chapter v magnetic properties of la 1-xsr xmno 3 : nanoparticles...

Chapter V Magnetic properties of La1-xSrxMnO3 : Nanoparticles

and Superparamganetism

Department of Physics, Shivaji University, Kolhapur. 113

5.1 Introduction

A lot of information about the magnetic properties of a material can be

obtained by studying its hysteresis loop. Hysteresis is a well known

phenomenon in ferromagnetic materials. A wide range of applications, from

electric motors to transformers, permanent magnets, various types of

electronic devices, magnetic recording devices, magnetic refrigeration etc. and

many more rely heavily on various aspects of hysteresis. The lanthanum

manganites for a particular application are dependent on its magnetic

properties. Magnetic properties are strongly dependent on chemical

composition, sintering temperature, grain size and crystal structure and

porosity of the material [1]. Hence, in this chapter we have reports on the

magnetic properties for different composition of LSMO.

5.2 Theoretical background

In a physical phenomenon an electric current loop generates a region of

magnetic field represented by a magnetic flux line. Figure 5.1 shows a simple

illustration of this phenomenon. The magnetic field vector at any given point

near to the electric current loop is given by H, vector quantity. There are also

other type of materials that are inherently magnetic, that is, they can generate

magnetic field without a macroscopic electric current [2]. Figure 5.2 shows a

magnetic bar as an example which exhibits a particular dipole North–South

that give the orientation. The best utility of magnetism is the force of attraction

that can provide many of our present technology devices based on magnetism

of magnetic materials; these include electrical power generators and

transformers, electric motors, radio, television, telephone and magnetic

refrigerator.

There are five basic types of magnetism have been observed and

classified on the basis of magnetic behavior of the material with respect to

applied magnetic field. These types are Diamagnetism, Paramagnetism,

Ferromagnetism, Anti-ferromagnetism and Ferrimagnetisms. If the particle




size of some magnetic materials is in nanometer range then materials shows

superparamganetism.

Figure 5.1 Magnetic field generated around an electrical current loop.

Figure 5.2 Magnetic material can generate a magnetic field

without an electric current.




5.2.1 Nanomaterials

Nanomaterials are defined as having at least one dimension less than

100 nm and typically are engineered to have unique properties which make

them desirable for commercial applications. The two main reasons why

materials at the nano scale can have different properties are increased relative

surface area and quantum effects. Nanomaterials have a much greater surface

area to volume ratio than their conventional forms, which can lead to greater

chemical reactivity and affect their strength. Also at the nano scale, quantum

effects can become much more important in determining the materials

properties and characteristics, leading to novel optical, electrical and magnetic

behaviours.

Nanomaterials can be nanoscale in one dimension, two dimensions and

three dimensions. They can exist in single, fused, aggregated or agglomerated

forms with spherical, tubular, and irregular shapes. The common types of

nanomaterials include nanotubes, dendrimers, quantum dots and fullerenes.

Products containing engineered nanomaterials are already in commercial use,

with some have been available for several years or decades. The range of

commercial products available today is very broad, including stain-resistant

and wrinkle-free textiles, cosmetics, sunscreens, electronics, paints and

varnishes.

Nanotechnology is a multidisciplinary grouping of physical, chemical,

biological, engineering, and electronic processes, materials, applications, and

concepts in which the defining characteristic is size. In this policy,

nanotechnology is a generic term that encompasses the manipulation of

matter at atomic or near atomic scales to produce new materials, structures,

or devices. The size of the nanoparticle grains strongly affects the property

changes in the bulk material. For instance, the overlapping of different grain

sizes affects the physical strength of the material [3].

The nanotechnology is as well as evolutionary as revolutionary in

nature. Evolutionary are the many applications where the same material is




incrementally improved by using nanotechnology. Revolutionary it can be

called where new properties originate from nanotechnology like for example

in quantum dots. Those new properties can be divided as

• Properties based on the fact that the surface is large compared to the

weight/volume.

• In addition to size, low energy dissipation and high processing speeds

are important.

• New properties not found in bulk or micro sized particles.

5.2.2 Classification of nanomaterials

Nanomaterials consisting of nanometer sized crystallites or grains and

interfaces may be classified according to their chemical composition and

shape. According to the shape of the crystallites or grains we can broadly

classify nanomaterials into four categories and shown in Figure 5.3.

1. Clusters or powders (MD=0)

2. Multilayers (MD=1)

3. Ultrafine grained overyaers or buried layers (MD=2)

4. Nanomaterials composed of equiaxed nanomter-sized grains (MD=3)

Figure.5.3 Classification of nanomaterials according to dimensions.




5.2.3 Properties of nanomaterials

As we know the properties of materials are drastically changes

with size. In case of magnetic nanomaterials particles changes then their

mechanical, optical and magnetic properties changes accordingly.

5.2.3.1 Mechanical properties

Due to the nanometer size, many of the mechanical properties of the

nanomaterials are modified to be different from the bulk materials including

the hardness, elastic modulus, fracture toughness, scratch resistance and

fatigue strength etc. An enhancement of mechanical properties of

nanomaterials can result due to this modification, which are generally

resultant from structural perfection of the materials [4, 5]. The small size

either renders them free of internal structural imperfections such as

dislocations, micro twins, and impurity precipitates or the few defects or

impurities present cannot multiply sufficiently to cause mechanical failure.

The imperfections within the nano dimension are highly energetic and will

migrate to the surface to relax themselves under annealing, purifying the

material and leaving perfect material structures inside the nanomaterials.

Moreover, the external surfaces of nanomaterials also have less or free of

defects compared to bulk materials, serving to enhance the mechanical

properties of nanomaterials [19]. The enhanced mechanical properties of the

nanomaterials could have many potential applications both in nano scale such

as mechanical nano resonators, mass sensors, microscope probe tips and nano

tweezers for nano scale object manipulation and in macro scale applications

structural reinforcement of polymer materials, light weight high strength

materials, flexible conductive coatings, wear resistance coatings, tougher and

harder cutting tools etc.

5.2.3.2 Optical properties

The optical properties are very much sensitive to size of the particles.

The applications based on optical properties of nanomaterials include optical




detector, laser, sensor, imaging, phosphor, display, solar cell, photocatalysis,

photo electrochemistry and biomedicine. Many of the underlying principles

are similar in these different technological applications that span a variety of

traditional disciplines including chemistry, physics, biology, medicine,

materials science and engineering, electrical and computer science and

engineering.

The optical properties of nanomaterials depend on parameters such as

feature size, shape, surface characteristics, and other variables including

doping and interaction with the surrounding environment or other

nanostructures. The simplest example is the well-known blue-shift of

absorption and photoluminescence spectra of semiconductor nanoparticles

with decreasing particle size, particularly when the size is small enough. For

semiconductors, size is a critical parameter affecting optical properties. By

simply controlling the physical dimensions, one can generate gold

nanostructures with absorption covering the entire visible and near IR regions

of the optical spectrum.

5.2.3.3 Magnetic properties

For a single isolated particle, coercivity or remanence as function of the

particle size, are sketched in the Figure 5.4. The large magnetic particles are

subdivided by Bloch walls into magnetic domains. Therefore, remanence and

coercivity are independent of the particle size. Decreasing particle size leads to

a size range, where the particles consist of only a single magnetic domain.

Therefore, coercivity and remanence are increasing drastically. This range of

particle sizes is used for magnetic data storage. Further decrease of the

particle size leads to a decrease of remanence and coercitivity to zero.

However, this step depends on the measurement time. The shorter the time

constant of the measuring method τm the more is this step shifted to smaller

particle sizes. Magnetic properties of a single isolated particle are strongly

influenced by the particle size. This phenomenon, called




"Superparamganetism" is observed, when the thermal energy of the particle

kT is larger than the energy of magnetic anisotropy KV.

Figure 5.4 Coercivity as a function of the particle size.

5.3 Coercivity and Superparamganetism

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

importance from both fundamental considerations and application point of

view. A brief discussion of magnetic behavior of low dimensional systems is

focused based on literature. Magnetic nanoparticles exhibit specific properties

such as coercivity and superparamganetism, generally attributed to reduced

size.

Coercivity:

The coercivity is also called the coercive field or coercive force. It is

defined for a ferromagnetic material as the intensity of the applied magnetic

field required to reduce the magnetization of that material to zero after the

magnetization of the sample has been driven to saturation.

The coercivity of fine particles has a striking dependence on their size.

Figure 5.5 shows dependence of coercivity on size schematically and how the




size range is divided according to the variation of coercivity with particle

radius r.

The following regions can be distinguished:

(i) Multi-domain (MD): It is observed for r >rc and in this region, the

coercivity decreases as the particle size increases and the coercivity

(Hc) is found to vary with size as ~ 1/ rn.

(ii) Single-domain (SD): For r0< r <rc, the particles become single

domain and in this size range, the coercivity reaches a maximum.

(iii) Superparamagnetic (SP): Below a critical size r0, the coercivity is

zero because of thermal effect, which is strong enough to

spontaneously demagnetize the assembly of magnetic particles.

Figure 5.5 Schematic of the size dependence of coercivity

Superparamganetism:

Superparamganetism is a form of magnetism, which appears in

small ferromagnetic or ferrimagnetic nanoparticles (1-10nm). In sufficiently

small nanoparticles, magnetization can randomly flip direction under the

influence of temperature. The typical time between two flips is called the Néel




relaxation time. In the absence of external magnetic field, when the time used

to measure the magnetization of the nanoparticles is much longer than

the Néel relaxation time, their magnetization appears to be in average zero

and they are said to be in the superparamagnetic state. In this state, an

external magnetic field is able to magnetize the nanoparticles, similarly to

a paramagnet. However, their magnetic susceptibility is much larger than the

one of paramagnets.

The effective magnetic moment of a ferromagnetic particle is

determined by its size. A ferromagnetic sample with a volume greater than a

critical value V divides into multiple magnetic domains, each magnetized along

the local easy axis but in one of two opposite directions. The multiple domain

structure is, however, no longer favorable below the critical volume, and the

particle becomes a single domain with ferromagnetic alignment of all its

moments along the easy axis in the same direction. Thermal fluctuations of the

moment exist on a microscopic scale, but to reverse the direction of the single

domains magnetization requires an energy ΔE to overcome the crystal-field

anisotropy. If single domain particles become small enough, KV would become

so small that thermal fluctuations could overcome the anisotropy forces and

spontaneously reverse the magnetization of a particle from one easy direction

to the other, even in the absence of an applied field. Each particle has a

magnetic moment μ = MsV and, if a field is applied, the field will tend to align

the moments of the particles and the thermal energy will tend to disalign

them. This is called superparamganetism. The probability of such a reversal by

thermal activation is proportional to exp (-ΔE/kT). This differs from

conventional paramagnetism because the effective moment of the particle is

the sum of its ionic particles, which can be several thousand spins in a

ferromagnetic particle small enough to show superparamganetism [6].

Very fine ferromagnetic particles have very short relaxation times even

at room temperature and behave superparamagnetically; and their behavior is

paramagnetic but magnetization values are typical of ferromagnetic




substances. The individual particles have normal ferromagnetic moments but

very short relaxation times so that they can rapidly follow directional changes

of an applied field and on removal of the field, do not hold any remanent

moment.

Superparamganetism is characterized by two experimental features

1. There is no hysteresis; (i.e., both the retentivity and the coercivity are

zero) in the field dependence of magnetization.

2. Magnetization curves measured at different temperatures superimpose

when magnetization (M) is plotted as a function of field (H) temperature

(T).

Superparamganetism can be destroyed by cooling. This follows because the

characteristic fluctuation time for a particle's moment varies exponentially

with temperature, so the magnetization appears to switch sharply to a stable

state as the temperature is reduced. The temperature at which this occurs is

called the blocking temperature (TB) and it depends linearly on the sample's

volume and on the magnitude of the crystal field anisotropy.

In the case of superparamagnetic materials, the magnetization shows

temperature and path dependence which is shown schematically in Figure 5.6.

Figure 5.6 Schematic diagram of ZFC and FC magnetization curves as a

function of temperature taken in an applied field H.




The two curves zero field cooled (ZFC) and field cooled (FC) show different

behavior at low temperatures. As the temperature increases the magnetic

moment in the FC curve decreases. However, as the temperature begins to rise

from 5K, the moment in the ZFC curve begins to increase. At a certain

temperature, the ZFC curve reaches a peak and this temperature is called the

blocking temperature (TB). The divergence of ZFC and FC curve and the

blocking temperature depend on the particle size and its distribution. The

blocking temperature of a substance should decrease with increasing applied

field and eventually disappear when the field reaches a critical value. The

higher field is expected to lower the barriers between the two easy axis

orientations.

For a particle of constant size below the blocking temperature TB, the

magnetization will be stable and shows hysteresis. It refers to particles which

have relaxation time for demagnetization longer than 100 sec. For uniaxial

particles using the same criterion for stability gives,

TB = ��

�� …..(5.1)

where K = Anisotropy constant

V = Volume of the particle

k = Boltzmann’s constant (1.38 × 10-23 JK-1)

If one considers Ni as a classical example with an anisotropy constant K = 4.5 ×

103 Jm-3 then for a size 20 nm, the particle will show a blocking temperature

(TB) at ~ 55 K by using Eq. 5.1. Below TB, the magnetization will have relatively

stable and shows ferromagnetic behavior. While above TB, the thermal energy

will be sufficient to suppress the ferromagnetic behavior and thus the particles

become superparamagnetic.

5.2.5 Applications of nanomaterials

Since the properties of nanomaterials all together different from bulk,

there is wide scope of applications of nanomaterials. By using nanomaterials,




everyday consumer products may be made lighter, stronger, cleaner, less

expensive, more efficient, more precise, or more aesthetic. Products containing

nanomaterials may improve our quality of life through more efficient target

driven pharmaceuticals, better medical diagnosis tools, faster computers,

cleaner energy production, etc. Several of the consumer end products available

today that utilize nanomaterials have been developed from existing products,

for example by the incorporation of nanomaterials into solid, viscous or liquid

matrices. About one third of the products are sunscreen lotions or cosmetics

such as skin-care and colorant products. For sunscreens, titanium dioxide and

zinc oxide nanoparticles are used as they absorb and reflect ultraviolet rays

but are still transparent to visible light and, thus, the resulting sunscreen

becomes both more appealing to the consumer and is claimed to be more

effective. Also liposomes, i.e. tiny vesicles made out of the same material as cell

membranes, are known to be used in the cosmetics industry. In fact, there are

very many cosmetic products that have nanomaterial content and the frequent

use of nanoparticles in cosmetics has indeed raised a number of concerns

about consumer safety, since they are applied directly on the human body.

The area where nanotechnology has a considerable impact, include these are

• Medical and pharmaceutical sector

• Bo-nanotechnology, bio-sensors

• Energy sector, including fuel cells, batteries and photovoltaics

• Environment sector including water remediation

• Construction sector, including reinforcement of materials

• Electronics and optoelectronics, photonics

5.3 Experimental

The La1-xSrxMnO3 (x=0.1, 0.2, 0.3 and 0.35) samples were synthesized by

solution combustion method with polyvinyl alcohol as a fuel. The LSMO

samples were structurally and morphologically characterized by x-ray

diffraction and Scanning electron microscope. The microstructure was




determined by Transmission electron microscopy. The magnetization vs

temperature of all the LSMO sample heat treated at 600oC, 900oC and 1200oC

were measured by using Vibrating Sample Magnetometer (Lakeshore-7307

Model) at room temperature. The powder samples filled into a sample holder

(plastic tube) having 2mm diameter. The weight of plastic tube before filling

LSMO powder and after filling powder were taken and weight of LSMO powder

calculated by subtracting these weights. The sample holder is hanged at the

central part of two magnetic poles. At room temperature, magnetization (emu)

in the LSMO samples were measured with an applied magnetic field -9 kOe to

+9 kOe. The temperature dependence of magnetization such as the field-

cooled (FC) and zero-field cooled magnetization for all the LSMO sample

measured in temperature range 100-350K, at the constant field of 500 Oe was

carried with a SQUID magnetometer (Quantum Design MPMS).

5.3 Results and discussion

The nanocrystalline nature was confirmed by x-ray diffraction study

and Transmission electron microscopy. The detail descriptions are described

elsewhere in section 3.4.2 and 3.4.5.

5.4.1 Study of superparamganetism of La1-xSrxMnO3(x=0.1-0.35)

at room temperature

The field dependence magnetization (emu/gm) of sample

La0.9Sr0.1MnO3, La0.8Sr0.2MnO3, La0.7Sr0.3MnO3 and La0.65Sr0.35MnO3 heat treated

at 600 oC, 900oC and 1200oC are as shown in Figure 5.7 and Figure 5.8. The S-

shaped nature of loop shows the LSMO particles shows superparamagnetic

behavior at room temperature. The magnetic hysteresis loss is very low or

almost zero which is beneficial for room temperature magnetic refrigeration.

From Figure 5.7 (a) it is observed that as annealing temperature increases,

increase in saturation magnetization. This is attributed to increase in

crystallite size with annealing temperature from 600oC to 1200oC. Similarly for




other composition of LSMO heat treated at 600oC, 900oC and 1200oC shows

same behavior from hysteresis loop as shown in Figure 5.7 (b) and Figure 5.8

(a, b).

Figure 5.7 Room temperature hysteresis loops (a) sample M1,

M2 and M3 (b) sample M4, M5 and M6.

-8000 -6000 -4000 -2000 0 2000 4000 6000 8000

-60

-40

-20

0

20

40

60

M3

M2

M1

Sa

tura

tion

Ma

gn

eti

zati

on

(em

u/g

m)

M agnetic field (O e)

(a)

-9000 -6000 -3000 0 3000 6000 9000

-60

-40

-20

0

20

40

60

M6

M5

Satu

rati

on

Magn

eti

zati

on

(em

u/g

m)

Magnetic Field (Oe)

M4

(b)




Figure 5.8 Room temperature hysteresis loops (a) sample M7, M8 and M9

(b) sample M10, M11 and M12.

-9000 -6000 -3000 0 3000 6000 9000

-60

-40

-20

0

20

40

60

M12

M11

Magnetic Field (Oe)

Sa

tura

tio

n M

ag

ne

tizati

on

(em

u/g

m)

M10

(d)

-9000 -6000 -3000 0 3000 6000 9000

-60

-40

-20

0

20

40

60M9

M8

Sa

tura

tio

n M

ag

net

iza

tio

n (

emu

/gm

)

Magnetic Field (Oe)

M7

(c)




The room temperature field dependent saturation magnetization for La1-

xSrxMnO3 with x=0.1, 0.2, 0.3 and 0.35 samples heat treated at 1200oC shown in

Figure 5.9. There is not enough change in saturation magnetization with

variation of strontium doping for x=0.1 to 0.35. The LSMO saturates above the

2000 Oe and remains parallel to applied field axis. The saturation

magnetization and coercivity for the samples M1, M2 and M3 are shown in

Figure 5.10 and for M3, M6, M9 and M12 are shown in Figure 5.11.

The hysteresis parameter such as saturation magnetization (Ms),

remanant magnetization (Mr), coercive field (Hc), remanent magnetization and

Bohr magneton (nB) are listed in Table 5.1 for samples M1-M3 and for M3, M6,

M9 and M12 samples heat treated at 1200oC in Table 5.2. From Figure 5.9 it is

observed that the saturation magnetization increases with annealing

temperature.

Figure 5.9 Room temperature hysteresis loop for sample M3, M6,

M9 and M12 heat treated at 1200oC.

-9000 -6000 -3000 0 3000 6000 9000

-60

-40

-20

0

20

40

60

Magnetic Field (Oe)

Sa

tura

tio

n M

ag

ne

tiza

tio

n (

em

u/g

m)




The coercive field increases first and then decreases with annealing

temperature. The substitution of Strontium in La site from x=0.1-0.35 increase

the saturation magnetization linearly from 52.10 to 57.27 emu/gm while the

coercivity decreases. Hence, we can say annealing plays an important role in

magnetic properties of LSMO.

Figure 5.10 Variation of saturation magnetization and coercive

field with annealing temperature of LSMO.

Figure 5.11 Variation of saturation magnetization and coercive

field for La1-xSrxMnO3 with x=0.1, 0.2, 0.3 and 0.35.

600 700 800 900 1000 1100 1200

10

20

30

40

50

60

Temperature o

C

Satu

riza

tio

n m

ag

ne

tiza

tio

n (

em

u/g

m)

10

12

14

16

18

20

22

24

26

28

Co

erc

ivity

(Oe

)

0.1 0.2 0.3

52

53

54

55

56

57

58

Sa

turi

za

tio

n m

ag

ne

tiza

tio

n (

em

u/g

m)

Concentration of Sr

6

7

8

9

10

11

Co

erc

ivity

(Oe

)




The magnetic moment per formula unit in Bohr magneton (nB) was calculated

by using the following relation [7],

��

�� ……(5.2)

where, M is the molecular weight of particular composition and Ms is

saturation magnetization (emu/gm).

Table 5.1: Room temperature saturation magnetization (MS), remnant

magnetization (Mr), Bohr magneton (nB) and coercivity (Hc) for LSMO

samples heat treated at 600oC (M1), 900oC (M2) and 1200oC (M3)

Sample Saturation

magnetization

(emu/gm)

Remanant

magnetization

(emu/gm)

Coercivity

(Oe)

Bohr

magneton

(emu/gm)

M1 10.52 0.03 18.62 0.44

M2 35.65 1.39 26.94 1.51

M3 52.02 0.32 10.87 2.21

Table 5.2: Room temperature saturation magnetization (MS), remnant

magnetization (Mr), Bohr magneton (nB) and coercivity (Hc) for LSMO

samples heat treated at 1200oC.

Sample Saturation

magnetization

(emu/gm)

Remanant

magnetization

(emu/gm)

Coercivity

(Oe)

Bohr

magneton

(emu/gm)

M3 52.02 0.32 10.87 2.21

M6 54.20 0.55 10.84 2.24

M9 56.32 0.39 7.78 2.28

M12 57.27 0.37 7.30 2.30




The net magnetic moment (nB) increases with increase in annealing

temperature for sample M1, M2 and M3 and shown in Table 5.2. However,

doping of Sr increases, the magnetic moment increases. Thus we can control

the saturation magnetization and coercivity by controlling the annealing

temperature and doping level of Strontium.

5.5.2 Effect of Sr doping on Curie temperature of La1-xSrxMnO3

(x=0.1-0.35)

The temperature dependence of magnetization such as the field-cooled

(FC) and zero-field cooled magnetization for the La0.65Sr0.35MnO3 sample heat

treated at 600oC, measured at the constant field of 500 Oe was carried with a

SQUID magnetometer (Quantum Design MPMS) and shown in Figure 5.12. It is

observed that the smooth variation of magnetic moment with temperature

from 200K-375K.

Figure 5.12 Temperature dependence of the magnetization in the FC

and ZFC process upon an application of 500 Oe for La0.65Sr0.35MnO3

heat treated at 600oC.




Figure 5.13 dM/dT Vs T plot for sample M1

Figure 5.14 Field cooled temperature dependence of the

magnetization of La0.9Sr0.1MnO3 in magnetic field 100Oe, 500Oe,

1kOe and 10kOe.

160 240 320 400 4800.05

0.00

-0.05

-0.10

-0.15

-0.20

-0.25

-0.30

dM

/dT

(em

u/g

m.K

)

Temperature (K)

Tc=295K

100 150 200 250 300 350

0

10

20

30

40

Ma

gn

etiz

ati

on

(em

u/g

m)

Temperature (K)

ZFC 100 Oe

FC 100 Oe

ZFC 500 Oe

FC 500 Oe

ZFC 1 kOe

FC 1 kOe

ZFC 10 kOe

FC 10 kOe




The FC and ZFC superimpose above 260K which confirms the La0.65Sr0.35MnO3

shows superparamganetism at room temperature. The Curie temperature (Tc),

defined by the maximum in the ‘‘absolute value’’ of dM/dT. Figure 5.13 shows

dM/dT-T curve for the determination of Curie temperature (Tc) and has been

found to be 295 K.

The M-T plot for the La0.9Sr0.1MnO3 (M1) heat treated at 600oC is shown

in Figure 5.14. It contains the M-T (Field cooled and Zero Field cooled) plot at

field 100Oe, 500Oe, 1kOe and 10kOe. The effect of applied magnetic field on

magnetization and transition temperature very much. The magnetization and

transition or Curie temperature increases with increase in applied magnetic

field. Below the Curie temperature one may distinguish the prominent split

between the field cooled (FC) and Zero Field cooled (ZFC) curve 100Oe and

500Oe. Above the Curie temperature at magnetic field 1 kOe and 10kOe the

field-cooled (FC) and zero-field cooled (ZFC) curves coincides. This type of

irreversibility behavior suggests that the magnetic anisotropy is not large in

the sample. The calculated value of Tc of the sample M1 is 248K .

Figure 5.15 Temperature dependence of the magnetization in the FC and ZFC

process upon an application of 500Oe for La0.8Sr0.2MnO3 (M4).

100 150 200 250 300 3500

3

6

9

12

15

18

Ma

gn

etiz

ati

on

(em

u/g

m)

Temperature (K)

ZFC 500 Oe

FC 500 Oe




Figure 5.16 Temperature dependence of the magnetization in the FC

and ZFC process upon an application of 500 Oe for La0.7Sr0.3MnO3 (M7).

The MT plot for samples La0.8Sr0.2MnO3 (M4), La0.7Sr0.3MnO3 (M7)is

shown in Figure 5.15 and Figure 5.16 respectively and Tc observed are 254K

and 257K. The variations in compositional homogeneity or oxygen

stoichiometry could cause variations of the Curie temperature on a narrow

scale and thereby a spreading of the overall Curie temperature. The Curie

temperature may be affected by the oxygen deficiency and the partially

disordered structure of grain boundaries in the sample.

The broadening of the magnetic transition for La0.9Sr0.1MnO3 (M1),

La0.8Sr0.2MnO3 (M4), La0.7Sr0.3MnO3 (M7) and La0.65Sr0.35MnO3 (M10) could be

attributed to the decreasing grain size associated with the increased Sr

content. A smaller grain size gives a larger proportion of surface-near spins,

which may be weaker ferromagnetically coupled than spins in the bulk of the

grains. This could give a distribution of Curie temperatures and thus a

broadened magnetic transition. A grain size dependent broadening of the

Curie temperature has previously been observed in granular Sr-doped

lanthanum manganites [8, 9]. In perovskite manganites, the mechanisms

100 150 200 250 300 3500

10

20

30

ZFC 500 Oe

FC 500 Oe

Ma

gn

etiz

ati

on

(em

u/g

m)

Temperature (K)




which govern the magnetic property of the material are the antiferromagnetic

super-exchange interaction in Mn3+–O2−Mn3+/Mn4+–O2−Mn4+ bonds and the

ferromagnetic double-exchange interaction in Mn3+–O2−Mn4+ bonds. The

relative strength of these two interactions can be strongly influenced by

replacing La-ion by divalent/monovalent ion of different size and oxidation

states which results in change of Mn3+/Mn4+ ratio as well as Mn–O bond length

and Mn–O–Mn bond angle. In case of polycrystalline La1−xSrxMnO3 samples,

substitution of smaller La3+ ion (ionic radii ∼ 0.121nm ) by larger Sr2+ ion

(ionic radii ∼ 0.131nm replace ) increases the average ionic radius of La-site,

thereby introduces crystallographic distortion and an increase in Mn–O–Mn

bond angle [10]. This results in increased double-exchange interaction and a

corresponding increase in TC of the system [11].

5.6 Conclusions

The magnetic properties of LSMO are found to be depends on Sr doping

and heat treatment. It is observed that saturation magnetization varies linearly

with increase in annealing temperature and Sr content. The Tc observed for all

LSMO samples investigated is in the range of 230-300K. The variation in Curie

temperature observed may be due to the oxygen deficiency and the partially

disordered structure of grain boundaries in the sample. It is revealed, Tc of

lanthanum manganites can be vary with substitution of Strontium and hence

LSMO can be tuned such that it may be used as refrigerant material for room

temperature magnetic refrigeration.




References

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3. R.A. Andrievski, and A.M. Glezer, Elsevier Science Ltd. 44 (2000) 1621-

1623.

4. G. Cao “Nanostructures & Nanomaterials: Synthesis, Properties &

Applications” Imperial College Press, 2004.

5. C. Herring and J. K. Galt, Phys. Rev. 85 (1952)1060–106

6. R.C.O’Handley, Modern Magnetic Materials Principles and Applications,

John Wiley and Sons, Inc., pp 435, (2000).

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Magn.Magn.Mater.195 (1999) 112.

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(1995) 12522.

chapter v magnetic properties of la 1-xsr xmno 3 : nanoparticles...

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