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Spin transport in normal and superconducting nanowires
KTH, School of Engineering Sciences (SCI), Applied Physics.
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Todays conventional electronic devices are based on electron charge transport in semiconductor channels. Spintronics is a rapidly emerging technology, which exploits the spin degree of freedom as well as the charge of the electrons. It is believed that extending conventional electronics to spin-electronics can yield devices with new functionality and result in new large scale applications. Examples of already existing spintronic technology are the magnetic random access memory, magneto-resistive read heads in hard drives and various magnetic field sensors. The fundamental requirement for a working spintronic device is the ability to generate, transport and detect spin currents, which are the subject of this thesis.

A current, spin polarized by a ferromagnet and injected into a non-magnetic material remains polarized for the duration of the spin relaxation time. This relaxation time, and consequently the useful distance the injected non-equilibrium spin can be transported in the non-magnetic transport channel, is dependent on the underlying spin relaxation mechanisms in the material. Furthermore, the transport channel can be deviced to exploit the spin-orbit scattering within the channel with the aim to achieve novel spin transport effects, such as the Spin Hall effect. We study such mechanisms and effects in normal and superconducting nanowires. The main results of the work are the following:

In thin film devices, the thickness of the electron transport channel can be comparable to the electron's mean free path, which makes the surface scattering the dominant scattering mechanism. To investigate how the additional surface momentum scattering affects spin relaxation, the thickness dependence of the spin relaxation parameters was analyzed. Using spin injection into Al nanowires of various thickness, it was found that the spin flip scattering at the surfaces is substantially weaker compared to that within the bulk of Al.

A five terminal device having a pair of spin sensitive detector electrodes placed symmetrically about the injection point was used to directly demonstrate the decoupling of spin and charge currents in a one-dimensional transport channel. The spin accumulation is shown to be strictly symmetric about the injection point and independent of the direction of the charge current.

For superconducting nanowires, it is found that the spin accumulation is enhanced by up to 5 orders of magnitude compared to that in the normal state of the wire. In contrast, the spin diffusion length is found to decrease by an order of magnitude on transition in to the superconducting state. This is interpreted as due to magnetic impurity rather than spin-orbit dominated spin-flip scattering in the nanowires studied. We additionally observe a giant spin Hall effect in superconductors, which is more than 5 orders of magnitude stronger than the values reported recently for Al nanowires in the normal state.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , v, 82 p.
Series
Trita-FYS, ISSN 0280-316X ; 2007:65
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-4486ISBN: 978-91-7178-750-7 (print)OAI: oai:DiVA.org:kth-4486DiVA: diva2:12507
Public defence
2007-09-28, FR32, Albanova, Roslagstullsbacken 21, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20100813Available from: 2007-09-10 Created: 2007-09-10 Last updated: 2011-10-19Bibliographically approved
List of papers
1. Spin-flip scattering at Al surfaces
Open this publication in new window or tab >>Spin-flip scattering at Al surfaces
2006 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 99, no 8, 08H701- p.Article in journal (Refereed) Published
Abstract [en]

Nonlocal measurements are performed on a multiterminal device to in situ determine the spin-diffusion length and in combination with resistivity measurements also the spin-relaxation time in Al films. By varying the thickness of Al we determine the contribution to spin relaxation from surface scattering. From the temperature dependence of the spin-diffusion length it is established that the spin relaxation is impurity dominated at low temperature. A comparison of the spin- and momentum-relaxation lengths for different thicknesses reveals that the spin-flip scattering at the surfaces is weak compared to that within the bulk of the Al films.

Keyword
Aluminum; Diffusion; Electronic equipment; Scattering; Surface treatment; Thermal effects; Al films; Spin-flip scattering; Spin-relaxation time; Surface scattering; Thin films
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-7469 (URN)10.1063/1.2159395 (DOI)000237404200426 ()2-s2.0-33646782968 (Scopus ID)
Note
QC 20100813Available from: 2007-09-10 Created: 2007-09-10 Last updated: 2010-08-13Bibliographically approved
2. Direct demonstration of decoupling of spin and charge currents in nanostructures
Open this publication in new window or tab >>Direct demonstration of decoupling of spin and charge currents in nanostructures
2006 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 6, no 4, 871-874 p.Article in journal (Refereed) Published
Abstract [en]

The notion of decoupling of spin and charge currents is one of the basic principles underlying the rapidly expanding field of spintronics. However, no direct demonstration of the phenomenon exists. We report a novel measurement in which a nonequilibrium spin population is created by a pointlike injection of current from a ferromagnet across a tunnel barrier into a one-dimensional spin channel and detected differentially by a pair of ferromagnetic electrodes placed symmetrically about the injection point. We demonstrate that the spin current is strictly isotropic about the injection point and, therefore, completely decoupled from the unidirectional charge current.

Keyword
Electric charge; Electric currents; Electrodes; Ferromagnetic materials; Nanostructures; Spin current; Spintronics; Nanostructured materials; nanomaterial; article; chemical model; chemistry; computer simulation; electricity; electrochemistry; electromagnetic field; evaluation; impedance; materials testing; methodology; radiation exposure; spin labeling; Computer Simulation; Electric Impedance; Electrochemistry; Electromagnetic Fields; Electrostatics; Materials Testing; Models, Chemical; Nanostructures; Spin Labels
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-7470 (URN)10.1021/nl052075c (DOI)000236916200054 ()16608301 (PubMedID)2-s2.0-33646419003 (Scopus ID)
Note
QC 20100813Available from: 2007-09-10 Created: 2007-09-10 Last updated: 2010-09-24Bibliographically approved
3. Enhanced spin accumulation in superconductors
Open this publication in new window or tab >>Enhanced spin accumulation in superconductors
2006 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 99, no 8, 08M513- p.Article in journal (Refereed) Published
Abstract [en]

A lateral array of ferromagnetic tunnel junctions is used to inject and detect nonequilibrium quasiparticle spin distribution in a superconducting strip made of Al. The strip width and thickness are kept below the quasiparticle spin diffusion length in Al. Nonlocal measurements in multiple parallel and antiparallel magnetic states of the detectors are used to in situ determine the quasiparticle spin diffusion length. A very large increase in the spin accumulation in the superconducting state compared to that in the normal state is observed and is attributed to a diminishing of the quasiparticle population by the opening of the gap below the transition temperature.

Keyword
Aluminum; Diffusion; Ferromagnetic materials; Quasicrystals; Superconducting transition temperature; Tunnel junctions; Antiparallel magnetic states; Quasiparticle population; Quasiparticle spin diffusion; Spin accumulation; Superconducting materials
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-5421 (URN)10.1063/1.2176910 (DOI)000237404200555 ()2-s2.0-33646807508 (Scopus ID)
Note
Uppdaterad från accepted till published: 20100924. QC 20100924Available from: 2006-03-08 Created: 2006-03-08 Last updated: 2017-11-21Bibliographically approved
4. Spin injection and relaxation in a mesoscopic superconductor
Open this publication in new window or tab >>Spin injection and relaxation in a mesoscopic superconductor
Show others...
2008 (English)In: Physical Review Letters, ISSN 0031-9007, Vol. 100, no 13, 136601- p.Article in journal (Refereed) Published
Abstract [en]

We study spin transport in a superconducting nanowire using a set of closely spaced magnetic tunnel contacts. We observe a giant enhancement of the spin accumulation of up to 5 orders of magnitude on transition into the superconducting state, consistent with the expected changes in the density of states. The spin relaxation length decreases by an order of magnitude from its value in the normal state. These measurements, combined with our theoretical model, allow us to distinguish the individual spin-flip mechanisms present in the transport channel. Our conclusion is that magnetic impurities rather than spin-orbit coupling dominate spin-flip scattering in the superconducting state.

Keyword
Density (specific gravity); Nanowires; Scattering; Spin dynamics; Density of states; Magnetic tunnel contacts; Superconducting materials
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-7472 (URN)10.1103/PhysRevLett.100.136601 (DOI)000254670300061 ()2-s2.0-41749096336 (Scopus ID)
Note
QC 20100813. Uppdaterad från Submitted till Published 20100813.Available from: 2007-09-10 Created: 2007-09-10 Last updated: 2010-08-13Bibliographically approved
5. Giant spin Hall effect in a mesoscopic superconductor
Open this publication in new window or tab >>Giant spin Hall effect in a mesoscopic superconductor
(English)Manuscript (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-7473 (URN)
Note
QC 20100813Available from: 2007-09-10 Created: 2007-09-10 Last updated: 2010-08-13Bibliographically approved

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