Spin based electronics or spintronics is a field having the electron's spinuddegree of freedom as a subject. It is about how to write, transfer and readudinformation using the electron spin. The birth of spintronics is consideredudto be the discovery of the giant magnetoresistance (GMR) in 1988 [1] andudsince then a major progress has been achieved in the field [2, 3]. The bestudexample of this progress is the development of so called spin-valves. Modernudday spin-valves are based on the GMR and they are used for measuring smalludmagnetic fields. Their most common application is as sensors in hard diskudreading heads.udSpintronics can conceptually be divided in two parts. The first one isudabout generating and detecting spin polarized electrons, which is normallyuddone using ferromagnetic materials, but can also be done using optical methodsud[3]. The latter part is about coherent transfer of spin information. Itudis of fundamental importance to understand how spin infomation can beudtransfered coherently over larger distances.udIn recent years new nanoscale allotropes of carbon have been discovered.udIn 1985 the first fullerene, the buckyball was discovered [4] and 1991 carbonudnanotubes (CNT) were discovered by Sumio Iijima [5]. CNTs behave as onedimensionaludconductors and the coherence length of the electron in them isudvery long, especially in individual SWCNT, where the electrons have beenudfound to be coherent over the distance of 3 �m [6]. Moreover, carbon is believedudto have long spin coherence length, due to low spin orbit coupling andudno nuclear spin of its main isotope 12C . This all makes CNTs an interestingudplatform for spin transport studies.udThe first work on CNT spin-valve devices was done on multiwall carbonudnanotubes (MWCNTs) contacted by Co electrodes [7]. By applying magneticudfield to the device the magnetization of the Co electrodes can be changed betweenudparallel and antiparallel mutual orientation. The resistance for parallelududand antiparallel mutual orientation, RP and RA respectively, are measuredudand the TMR, which is defined as followsudTMR = (RA - RP)/udis calculated. The TMR of this first CNT spin-valve was 9% at maximumudand it was positive (i.e RA > RP ) [7, 8].udNegative TMR signal was later measured in similar devices, i.e. MWCNTsudcontacted with Co electrodes. The maximal size of the TMR signal in theseuddevices was 36% for a low current bias, but higher current bias resulted inudlower TMR signals [9]. The origin of the di�erent sign of the TMR was notudclear by then.udThe first CNT spin devices fabricated in our lab wereMWCNTs contactedudby Pd1-xNix (x ~ 0:7) 1. These ferromagnetic contacts were transparent,udhaving room temperature resistance of 5:6 k[omega]. What was new about theseuddevices was that they were equipped with a back gate and could be tunedudbetween di�erent transport regimes [10]. More importantly it was shown thatudTMR was dependent on the back gate voltage [11]. Further studies revealedudthat the TMR signal was either negative or positive dependent on appliedudgate voltage, but the origin of this behavior was not well understood [12].udWhen the signal changes in TMR were studied single wall carbon nanotubesud(SWCNT) grown in-house by chemical vapor deposition (CVD) usingudmethane as a carbon source became available. The CVD growing processudhad been optimize to produce individual SWCNT [13]. Individual CVDudgrown SWCNTs were connected with PdNi contacts. In such device it wasudshown that the TMR signal was correlated with the coulomb oscillations ofudthe quantum dot which is formed in the SWCNT between the contacts. InudSWCNT the quantum dot behavior is much simpler than in MWCNT andudthe TMR could be tuned smoothly from positive to negative values by theudgate voltage [12, 14]. This work demonstrated for the first time the controludof spin transport in a three terminal device.udThere are still many open questions concerning SWCNT spin devices.udThere are mainly two issues that one should be concerned about when constructinguda SWCNT spin valve device. The first one is the switching characteristicsudof the electrodes. The switching in the devices contacted with PdNiudcontacts is not always clear indicating that the electrode consists of manyudmagnetic domains.udThe latter one is due to spurious effects in the SWCNT spin-valves. Suchudeffects could be magneto-coulomb effect [15] or tunnelling anisotropic magnetoresistanceud(TAMR). Spurious effects could cause a "false TMR signal",ududi.e. a switching behavior in the signal as a function of applied field that thatuddoes not originate from transport of spin.udThe focus of the this work was mainly to address these issues but someudwork was also done on how to process of individual SWCNT devices. PdNiudelectrodes were studied in order to understand their switching behavior better.udWe worked to optimize the switching characteristics of the spin-valveuddevices, by trying other contact materials on the SWCNTs.udOne way of avoiding spurious e�ects is to make multi-terminal devices.udIt has been shown in metallic nanostructures that by measuring non-localudspin signals, artefacts can be avoided. Non-local spin transport measurementsudhave been done on SWCNT contacted by four Co contacts [16]. Theudmultiterminal devices made in this work have two normal contacts and twoudferromagnetic contacts. They are gateable with a back-gate enabling it toudstudy the behavior of the three quantum dots that are formed in each segmentudof the tube between the contacts.udOutline of this thesisud- Chapter 2 is on the basics of spintronics. It includes a short descriptionudon ferromagnetism and on anisotropic magnetoresistance (AMR)udand for historical resons giant magnetoresistance (GMR) is briefly discussed.udThe tunnelling magnetoresistance is explained and Julliére'sudmodel.ud- Chapter 3 is on carbon nanotubes. It is focused on single wall carbonudnanotubes (SWCNT), their structure and their electronic properties.ud- Chapter 4 is on processing of SWCNT devices. The first part of theudchapter is on SWCNT production and characterization of the SWCNTudmaterial. A lot of time was invested in the lab in finding the best wayudto obtain individual SWCNT for our nanotube project. Both mainudapproaches tested, i.e spreading tubes from suspension solution andudCVD growth are described. In the latter part it is generally describedudhow to make SWCNT devices.ud- Chapter 5 is on SWCNT based spin valves. The idea behind theudSWCNT is discussed (the statement of the problem) and then measurementsudusing different ferromagnetic contact materials are discussed.udTemperature dependence on TMR in SWCNT is discussed in the lastudsection of the chapter.ud- Chapter 6 is on measurements on multiterminal devices. Non-localudand semi-nonlocal measurements are shown and discussed.ud- Chapter 7 is a summary of the thesis.udDetails on experimental setups and recipes can be found in appendices.udud
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