Electrical Spin Injection into High Mobility 2D Systems

Abstract

We report on spin injection into a high mobility 2D electron system confined at an ðAl; GaÞAs=GaAs interface, using (Ga,Mn)As Esaki diode contacts as spin aligners. We measured a clear nonlocal spin valve signal, which varies nonmonotonically with the applied bias voltage. The magnitude of the signal cannot be described by the standard spin drift-diffusion model, because at maximum this would require the spin polarization of the injected current to be much larger than 100%, which is unphysical. A strong correlation of the spin signal with contact width and electron mean free path suggests that ballistic transport in the 2D region below ferromagnetic contacts should be taken into account to fully describe the results. DOI: 10.1103/PhysRevLett.113.236602 PACS numbers: 72.25.Dc, 72.25.Hg, 72.25.Mk, 73.40.Kp All-electrical spin injection and detection, one of the key ingredients for functional spintronics devices, has been successfully realized in bulk semiconductors like GaAs [1,2] and Si [3,4]. To implement the spin-transistor functionality, proposed by Datta and Das [5], one needs the capacity to controllably rotate the spin of electrons on the way from source to drain. It has been suggested [5] to employ Rashba spin-orbit interaction, due to which the k vector of an electron in a two-dimensional electron gas (2DEG) is connected to an effective magnetic field, which is in turn tunable by an electric field. Thus, the precession angle in a sufficiently narrow channel can be coherently controlled by a gate voltage, provided transport between source and drain is ballistic. Hence, high mobility 2D systems are needed for these experiments. Although spin injection into graphene, a truly 2D system, has already been realized [6,7], no experiments exist in the ballistic regime where the injector or detector widths or their separation is significantly smaller than the electron mean free path l mf . Spin injection into semiconductor-based 2DEGs turns out to be difficult and only a few reports are available [8][9][10][11]. The analysis of experiments is also complicated by the lack of a corresponding ballistic theory. Although different aspects of ballistic spin injection and spin transport have been discussed theoretically [12][13][14][15][16][17], there is no comprehensive theory that would allow us to describe the experimental outcome in this regime in a way in which the spin drift-diffusion theory describes experiments on devices with bulk 3D channels [1,2].In this Letter we provide clear evidence of spin injection into a high mobility 2DEG confined in an inverted ðAl; GaÞAs=GaAs heterojunction, employing the ferromagnetic (FM) semiconductor (Ga,Mn)As in an Esaki diode configuration [18-21] as a spin aligner. We observed a dramatically enhanced nonlocal spin valve (NLSV) signal at negatively biased Esaki junctions when electrons can tunnel directly from (Ga,Mn)As into 2DEG states. At maximum the amplitude of the signal is much larger than predicted by the standard spin drift-diffusion model. The signal is especially l...

Hanle spin precession in a two-dimensional electron system

“…In the samples presented below λ sf is typically between 2 and 12 μm. At low bias we obtain for our devices P inj = 75 ± 10%, being in a good agreement with previous reports using similar spin Esaki diodes [5,11,23].…”

“…This is not particularly desirable, as a large B can affect the magnetization of the spin-aligning contacts, thus distorting Hanle measurements. Recently, we briefly discussed Hanle curves measured in a 2D channel by means of a fully electrical setup, which were found to be strongly bias dependent [11]. What is more, from the measurements we extracted a spin-relaxation time of τ s = 5 ns, larger than expected from a pure Dyakonov-Perel spin-relaxation mechanism [1,17,18], dominating in semiconductor 2DESs.…”

“…Whereas spin injection into bulk semiconductors has been realized in many systems [3][4][5][6][7][8], there are few reports of successful experiments in 2D systems [9], particularly in those with high mobility carriers. [10,11] Therefore, many spin properties of semiconductor 2DESs have been investigated so far using optical methods, taking advantage of optical orientation in such systems [12][13][14]. Recent successful experiments demonstrating spin injection in 2DESs in InGaAs quantum wells and at a GaAs/(Al,Ga)As interface [9,11] open up novel ways to study these phenomena in a lateral geometry, typical for potential devices.…”

“…[10,11] Therefore, many spin properties of semiconductor 2DESs have been investigated so far using optical methods, taking advantage of optical orientation in such systems [12][13][14]. Recent successful experiments demonstrating spin injection in 2DESs in InGaAs quantum wells and at a GaAs/(Al,Ga)As interface [9,11] open up novel ways to study these phenomena in a lateral geometry, typical for potential devices. One of the most important spin parameters of the transport channel is the spin-relaxation time τ s , which determines how far and how efficiently spin information can be transported along the channel.…”

Thermoelectric and thermospin transport in a ballistic junction of graphene

“…35,36 One may observe recently an increasing interest in the ballistic transport regime. 37,38 This is due to the possibility of a long mean free path ℓ in 2D electron gas 38 , where ℓ ≃ 3 µm has been already reached, 39 and in graphene nanoribbons, where ℓ > 10 µm has been reported. 40 The main objective of this paper is a theoretically description of thermoelectric and thermospin transport properties of ballistic graphene junctions.…”