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Charge-spin current conversion in high quality epitaxial Fe/Pt systems: Isotropic spin Hall angle along different in-plane crystalline directions

Abstract : We report the growth of MgO[001]//Fe(6 nm)/MgO(7 nm) and MgO[001]//Fe(6 nm)/Pt(6 nm) by molecular beam epitaxy and show that the full characterization by spin-orbit ferromagnetic resonance (SO-FMR) allows the determination of magnetic anisotropies by classical FMR-only studies. The spin mixing conductance of the epitaxial Fe/Pt interface was measured to be g "# effect ¼ 2:660:5 Â 10 19 m 2 , and the effective spin Hall angle (SHA) h effect SHE was estimated at different in-plane crystalline directions. It was found that h effect SHE is the same in all directions. When taking into account high enough excitation frequencies to achieve uniform precession of magnetization, the effective SHA for epitax-ial Pt in Fe/Pt is h effect SHE ¼ 0:05160:005. We address about the proper conditions to determine those relevant spintronic parameters. Published by AIP Publishing. https://doi.org/10.1063/1.5079236 The conversion of spin current into charge current and vice versa plays key roles in new research efforts in spin-tronics and related applications. This interconversion can be achieved without any external magnetic field or magnetic material in 3-dimensional systems that exhibit strong spin-orbit coupling, 1 namely, the spin Hall effect (SHE). 2-4 The quantification of the efficiency of such interconversion is called the spin Hall angle (SHA). The spin Hall angle determination is thus relevant to find out new materials for applications like in magnetic memories because it will allow reducing power consumption. Large spin Hall angles have been found in heavy metals like Pt, 5-8 Ta, 9 and W 10,11 and alloys like CuBi, 12 AuW, and AuTa. 13,14 In a heavy metal or alloy layer which is in contact with a layer of a different material, an injected spin current might decrease through the interface due to interfacial interactions and the spin Hall angle becomes an effective spin Hall angle h effect SHE. 8,15,16 There are several techniques available to evaluate the effective spin Hall angle or effective spin orbit torque (SOT), like spin pumping ferromagnetic resonance, 7,17-20 spin-orbit ferromagnetic resonance (SO-FMR), 6,21-24 no-local injection in lateral spin valves, current-induced magnetization switching , harmonic measurements, spin Hall magnetoresistance, and so on. In all these experiments, in the measurements as well as in the analysis, there are many details and approximations that are often overlooked. So far, the SHA quantification has been evaluated mostly on sputtered polycrystalline samples. There is a lack of such experimental evaluation in epitax-ial samples considering different crystalline axes. In this paper, we focus on the spin-orbit or spin-torque ferromagnetic resonance (SO-FMR, ST-FMR) technique to study epitaxial samples. We have patterned bars along different crystallographic directions. The results show isotropic values for the thickness level in this study, Fe(6 nm)/Pt(5 nm). Epitaxial s//Fe(6 nm)/Pt(5 nm) bilayers were grown by molecular beam epitaxy, where s stands for the crystalline [001]MgO substrate. The charge-spin current conversion was evaluated by SO-FMR. For comparison, a single Fe layer capped with 7 nm of MgO was grown and characterized by the classical FMR method with magnetic field H dc applied in-plane along different crystalline directions. The Fe layer was grown simultaneously for the s//Fe/ MgO reference sample as for the s//Fe/Pt bilayer as shown in Fig. 1. After the deposition of the Fe layer, half of the sample was covered to deposit 7 nm of MgO, and then, the operation is reversed to deposit the Pt layer. In such a way, we have a true Fe reference layer to estimate, for instance, the effective spin mixing conductance at the Fe/Pt interface. The 2D growth and quality of the sample were monitored in-situ by reflection high-energy electron diffraction (RHEED) as shown in Fig. 1(b). The Fe crystalline cell grown rotated by 45 on top of the [001]MgO crystalline cell because of their lattice parameters, that is, the direction [110]Fe jj [100]MgO and the direction [100]Fe jj [110]MgO. 25 In the following, we will refer only to the MgO crystalline axes. Small pieces of about few mm 2 were extracted from the center region of the s//Fe/MgO sample to avoid edge deposi-tion issues and measured by FMR. A grounded coplanar wave guide (GCPW) was used as shown in Fig. 2(a). Hence, the FIG. 1. (a) Schematic of the full stack that has been grown by MBE. After the deposition of the Fe layer, half of the sample was cover to deposit 7 nm of MgO or 5 nm of Pt. (b) RHEED pattern showing the good quality and 2D growth of Fe as well as the Pt layers. (c) The cubic Fe cell grown rotated by 45 on top of the cubic cell of MgO. a)
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Submitted on : Wednesday, November 11, 2020 - 12:32:11 PM
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C Guillemard, S. Petit-Watelot, S. Andrieu, J.-C. Rojas-Sánchez. Charge-spin current conversion in high quality epitaxial Fe/Pt systems: Isotropic spin Hall angle along different in-plane crystalline directions. Applied Physics Letters, American Institute of Physics, 2018, 113, ⟨10.1063/1.5079236⟩. ⟨hal-02999961⟩

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