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▼ [15p-501-19] Spin-torque generator engineered by natural oxidation of Cu
Keywords:spin Hall effect, spin-torque ferromagnetic resonance, oxidation
The spin Hall effect (SHE) is a spin-orbit coupling phenomenon, which enables electric generation and detection of spin currents. It has been believed that heavy metals, which have large nuclear potential, are indispensable for the spin-torque generation. Here, we show that the SHE in Cu, a light metal with weak spin-orbit coupling, is significantly enhanced through natural oxidation. We demonstrate that the spin-torque generation efficiency of a Cu/Ni81Fe19 bilayer is enhanced by over two-orders of magnitude by tuning the surface oxidation, reaching the efficiency of Pt/ferromagnetic metal bilayers. This finding illustrates a crucial role of oxidation in the SHE, opening a route for engineering the spin-torque generator by oxygen control and manipulating magnetization without using heavy metals.
SHE can be used to excite dynamics in an ordinary ferromagnetic metal. We study naturally-oxidized Cu/ Ni81Fe19 bilayer films with a microwave-frequency charge current applied in the film plane. An oscillating transverse spin current, which is generated in the naturally-oxidized Cu by the SHE and injected into the adjacent Ni81Fe19, transfer the angular momentum to magnetization via conduction electron. Spin current thereby exerts an oscillating spin-torque on the Ni81Fe19 that induces magnetization precession, which is called spin-torque ferromagnetic resonance (ST-FMR).
Conversion between the current and the spin current, which is generated by the spin-orbit coupling, has not only a bulk effect like a SHE but also an interface or surface effect. This is known as Rashba spin-orbit interaction, which causes the splitting of up spin, and down spin electron energy at zero magnetic field. When an electric current is applied to the Rashba system, the direction of the electron spin is aligned in one direction due to the coupling of the electron momentum and the spin. Therefore, the spin-polarized electron at the interface is exchange-coupled with the magnetization. As a result, even in a system without a SHE, spin-torque is added to magnetization by this exchange coupling.
The spin-torque also appears in the system of Py/CuOx with increasing the homogeneous oxidation level. We find such an efficient spin-torque generation arises not from the SHE in the CuOx layer but from the Rashba effect. The difference in the naturally oxidezed Cu/Ni81Fe19 bilayer and the Ni81Fe19/CuOx bilayer is the interface oxidation and the homogeneity of the oxidation level in the Cu layer. From comparison of the results of the naturally oxidized Cu and homogeneously oxidized one, we demonstrate that the natural oxidation plays the crucial role in the significant enhancement of the SHE. This finding is clearly different from recent observations of the interfacial Rashba spin–orbit torque affected by oxygen incorporation.
SHE can be used to excite dynamics in an ordinary ferromagnetic metal. We study naturally-oxidized Cu/ Ni81Fe19 bilayer films with a microwave-frequency charge current applied in the film plane. An oscillating transverse spin current, which is generated in the naturally-oxidized Cu by the SHE and injected into the adjacent Ni81Fe19, transfer the angular momentum to magnetization via conduction electron. Spin current thereby exerts an oscillating spin-torque on the Ni81Fe19 that induces magnetization precession, which is called spin-torque ferromagnetic resonance (ST-FMR).
Conversion between the current and the spin current, which is generated by the spin-orbit coupling, has not only a bulk effect like a SHE but also an interface or surface effect. This is known as Rashba spin-orbit interaction, which causes the splitting of up spin, and down spin electron energy at zero magnetic field. When an electric current is applied to the Rashba system, the direction of the electron spin is aligned in one direction due to the coupling of the electron momentum and the spin. Therefore, the spin-polarized electron at the interface is exchange-coupled with the magnetization. As a result, even in a system without a SHE, spin-torque is added to magnetization by this exchange coupling.
The spin-torque also appears in the system of Py/CuOx with increasing the homogeneous oxidation level. We find such an efficient spin-torque generation arises not from the SHE in the CuOx layer but from the Rashba effect. The difference in the naturally oxidezed Cu/Ni81Fe19 bilayer and the Ni81Fe19/CuOx bilayer is the interface oxidation and the homogeneity of the oxidation level in the Cu layer. From comparison of the results of the naturally oxidized Cu and homogeneously oxidized one, we demonstrate that the natural oxidation plays the crucial role in the significant enhancement of the SHE. This finding is clearly different from recent observations of the interfacial Rashba spin–orbit torque affected by oxygen incorporation.