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Spin-dependant transport in lateral nano-devices based on magnetic tunnel junctions
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
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 [en]
Spin diffusion, magnetic tunnel junctions
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-3866ISBN: 91-7178-278-8 (print)OAI: oai:DiVA.org:kth-3866DiVA: diva2:9783
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
List of papers
1. Magnetoresistance in Co/AlOx/Co tunnel junction arrays
Open this publication in new window or tab >>Magnetoresistance in Co/AlOx/Co tunnel junction arrays
2002 (English)In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 249, 513-518 p.Article in journal (Refereed) Published
Abstract [en]

Lateral arrays of Co/AlOx/Co junctions with dimensions down to 60 nin and inter-junction separations similar to60-100 nm have been fabricated and analyzed for possible coherent tunneling effects. Extra attention is paid to avoid uncertainties due to inconsistencies in switching and/or resistance of successive barriers. We observe similar to10% magnetoresistance enhancement at moderate bias in double junctions that cannot be accounted for by a simple model of two resistsors in series.

Keyword
magnetoresistance, magnetic nanostructure, tunnel junction array, Co, AlOx
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-5414 (URN)10.1016/S0304-8853(02)00457-2 (DOI)000179065800022 ()
Note
QC 20100924Available from: 2006-03-08 Created: 2006-03-08 Last updated: 2017-11-21Bibliographically approved
2. Magnetic switching and magnetoresistance in nanoscale spin tunnel junctions
Open this publication in new window or tab >>Magnetic switching and magnetoresistance in nanoscale spin tunnel junctions
2002 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 92, no 10, 6062-6065 p.Article in journal (Refereed) Published
Abstract [en]

Co/AlOx/Co magnetic tunnel junctions in both multijunction arrays and double-tunnel junction geometries have been studied. The junctions exhibit magnetoresistance (MR) and change their resistance by similar to10% depending on the relative magnetic orientation of the tunnel junction electrodes. MR measurements show a strong dependence on the device geometry. We find that it is necessary to form tunnel junctions with electrode width similar to70 nm for the magnetic switching at the tunnel junction to be clean and single domain like.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-5415 (URN)10.1063/1.1515099 (DOI)000178987200074 ()
Note
QC 20100924Available from: 2006-03-08 Created: 2006-03-08 Last updated: 2017-11-21Bibliographically approved
3. Spin injection in ferromagnet-superconductor/ normal-ferromagnet structures
Open this publication in new window or tab >>Spin injection in ferromagnet-superconductor/ normal-ferromagnet structures
2004 (English)In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 272-276/ Suppl, 1469-1470 p.Article in journal (Refereed) Published
Abstract [en]

We have fabricated multi-terminal devices consisting of three parallel ferromagnetic ( FM) electrodes with tunnel junction contacts to a 100 nm wide Al strip. A spin polarized current injected from one FM electrode into the Al strip can be detected at the other FM electrodes. The spin diffusion length and polarization in Al are measured at room temperature (RT), and at 4 K: No magnetoresistance (MR) over two junctions in series could be measured with the strip in the normal state. A weak MR is observed in the superconducting (S) state.

Keyword
spin injection, magnetoresistance
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-5416 (URN)10.1016/j.jmmm.2003.12.408 (DOI)000202897200568 ()2-s2.0-10944228938 (Scopus ID)
Note
QC 20100924Available from: 2006-03-08 Created: 2006-03-08 Last updated: 2017-11-21Bibliographically approved
4. 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
5. 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
6. Suppression of superconductivity due to spin imbalance in Co/Al/Co single electron transistor
Open this publication in new window or tab >>Suppression of superconductivity due to spin imbalance in Co/Al/Co single electron transistor
2003 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 93, 8650-8652 p.Article in journal (Refereed) Published
Abstract [en]

Transport properties of ferromagnetic/nonmagnetic/ferromagnetic single electron transistors are investigated as a function of external magnetic-field, temperature, bias, and gate voltage. By designing the magnetic electrodes to have different switching fields, a two-mode device is realized having two stable magnetization states, with the electrodes aligned in parallel and antiparallel. Magnetoresistance of approximately 100% is measured in Co/AlOX/Al/AlOX/Co double tunnel junction spin valves at low bias, with the Al spacer in the superconducting state. The effect is substantially reduced at high bias and temperatures above the T-C of the Al. The experimental results are interpreted as due to spin imbalance of charge carriers resulting in suppression of the superconducting gap of the Al island.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-5419 (URN)10.1063/1.1556284 (DOI)000182822600333 ()
Note
QC 20100924Available from: 2006-03-08 Created: 2006-03-08 Last updated: 2017-11-21Bibliographically approved
7. Evidence for Suppression of Superconductivity by Spin Imbalance in Co-Al-Co Single-Electron Transistors
Open this publication in new window or tab >>Evidence for Suppression of Superconductivity by Spin Imbalance in Co-Al-Co Single-Electron Transistors
2003 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 91, no 14, 149701- p.Article in journal (Refereed) Published
Keyword
junctions, magnetoresistance, valve
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-5420 (URN)10.1103/PhysRevLett.91.149701 (DOI)000185719500058 ()
Note
QC 20100924Available from: 2006-03-08 Created: 2006-03-08 Last updated: 2017-11-21Bibliographically approved
8. 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

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Output format
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