A study on fe induced structural, magnetic and transport properties in colossal magnetoresistive nd0 67sr0 33mno3 polycrystalline bulk and films 2

2 Sample Preparation and Characterization Techniques 2.1 Sample Preparation In this work, the mixed-valence manganites have been prepared in the form of polycrystalline ceramic and thin

2 Sample Preparation and Characterization Techniques

2.1 Sample Preparation

In this work, the mixed-valence manganites have been prepared in the form of

polycrystalline ceramic and thin films The two synthesis methods employed are

solid-state reaction and pulsed laser deposition Solid-solid-state reaction method is used in an

attempt to produce the polycrystalline ceramic Nd0.67Sr0.33MnO3 target The samples are

usually composed of micrometer sized grains Pulsed laser deposition is used to produce

both epitaxial and polycrystalline Nd0.67Sr0.33MnO3 thin films Besides the above

mentioned methods, wet chemical methods [68] which produce a precursor gel of

intimately mixed and hydrated oxides by means of chemical co-precipitation [69, 70]

technique are among other techniques that have been used to prepare the target material

However, only the solid-state reaction and pulsed laser deposition techniques will be

briefly described in this chapter

2.1.1 Solid-state reaction for the preparation of polycrystalline ceramic

single-phase material is achieved [71] In this work, polycrystalline Nd0.67Sr0.33MnO3

bulk is prepared in the following five simple steps and figure 2 – 1 shows the flow chart

for the standard solid-state reaction preparation procedure for Nd0.67Sr0.33MnO3 bulk as

below:

Step 1: Stoichiometric proportions of fine and analytical grade purity powders of oxides,

Nd2O3, SrCO3 and MnO2 were separately weighed and prepared

Step 2: The stoichiometric portions were mixed together thoroughly in acetone using the

FRITSCH centrifugal ball milling machine with zirconium oxide balls for several

hours until the mixture became homogeneous

Step 3: The slurry was dried and the residue was calcined or pre-sintered in an alumina

crucible at 1000 °C for 15 hrs in air to obtain better crystallization

Step 4: The calcined powder thus obtained was reground The reground powder must be

of a very fine form as the solid state reaction depends on the interdiffusion

between the powders Steps 1, 2, 3 and 4 were repeated several times until a

single perovskite phase was obtained

Step 5: The final powder was pressed into a pellet of dimension 5 x 10 x 2 mm3 and

sintered at 1250 °C for 24 hrs in air Finally the desired stoichiometry is cooled

down to room temperature

Nd 2 O 3 + MnO 2 + SrCO 3

Sintering

Target

Grinding Powder Pressing

Figure 2 - 1 Flow chart for standard preparation procedure of ceramic

Nd0.67Sr0.33MnO3 target by solid-state reaction method

Although the wet chemical methods such as sol-gel and chemical co-precipitation

techniques can be used to prepare the target material, these methods have found

application only in certain specialized areas due to the higher manufacturing costs

involved and the necessity of disposing large quantities of aqueous solutions Solid-state

reaction method is widely versatile and quite useful in producing pure materials which

are instrumental in the study of the structure and magnetic properties of perovskite

manganites A more detailed description of this method can be found in a book by

Buchanan [72] with particular reference to ferrites

2.1.2 Pulsed laser deposition for the preparation of Nd0.67Sr0.33MnO3 thin

film

Manganese oxide thin films have been prepared by several methods such

as sputtering, molecular beam epitaxy (MBE) or pulsed laser deposition (PLD) from the

sintered ceramic targets Pulsed laser deposition (PLD) is one of the physical vapor

deposition (PVD) methods for thin film fabrication The laser age began with the birth of

the first laser in 1960 However, this technique went virtually unnoticed until Venkatesan

and coworkers [73, 74] demonstrated the growth of newly discovered high-temperature

superconductors of bulk Y-Ba-Cu material followed by annealing in air or oxygen in

1987 Since then, it had been extensively developed for cuprate superconductors and later

adapted for manganites In this project, an excimer laser KrF beam operating in the

ultraviolet range with wavelength, λ = 248 nm, is employed Oxygen gas is employed in

the chamber A simplified schematic arrangement for preparing the manganese thin films

by PLD is presented in figure 2 - 2

Figure 2 - 2 A simplified diagram of the pulsed laser deposition (PLD) system

The KrF excimer laser is focused through a UV-transmitting window onto

a rotating ceramic Nd0.67Sr0.33MnO3 target, prepared by the method as described in

section 2.1.1, by focusing lens The energy per pulse or fluence density used is about 2

J/cm2 When the laser beam hits the target, plasma which takes the form of a plume,

expanding several centimeters long normal to the target towards the substrates is created

The plasma plume contains both ground and excited state neutral atoms and ions, as well

as electrons These undergo collisions in the high-density plasma region near the target,

resulting in a highly directional flow in front of the target surface and deposition on the

substrate surface Thus a layer of thin film is built up

The critical variables for thin film deposition are the gas pressure in the chamber (in this case, the oxygen pressure), the substrate temperature and thin film

annealing time/gas pressure Compounds such as La1-xSrxMnO3 [1], La0.67Ba0.33MnO3

[75], La1-xCaxMnO3 [76, 77], Nd1-xSrxMnO3 with 0 < x < 1 [78] and Pr0.7Sr0.3MnO3 [79]

have been deposited on different substrates such as LaAlO3 (LAO), MgO, SrTiO3 (STO)

and Si Optimal conditions for the growth of high quality manganites thin films as

recorded in the literature are temperature ranging from 600 – 800 °C and an oxygen

pressure of 2 – 5 × 10-1 mbar, followed by proper annealing under oxygen pressure With

careful selection of the type of substrate, epitaxial and polycrystalline thin films can be

fabricated For example, we can fabricate (001)-oriented epitaxial manganite thin film on

a (001)-oriented 5 × 10 mm2 SrTiO3 substrate Polycrystalline manganite thin film can be

fabricated on a Si substrate In this work, high quality epitaxial Nd0.67Sr0.33MnO3

monolayered and bilayered La0.67Sr0.33MnO3/Nd0.67Sr0.33MnO3 thin films were deposited

on LAO and STO substrates

2.2 Sample Characterization Techniques

This section aims to review a number of techniques used to analyze the structural,

magnetic and electrical transport properties of a material Among the wide variety of

techniques that are available to measure and analyze these quantities, we illustrate the

most common methods by examples involving the samples we have fabricated in this

project X-ray diffraction (XRD) is the conventional bulk sensitive technique used for

structural characterization Other techniques such as scanning electron microscopy

(SEM), atomic force microscope (AFM), and magnetic force microscope (MFM) are

employed for surface morphologies characterizations To determine the magnetic

properties of these half-metallic ferromagnetic manganese oxides, vibrating sample

magnetometer (VSM) is employed The four-point probe technique is used to measure

the electrical transport property of the material X-ray photoelectron microscope (XPS) is

used to analyze the surface concentration and the chemical environment of each element

in the outer surface of ∼100 Å of the samples

2.2.1 X-ray Diffraction

X-rays were discovered by W C Roentgen in 1895 [80] X-ray diffraction

(XRD) is a non-destructive technique widely applied for identification of compounds by

their diffraction pattern Among the various kinds of micro- and nano-crystalline

materials for characterization are inorganics, organics, drugs, minerals, catalysts,

ceramics or metals The physical state of the materials can be loose powders, thin films or

bulk materials In this work, the samples investigated are ceramic Nd0.67Sr0.33MnO3,

Fe-doped Nd0.67Sr0.33MnO3, La0.67Sr0.33MnO3 bulk and thin films

Collimating

Receiving slit

θ

Sample Holder

X-ray Tube Tower

Figure 2 - 3 Photograph of the Phillips PW1710 X-ray Diffractometer with the

important parts labeled

Figure 2 - 3 above shows the Phillips PW1710 X-ray Diffractometer used

in our laboratory Striking a pure anode of a particular metal with high-energy electrons

in a sealed vacuum tube (x-ray tube as indicated in the picture) generates X-rays usable

for x-ray diffraction Here we employed the Copper (Cu) Kα (1.54056 Å) radiation

source A coherent beam of monochromatic CuKα from the x-ray tube is directed at the

sample Interaction of X-rays with sample creates secondary diffracted beams of X-rays

which are related to the interplanar spacings in the crystalline materials according to

Bragg's Law [81]:

nλ = 2dsinθ where n is an integer, d is the interplanar spacing in the crystalline phase, θ is the

incidence angle and λ is the wavelength of the incident x-ray The diffracted beams are

then collected by the detector through the receiving slit The diffraction peaks are

measured along a 2θ diffractometer circle whereby the x-ray tube is fixed and the

specimen moves at half the angular rate of the detector to maintain the θ-2θ geometry

The angle of the diffraction is related to the interplanar spacing, d and the intensity of the

diffraction peak is related to the strength of those diffractions in the specimen Figure 2 -

4 shows the X-ray diffraction pattern for Nd0.67Sr0.33MnO3 target by solid-state reaction

method recorded in terms of 2θ using a computer running PC-APD analytical software

0 50 100 150 200 250 300 350 400

Based on figure 2 - 4, by using fitting program, Winfit, adopted freely from Stefan

Krumm [82], the unit cell parameters and microstructural parameter (grain size) can be

obtained The polycrystalline ceramic Nd0.67Sr0.33MnO3 target can be indexed according

to a tetragonal structure with lattice parameters, a = 8.635 Å and c = 3.848 Å One can

also refer to Powder diffraction file [83] available in the market for determining the

structural phase of the sample

In this day of automated data collection and analysis, the preparation of sample is

usually the most critical factor influencing the quality and reliability of the collected

analytical data The three parameters of special interest in a diffraction pattern are (1) the

position of the diffraction peaks, (2) the peak intensities, and (3) the intensity distribution

as a function of diffraction angle How accurate these experimental results represent the

sample in terms of these parameters determine whether the results are useful for phase

identification, or more detailed analyses like crystallite size and distribution, stress and

strain or quantitative determination of different phases in a multi-phase sample

Therefore, to be able to see all the diffraction peaks, the powdered sample must present a

large number of crystallites in a random orientation to the incident beam For thin film

analysis, it is important to mount the film on a zero-background plate to ensure that the

surface of the film is parallel to the reference plane in order to fulfill Bragg’s diffraction

condition

2.2.2 Vibrating Sample Magnetometer (VSM)

Vibrating sample magnetometer (VSM) has become widely accepted as a

standard technique for magnetic measurements This technique was first highlighted by

Foner [84] in 1959 It is a relative measurement giving the total magnetic moment of a

sample A VSM is an example of an induction technique whereby the AC field produced

by an oscillating sample moment located in the center of two counter-wound coils spaced

a small distance apart is measured The motion of the magnetic sample creates a changing

flux in the coils which induces a voltage across the pick up coils as shown in figure 2 - 5

Audio Amplifier Transducer

Magnet

XY Recorder

Lock-in Amplifier

Transducer Driver

Magnet Power Supply Pick up coils

Figure 2 - 5 A simplified diagram of the VSM system

The pair of pick up coils is wound in opposition because by doing so, the

induced voltage from one coil due to the vibrating sample is added to that from the other

coil The voltages from the changing background field are equal in magnitude but

opposite in direction and will cancel each other out However, it is important to calibrate

the voltage against a sample of known moment using calibration materials such as nickel,

palladium or iron The signals from the pick up coils are sent to two variable gain

amplifiers through a balanced secondary microphone transformer and then amplified by a

lock-in amplifier Lastly the signals are processed and data collected by a computer

0 1 2 3 4 5 6 7

Figure 2 - 6 The temperature dependent magnetization, M(T) for Nd0.67Sr0.33MnO3 and

La0.67Sr0.33MnO3 at a fixed magnetic field of 200 Oe from 77 to 470 K

In this work, we employed a computer controlled MagLab VSM system

from the Oxford Instruments Common features of this MagLab VSM system include

automated systems for changing the temperature and magnetic field applied to the

sample The automated MagLab VSM system operates in a temperature from boiling

point of liquid helium (5 K) to 300 K and a maximum magnetic field up to 9 Tesla In

this work, both the isothermal magnetic hysteresis loop, M(H) and the temperature

dependent magnetization, M(T) at a fixed field were evaluated The covered range of

temperature was from 77 to 300 K and the fixed magnetic field adopted for the M(T)

curves ranged from 80 to 2000 Oe

It is important to note that the measured moment of the sample is the sum

of the moments of the sample itself, the substrate (for example thin film), the sample

holder and any medium which is moving inside the sample region Therefore, in order to

determine the characteristics of the sample, it is necessary to minimize or subtract the

contributions from the outer components There may be little option in choosing the

substrate as this has an important bearing on the sample preparation itself Its contribution

to the total signal is best determined by a separate measurement of the substrate material

itself In addition, the sample holder can be constructed from a diamagnetic material and

shaped so as to be as symmetrical as possible about the centre of the coil system in order

to eliminate the contribution from it

2.2.3 Scanning Electron Microscope (SEM)

The scanning electron microscope (SEM) is used to study the surface, or

near surface structure of bulk specimens Unlike optical microscope which uses light

beam, SEM employs a beam of electrons directed at the specimen An electron gun,

usually of the tungsten filament thermionic emission type, produces electrons, and

accelerates them to an energy of about 2 - 40 keV Two or three condenser lenses then

demagnify the electron beam until, as it hit the specimen, it may have a diameter of only

2 – 10 nm The fine beam of electrons is scanned across the specimen by the scan coils,

while a detector counts the number of low energy secondary electrons, or other radiation,

given off from each point on the surface At the same time, the spot of a cathode ray tube

(CRT) is scanned across the screen, while the brightness of the spot is modulated by the

amplified current from the detector The electron beam and CRT spot are both scanned in

a rectangular set of straight lines known as a raster The mechanism by which the image

is magnified involves no lenses at all It can easily be obtained by making the raster

scanned by the electron beam on the specimen smaller than the raster displayed on the