perovskite

oxides have been studied by Sahu and Roul (2008). The magnetisation of

nanophase materials is prominently dependent on the particle size, whereas

the transport and magnetoresistive properties are strongly dependent on the

sintering temperature. Annealing improves the magnetic homogeneity of the

grain and grain boundaries. These improvements are favourable to enhance

30

the intrinsic properties of the compound, especially the decrease in resistivity

that induces the MR ratio to increase.

Size induced crystallographic structure in nanoparticles of

La0.5Ca0.5MnO3 has been studied by Sarkar et al. (2008). It is reported that the

orthorhombic distortion in nanoparticles at room temperature freezes at higher

temperature. The broad transition temperature range and low field MR in

La0.7Ca0.3MnO3:nano-ZnO composites has been explored through temperature

dependence ac conductivity measurements (Siwach et al. 2009). The effect of

strontium doping level, electrical transport and magnetic properties of

La1─xSrxMnO3 (x = 0.1, 0.2 and 0.3) nanoparticles has been studied. The TC

and TMI of nano perovskites increase with the doping level. The maximum

values of TC (377 K) and TMI (264 K) occur at x = 0.3 (Cuong and Kim 2009).

Crystallite size dependence of superparamagnetic behaviour having saturated

magnetisation (~20–47 emu g-1) and the coercive field (~10–40 Oe) of

La1─xSrxMnO3 (x = 0.1, 0.2, 0.3, 0.4 and 0.5) nanoparticles has been studied

(Daengsakul et al. 2009).

Magnetisation measurements are performed on La1─xPbxMnO3

nanowires from low temperature to room temperature to reveal the cluster

glass behaviour of the nanowires below the TC. Room temperature hysteresis

measurements show PM-like behaviour (Babu et al. 2010). The presence of

short range magnetic ordering and depressed magnetic entropy in

paramagnetic phase in La0.9Pb0.1MnO3 perovskites has been studied through

the magnetisation (M2) isotherms (Tozri et al. 2010). The variation in

resistivity of La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 nanocomposites with

temperature has been studied (Kim et al. 2010) and shows a semiconducting

behaviour. It is remarkable to note that an enhanced MR effect is found for

the composites over a wide temperature range from low temperature to room

temperature in an applied magnetic field of 0.5 T. The spin-polarised

31

tunnelling and the spin dependent scattering may be attributed to the

enhanced low field MR effect (Kim et al. 2010).

A high saturation moment of ~ 47 emu g-1 has been achieved at 10

K with a good square hysteresis loop, and room temperature magnetisation of

~15 emu gm-1 obtained in La0.7Sr0.3MnO3 nanoparticles (Ali et al. 2011). An

attempt has been made to study the local structure and magnetic

inhomogeneity of nano sized La0.7Sr0.3MnO3 manganites (Ulyanov et al.

2011). The decrease in grain size leads to weakening of DE interactions. The

absence of CO and phase transition temperatures of La1─xSrxMnO3 (x = 0.28,

0.31 and 0.36) has been explored through on-line ultrasonic measurements

(Sankarrajan et al. 2011). The semiconducting behaviour of grain size

dependent La0.9Sr0.1MnO3 nanoparticles has been studied by dc resistivity

measurement (Shinde et al. 2012). A recent study on the La0.8Sr0.2MnO3 nano

perovskites reveals better catalytic activity for oxygen reduction, higher

discharge plateau and specific capacity (Fu et al. 2012).

1.4.2 Maraging Steel

The percentage elongation of M250 maraging steel has been

investigated under uniaxial tensile conditions in the temperature range from

room temperature to 823 K. The observed results reveal that the elongation

remains constant at all strain rates up to 673 K. An elongated structure is

observed at temperature above 773 K. XRD studies on maraging steel reveal

the presence of reverted austenite at 823 K (Venkatanarayana et al. 1996).

The effect of ageing on the microstructure, tensile properties, fracture

toughness and fractographic features of a cobalt-free 18% Ni 250-grade

maraging steel has been investigated (Sinha et al. 1998). This investigation

confirms that the Ni3Ti phase precipitation has an excellent resistance to

coarsening at the normal ageing temperatures. The evolution of precipitates in

maraging steel of grade 350 has been studied using small angle X-ray

32

scattering and transmission electron microscopy (TEM); Tewari et al. 2000).

These studies reveal that ageing the steel at 703 K results in a rhombohedral

distortion of the supersaturated body centered cubic martensite which effects

in the nucleation and growth of Ni3(Ti, Mo) precipitations.

A sustained stress of approximately 40% of the ultimate tensile

stress after ageing at 11,500 h in maraging steel has been investigated through

XRD and optical studies (Reddy et al. 2001). Christopher et al. (2004) have

studied the development of a failure assessment diagram to maraging steels

and its validity is verified by considering the maraging steel fracture data of

surface crack tension specimens and pressure vessels having axial surface

cracks. Fracture strength/failure pressure estimates based on this investigation

are found to be in reasonably good agreement with the other experimental

results.

The magnetic Barkhausen emission measurement has been

performed on maraging steel using an encircling pickup coils (Rajkumar et al.

2007). The root mean square voltage of magnetic Barkhausen emission peak

is used as a tool to correlate the microstructural changes in maraging steel.

Ultrasonic velocity measurement has been performed on maraging steel aged

at different temperatures (Yeheskel 2009). The sound velocity measurement is

used to explain the intermediate stages of ageing in maraging steel through

depletion of Ni from the martensitic matrix due to Ni3Ti and Ni3Mo

formations. Fatigue crack growth tests performed on maraging steel reveal

that the reduction in effective crack driving forces by the interlocking and

friction of the asperities of the crack surface (Doquet et al. 2010). The

corrosion inhibition of the aged 250 grade maraging steel has been

investigated by potentiodynamic polarisation and electrochemical impedance

spectroscopy techniques. The inhibition efficiency increased with an increase

in inhibitor concentration and decreased with an increase in ageing

33

temperature (Poornima et al. 2011). Rao et al. have attempted to measure the

hardness, deformation and residual stress in maraging steel by magnetic

Barkhausen noise.

1.5 OBJECTIVES AND SCOPE OF THE PRESENT

INVESTIGATION

It is evident from exclusive review of literature that most research

on manganite is concerned with alkaline earth elements such as Sr, Ca and Ba

or a combination of these elements in bulk samples. The effects of grain size

on magnetic, transport and structural properties of manganites have been

extensively studied by many investigators and suggest that particle size, the

doping level in the lanthanum site and oxygen content are important in the

physico-chemical properties of these oxides. The complete characterisation of

prepared perovskite samples is done by studies such as density, XRD, energy

dispersive analysis of an X-ray spectrometer (EDX), Fourier transform

infrared spectroscopy (FTIR), scanning electron microscope (SEM), TEM and

surface area measurement (SBET). Efforts are also made to emphasise the

structural behaviour of nano perovskites in comparison with their bulk

counterparts. However, the effect of size on the behaviour of phase transition

temperature through on-line ultrasonic measurements in bulk and

nanostructured La1─xCaxMnO3 (x = 0.75, 0.80 and 0.85), La1─xPbxMnO3 (x =

0.3, 0.4 and 0.5) and La1─xSrxMnO3 (x = 0.32, 0.35 and 0.37) perovskite

manganite materials is found to be minimal.

Further, It is inferred from a brief review on maraging steels that

most of the studies used for the evaluation of microstructural changes during

ageing of maraging steel are destructive, time consuming and off-line in

nature. Therefore, the temperature dependent on-line microstructural

characterisation of maraging steel over a wide range of temperature is

34

advantageous to understand the microstructural changes that occur in

maraging steels during ageing.

The specific objectives of the present investigation are:

• To synthesise the both bulk and nano La1─xCaxMnO3 (x = 0.75,

0.80 and 0.85), La1─xPbxMnO3 (x = 0.3, 0.4 and 0.5) and

La1─xSrxMnO3 (x = 0.32, 0.35 and 0.37) perovskite manganite

materials.

• To elucidate the structural, optical and morphological properties

of the bulk and nano perovskites using the characterisation

studies such as XRD, EDX, FTIR, SEM, TEM and BET.

• To design and fabricate an indigenous experimental set-up for

on-line ultrasonic velocities/attenuations measurements from

room temperature (300 K) to 1200 K by overcoming the

drawbacks of the already designed experimental set-up at the

laboratory.

• To explore temperature dependent phase transitions existing in

prepared perovskite manganite samples on basis of the

ultrasonic measurements.

• To determine and compare the elastic constant of both bulk and

nano perovskite manganite samples.

• To explicate the behaviour of temperature dependent transition

under influence of the grain size at the transition temperature.

• To prepare a maraging steel specimen with a suitable heat

treatment.

• To investigate the microstructural changes that occurs in

maraging steels during ageing.

• To correlate the variation in ultrasonic measurements with

structural/microstructural changes during ageing.

35

The systematic research made on the samples helps to understand

the structural properties of perovskites, like surface spin, structural disorders,

half-metallicity and their possible influence on magnetism. It is essential

while using the manganite oxides for a wide range of applications. The

influence of grain boundary on the transport properties and spin-dependent

tunnelling mechanism in perovskites is explored through the present

investigation. The on-line ultrasonic measurements during thermal treatments

facilitate us to explore the precise information about the temperature

dependent structural and phase transformation of bulk and nano perovskite

samples and maraging steel. Further, this study is used to draw the phase

diagram as a function divalent composition (x).

1.6 ORGANISATION OF THE THESIS

The thesis consist of four chapters namely Introduction, Materials &

Methods, Results & Discussion and Summary & Conclusion. In this first

chapter of the thesis, a brief introduction about the bulk and nanostructured

lanthanum based perovskites and M250 grade maraging steel are discussed

along with an exclusive review. The preparation of sample, procedures for

materials characterisation techniques and design and fabrication of an

indigenous experimental set-up for on-line ultrasonic velocities/attenuations

measurements are discussed in detail in Chapter 2. The obtained result and

discussions for all the prepared samples are given in Chapter 3. Chapter 4

deals with summary and conclusions of the present investigation along with

the future scope of the thesis.