Thesis (MSc)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: In this investigation the formation of thin film manganites and their electrical characteristics
is studied. In order to see the effect of oxidation states on magneto-resistivity,
80% of Mn is replaced by Fe. Pulsed laser deposition (3 J/cm2), carried out in oxygen
partial pressures ranging from 0.01 mbar to 1.00 mbar was used to fabricate the thin films
from two target compositions, namely La2CaMn2.94Feo.0609 and La2CaMno.6Fe2.409.
Films were deposited on Si< 100 >, MgO< 100 >, SrTi03< 100 > and LaAl03< 100 >
single crystal substrates. Samples were characterized by RBS, AFM, SEM, and XRD.
Electrical measurements were also carried out.
One of the main characterization techniques in this investigation is Rutherford Backscattering
Spectrometry (RBS). It has been shown that RBS is a very powerful characterization
technique when used in conjunction with the RUMP simulation program. The effect
of various parameters can be determined beforehand by RUMP simulation of the thin
film structures to be investigated. Simulation shows that RBS is an excellent characterization
tool for determining film thickness and stoichiometry. The role of oxygen uptake
in La2CaMn3_xFexOg was investigated as the oxidation states of elements in manganite
materials have a large effect on their magnetoresistive properties. The height of the La
signal can be used as a measure of the oxygen content. RBS spectra of films deposited on
single crystal silicon substrates at different ambient pressures show that the fit between
simulated and measured RBS spectra improves with higher oxygen pressures, thereby
indicating better quality manganite material. The RBS spectra also show that the films
have good stoichiometry.
Atomic force microscopy was used to determine the roughness of the thin films. The annealed
film (average roughness 4.5 nm) shows a surface smoother than the non-annealed
film (average roughness 5.3 nm). SEM measurements show that in the case of samples
having a high Fe content, the crystallite size varies between about 0.04 11m and 0.10 11m,
while for samples with high manganese content, the crystallinity varies between 0.03 jJ,m
and 0.06jLm. Manganites were analyzed using Bragg-Brentano (28) X-ray diffraction.
Measurements show that manganite films cannot be grown epitaxially on Si< 100 > and
MgO< 100 > single crystals due to a large lattice mismatch. In the case of SrTi03 and
LaAl03 several reflections and sharp peaks from the film can be seen, indicating reasonable
epitaxial growth. SEM measurements of the samples however show polycrystallinity.
Complete epitaxy has thus not occurred, but many grains have an epitaxial orientation.
Resistance versus temperature (the room temperature to about 100 K) in zero magnetic
field was measured for a La2CaMno.06Fe2.409 thin film and maximum resistance corresponding
to about 108 K was found. At higher temperatures the resistance decreases
as temperature increases. The manganite thin film therefore shows semiconductor behaviour.
Resistance measurements carried out at different magnetic fields (0 - 1 T) show
a small positive magnetoresistance of 0.83 %. Usually the magnetoresistance phenomenon
is measured at higher magnetic fields and this could be the reason for our low value as
well as the fact that the iron content could be too high. / AFRIKAANSE OPSOMMING: In hierdie ondersoek is die formasie en karakterisering van dunlagie manganiete
ondersoek. Om die effek van oksidasie-toestand op magnetoresistiwiteit te bepaal, is
80% van die Mn verplaas deur Fe. Pulseerde laser deposissie(3 J/cm2), is uitgevoer by
'n parsiële suurstof druk tussen 0.10 en 1.00 mbar deur gebruik te maak van La2Ca
Mn2.94Feo.o609 en La2CaMno.6Fe2.409 teiken skywe. Dunlagies was gedeponeer op
Si<IOO>, MgO<IOO>, SrTi03<100> en LaAl03<100> enkelkristal substrate. Die
dunlagies is daarna ge-karakteriseer met behulp van Rutherford terugverstrooing
(RBS), atoom krag mikroskopie(AFM), skandeer elektronmikroskopie (SEM) en xstraal
diffraksie(XRD). Elektriese metings is ook uitgevoer.
Een van die hoof tegnieke wat gebruik is in hierdie ondersoek is Rutherford
terugverstrooing (RBS) van 2 Mev alfa-deeltjies. In hierdie navorsing is aangetoon
dat RBS saam met spektra simulasie(RUMP), 'n besondere kragtige metode is om die
stoichiometrie en dikte van manganiet lagies te bepaal. Die rol van die opname van
suurstof in die dunlagies was ondersoek, aangesien die oksidasie toestand van
manganiet lagies 'n groot effek het op hulle magnetoresistiwiteit. Die hoogte van die
La sein is gebruik as 'n maatstaf van suurstof inhoud. RBS spektra van dunlagies
gevorm op enkelkristal silikon substrate by verskillende parsiële suurstof drukke wys
dat die passing tussen gemete en gesimuleerde spektra verbeter by hoër suurstof
drukke, wat beter kwaliteit manganiet materiaal aandui. Die RBS spektra het ook
aangetoon dat die stoichiometrie van die lagies uitstekend is.
Atoom krag mikroskopie(AFM) is gebruik om die grofheid van die oppervlaktes van
die dunlagies te bepaal. Lagies wat by 750 grade celsius uitgegloei is ( gemiddelde
gladheid van 4.5 nm) was gladder as films wat nie na ablasie uitgegloei is nie
(gemiddelde gladheid van 5.3 nm). SEM metings toon ook dat dunlagies met 'n hoë
Fe inhoud 'n kristalliet deursnit het van 0.04 tot 0.10 mikrometer en die met 'n hoë
mangaan inhoud 'n poli-kristalliniteit het van tussen 0.03 en 0.06 mikrometer het.
Bragg-Brentano(twee-theta) X-straal diffraksie meting wys dat manganiet films nie
epitaksieël op Si<IOO> en MgO<IOO> enkelkristal substrate gevorm kan word nie,
weens 'n groot verskil in die kristal-rooster parameters. SEM metings van die
monsters wys polikristalliniteit. Algehele epitaksie het dus nie plaasgevind nie, maar
verskeie kristalliete het 'n epitaksiële orientasie. Weerstand metings is gemaak by
temperature so laag as 100 Kelvin vir La2CaMno.o6Fe2.409dunlagies en 'n maksimum
weerstand is by 108 Kelvin gevind. By hoër temperature het die weerstand afgeneem
soos die temperatuur toeneem, wat halfgeleier gedrag aandui. Weerstand metings by
verskillende magneetvelde (0 tot I Tesla) wys 'n klein magnetoresistiwiteits effek van
0.83%. Gewoonlik word magnetoresistiwiteit gemeet by hoë magneet velde (ongeveer
6 Tesla). Dit, sowel as die hoë Fe samestelling van die monsters kan die rede wees vir
die lae magnetoresistiwiteit wat waargeneem word.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52852 |
| Date | 12 1900 |
| Creators | Nsengiyumva, Schadrack |
| Contributors | Pretorius, R., Stellenbosch University. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 132 pages : illustrations |
| Rights | Stellenbosch University |
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