Formation and characterization of pulsed laser ablated magnetoresistive material

[摘要] ENGLISH ABSTRACT: In this investigation the formation of thin film manganites and their electrical characteristicsis 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 oxygenpartial pressures ranging from 0.01 mbar to 1.00 mbar was used to fabricate the thin filmsfrom 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 BackscatteringSpectrometry (RBS). It has been shown that RBS is a very powerful characterizationtechnique when used in conjunction with the RUMP simulation program. The effectof various parameters can be determined beforehand by RUMP simulation of the thinfilm structures to be investigated. Simulation shows that RBS is an excellent characterizationtool for determining film thickness and stoichiometry. The role of oxygen uptakein La2CaMn3_xFexOg was investigated as the oxidation states of elements in manganitematerials have a large effect on their magnetoresistive properties. The height of the Lasignal can be used as a measure of the oxygen content. RBS spectra of films deposited onsingle crystal silicon substrates at different ambient pressures show that the fit betweensimulated and measured RBS spectra improves with higher oxygen pressures, therebyindicating better quality manganite material. The RBS spectra also show that the filmshave good stoichiometry.Atomic force microscopy was used to determine the roughness of the thin films. The annealedfilm (average roughness 4.5 nm) shows a surface smoother than the non-annealedfilm (average roughness 5.3 nm). SEM measurements show that in the case of sampleshaving 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,mand 0.06jLm. Manganites were analyzed using Bragg-Brentano (28) X-ray diffraction.Measurements show that manganite films cannot be grown epitaxially on Si< 100 > andMgO< 100 > single crystals due to a large lattice mismatch. In the case of SrTi03 andLaAl03 several reflections and sharp peaks from the film can be seen, indicating reasonableepitaxial 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 magneticfield was measured for a La2CaMno.06Fe2.409 thin film and maximum resistance correspondingto about 108 K was found. At higher temperatures the resistance decreasesas temperature increases. The manganite thin film therefore shows semiconductor behaviour.Resistance measurements carried out at different magnetic fields (0 - 1 T) showa small positive magnetoresistance of 0.83 %. Usually the magnetoresistance phenomenonis measured at higher magnetic fields and this could be the reason for our low value aswell as the fact that the iron content could be too high.

[发布机构] Stellenbosch University