Evidence of interfacial asymmetric spin scattering at ferromagnet-Pt interfaces

Letter

Evidence of interfacial asymmetric spin scattering at ferromagnet-Pt interfaces

Van Tuong Pham, Maxen Cosset-Chéneau, Ariel Brenac, Olivier Boulle, Alain Marty, Jean-Philippe Attané, and Laurent Vila

Phys. Rev. B 103, L201403 – Published 18 May 2021

Abstract

We measure the spin-charge interconversion by the spin Hall effect in various ferromagnetic/Pt nanodevices. The extracted effective spin Hall angles of Pt evolve drastically with the ferromagnetic (FM) materials (CoFe, Co, and NiFe), when assuming transparent interfaces and a bulk origin of the spin injection/detection by the FM elements. By carefully estimating the interface resistance, we show that it is quite large and cannot be neglected. We then evidence that the spin injection/detection at the FM/Pt interfaces are dominated by the spin polarization of the interfaces. We show that interfacial asymmetric spin scattering becomes the driving mechanism of the spin injection in our samples.

Received 16 November 2020
Revised 20 April 2021
Accepted 4 May 2021

DOI:https://doi.org/10.1103/PhysRevB.103.L201403

Physics Subject Headings (PhySH)

Electrical generation of spin carriers Spin accumulation Spin current Spin diffusion Spintronics

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Van Tuong Pham , Maxen Cosset-Chéneau, Ariel Brenac, Olivier Boulle, Alain Marty, Jean-Philippe Attané^*, and Laurent Vila^†

University Grenoble Alpes, CEA, CNRS, Spintec, F-38000 Grenoble, France

^*jean-philippe.attane@cea.fr
^†laurent.vila @cea.fr

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Issue

Vol. 103, Iss. 20 — 15 May 2021

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Images

Figure 1
(a) Sketch of direct SHE measurement using the FM/SOC nanostructure. Left: Two FM electrodes probe the spin accumulation (SA) induced at the top surface of a SOC wire by the direct SHE. The red volume is spin-up rich ( $+ x$ polarization) while the gray one is spin-down rich ( $- x$ polarization). The black arrows indicate the vector magnetizations of the electrodes $(+ m_{x} / - m_{x})$ . Right: electrochemical potential landscape within the SOC material. (b) Electrochemical potential landscape at one FM/SOC interface of the device, in the case of a transparent interface (i), and in the case of a resistive interface with an interfacial spin asymmetry (ii). The red and black curves correspond to the electrochemical potential of electrons whose spins are polarized along $x$ and $- x$ , respectively. In (ii), $Δ μ_{int}$ is an additional electrochemical potential due to the resistive interface contribute to spin detections. (c) Scanning electron microscopy (SEM) image of a CoFe/Pt sample with a sketch of direct SHE measurement configuration. (d), (e), and (f) are the experimental field sweep loops measured along $x$ on CoFe/Pt, Co/Pt, and NiFe/Pt samples, respectively, with identical geometrical parameters: nanowire widths of $w_{Pt} = 400 nm$ and $w_{FM} = 50 nm$ and thicknesses $t_{Pt} = 7 nm$ and $t_{FM} = 15 nm$ . The black arrows indicate the configuration of the magnetic electrodes when sweeping the magnetic field.
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Figure 2
(a) SEM image of a NiFe/Pt cross, and sketch of the electrical setup for interface resistance measurements. (b) 3D-FEM simulation results of charge transport through the interface: (i) and (ii) are the distributions of the charge-current density ( $J_{C}$ ) and potential ( $μ_{C}$ ) landscape, (iii) and (iv) show the charge current distribution in the interface plane, for a transparent interface (iii) and an interface resistance of $29 f Ω m^{2}$ (iv). (c), (d), and (e) are the expected resistance as a function of the interfacial RA of CoFe/Pt, Co/Pt, and NiFe/Pt systems, respectively. From the simulation curves we deduced the interface RA from the measured $V_{int} / I$ value. The measured $V_{int} / I$ value is an average of measurements obtained on three to five different nanostructures.
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Figure 3
(a) Example of FEM simulation for the SCIC device in the inverse SHE configuration with the geometrical parameters of Fig. 1. A charge current $I_{C}$ is applied in between the ferromagnetic electrodes, which possess antiparallel magnetizations to inject spin-current density ( $J_{S}$ ) into Pt. The inverse SHE converts the spin current flowing in the $z$ direction ( $J_{S}$ ) into a charge current along the Pt wire ( $y$ direction). The SA profile is presented in SM, Fig. S1 [17]. (b) Spin signal amplitudes as a function of the RA for CoFe/Pt. The simulation indicates three regions corresponding to three regimes of the interface: transparent (i), intermediate (ii), and resistive (iii) (See the results for Co/Pt and NiFe/Pt system in SM, Fig. S2 [17]). (c) x-z cross section at the sample center of the electrochemical potential distribution (proportional to the spin density). It presents the SA profile in the direct SHE configuration [Fig. 1], for values of RA corresponding to three cases (i, ii, and iii). The applied charge current density $(J_{C})$ is in the $- y$ direction (red-black dots in different size shows the different local $J_{C}$ due to the electrical shunting by FM). The SHE creates a spin current in the $z$ direction ( $J_{S}$ ) (small black arrows in Pt; the different size corresponds to different produced local $J_{S}$ ). $m_{1}$ and $m_{2}$ are the magnetizations of FMs. (d) Plots of the geometry factor (G) (see more in SM, note S3 [17]) and of the effective spin-charge conversion rate ( $λ_{eff}$ ) as a function of RA, for $γ = p_{F}$ , i.e., the spin polarization of FM electrode is kept at the interface.
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Figure 4
Simulated and experimental values of the amplitude of SHE signals vs the Pt wire width in CoFe/Pt (a), Co/Pt (b), and NiFe/Pt (c) systems. The simulations are performed assuming the same Pt SHE efficiency ( $θ_{SH} λ_{Pt} = 0.19 nm$ ). $t_{FM}$ , $t_{Pt}$ , and $w_{FM}$ are the same as in Figs. 1, 1, and 1.
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