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Electrical spin injection and transport in Germanium  [PDF]
Yi Zhou,Wei Han,Li-Te Chang,Faxian Xiu,Minsheng Wang,Michael Oehme,Inga A. Fischer,Joerg Schulze,Roland. K. Kawakami,Kang L. Wang
Physics , 2011, DOI: 10.1103/PhysRevB.84.125323
Abstract: We report the first experimental demonstration of electrical spin injection, transport and detection in bulk germanium (Ge). The non-local magnetoresistance in n-type Ge is observable up to 225K. Our results indicate that the spin relaxation rate in the n-type Ge is closely related to the momentum scattering rate, which is consistent with the predicted Elliot-Yafet spin relaxation mechanism for Ge. The bias dependence of the nonlocal magnetoresistance and the spin lifetime in n-type Ge is also investigated.
Electrical and thermal spin accumulation in germanium  [PDF]
A. Jain,C. Vergnaud,J. Peiro,J. C. Le Breton,E. Prestat,L. Louahadj,C. Portemont,C. Ducruet,V. Baltz,A. Marty,A. Barski,P. Bayle-Guillemaud,L. Vila,J. -P. Attané,E. Augendre,H. Jaffrès,J. -M. George,M. Jamet
Physics , 2012, DOI: 10.1063/1.4733620
Abstract: In this letter, we first show electrical spin injection in the germanium conduction band at room temperature and modulate the spin signal by applying a gate voltage to the channel. The corresponding signal modulation agrees well with the predictions of spin diffusion models. Then by setting a temperature gradient between germanium and the ferromagnet, we create a thermal spin accumulation in germanium without any tunnel charge current. We show that temperature gradients yield larger spin accumulations than pure electrical spin injection but, due to competing microscopic effects, the thermal spin accumulation in germanium remains surprisingly almost unchanged under the application of a gate voltage to the channel.
Lateral Spin Injection in Germanium Nanowires  [PDF]
En-Shao Liu,Junghyo Nah,Kamran M. Varahramyan,Emanuel Tutuc
Physics , 2010, DOI: 10.1021/nl1008663
Abstract: Electrical injection of spin-polarized electrons into a semiconductor, large spin diffusion length, and an integration friendly platform are desirable ingredients for spin-based devices. Here we demonstrate lateral spin injection and detection in germanium nanowires, by using ferromagnetic metal contacts and tunnel barriers for contact resistance engineering. Using data measured from over 80 samples, we map out the contact resistance window for which lateral spin transport is observed, manifestly showing the conductivity matching required for spin injection. Our analysis, based on the spin diffusion theory, indicates that the spin diffusion length is larger than 100 {\mu}m in germanium nanowires at 4.2 K.
Crossover from spin accumulation into interface states to spin injection in the germanium conduction band  [PDF]
A. Jain,J. -C. Rojas-Sanchez,M. Cubukcu,J. Peiro,J. C. Le Breton,E. Prestat,C. Vergnaud,L. Louahadj,C. Portemont,C. Ducruet,V. Baltz,A. Barski,P. Bayle-Guillemaud,L. Vila,J. -P. Attané,E. Augendre,G. Desfonds,S. Gambarelli,H. Jaffrès,J. -M. George,M. Jamet
Physics , 2012, DOI: 10.1103/PhysRevLett.109.106603
Abstract: Electrical spin injection into semiconductors paves the way for exploring new phenomena in the area of spin physics and new generations of spintronic devices. However the exact role of interface states in spin injection mechanism from a magnetic tunnel junction into a semiconductor is still under debate. In this letter, we demonstrate a clear transition from spin accumulation into interface states to spin injection in the conduction band of $n$-Ge. We observe spin signal amplification at low temperature due to spin accumulation into interface states followed by a clear transition towards spin injection in the conduction band from 200 K up to room temperature. In this regime, the spin signal is reduced down to a value compatible with spin diffusion model. More interestingly, we demonstrate in this regime a significant modulation of the spin signal by spin pumping generated by ferromagnetic resonance and also by applying a back-gate voltage which are clear manifestations of spin current and accumulation in the germanium conduction band.
Surface Chemistry and Electrical Properties of Germanium Nanowires  [PDF]
Dunwei Wang,Ying-Lan Chang,Qian Wang,Jien Cao,Damon Farmer,Roy Gordon,Hongjie Dai
Physics , 2004,
Abstract: Germanium nanowires with p- and n-dopants were synthesized by chemical vapor deposition and used to construct complementary field effect transistors . Electrical transport and x-ray photoelectron spectroscopy data are correlated to glean the effects of Ge surface chemistry to the electrical characteristics of GeNWs. Large hysteresis due to water molecules strongly bound to GeO2 on GeNWs is revealed. Different oxidation behavior and hysteresis characteristics and opposite band bending due to Fermi level pinning by interface states between Ge and surface oxides are observed for p- and n-type GeNWs. Vacuum annealing above 400C is used to remove surface oxides and eliminate hysteresis in GeNW FETs. High-k dielectric HfO2 films grown on clean GeNW surfaces by atomic layer deposition (ALD) using an alkylamide precursor is effective serving as the first layer of surface passivation. Lastly, the depletion length along the radial direction of nanowires is evaluated. The result suggests that surface effects could be dominant over the bulk properties of small diameter wires.
Electrical spin injection and detection in a semiconductor. Is it feasible?  [PDF]
A. T. Filip,B. H. Hoving,F. J. Jedema,B. J. van Wees
Physics , 2000,
Abstract: The electrical injection of spin polarized electrons in a semiconductor can be achieved in principle by driving a current from a ferromagnetic metal, where current is known to be significantly spin polarized, into the semiconductor via ohmic conduction. For detection a second ferromagnet can be used as drain. We studied submicron lateral spin valve junctions, based on high mobility InAs/AlSb two-dimensional electron gas (2DEG), with Ni, Co and Permalloy as ferromagnetic electrodes. In the standard geometry it is very difficult to separate true spin injection from other effects, including local Hall effect, anomalous magnetoresistance (AMR) contribution from the ferromagnetic electrodes and weak localization/anti-localization corrections, which can closely mimic the signal expected from spin valve effect. The reduction in size, and the use of a multiterminal non-local geometry allowed us to reduce the unwanted effects to a minimum. Despite all our efforts, we have not been able to observe spin injection. However, we find that this 'negative' result in these systems is actually consistent with theoretical predictions for spin transport in diffusive systems.
Dynamical Spin Injection into p-type Germanium at Room Temperature  [PDF]
Mariko Koike,Eiji Shikoh,Yuichiro Ando,Teruya Shinjo,Shinya Yamada,Kohei Hamaya,Masashi Shiraishi
Physics , 2012, DOI: 10.7567/APEX.6.023001
Abstract: We demonstrate dynamical spin injection into p-type germanium (Ge) at room temperature (RT) using spin pumping. The generated pure spin current is converted to a charge current by the inverse spin-Hall effect (ISHE) arising in the p-type Ge sample. A clear electromotive force due to the ISHE is detected at RT. The spin-Hall angle for p-type Ge is estimated to be {\theta}SHE = 2.6x10-3 at RT, which is much larger than that for p-type Si.
Spin injection, accumulation and spin precession in a mesoscopic non-magn etic metal island  [PDF]
M. Zaffalon,B. J. van Wees
Physics , 2004, DOI: 10.1103/PhysRevB.71.125401
Abstract: We experimentally study spin accumulation in an aluminium island with all dimensions smaller than the spin relaxation length, so that the spin imbalance throughout the island is uniform. Electrical injection and detection of the spin accumulation are carried out in a four terminal geometry by means of four cobalt electrodes connected to the island through tunnel barriers. We model the system theoretically and we investigate the role of the ferromagnetic electrodes on the spin accumulation at the limit at which the electron diffusion time can be neglected. We present measurements of spin accumulation at room temperature and at 4.2 K: in both cases the spin accumulation signal is larger than the Ohmic resistance of the aluminium island. From magnetisation precession measurements at room temperature, we extract a spin relaxation time of 60 ps and a polarisation P = 8% for tunnel barriers with resistances as low as 20 \Omega\mu m^2.
Spin injection and spin accumulation in permalloy-copper mesoscopic spin valves  [PDF]
F. J. Jedema,M. S. Nijboer,A. T. Filip,B. J. van Wees
Physics , 2001,
Abstract: We study the electrical injection and detection of spin currents in a lateral spin valve device, using permalloy (Py) as ferromagnetic injecting and detecting electrodes and copper (Cu) as non-magnetic metal. Our multi-terminal geometry allows us to experimentally distinguish different magneto resistance signals, being 1) the spin valve effect, 2) the anomalous magneto resistance (AMR) effect and 3) Hall effects. We find that the AMR contribution of the Py contacts can be much bigger than the amplitude of the spin valve effect, making it impossible to observe the spin valve effect in a 'conventional' measurement geometry. However, these 'contact' magneto resistance signals can be used to monitor the magnetization reversal process, making it possible to determine the magnetic switching fields of the Py contacts of the spin valve device. In a 'non local' spin valve measurement we are able to completely isolate the spin valve signal and observe clear spin accumulation signals at T=4.2 K as well as at room temperature. We obtain spin diffusion lengths in copper of 1 micrometer and 350 nm at T=4.2 K and room temperature respectively.
Comment: "Electrical injection and detection of spin accumulation in silicon at 500 K with magnetic metal / silicon dioxide contacts" [Nature Commun. 2:245 doi:10.1038/ncomms125 (2011)]  [PDF]
C. H. Li,O. M. J. van 't Erve,B. T. Jonker
Physics , 2011, DOI: 10.1038/ncomms125
Abstract: In a recent publication, we demonstrated electrical spin injection and detection in n-type silicon at temperatures up to 500K using ferromagnetic metal / SiO2 tunnel barrier contacts in a three-terminal geometry (Nature Commun. 2:245 doi:10.1038/ncomms125 (2011)). In comparing our measured spin-voltage signal with the value predicted by theory, we followed the analysis of Tran et al, (Phys. Rev. Lett. 102, 036601 (2009)), and inadvertently propagated an error found therein. As they note in a recent erratum (arXiv:0810.4770v2), the correct expression for the spin resistance area product from the theory for a sample with a spin diffusion length LSD much less than the contact width or channel thickness (our experimental situation) is given by the product {gamma}^2 {rho} LSD, where {gamma} is the tunneling spin polarization, and {rho} is the resistivity of the semiconductor transport channel. With this correction, our measured spin voltages are much larger than those predicted by theory, rather than in good agreement as we stated. We emphasize that the basic conclusions of our paper are the same - the systematic decrease in electron spin lifetime with increasing electron density demonstrates spin accumulation in the Si channel rather than in interface states. We further show that the measured spin lifetimes are essentially independent of the tunnel barrier material (SiO2, Al2O3, MgO) or the magnetic metal used (Fe, CoFe, NiFe), demonstrating clear correlation of the measured spin lifetime with the character of the Si, and little correlation with the tunnel barrier / interface or magnetic metal.
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