Improved Spin-Orbit Torque Switching Efficiency by He<SUP>+</SUP> Ion Irradiation

다국어 초록 (Multilingual Abstract)

The magnetic thin films with inversion symmetry breaking have been investigated because of its great potential for application of magnetic random access memory (MRAM) devices and scientific interests about spin-orbit coupling (SOC) phenomena. Especially because Heavy-metal (HM) / Ferromagnet (FM) / Oxide (MgO) structure has strong SOC at the HM/FM interface and inversion symmetry breaking system, resulting in strong perpendicular magnetic anisotropy (PMA) [1], spin-orbit torque (SOT) [2] and Dzyaloshinskii-Moriya interaction (DMI) [3,4]. Among them, the SOT driven magnetization switching is a perspective phenomenon because of its application to the spintronic devices. For more efficient SOT based spin devices, many researchers try to reduce critical switching current by HM layer modulation [5], interface modifying [6, 7], and ion irradiation [8, 9].
Here, we report that local magnetic properties are modulated by He⁺ ion irradiation in Pt/Co/MgO structure. The Pt(5)/Co(0.8)/MgO(2) structure is irradiated by He⁺ ion with the dose range from 0 to 30 ions/nm², then the reduction of switching current is shown about ~ 30% at dose amount of 30 ions/nm² under external magnetic field of 3 kOe. The reduction of switching current is related with effective PMA field and spin Hall angle (SHA), we analyze each phenomenon using generalized Sucksmith-Thompson method and harmonic Hall analysis, respectively. The result show the decrease of PMA field from 13.7 kOe to 8.5 kOe (~ 38%) and increase of SHA from 0.096 to 0.132 (~ 27%). And then, for understanding the reason of improvement of SHA, we conduct the resistivity measurement with temperature variation from 5 K to 225 K in 5 nm thickness single Platinum layer. The results show gradual increasing tendency in resistivity and decreasing tendency in residual-resistivity ratio (ρ_300K/ρ_5K) at higher dose amount. These imply that the He⁺ ion irradiation makes extra scattering sources in HM layer and result in increasement of resistivity and SHA.