Spin relaxation in graphene with self-assembled cobalt porphyrin molecules

Abstract

In graphene spintronics, interaction of localized magnetic moments with the electron spins paves a new way to explore the underlying spin relaxation mechanism. A self-assembled layer of organic cobalt-porphyrin (CoPP) molecules on graphene provides a desired platform for such studies via the magnetic moments of porphyrin-bound cobalt atoms. In this work a study of spin transport properties of graphene spin-valve devices functionalized with such CoPP molecules as a function of temperature via non-local spin-valve and Hanle spin precession measurements is reported. For the functionalized (molecular) devices, we observe a slight decrease in the spin relaxation time (τs), which could be an indication of enhanced spin-flip scattering of the electron spins in graphene in the presence of the molecular magnetic moments. The effect of the molecular layer is masked for low quality samples (low mobility), possibly due to dominance of Elliot-Yafet (EY) type spin relaxation mechanisms. Graphene, one atom thick layer of sp 2 carbon atoms, has potential for spintronic applications due to theoretically predicted high spin relaxation time (τ s ≈ 100 ns) and long spin diffusion length (λ s ≈ 100 µm) 1,2 . These exceptional properties are attributed to negligible spin orbit coupling and weak hyperfine interaction due to the low atomic mass of carbon 3 . However, the maximum reported experimental values demonstrate λ s of about 12 µm 4 for encapsulated graphene and τ s about 2.7 ns for the hydrogenated graphene 5 , which although remarkable when compared with other metals and semiconductors, are still lower by more than an order in magnitude than the theoretically predicted values. A mismatch between theory and experiments suggests towards external factors such as impurities/defects present near the graphene lattice, which dominate the spin relaxation process and result in a lower value for λ s .In order to probe the role of impurities on spin transport, one can systematically introduce them to graphene. In recent years, different research groups have demonstrated several ways of introducing impurities (magnetic and non-magnetic) in graphene such as doping with adatoms, introducing defects and chemicalfunctionalization 6-10 , each method introducing a different spin relaxation source. For example heavy metal atoms such as Au can change the spin transport properties in graphene via spin orbit coupling 11 . On the other hand, light metal (Mg) ions can introduce charge impurity scattering of spins in graphene 12 , although the experimental study rules out the role of this mechanism 13 . A significant change in the spin transport properties of graphene was reported in the presence of magnetic moments 14 , which can be introduced via hydrogenation or by introducing vacancies in the graphene lattice. Remarkably, recent weak localization measurements on graphene 15 also show that magnetic impurities could play a key role in limiting the spin relaxation time in graphene. As proposed by Fabian et al. 16 , if the localized moments are presen...

Spin relaxation in disordered graphene: Interplay between puddles and defect-induced magnetism

“…The small relaxation times observed in Ref. 7 were confirmed by other experiments with different setups [8][9][10][11][12][13][14] . A large theoretical effort has been devoted to unravel this puzzle 4,[15][16][17][18][19][20][21][22][23][24][25][26][27] and different mechanisms have been proposed for the enhanced spin relaxation, such as charge puddles 15,23,24 , impurityinduced enhancement of spin orbit interaction [17][18][19][20] and magnetic impurities 4,[21][22][23] .…”

“…In fact, the findings in Ref. 10 enforce the conjecture that outlying magnetic π-like scatterers rule the spin relaxation process since the effect of the introduction of CoPP on τ s (ε) is reported to only be significant in the better quality samples, and unnoticible for the low-quality ones, which are the ones with a higher chance for having π−like spin scatterers be dominating the relaxation process.…”

“…This kind of effect seems to be a unique feature of the unusual defect induced π-like magnetism in graphene, as shown in a recent study 10 of a graphene spin valve device with cobalt porphyrin (CoPP) molecules. These molecules possess a magnetic moment, which is extrinsic to graphene.…”

Two-channel model for spin-relaxation noise

“…Signal fluctuations with 1/f power spectral density are believed to originate from a broad distribution of time scales related with the measured quantity [1,2], which for electronic transport is associated to the trappingdetrapping times of charge carriers via impurities [1,3]. A two dimensional sheet of graphene, owing to its surface sensitivity [4][5][6][7][8] and superior spin transport properties [9,10], offers a unique platform to study the interaction of impurities with the electron spin via the universally observed phenomenon of 1/f noise. Such an approach leads to the expectation of a spin-dependent 1/f noise in the average spin accumulation [11], and to the fundamental question of what is the origin of this noise.…”

Spin Manipulation in Graphene by Chemically Induced Pseudospin Polarization

“…The sources of spin relaxation turn out to be diversified in graphene, and extrinsic disorder driven by ad-atom impurities can significantly enhanced spin-orbit interaction around defects 1 , or create local magnetism 22 ; both effects usually reducing spin lifetimes, even in the dilute limit [23][24][25][26][27][28][29][30][31] .…”