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Register a new userEnhancement of spin relaxation time in hydrogenated graphene spin valve devices
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Added by
Magdalena Wojtaszek
Publication date
2012
fields
Physics
and research's language is
English
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Hydrogen adsorbates in graphene are interesting as they are not only strong Coulomb scatterers but they also induce a change in orbital hybridization of the carbon network from sp^2 into sp^3. This change increases the spin-orbit coupling and is expected to largely modify spin relaxation. In this work we report the change in spin transport properties of graphene due to plasma hydrogenation. We observe an up to three-fold increase of spin relaxation time tau_S after moderate hydrogen exposure. This increase of tau_S is accompanied by the decrease of charge and spin diffusion coefficients, resulting in a minor change in spin relaxation length lambda_S. At high carrier density we obtain lambda_S of 7 microns, which allows for spin detection over a distance of 11 microns. After hydrogenation a value of tau_S as high as 2.7 ns is measured at room temperature.
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Graphene - a single atomic layer of graphite - is a recently-found two-dimensional form of carbon, which exhibits high crystal quality and ballistic electron transport at room temperature. Soft magnetic NiFe electrodes have been used to inject polarized spins into graphene and a 10% change in resistance has been observed as the electrodes switch from the parallel to the antiparallel state. This coupled with the fact that a field effect electrode can modulate the conductivity of these graphene films makes them exciting potential candidates for spin electronic devices.
A long spin relaxation time (tausf) is the key for the applications of graphene to spintronics but the experimental values of tausf have been generally much shorter than expected. We show that the usual determination by the Hanle method underestimates tausf if proper account of the spin absorption by contacts is lacking. By revisiting series of experimental results, we find that the corrected tausf are longer and less dispersed, which leads to a more unified picture of tausf derived from experiments. We also discuss how the correction depends on the parameters of the graphene and contacts.
The carbon isotope $^{13}$C, in contrast to $^{12}$C, possesses a nuclear magnetic moment and can induce electron spin dephasing in graphene. This effect is usually neglected due to the low abundance of $^{13}$C in natural carbon allotropes ($sim$1 %). Chemical vapor deposition (CVD) allows for artificial synthesis of graphene solely from a $^{13}$C precursor, potentially amplifying the influence of the nuclear magnetic moments. In this work we study the effect of hyperfine interactions in pure $^{13}$C-graphene on its spin transport properties. Using Hanle precession measurements we determine the spin relaxation time and observe a weak increase of $tau_{s}$ with doping and a weak change of $tau_{s}$ with temperature, as in natural graphene. For comparison we study spin transport in pure $^{12}$C-graphene, also synthesized by CVD, and observe similar spin relaxation properties. As the signatures of hyperfine effects can be better resolved in oblique spin-valve and Hanle configurations, we use finite-element modeling to emulate oblique signals in the presence of a hyperfine magnetic field for typical graphene properties. Unlike in the case of GaAs, hyperfine interactions with $^{13}$C nuclei influence electron spin transport only very weakly, even for a fully polarized nuclear system. Also, in the measurements of the oblique spin-valve and Hanle effects no hyperfine features could be resolved. This work experimentally confirms the weak character of hyperfine interactions and the negligible role of $^{13}$C atoms in the spin dephasing processes in graphene.
By successive oxygen treatments of graphene non-local spin-valve devices we achieve a gradual increase of the contact resistance area products ($R_cA$) of Co/MgO spin injection and detection electrodes and a transition from linear to non-linear characteristics in the respective differential dV-dI-curves. With this manipulation of the contacts both spin lifetime and amplitude of the spin signal can significantly be increased by a factor of seven in the same device. This demonstrates that contact-induced spin dephasing is the bottleneck for spin transport in graphene devices with small $R_cA$ values. With increasing $R_cA$ values, we furthermore observe the appearance of a second charge neutrality point (CNP) in gate dependent resistance measurements. Simultaneously, we observe a decrease of the gate voltage separation between the two CNPs. The strong enhancement of the spin transport properties as well as the changes in charge transport are explained by a gradual suppression of a Co/graphene interaction by improving the oxide barrier during oxygen treatment.
We discuss the influence of the magneto-coulomb effect (MCE) on the magnetoconductance of spin valve devices. We show that MCE can induce magnetoconductances of several per cents or more, dependent on the strength of the coulomb blockade. Furthermore, the MCE-induced magnetoconductance changes sign as a function of gate voltage. We emphasize the importance of separating conductance changes induced by MCE from those due to spin accumulation in spin valve devices.
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