Long Spin Diffusion Length in Few-Layer Graphene Flakes

W. Yan, L. C. Phillips, M. Barbone, S. J. Hämäläinen, A. Lombardo, M. Ghidini, X. Moya, F. Maccherozzi, S. van Dijken, S. S. Dhesi, A. C. Ferrari, and N. D. Mathur

Phys. Rev. Lett. 117, 147201 – Published 27 September 2016

Abstract

We report a spin valve with a few-layer graphene flake bridging highly spin-polarized ${La}_{0.67} {Sr}_{0.33} {MnO}_{3}$ electrodes, whose surfaces are kept clean during lithographic definition. Sharp magnetic switching is verified using photoemission electron microscopy with x-ray magnetic circular dichroism contrast. A naturally occurring high interfacial resistance $\sim 12 M Ω$ facilitates spin injection, and a large resistive switching ( $0.8 M Ω$ at 10 K) implies a $70 - 130 μ m$ spin diffusion length that exceeds previous values obtained with sharp-switching electrodes.

Received 11 April 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.147201

Physics Subject Headings (PhySH)

Spin current Spin diffusion Spin relaxation

Graphene

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

W. Yan^1,*, L. C. Phillips¹, M. Barbone², S. J. Hämäläinen³, A. Lombardo², M. Ghidini^1,4, X. Moya¹, F. Maccherozzi⁵, S. van Dijken³, S. S. Dhesi⁵, A. C. Ferrari², and N. D. Mathur^1,†

¹Department of Materials Science, University of Cambridge, Cambridge CB3 0FS, United Kingdom
²Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, United Kingdom
³NanoSpin, Department of Applied Physics, Aalto University School of Science, P.O. Box 15100, FI-00076 Aalto, Finland
⁴DiFeST, University of Parma, viale G.P. Usberti 7/A, 43124 Parma, Italy
⁵Diamond Light Source, Chilton, Didcot, Oxfordshire OX11 0DE, United Kingdom

^*Present address: CIC nanoGUNE Consolider, Tolosa Hiribidea 76, E-20018 Donostia—San Sebastian, Spain.
^†ndm12@cam.ac.uk

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Vol. 117, Iss. 14 — 30 September 2016

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Images

Figure 1
Magnetic switching in LSMO (110) electrodes $A - D$ at 150 K. (a)–(h) PEEM images at remanence after applying a magnetic field $μ_{0} H_{pulse}$ along the LSMO easy axis $∥ {[001]}_{STO}$ (red arrow). Field magnitudes represent upper bounds on the fields at which the observed switching occurs. XMCD asymmetry represents the projection of the in-plane surface magnetization on the in-plane projection of the grazing-incidence beam direction (green arrow). Blue (red) depicts magnetization parallel (antiparallel) to the green arrow. Transport is measured between $B$ and $C$ using similar electrodes.
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Figure 2
LSMO-5LG-LSMO spin-valve characterization. MOKE microscopy measurements of magnetic switching in LSMO (110) electrode $A$ at 10 K. The applied magnetic field $μ_{0} H$ and the measured component of magnetization $M$ are collinear with the LSMO easy axis and ${[001]}_{STO}$ . $M_{s}$ is the saturation magnetization. The AFM image ( $30 μ m \times 30 μ m$ ) (upper inset) and phase-contrast AFM image (lower inset) show the FLG flake.
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Figure 3
(a,b) Raman spectra before and after 5LG transfer to LSMO electrodes separated the by NGO-backfilled STO substrate (BSTO). The Raman measurements after transfer are performed on the 5LG flake directly above the BSTO. The G and 2D peaks are visible for the flake on ${SiO}_{2}$ (blue) and after subsequent transfer to the LSMO electrodes separated by BSTO (red). A point-by-point subtraction of the BSTO background (green) yields the spectrum for the graphene flake on BSTO (black), where the D peak is absent.
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Figure 4
LSMO-5LG-LSMO spin-valve magnetotransport at 10 K. Resistance $R_{B C}$ and magnetoresistance ${MR}_{B C}$ on decreasing (blue) and increasing (red) magnetic field $H$ along the LSMO easy axis collinear with ${[001]}_{STO}$ . Dashed lines indicate $R_{B C} = 24.8 M Ω$ and $R_{B C} + Δ R_{B C} = 25.6 M Ω$ for parallel and antiparallel electrode magnetizations, respectively. The antiparallel configuration is indicated by gray shading in $12.5 mT < | μ_{0} H | < 34.5 mT$ . Data obtained by averaging three up and down sweeps (Supplemental Fig. S4). $Bias voltage = 80 mV$ . 5LG bridges a $2 μ m$ gap between electrodes $B$ and $C$ , whose subscripts are dropped for analysis.
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Figure 5
LSMO-5LG-LSMO spin-valve parameters at 10 K. Contours and grayscale show MR( $γ, l_{sf}^{G}$ ) from the drift-diffusion model [23]. For $MR \sim 3.0 %$ and $0.95 > γ > 0.54$ , we find $70 μ m < l_{sf}^{G} < 130 μ m$ (red contour). The red dot denotes MR $(γ = 0.8, l_{sf}^{G} = 85 μ m) = 3.0 %$ for the experimental value of $R_{P} = 24.8 M Ω$ . Inset: $MR (R_{P})$ for $γ = 0.8$ and $l_{sf}^{G} = 85 μ m$ .
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