Novel Magnetoresistive Structures Using Self-Assembly and Nanowires on Si
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Novel Magnetoresistive Structures Using Self-Assembly and Nanowires on Si Mazin Maqableh*, Xiaobo Huang and Bethanie J. H. Stadler Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455. ABSTRACT Anodic Aluminum Oxide (AAO) was grown both as free-standing membranes and as integrated layers on Si as templates for arrays of magnetoresistive nanowires. The barrier layer was completely removed in both cases and Co/Cu multilayered nanowires were successfully grown in these templates by DC electrodeposition. Magnetic hysteresis loops and currentperpendicular-to-plane giant magnetoresistance (CPP-GMR) up to 25% were measured for nanowires grown in free standing AAO templates and in templates grown on Si. Spin transfer torque (STT) switching was also measured for multilayers grown in free standing templates with a switching current density of 2.7 x 108 A/cm2. INTRODUCTION Anodic Aluminum Oxide (AAO) is a promising template material for fabricating nanowires because of its self-assembled nanopores whose dimensions can be precisely controlled by tuning the different anodization parameters [1]. As well as having free standing AAO templates, the AAO can be integrated onto Si substrate [2-5] to open the road of making devices such as MRAM and catalysts that benefit from the combination of silicon processing and the self assembly properties of AAO. AAO templates can be made using two-step anodization which results in highly ordered and straight nanopores [6]. A major concern for integrated nanowires is the removal of the barrier layer, which is a thin aluminum oxide layer existing at the bottom of the pores. This must be removed or thinned before efficient electrochemical deposition of nanowires can occur. Several methods have been used to remove this barrier. One method involves pore widening by phosphoric or chromic acid which will result in removal of the barrier layer as well as widening the pores [3, 5]. This method is disadvantageous in the sense that pore size is not preserved. Another method uses Ar ion-milling to break the barrier layer [7]. This method has two disadvantages. It requires an ultra thin AAO template so that Ar ions can reach the bottom of the pores with sufficient energy to break the barrier layer. It also damages the surface of the AAO as well as etching it, so the AAO thickness in this method is not preserved. A third method is to perform the second anodization for a very long time. A spike in the time dependence current curve during this step is used as a sign to manually stop the anodization process [3, 4]. Metallic nanowires can be grown in these templates by DC [3, 4, 8] and AC [9, 10] electrochemical deposition. Co/Cu multilayered nanowires have also been electrodeposited in free standing AAO templates using a mixture electrolyte that contains both Co and Cu cations [8, 12-14]. These electrochemically deposited multilayered nanowires have shown currentperpendicular-to-plane giant magnetoresistance (GMR) [8, 11] as well as a spin
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