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Magnetoresistance Effects in Multilayer Graphene as Grown on Nickel
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- Author / Creator
- Bodepudi, Srikrishna C
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Magnetoresistance (MR), the change in electrical resistance of a solid-state system due to an external magnetic field, is a key effect in condensed matter physics, both for fundamental understanding of charge transport phenomena, as well as immense commercial implications. Artificial layered structures often exhibit strong magnetoresistance (MR) effects that are exploited in various data storage and magnetic field sensing technologies. Graphite is a naturally occurring layered structure in which single graphitic layers (or “graphene”) are stacked up on each other. Magnetoresistance (MR) in graphitic systems (single to few layers of graphene and bulk graphite) has drawn significant attention in recent years. It has been theoretically predicted that multilayer graphene (MLG) on Ni can potentially exhibit large magnetoresistance values due to spin filtering effect. However, experimental work in this area is rare. The purpose of this work is to explore various magnetoresistance effects in MLG/Ni systems. Multilayer graphene (MLG) stacks with various thicknesses have been grown on polycrystalline Ni substrates using a standard chemical vapor deposition (CVD) recipe. These samples exhibit a large, negative MR effect in current-perpendicular-to-plane (CPP) geometry with the magnetic field normal to the plane. A negative magnetoresistance effect ~ 104% has been observed, which persists even at room temperature. The observed magnetoresistance is extremely high as compared to other known materials systems for similar temperature and field range. This effect is correlated with the shape of the 2D peak as well as with the absence of D peak in the Raman spectrum. The observed data is qualitatively consistent with the “interlayer magnetoresistance” (ILMR) mechanism in which interlayer charge transfer occurs between the zero mode Landau levels of weakly coupled graphene layers. To further understand ILMR effect, angle and thickness dependent studies have been performed in as-grown MLG on Ni samples. Angular dependences of ILMR effect in as-grown MLG agree well with theory. However, the angular response is sharper than expected and is related to the additional sources of positive MR present in the system. In addition, the ILMR effect persists and becomes stronger as thickness of MLG is increased. Interestingly, for larger thickness samples, magnitude of the MR effect is relatively insensitive to temperature. To further verify this, CPP MR measurements have been performed in as-grown MLG samples with different thicknesses. In the next stage of this thesis, as-grown MLG samples have been tested in spin valve configuration in order to investigate spin-related magnetoresistance effects and its implications for graphene spin filters. However these devices only show weak localization and ILMR but no spin filtering. Based on above observations, we are planning to explore the following subprojects in future: (1) MR effects in as-grown MLG on cobalt (Co), (2) MR effects in functionalized graphene/Ni (111) and exploring spin filtering effect in this system, and (3) MR effects in transferred MLG on flexible substrates. Due to large MR value and its persistence at room temperature, this ILMR effect in as-grown MLG samples is expected to have commercial implications and encourage further research on MLG physics and MLG growth mechanisms on ferromagnetic substrate. Further, intrinsic compatibility of MLG with flexible electronics and sensorics makes ILMR an exciting platform for future magnetic sensing and data storage technologies.
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- Graduation date
- Spring 2016
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.