Abhay Singh

High Density Integration of Carbon Nanotube Spin Valves

In this work we report fabrication and characterization of short-channel multi-walled carbon nanotube (MWCNT) based spin valves. These devices are embedded in hexagonally ordered nanopores in an anodic alumina template and show spin relaxation length of 0.28 μm at 8K.

Spin independent magnetoresistance effects in vertical graphene spin valves

Vertical spin valve device configuration with multilayer graphene (MLG) as spacer has drawn significant attention in recent years because of its potential to produce large magnetoresistance (MR) effect due to perfect spin filtering. However, demonstration of this effect has remained elusive so far. Here we consider MLG vertical spin valve structures and show that they exhibit spin independent MR effects, which are orders of magnitude stronger than the spin dependent effects reported to date. These effects manifest within a moderate field range of 10 kG and depend on various factors such as hybridization near the top graphene surface, doping, defects and interlayer coupling. These effects highlight the rich spectrum of physical phenomena that manifest in such systems, which could be exploited in low to moderate magnetic field sensing applications.

Thickness Dependent Interlayer Magnetoresistance in Multilayer Graphene Stacks

Chemical Vapor Deposition grown multilayer graphene (MLG) exhibits large out-of-plane magnetoresistance due to interlayer magnetoresistance (ILMR) effect. It is essential to identify the factors that influence this effect in order to explore its potential in magnetic sensing and data storage applications. It has been demonstrated before that the ILMR effect is sensitive to the interlayer coupling and the orientation of the magnetic field with respect to the out-of-plane (c-axis) direction. In this work, we investigate the role of MLG thickness on ILMR effect. Our results show that the magnitude of ILMR effect increases with the number of graphene layers in the MLG stack. Surprisingly, thicker devices exhibit field induced resistance switching by a factor of at least ~107. This effect persists even at room temperature and to our knowledge such large magnetoresistance values have not been reported before in the literature at comparable fields and temperatures. In addition, an oscillatory MR effect is observed at higher field values. A physical explanation of this effect is presented, which is consistent with our experimental scenario.

Magnetoresistance effects in multilayer graphene as grown on ferromagnetic substrates and implications for spin filtering

Multilayer graphene (MLG) or thin graphitic films as grown on nickel (Ni) or cobalt (Co) has been recently proposed as a promising platform for realizing highly efficient spin filters. However, graphene forms chemisorption interface with Ni and Co, which significantly affects the electronic properties of the interfacial layers as well as the growth of the subsequent graphene layers. Such systems can give rise to various types of magnetoresistance (MR) effects that are completely unrelated to spin filtering. It is, therefore, important to understand these MR effects in order to identify the spin filtering related signal. In this work we highlight on the various MR effects that are observed in Ni/MLG systems and that are also unrelated to spin filtering. In particular, an “interlayer magnetoresistance” (ILMR) effect manifests in these systems, which can result in large MR values that are comparable to state-of-the-art magnetic tunnel junctions at similar operating conditions. Preliminary measurements on Co/MLG samples also indicate presence of ILMR effect.

Synthesis of Amorphous InSb Nanowires and a Study of the Effects of Laser Radiation and Thermal Annealing on Nanowire Crystallinity

Although various synthesis and characterization strategies have been employed for the synthesis of crystalline nanowires, there is very little work done on development of low-dimensional amorphous semiconductors. This paper presents a simple strategy to grow amorphous InSb (a-InSb) nanowires (NWs) in a chemical vapor deposition (CVD) system. The NWs were grown on Si substrate coated with indium film and the lack of crystallinity in the as-grown stoichiometric NWs was ascertained by Raman spectroscopy and electron transport measurements. A model proposed to explain the amorphous NW growth mechanism takes into account the fact that NW growth was carried out at the high temperature ramp-up rate of 75 ∘C/min. This high rate is believed to affect the growth kinematics and determine the arrangement of atoms in the growing NW. Raman spectrum of the as-grown sample shows a broad peak around 155 cm−1, indicative of the presence of high density of homopolar Sb-Sb bonds in the amorphous matrix. It was also found that high intensity laser light induces localized crystallization of the NW, most likely due to radiation-stimulated diffusion of defects in a-InSb. The nonlinear trend of the current-voltage characteristics for individually contacted a-InSb NWs was analyzed to prove that the non-linearity is not induced by Schottky contacts. At high bias fields, space charge limited conduction was the proposed electron transport mechanism. Post-growth annealing of the as-grown a-InSb NWs was found to be very effective in causing the NWs to undergo a phase transition from amorphous to crystalline.