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Direct demonstration of decoupling of spin and charge currents in nanostructures
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0003-2339-1692
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0001-8534-6577
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.

Place, publisher, year, edition, pages
2006. Vol. 6, no 4, 871-874 p.
Keyword [en]
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: urn:nbn:se:kth:diva-7470DOI: 10.1021/nl052075cISI: 000236916200054PubMedID: 16608301Scopus ID: 2-s2.0-33646419003OAI: oai:DiVA.org:kth-7470DiVA: diva2:12503
Note
QC 20100813Available from: 2007-09-10 Created: 2007-09-10 Last updated: 2010-09-24Bibliographically approved
In thesis
1. Spin transport in normal and superconducting nanowires
Open this publication in new window or tab >>Spin transport in normal and superconducting nanowires
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:nbn:se:kth:diva-4486 (URN)978-91-7178-750-7 (ISBN)
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
2. Spin-dependant transport in lateral nano-devices based on magnetic tunnel junctions
Open this publication in new window or tab >>Spin-dependant transport in lateral nano-devices based on magnetic tunnel junctions
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This thesis is an experimental study of spin dependent transport in nanoscale ferromagnetic tunnel junction arrays and lateral multi-terminal devices with normal metal and superconducting spin transport channels.

Two-, three-, and five-junction arrays have been fabricated in the form of lateral circuits and characterized using variable temperature magneto-transport measurements. The smallest inter-junction separation achieved was 65 nm. No significant enhancement in the sequential magneto-resistance (MR) was observed, which is attributed to the combined effect of short spin diffusion length in the ferromagnetic electrodes and high resistance of the tunnel barriers used. A substantially weaker bias dependence of the MR is observed for double junctions than for single junctions, consistent with the theoretical expectations.

Spin diffusion and relaxation in one-dimensional normal metal channels is studied using a novel multi-terminal device. The device has multiple ferromagnetic detector electrodes for an in-situ determination of the spin transport parameters. Such configuration has a great advantage as it eliminates sample-to-sample uncertainties in the physical properties studied. A three terminal device having a pair of detector electrodes placed symmetrically about the injection point is used to directly demonstrate decoupling of spin and charge current in nanostructures. Furthermore, by varying the thickness of the normal metal channel on the scale of the mean free path the surface contribution to spin relaxation is measured and compared to the bulk spin scattering rate. It is found that for Al surface scattering makes a weak contribution to the overall spin relaxation rate, the result that should be important for a number of proposed thin film spin-based devices.

The interplay between non-equilibrium magnetism and superconductivity is studied in a ferromagnetic/superconductor single electron transistor. Spin imbalance in the base is controlled by the bias voltage applied to the magnetic emitter/collector as well as the relative orientation of their magnetic moments. A strong magneto-transport effect is observed and attributed to a suppression of the superconducting gap in the center electrode by the spin imbalance in the antiparallel state of the device. The intrinsic spin relaxation parameters for the center electrode, important for interpreting the data are studied in a separate experiment using spin injection into a one-dimensional superconducting channel. It is found that the spin accumulation increases substantially on transition into the superconducting state while the spin diffusion length is reduced. These results represent a new way of combining magnetism and superconductivity on the nano-scale.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. iv, 86 p.
Series
Trita-FYS, ISSN 0280-316X ; 2006:08
Keyword
Spin diffusion, magnetic tunnel junctions
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-3866 (URN)91-7178-278-8 (ISBN)
Public defence
2006-03-17, FR 4, Oskar Klein, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100924Available from: 2006-03-08 Created: 2006-03-08 Last updated: 2012-02-10Bibliographically approved

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