Tuning spin-charge interconversion with quantum confinement in ultrathin bismuth films

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Tuning spin-charge interconversion with quantum confinement in ultrathin bismuth films

C. Zucchetti, M.-T. Dau, F. Bottegoni, C. Vergnaud, T. Guillet, A. Marty, C. Beigné, S. Gambarelli, A. Picone, A. Calloni, G. Bussetti, A. Brambilla, L. Duò, F. Ciccacci, P. K. Das, J. Fujii, I. Vobornik, M. Finazzi, and M. Jamet

Phys. Rev. B 98, 184418 – Published 16 November 2018

Abstract

Spin-charge interconversion (SCI) phenomena have attracted a growing interest in the field of spintronics as a means to detect spin currents or manipulate the magnetization of ferromagnets. The key ingredients to exploit these assets are a large conversion efficiency, the scalability down to the nanometer scale, and the integrability with optoelectronic and spintronic devices. Here, we show that, when an ultrathin Bi film is epitaxially grown on a Ge(111) substrate, quantum size effects arising in nanometric Bi islands drastically boost the SCI efficiency, even at room temperature. Using x-ray diffraction, scanning tunneling microscopy, and spin- and angle-resolved photoemission, we obtain a clear picture of the film morphology, crystal, and electronic structures. We then directly probe SCI with three different techniques: magneto-optical Kerr effect to detect the charge-to-spin conversion generated by the Rashba-Edelstein effect (REE), optical spin orientation, and spin pumping to generate spin currents and measure the spin-to-charge conversion generated by the inverse Rashba-Edelstein effect (IREE). The three techniques show a sizable SCI only for 1–3-nm-thick Bi films corresponding to the presence of bismuth nanocrystals at the surface of germanium. Due to three-dimensional quantum confinement, those nanocrystals exhibit a highly resistive volume separating metallic surfaces where SCI takes place by (I)REE. As the film size increases, the Bi film becomes continuous and semimetallic leading to the cancellation of SCIs occurring at opposite surfaces, resulting in an average SCI that progressively decreases and disappears. These results pave the way for the exploitation of quantum size effects in spintronics.

Received 27 May 2018
Revised 19 July 2018

DOI:https://doi.org/10.1103/PhysRevB.98.184418

Physics Subject Headings (PhySH)

Dynamic spin injection Optical spin injection Rashba coupling Spin current Spin diffusion Spintronics Surface states

Single crystal materials Surfaces Ultrathin films

Angle-resolved photoemission spectroscopy Ferromagnetic resonance Magneto-optical Kerr effect Molecular beam epitaxy Scanning tunneling microscopy Spin-resolved photoemission spectroscopy X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

C. Zucchetti¹, M.-T. Dau², F. Bottegoni¹, C. Vergnaud², T. Guillet², A. Marty², C. Beigné², S. Gambarelli³, A. Picone¹, A. Calloni¹, G. Bussetti¹, A. Brambilla¹, L. Duò¹, F. Ciccacci¹, P. K. Das⁴, J. Fujii⁴, I. Vobornik⁴, M. Finazzi¹, and M. Jamet^2,*

¹LNESS-Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
²Univ. Grenoble Alpes, CEA, CNRS, Grenoble INP (Institute of Engineering Univ. Grenoble Alpes), INAC-Spintec, 38000 Grenoble, France
³Univ. Grenoble Alpes, CEA, INAC-SYMMES, 38000 Grenoble, France
⁴CNR-IOM Laboratorio TASC, 34149 Trieste, Italy

^*matthieu.jamet@cea.fr

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Vol. 98, Iss. 18 — 1 November 2018

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Images

Figure 1
Morphology of Bi films grown on Ge(111) as a function of the film thickness $t$ . Typical STM images of (a) pseudocubic Bi nanocrystals (3D PC) for $t < 3.5$ nm, (b) percolated pseudocubic Bi nanocrystals forming a 2D layer (2D PC) for $t = 3.8$ nm, and (c) continuous Bi film with (111) orientation for $t = 8$ nm. The thickness $t$ is calculated starting from the Bi/Ge(111)- $(\sqrt{3} \times \sqrt{3}) R 30^{\circ}$ wetting layer.
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Figure 2
Thickness evolution of the band structure of Bi/Ge(111) as obtained by ARPES measurements. (a)–(e) Band structure along the $\bar{K} − \bar{Γ} − \bar{M}$ directions for $t = 1, 2, 3, 5$ , and 9 nm of Bi deposited on the Bi/Ge(111)- $(\sqrt{3} \times \sqrt{3}) R 30^{\circ}$ wetting layer. $\bar{K}$ , $\bar{Γ}$ , and $\bar{M}$ are high symmetry points of the Ge(111) SBZ shown in the inset of panel (a). BE and $k$ are the electron binding energy and momentum, respectively. Different band structures can be identified depending on the structural phase which are: PC Bi nanocrystals for $t = 1 - 3$ nm (3D PC phase), a PC Bi film (2D PC phase) for $t = 5$ nm (HEX grains do not give a visible signal), and a (111)-oriented Bi film (HEX phase) for $t = 9$ nm. (f) Schematics of the Bi(110) 2D Fermi surface according to Ref. [44]. ${\bar{M}}^{'}$ , ${\bar{Γ}}^{'} = \bar{Γ}$ , and ${\bar{X_{1}}}^{'}$ and ${\bar{X_{2}}}^{'}$ are the high symmetry points of the Bi(110) SBZ. Blue lines (around $\bar{Γ}$ and ${\bar{M}}^{'}$ ) correspond to hole states while orange ones (near ${\bar{X_{1}}}^{'}$ ) correspond to electron states. They are also reported in (d). $k_{[\bar{1} 10]}$ , $k_{[\bar{1} \bar{1} 2]}$ , and $k_{[111]}$ are the basis vectors of the reciprocal space. (g) Experimental 2D Fermi surface for $t = 5$ nm reproduced (in orange and blue lines) by superimposing six times the Fermi surface of bulk Bi(110) in (f). (h) 2D Fermi surface of single crystalline Bi(111) for $t = 9$ nm.
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Figure 3
Spin texture of metallic surface states. (a)–(d) Spin polarization recorded along the $\bar{Γ}$ states showing the helical spin texture, for $t = 2.5$ nm. The numbers are the net spin polarization values. (e) Band structure along $\bar{Γ} − \bar{K}$ for $t = 5$ nm. Red solid lines are $\bar{Γ}$ ( $k_{[\bar{1} 10]} \approx 0.1 Å^{- 1}$ ) and ${\bar{M}}^{'}$ ( $k_{[\bar{1} 10]} \approx 0.7 Å^{- 1}$ ) hole states of Bi(110), respectively. Dotted lines crossing the Fermi level correspond to the electron pockets near the ${\bar{X_{1}}}^{'}$ point shown in Figs. 2 and 2. (f),(g) Spin polarization of ${\bar{M}}^{'}$ states.
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Figure 4
Charge-to-spin conversion in Bi films probed by MOKE. (a) Schematics of the experimental setup for charge-to-spin conversion. An electrical current flows into or at the surface of the Bi layer and is converted into a spin accumulation at the top and bottom surfaces. The spin accumulation at the bottom surface is detected by longitudinal Kerr effect. (b) Kerr angle $ϑ_{k}$ detected as a function of the Bi thickness.
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Figure 5
Spin-to-charge conversion efficiency probed by either optical or electrical spin injection. (a),(b) Schematic drawings showing the experimental geometries used for optical spin orientation and spin pumping measurements, respectively (for more details, see Secs. F and G of the Supplemental Material [30]). (c) Bi-thickness dependence of the spin-to-charge conversion efficiency $I_{C} / Φ_{ph}$ at room temperature using optical spin orientation. $I_{C} = Δ V / R$ is the generated charge current where $Δ V$ is the voltage measured in open circuit conditions upon illumination with circularly polarized light, and $R$ is the electrical resistance between the two contacts estimated using four-probe resistance geometry. The charge current is normalized to the excitation signal, i.e., the photon flux $Φ_{ph}$ . Inset: $λ_{IREE}$ values deduced at room temperature. (d) Bi-thickness dependence of the spin-to-charge conversion efficiency $I_{C} / H_{rf}^{2}$ at 30 K and room temperature using spin pumping. $I_{C} = Δ V / R$ is the generated charge current where $Δ V$ is the voltage measured in open circuit conditions at the ferromagnetic resonance of the Co electrode and $R$ is the electrical resistance between the two contacts. The charge current is normalized to the excitation signal, i.e., the radio frequency power proportional to $H_{rf}^{2}$ . Inset: $λ_{IREE}$ values deduced at room temperature.
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