| 1. |
- Johansson, Jan, et al.
(author)
-
Enhanced spin accumulation in superconductors
- 2006
-
In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 99:8, s. 08M513-
-
Journal article (peer-reviewed)abstract
- 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.
|
|
| 2. |
- Johansson, Jan, et al.
(author)
-
Suppression of superconductivity due to spin imbalance in Co/Al/Co single electron transistor
- 2003
-
In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 93, s. 8650-8652
-
Journal article (peer-reviewed)abstract
- 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.
|
|
| 3. |
- Poli, Ninos, et al.
(author)
-
Spin-flip scattering at Al surfaces
- 2006
-
In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 99:8, s. 08H701-
-
Journal article (peer-reviewed)abstract
- 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.
|
|
| 4. |
- Poli, Ninos, et al.
(author)
-
Spin injection and relaxation in a mesoscopic superconductor
- 2008
-
In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 100:13, s. 136601-
-
Journal article (peer-reviewed)abstract
- 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.
|
|
| 5. |
- Urech, Mattias, et al.
(author)
-
Direct demonstration of decoupling of spin and charge currents in nanostructures
- 2006
-
In: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 6:4, s. 871-874
-
Journal article (peer-reviewed)abstract
- 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.
|
|
| 6. |
|
|
| 7. |
- Urech, Mattias, et al.
(author)
-
Magnetic switching and magnetoresistance in nanoscale spin tunnel junctions
- 2002
-
In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 92:10, s. 6062-6065
-
Journal article (peer-reviewed)abstract
- 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.
|
|
| 8. |
- Urech, Mattias, et al.
(author)
-
Magnetoresistance in Co/AlOx/Co tunnel junction arrays
- 2002
-
In: Journal of Magnetism and Magnetic Materials. - 0304-8853 .- 1873-4766. ; 249, s. 513-518
-
Journal article (peer-reviewed)abstract
- 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.
|
|
| 9. |
- Urech, Mattias, 1973-
(author)
-
Spin-dependant transport in lateral nano-devices based on magnetic tunnel junctions
- 2006
-
Doctoral thesis (other academic/artistic)abstract
- 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.
|
|
| 10. |
- Urech, Mattias
(author)
-
Spin dependent transport in Co nano-scale tunnel junctions
- 2002
-
Licentiate thesis (other academic/artistic)abstract
- Ferromagnetic tunnel junctionshave been fabricated, measured and theoretically described. Anoverview of the Julliere model, and Slonczewski's model todescribe the resistance change due to di®erent orientationof the magnetization between the magnetic electrodes(magnetoresistance) is given. Then a double-junctionconfiguration is theoretically analyzed in the sense of localspin accumulation related to the length an electron can travelbefore relaxing its spin. The analysis shows that thee®ect of spin accumulation contributes with a very smallor negligiblechange of the magnetoresistance for adouble-junction with large resistive barriers, and biased witha small voltage compared with a simple two-resistors-in-seriesmodel. Single-, triple-, and five- junction arrays werefabricated and measured. In order to analyze the spinaccumulation e®ect a double-junction with three terminalswas fabricated and measured to enable measurement of eachjunction of the double junction individually. Thedouble-junction configuration did not show any additionalmagnetoresistance over the two-junctions-in-series value at lowvoltage bias. However, the magnetoresistance of adouble-junction exhibited a weaker dependence on the biasvoltage than that expected for two junctions in series.
|
|
