  The electronic properties of armchair graphene nanoribbons (AGNRs) can be
significantly modified from semiconducting to metallic states, by applying a
uniform perpendicular magnetic field (B-field). Here, we theoretically study
the bandgap modulation induced by a perpendicular B-field. The applied B-field
causes the lowest conduction subband and the top-most valence subband to move
closer to one another to form the n=0 Landau level. We exploit this effect to
realize a device relevant MR modulation. Unlike in conventional spin-valves,
this intrinsic MR effect is realized without the use of any ferromagnetic
leads. The AGNRs with number of dimers, Na=3p+1 [p=1,2,3,...] show the most
promising behavior for MR applications, with large conductance modulation and
hence, high MR ratio at the optimal source-drain bias. However, the MR is
suppressed at higher temperature due to the spread of the Fermi function
distribution. We also investigate the importance of the source-drain bias in
optimizing the MR. Lastly, we show that edge roughness of AGNRs has the
unexpected effect of improving the magnetic sensitivity of the device and thus
increasing the MR ratio.
