Spin accumulation in ferromagnetic/nonmagnetic devices
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0998-J05-17
Spin accumulation in ferromagnetic/nonmagnetic devices Oliver Posth, Mario Brands, and Günter Dumpich Experimental Physik, AG Farle, Universität Duisburg-Essen, Campus Duisburg, Lotharstr. 1, Duisburg, 47048, Germany ABSTRACT We have fabricated ferromagnetic/nonmagnetic (FM/NM) metal heterojunctions for the detection of the spin accumulation effect in different nonmagnetic metals. The polycrystalline heterojunctions are prepared by high resolution electron beam lithography (HR-EBL) and a special oblique evaporation technique. The ferromagnetic (FM) and the nonmagnetic (NM) metal are evaporated on top of each other in a single evaporation process to achieve a clean interface between the two metals. The spin accumulation effect is detected in nonmagnetic copper (Cu) and aluminum (Al), from which we determine the polarization of the interface between the ferromagnetic and nonmagnetic metal. INTRODUCTION A spin polarized current injected into a nonmagnetic metal causes a spin accumulation, which can be measured as a voltage between the nonmagnet and a second ferromagnetic electrode. The effect of spin accumulation first demonstrated by Johnson and Silsbee in Al [1]. Other studies have verified this effect in different polycrystalline heterojunctions to determine the spin diffusion length [2-4]. Usually, the heterojunction is fabricated by a two step electron beam lithography (EBL) process, where the interface between the two metals is cleaned by ion milling before the second metal is evaporated. In the present paper, we show that by evaporating the nonmagnetic metal onto the ferromagnetic metal by the technique of oblique evaporation [5], we obtain a high quality interface between the two metals. This allows us to observe the spin accumulation effect in several nonmagnetic materials. EXPERIMENTAL DETAILS We fabricate multiterminal heterojunctions consisting of two ferromagnetic and a nonmagnetic nanowire. The wires are prepared by high resolution electron beam lithography (HR-EBL) onto Te-doped GaAs substrates. A thick 600K type resist is exposed by a LEO secondary electron microscope (SEM) with RAITH beam control unit. The metals are evaporated in an UHV camber with a base pressure of pB = 8x10-9 mbar. Figure 1 schematically shows the developed resist structure used for the oblique evaporation. Cobalt wires are first prepared by evaporating Co at an angle of 7.5° to the surface normal, followed by - without breaking the vacuum evaporating the nonmagnetic metal at an angle of 7.5° from the opposite direction.
Figure 1. Exposed and developed structure in the PMMA resist. The metals are evaporated at an angle of 7.5° to the surface normal and 40° to the substrate edge into the resist Figure 2 shows a SEM image of the device structure used for the measurement of the spin accumulation effect. Two adjacent Co wires with different wire width are connected by a narrow nonmagnetic nanowire. The electrical current I flows from the wide Co wire (Co2) into the nonmagnetic wire (NM) and causes a spin accumulation eff
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