Spin injection and the local Hall effect in InAs quantum wells

Transcription

1 Spin injection and the local Hall effect in InAs quantum wells F.G. Monzon * and M.L. Roukes California Institute of Technology, Condensed Matter Physics , Pasadena CA We report on our efforts to directly inject and detect a spin polarization in a NiFe / (InAs quantum well) / NiFe system at 4.2 K. Numerous device geometries at inter-magnet spacings of between 750 nm and 64 µm are explored, often demonstrating marked hysteretic behavior consistent with the interaction of strong fringe magnetic fields with the low-density InAs quantum well. These local Hall voltages, interesting in themselves, have been investigated systematically and attempts have been made to minimize their role in smaller spin devices. The apparent lack of a strong and unambiguous spin-coupled signal in our smallest devices suggests that NiFe / InAs interfacial spin-scattering mechanisms are a crucial limiting factor. Keywords: spin injection, nanomagnet, InAs spin transistor * fgmonzon@cco.caltech.edu, phone: (626) , fax: (626)

2 Shortly after the operation of all-metallic spin transistor-like devices was demonstrated a decade ago, 1 a device utilizing an InAs two-dimensional electron gas (2DEG) was proposed. 2 This device has not materialized. The detection of a spin-coupled signal in a NiFe / InAs / NiFe system is complicated by strong fringe magnetic fields that induce local Hall voltages in the 2DEG. When the magnetization of the thin-film magnets changes so do these voltages, thereby obscuring any spin-coupled signal. In the course of our spin work we have provided the first characterization of this local Hall effect (LHE), 3 which may have technological potential in its own right. At first glance one would expect the spin injection phenomenon in an InAs 2DEG to be much larger than in the all-metal case. The magnitude of a spin-coupled signal can be described by the quantity R 4 spin which we estimate to be of order 200 mω for a device with a 1µm wide channel and contact separation, a spin-flip time τ sf of 1.75 ps 5, and a spin-polarized injection efficiency, η, of 5% (the polarization transfer from the injector F1 into the paramagnet P and from P into the detector F2). This presumes that d is less than the spin relaxation length, δ s. We find δ s 3µm for n s = cm -2, µ = 6 x 10 5 cm 2 /Vs, and τ sf as above. These estimates rely on the value for τ sf, really a spin-dephasing time, obtained from weak antilocalization experiments that are very sensitive to slight precessional effects. We expect that the actual nonequilibrium magnetization decay time is actually much longer than this dephasing time. 6 There are two problems with this estimate: 1) Its development presumes diffusive transport whereas our InAs devices are ballistic, or quasi-ballistic, and hence involve different spin-scattering mechanisms; 7 2) Interfacial non-idealities are lumped into a constant η, hiding important physics like interfacial resistance, interfacial spin-flips, and spin relaxation in the 2

3 ferromagnets. 8 These factors may not be constant but, rather, dependent upon the nonequilibrium spin population itself. The magnitudes of these factors, in the ballistic regime, are undetermined. Despite these difficulties, the magnitude of the estimate for R spin has motivated several groups to pursue spin injection into InAs 2DEGs. Our experiments are carried out using several different device geometries patterned by photolithography (PL) and electron beam lithography (EBL). Two PL geometries are shown in Figure 1. Measurements involve slowly stepping in-plane magnetic field while a current is sent through F1 and the voltage at F2 is lock-in detected. A signature of spin-coupled transport experiments in the diffusive regime is a dip in detector voltage with F1 and F2 anti-aligned. 1 Numerous devices at both small and large separations showed such signals, suggestive of spincoupled transport. However, as exemplified in Fig. 1(d), other hysteretic behavior was also observed. The data of Fig. 1(c,e) are likely LHE signals because they were not seen consistently, and were nonexistent in geometries where the influence of the LHE was smallest. Despite this, the anomalous appearance of several of our data, especially in the Fig. 1(b) geometry, was difficult to explain conclusively due to the large number of unknown parameters in our experiments. This was improved upon in EBL devices. We digress to mention the characteristics and device potential of the LHE. Elsewhere we detail our LHE device structure, 9 consisting of a thin film magnet deposited with one end centered over a cross-junction. Figure 2 shows data from electron beam evaporated NiFe magnets on crosses of 750 Å thick n+ GaAs (n cm -3 ). For narrower magnets the hysteresis loops are very square, indicative of quasi-single domain behavior. 9 Besides being potentially useful for their non-volatility and simplicity, LHE elements provide an effective magnetic field multiplication : a large field (>1 T near the NiFe) is switched by a much smaller field (tens or hundreds of Oe). 3 The average field in the cross, <B >, is reduced by 3

4 regions of the cross far from the magnet, so we see at most a multiplication of <B >/H c 1.3, but this could be optimized with narrower crosses, a thinner n+ layer, or a ferromagnetic film with smaller H c. Using the LHE we determined appropriate magnet sizes, in terms of coercivity and loop sharpness, for our smaller spin devices. As a final note, LHE devices are sensitive magnetometers, in principle able to measure several hundred Bohr magnetons at room temperature competitive with SQUID susceptometers 10 that operate at cryogenic temperatures. Figure 3 shows SEM micrographs of two EBL devices. Magnets were deposited in InAs windows where the GaSb cap was etched away, thereby avoiding the micromagnetic difficulties of our PL devices, in which the magnets were non-planar. Each chip had a set of cross-junctions where InAs and ferromagnet properties were measured. We found that our magnets were switching sharply (within a few Oe) and at well separated coercivities, due to their differing aspect ratios. Carrier mobility decreased somewhat due to ion milling of the cap layer 4 but the elastic mean free path was still above 2µm. In Fig. 3(c) we show data from a device similar to that of Fig. 3(a), with 750nm F1- F2 separation. The hysteresis loop has a 1 mω full-scale deflection that is the result of local Hall voltages. Any spin-coupled signal in this geometry needs to be larger than this signal level, but none has so far been seen. Data from a device with 1.5µm separation, similar to the one shown in Fig. 3(b), are displayed in Fig. 3(d). Here one does not expect to see a hysteresis loop even if the LHE is significant. The trace instead shows both a peak and a dip: inconsistent with expectations, based upon diffusive spin-coupled transport, of two features with the same polarity. However, the widths of the features are also inconsistent with the widths of the magnets coercive transitions. The origin of the hysteresis in this device has not been conclusively determined. Hanle effect 1 experiments on both of the above devices were also 4

5 attempted but proved ambiguous, in part because of the presence of a slowly varying magnetoresistive background. In conclusion, we are now able to study and control many aspects of the physics underlying spin-coupled transport, in situ, in submicron-scale devices. Progressive improvements, made in the course of investigating several hundred devices, include minimization of magnetic fringing fields in the low density conduction channels, engineering of nanomagnet switching properties (coercivity and transition width), elimination of multiple magnetic domains in the injection region, and careful on-chip characterization of the InAs quantum wires. These improvements enable suppression of the LHE by factors of up to several hundred, thereby unveiling hysteretic features which are interesting but not yet unambiguously attributable to spin-coupled transport. These features are much weaker than initially anticipated. In this regard, the NiFe / InAs interface is of great concern, since other work 11 has found magnetically dead layers at ferromagnet / semiconductor interfaces. Also, ballistic spin-coupled transport is currently not well understood: spin-injection from a diffusive ferromagnet into a high mobility 2DEG merits further theoretical investigation. We gratefully acknowledge the contributions of D.S. Patterson to the nanomagnet work, and M. Thomas, H. R. Blank, and H. Kroemer for providing the high-quality InAs material. This work is supported under ONR Grant No. N

6 (a) (b) R (Ω) (c) (d) (e) Fig. 1. Two geometries used in PL devices (a,b). Black areas denote contacts (NiFe on top of exposed InAs), densely spotted areas are NiFe, sparsely spotted areas are conducting mesas, and white regions show metal interconnections. Channel widths were either 3µm or 6µm and separations varied from 6µm to 64µm. In (c) and (d) are shown data for devices similar to that in (a), while (e) shows data for a device like that of (b).

7 R H (Ω) nm 125nm 175nm 250nm 350nm 500nm w = 1000nm Fig. 2. Hysteresis loops for NiFe magnets of thickness 500 Å, aspect ratio 10, and varying widths. Traces are offset vertically for clarity. All data were taken at room temperature. (a) (b) R H (Ω) (c) (d) Fig. 3 SEM micrographs of EBL devices (a,b). These devices have F1-F2 spacings of 1.5µm. Magnet dimensions are 500nm x 10 µm and 750 nm x 7.5 µm. Data from similar devices are shown in (c,d). 7

8 1 Mark Johnson and R.H. Silsbee, Phys. Rev. B 37, 5326 (1988). 2 S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990). 3 F.G. Monzon, Mark Johnson, and M.L. Roukes, Appl. Phys. Lett. 71, 3087 (1997). 4 F.G. Monzon and M.L. Roukes, submitted to Phys. Rev. B. 5 G.L. Chen, J. Han, T.T. Huang, S. Datta, and D.B. Janes, Phys. Rev. B 47, 4084 (1993). 6 In GaAs 2DEGs, τ sf is on the order of 10 ns (V.E. Zhitomirski, V.E. Kirpichev, A.I. Filin, V.B. Timofeev, B.N. Shepel, and K. v. Klitzing, JETP Lett. 58, 439 (1993)), resulting in a spin relaxation length of tens of microns using typical values for sheet density and mobility. This contrasts with the spin-dephasing time from antilocalization studies of 25 ps (P.D. Dresselhaus, C.M.A. Papavassiliou, R.G. Wheeler, and R.N. Sacks, Phys. Rev. Lett. 68, 106 (1992). 7 M.I. D'yakanov and V.I. Perel', Zh. Eskp. Teor. Fiz. 60, 1954 (1971) [Sov. Phys. JETP 33, 1053 (1971)]. 8 A. Fert and S.-F. Lee, Phys. Rev. B 53, 6554 (1996). 9 F.G. Monzon, D.S. Patterson, and M.L. Roukes, submitted to Appl. Phys. Lett. 10 M.B. Ketchen, D.D. Awschalom, W.J. Gallagher, A.W. Kleinsasser, R.L. Sandstrom, J.R. Rozen, and B. Bumble, IEEE Trans. Magn. 25, 1212 (1989). 11 A. Filipe, A. Schuhl, and P. Galtier, Appl. Phys. Lett 70, 129 (1997). 8