Magnetic nanostructures for lateral spin-transport devices

Magnetic nanostructures for lateral spin-transport devices     Dirk Grundler; Thomas M. Hengstmann; Haiko Rolff Institut für Angewandte Physik und Zentrum für Mikrostrukturforschung, Universität Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany     ABSTRACT We review our investigations on nanostructured ferromagnets fabricated on semiconductor heterostructures. We show that ballistic electrons in the semiconductor are in particular sensitive to local stray fields. The reversal of the ferromagnets give rise to characteristic magnetoresistance traces in the ferromagnet/semiconductor hybrid structures. We use these stray-field effects for detailed studies on the magnetic properties of rectangular micromagnets, nanodisks and nanorings. The nanostructures might be favorable candidates for ferromagnetic contacts in future spin-injection experiments in lateral feromagnet/semiconductor hybrid structures.     1 Introduction Injection of spin-polarized currents into semiconductors is a challenging research field [1-4]. High spin injection efficiencies would render spintronics with semiconductors relevant for technological applications [5]. One of the most controversial issues in experiment [6-9] and theory [10] is the all-electrical spin injection and spin detection in a high-mobility two-dimensional electron system (2DES) using ferromagnetic metals. We have recently developed a model to describe ballistic transport in ferromagnet/semiconductor hybrid structures [11-13]. In this transport regime spin injection is possible by means of spin filtering at the ferromagnet/semiconductor interface [12, 14]. Characteristic and pronounced spin-valve effects are predicted [13]. It has earlier been argued however that stray fields perpendicular to the plane of the 2DES could induce an additional magnetoresistance (MR) effect which might mimic a spin-valve effect [8]. This Lorentz-force induced MR effect is often called the local Hall effect. The latter is prominent in lateral ferromagnet/semiconductor hybrid structures rather than in the all-metal giant magnetoresistance (GMR) devices due to the smaller carrier density ns of the 2DES. An important prerequisite for the interpretation of transport phenomena in ferromagnet/semiconductor hybrid structures is therefore a good knowledge of the stray fields of the ferromagnetic contacts. We have investigated different nanostructured ferromagnets with respect to the local Hall effect. Here, we review our recent experimental results.   2 Experimental setup Mesoscopic 2DES which are sensitive to the local Hall effect [15, 16] have been fabricated in a GaAs/Al0.3Ga0.7As heterostructure using electron-beam lithography and photolithography followed by a wet etching process. The heterostructure has been grown by means of molecular beam epitaxy (MBE). Due to modulation doping it contains a high-mobility 2DES 90 nm below the surface. The electronic width of the etched Hall bar is in the order of 1mm. A typical device is shown in Fig. 1. Contacts are fabricated using evaporation and lift-off technique. The carrier density ns is 2.7·1011 cm–2 and the mobility m is 350,000 cm2/Vs at 4.2 K. Hence, the electron mean free path is le = 3 mm. All experiments are performed at 4.2 K such that the electron system is in the ballistic transport regime. The poly-crystalline permalloy (Ni80Fe20) micro- and nanomagnets are integrated on the cap layer of the heterostructure using electron beam lithography, evaporation and lift-off technique (Fig. 1). The 20 nm thick permalloy film has been capped by a 9 nm gold layer. The ferromagnetic nanostructures are electrically isolated from the 2DES. By this means, we are able to investigate in detail the stray field effects and to focus on Lorentz-force induced MR effects. For this, we have performed Hall and bend-resistance measurements on the mesoscopic 2DES. The bend resistance is a characteristic MR feature that originates from electrons travelling ballistically through the 2DES [17].     The external magnetic field B is applied in the plane of the 2DES, i.e., the observed hysteretic features are solely due to stray fields of the ferromagnets which are perpendicular to the 2DES. The field B is swept in small steps of a few tenth of a mT. The current I of amplitude 3 mA applied to the 2DES is modulated at 37 Hz. The voltage is detected using lock-in technique.   3 Experimental results and discussion In the following we present stray-field investigations performed on three 2DES with different ferromagnetic nanostructures on top: (I) a rectangular two-micromagnet system, (II) a nanodisk and (III) a nanoring. The first has been proposed for spin-injection and spin-detection experiments in lateral ferromagnet/semiconductor hybrid structures in Ref. [18]. Nanodisks and nanorings have been proposed earlier to be interesting for GMR spin-valve devices [19]. Rectangular micromagnets The Hall resistance measures the averaged magnetic flux in the 2DES [15, 16]. Recently we have shown that, in particular, the bend resistance is sensitive to the local details of the field pattern penetrating the 2DES and that it is useful to enhance considerably the lateral resolution of a 2DES Hall bar [20]. This is demonstrated on sample (I) in Fig. 2. Here, the bend-resistance trace is shown which is measured on a 2DES cross junction containing a two-micromagnet system (compare Fig. 1) with a separation d = 200 nm. The four contact leads are labelled with 1 to 4 (inset of Fig. 2). The current is injected between the leads 2 and 4 and the voltage is measured between the contacts 1 and 3. By this means, electrons are detected at contact 1 which transverse the cross junction ballistically [17] and which enter the voltage probe 1 without being deflected by the inhomogeneous stray field.     The bend resistance R24,13 shows a hysteretic 'butterfly'-like shape which is interesting by itself. Here it is important that the stray field gives rise to a characteristic and pronounced MR effect for the ballistic electrons in the mesoscopic 2DES. In micromagnetic simulations we have found that the change of the domain pattern in the two-micromagnet system occurs very local within sub-micron domains. The experimental result is that ballistic electrons in the 2DES are very sensitive to these local stray fields [20], i.e., the Lorentz-force induced MR effect reflects in detail the magnetic behavior of the ferromagnets on the nanometer scale. In Fig. 2 the bend resistance is nonzero for almost all magnetic fields. Near zero magnetic field, i.e., in the remanent state, the bend resistance is in particular either positive or negative depending on the magnetic history. As a result, we find that a ballistic 2DES beneath a rectangular two-micromagnet system shows a very complex MR caused by the stray fields. Nanodisks It has early been argued [19] that nanomagnets with a circular shape fab (...truncated)