Browsing by Author "Jammalamadaka, S. Narayana"

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    Dynamic response of exchange bias in graphene nanoribbons
    (American Institute of Physics, 2012) Jammalamadaka, S. Narayana; Rao, S.S.; Vanacken, J.; Moshchalkov, V.V.; Lu, Wei; Tour, J.M.; Smalley Institute for Nanoscale Science and Technology
    The dynamics of magnetic hysteresis, including the training effect and the field sweep rate dependence of the exchange bias, is experimentally investigated in exchange-coupled potassium split graphenenanoribbons (GNRs). We find that, at low field sweep rate, the pronounced absolute training effect is present over a large number of cycles. This is reflected in a gradual decrease of the exchange bias with the sequential field cycling. However, at high field sweep rate above 0.5 T/min, the training effect is not prominent. With the increase in field sweep rate, the average value of exchange bias field grows and is found to follow power-law behavior. The response of the exchange bias field to the field sweep rate variation is linked to the difference in the time it takes to perform a hysteresis loop measurement compared with the relaxation time of the anti-ferromagnetically aligned spins. The present results may broaden our current understanding of magnetism of GNRs and would be helpful in establishing the GNRs-based spintronic devices.
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    Ferromagnetism in Graphene Nanoribbons: Split versus Oxidative Unzipped Ribbons
    (American Chemical Society, 2012) Rao, S.S.; Jammalamadaka, S. Narayana; Stesmans, A.; Moshchalkov, V.V.; van Tol, J.; Kosynkin, D.V.; Higginbotham, A.; Tour, J.M.; Smalley Institute for Nanoscale Science and Technology
    Two types of graphene nanoribbons: (a) potassium-split graphene nanoribbons (GNRs), and (b) oxidative unzipped and chemically converted graphene nanoribbons (CCGNRs) were investigated for their magnetic properties using the combination of static magnetization and electron spin resonance measurements. The two types of ribbons possess remarkably different magnetic properties. While a low-temperature ferromagnet-like feature is observed in both types of ribbons, such room-temperature feature persists only in potassium-split ribbons. The GNRs show negative exchange bias, but the CCGNRs exhibit a ﾓpositive exchange biasﾔ. Electron spin resonance measurements suggest that the carbon-related defects may be responsible for the observed magnetic behavior in both types of ribbons. Furthermore, information on the proton hyperfine coupling strength has been obtained from hyperfine sublevel correlation experiments performed on the GNRs. Electron spin resonance finds no evidence for the presence of potassium (cluster) related signals, pointing to the intrinsic magnetic nature of the ribbons. Our combined experimental results may indicate the coexistence of ferromagnetic clusters with antiferromagnetic regions leading to disordered magnetic phase. We discuss the possible origin of the observed contrast in the magnetic behaviors of the two types of ribbons studied.