May, 21, 2014 Long, 140 ns electron spin lifetime in chemically synthesized graphene and related nanostructures and its strong interplay between the surface bound oxygen Bálint Náfrádi László Forró Mohammad Choucair

Spintronics Spintronics aims to utilize the spin of electrons for new forms of information storage and logic devices. Key material’s parameter: long spin lifetime τ S long spin diffusion length l S Detrimental: spin orbit coupling one needs light elements magnetic impurities nuclear moments … July, 8, 2015

Spintronics Spintronics aims to utilize the spin of electrons for new forms of information storage and logic devices. Key material’s parameter: long spin lifetime τ S long spin diffusion length l S Detrimental: spin orbit coupling one needs light elements τSτS <4 K Bulk metal~ps semiconductor100 ns dynamic random-access memory (DRAM) is about 20 ns July, 8, 2015

Spintronics Spintronics aims to utilize the spin of electrons for new forms of information storage and logic devices. Key material’s parameter: long spin lifetime τ S long spin diffusion length l S Detrimental: spin orbit coupling one needs light elements τSτS <4 K Bulk metal~ps semiconductor100 ns Nano metal150 ns semiconductor10μs dynamic random-access memory (DRAM) is about 20 ns July, 8, 2015

Spintronics Spintronics aims to utilize the spin of electrons for new forms of information storage and logic devices. Key material’s parameter: long spin lifetime τ S long spin diffusion length l S Detrimental: spin orbit coupling one needs light elements τSτS <4 K300 K Bulk metal~ps fs-ps semiconductor100 ns Nano metal150 ns10-40 ps semiconductor10μs1-4 ns dynamic random-access memory (DRAM) is about 20 ns July, 8, 2015

Carbon (graphene) Spintronics? Theoretical estimates: τ S =~μs l S = ~ K July, 8, 2015 High mobility~10 4 cm 2 V -1 s -1 Weak spin orbit coupling S I =0 for 12 C

Carbon (graphene) Spintronics? Theoretical estimates: τ S =~μs l S = ~ K July, 8, 2015 High mobility~10 4 cm 2 V -1 s -1 Weak spin orbit coupling S I =0 for 12 C M. Peplov Nature 522, , (2015)

Carbon (graphene) Spintronics? Experimental: τ S =0.2-2 ns l S = 3-8 μm Theoretical : τ S =~μ s l S = ~300 μm ?!? Optimistic (extrinsic) Metallic, feromagnetic contacts Substrate Ripples Finite size flakes Adatoms functionalization … Pessimistic (intrinsic) Broken inversion symmetry Multiple Dirac cones Valley dynamics … July, 8, 2015

Carbon (graphene) Spintronics? Experimental: τ S =0.2-2 ns l S = 3-8 μm Theoretical : τ S =~μ s l S = ~300 μm ?!? N. Tombros et al. Nature 448, , (2007) July, 8, 2015

Electron Spin Resonance (ESR) ESR pros: Contactless (few impurities) No substrate local (inhomogeneity in not a problem) Direct measure of τ S ESR cons: ~mg sample is required (~10 m 2 ) July, 8, 2015

Solvothermal Graphene ribbons solvothermal synthesis Gram scale production Catalist free 3D self supporting network of graphene nano ribbons approximates very well the assembly of graphene sheets CESR with τ S =65 ns T<50 K) M. Choucair et al. Nature Nanotech. 3, 30-33, (2009) B. Náfrádi et al. Carbon 74, , (2014) July, 8, 2015

Solvothermal Graphene ribbons M. Choucair et al. Nature Nanotech. 3, 30-33, (2009) B. Náfrádi et al. Carbon 74, , (2014) July, 8, 2015

CESR at 315 GHz Y. Kim et al. PRL, 110, , (2013) B. Náfrádi et al. Carbon 74, , (2014) T<50 K CESR + paramagnet τ S =65 ns July, 8, 2015

Motional narrowing by conduction electrons B. Náfrádi et al. Carbon 74, , (2014) T<50 K CESR, paramagnet τ S =65 ns e-e- T>50 K CESR + paramagnet coupled e-e- July, 8, 2015

Graphene Spintronics? There is a graphenic material which Approximates very well the assembly of graphene sheets. Spin lifetime of itinerant electrons in remarkably long 65 ns. It is <10% of the sample volume. But there is hope for graphene spintronics! Why is it so difficult to obtain long spin lifetime? already O 2 decreases τ S significantly July, 8, 2015

as grown treated at 1070 K Factor ~10 increase of τ S upon heat treatment. The change is completely reversible. O 2 sensitivity Dipole field of O 2 B. Náfrádi et al. Chemistry – A, 21, , (2015) July, 8, 2015 τ S =140 ns

O 2 sensitivity Thermogravitometry: 16 wt% is O B. Náfrádi et al. Chemistry – A, 21, , (2015) July, 8, 2015

O 2 sensitivity B. Náfrádi et al. Chemistry – A, 21, , (2015) July, 8, 2015

Thank you for your attention! July, 8, 2015

O 2 sensitivity B. Náfrádi et al. unpublished, (2014) Still not a perfectly homogeneous sample July, 8, 2015

Graphene Spintronics? July, 8, 2015

O 2 sensitivity XPS: There is a decrease in O from 16wt.% to <<1%wt. B. Náfrádi et al. Chemistry – A, 21, , (2015) July, 8, 2015

Motional narrowing by conduction electrons B. Náfrádi et al. Carbon 74, , (2014) July, 8, 2015

Motional narrowing by conduction electrons B. Náfrádi et al. Carbon 74, , (2014) χ Pauli = 3.1×10 -7 emu/g(4%) n e = 1.4×10 10 cm -2 July, 8, 2015

Motional narrowing by conduction electrons J. Main et al. N.Phys. 4, , (2008) B. Náfrádi et al. Carbon 74, , (2014) χ Pauli = 3.1×10 -7 emu/g(4%) n e = 1.4×10 10 cm -2 n pouddle = 1.5×10 11 cm -2 July, 8, 2015

CESR at 315 GHz B. Náfrádi et al. Carbon 74, , (2014) T<50 K CESR + paramagnet T S =65 ns July, 8, 2015

CESR at 315 GHz B. Náfrádi et al. Carbon 74, , (2014) T<50 K CESR + paramagnet T S =65 ns July, 8, 2015

Motional narrowing by conduction electrons Observed an almost perfect Lorentzian shape ΔH=0.043 mT χ=3.7×10 19 spin/g Calculated: r e-e = 1.3 nm ΔH dip-dip = 0.87 mT Linear broadening with 1.9×10 -4 mT/GHz B. Náfrádi et al. Carbon 74, , (2014) July, 8, 2015

Corrugation Depth Corrugation Period HE 11 Propagating Mode + TE 11 TM 11 = mm ESR Instrumentation GHz + + off-resonanceon-resonance B0B0 July, 8, 2015

ESR Instrumentation GHz P1 45 o Faraday Rotator Variable Polarizer Beamsplitter P2 P3 Sample Arm Phase Adjustment Mixer Oscillator Local Oscillator Arm Cryostat, Sample July, 8, 2015

ESR Instrumentation GHz July, 8, 2015