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Interfacial two-dimensional electron gas (2DEG) formed at the perovskite-type oxide, such as SrTiO3, has attracted significant attention due to its properties of ferromagnetism, superconductivity, and its potential application in oxide-based low-power consumption electronics. Recent studies have investigated spin-to-charge conversion at the STO interface with different materials, which could affect the efficiency of this 2DEG interface. In this report, we presented an Ar^+ ion milling method to create a 2DEG at STO directly by inducing oxygen vacancies. To quantify the spin-to-charge conversion of this interface, we measured the angular-dependent spin-torque ferromagnetic resonance (ST-FMR) spectra, revealing an unconventional spin polarization at the interface of Argon ion-milled STO and NiFe. Furthermore, a micromagnetic simulation for angular-dependent spin-torque ferromagnetic resonance (ST-FMR) has been performed, confirming the large unconventional spin polarization at the interface.

The impurity incorporation in host high-spin orbit coupling materials like platinum has shown improved charge-to-spin conversion by modifying the up-spin and down-spin electron trajectories by bending or skewing them in opposite directions. This enables efficient generation, manipulation, and transport of spin currents. In this study, we irradiate the platinum with non-focus dense plasma to incorporate the oxygen ion species. We systematically analyze the spin Hall angle of the oxygen plasma irradiated Pt films using spin torque ferromagnetic resonance. Our results demonstrate a 2.4 times enhancement in the spin Hall effect after plasma treatment of Pt as compared to pristine Pt. This improvement is attributed to the introduction of disorder and defects in the Pt lattice, which enhances the spin-orbit coupling and leads to more efficient charge-to-spin conversion without breaking the spin-orbit torque symmetries. Our findings offer a new method of dense plasma-based modification of material for the development of advanced spintronic devices based on Pt and other heavy metals.

Interfacial two-dimensional electron gas (2DEG), especially the SrTiO3-based ones at the unexpected interface of insulators, have emerged to be a promising candidate for efficient charge-spin current interconversion. In this article, to gain insight into the mechanism of the charge-spin current interconversion at the oxide-based 2DEG, we focused on conducting interfaces between insulating SrTiO3 and two types of aluminium-based amorphous insulators, namely SrTiO3/AlN and SrTiO3/Al2O3, and estimated their charge-spin conversion efficiency, {\theta}_cs. The two types of amorphous insulators were selected to explicitly probe the overlooked contribution of oxygen vacancy to the {\theta}_cs. We proposed a mechanism to explain results of spin-torque ferromagnetic resonance (ST-FMR) measurements and developed an analysis protocol to reliably estimate the {\theta}_cs of the oxide based 2DEG. The resultant {\theta}_cs/t, where t is the thickness of the 2DEG, were estimated to be 0.244 nm-1 and 0.101 nm-1 for the SrTiO3/AlN and SrTiO3/Al2O3, respectively, and they are strikingly comparable to their crystalline counterparts. Furthermore, we also observe a large direct current modulation of resonance linewidth in SrTiO3/AlN samples, confirming its high {\theta}_cs and attesting an oxygen-vacancy-enabled charge-spin conversion. Our findings emphasize the defects' contribution to the charge-spin interconversion, especially in the oxide-based low dimensional systems, and provide a way to create and enhance charge-spin interconversion via defect engineering.

Interfacial two-dimensional electron gases (2DEG), especially the SrTiO3-based ones at the unexpected interface of insulators, have emerged to be promising candidates for efficient charge–spin interconversion. Herein, to gain insight into the mechanism of the charge–spin interconversion, quasi-2DEG between insulating SrTiO3 and two types of aluminum-based amorphous insulators, namely SrTiO3/AlN and SrTiO3/Al2O3, are focused on and their charge-to-spin conversion efficiency is estimated. The two types of amorphous insulators are selected to probe the overlooked contribution of oxygen vacancy. A mechanism to explain the results of spin–torque ferromagnetic resonance measurements is proposed and an analysis protocol to reliably estimate in quasi-2DEG is developed. The resultant, thickness of the 2DEG, is estimated to be 0.244 and 0.101 nm−1 for SrTiO3/AlN and SrTiO3/Al2O3, respectively, which are strikingly comparable to their crystalline counterparts. Furthermore, a large direct current modulation of resonance linewidth in SrTiO3/AlN samples is developed, confirming and attesting an oxygen vacancy-enabled charge–spin conversion. The findings emphasize the defects' contribution-, especially in oxide-based low-dimensional systems, and provide a way to create and enhance charge–spin interconversion via defect engineering. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version U.S. and A.D. contributed equally to this work. Y.F. and H.A. would like toacknowledge JSPS Grant-in-Aid (KAKENHI no. 22K04198), KIOXIACorporation and Iketani Science and Technology Foundation. X.R.W.acknowledges support from Academic Research Fund Tier 2 (grant no.MOE-T2EP50220-0005) and Tier 3 (grant no. MOE2018-T3-1-002) fromSingapore Ministry of Education. R.M., S.G., and R.S.R. would like toacknowledge the support from the Ministry of Education, Singapore, grantno. MOE2019-T2-1-058 (ARC 1/19 RSR). R.M., S.G., R.S.R., and X.R.W.acknowledges support from the National Research Foundation (NRF),Singapore, under its 21st Competitive Research Programs (CRP grantno. NRF-CRP21-2018-0003).

Damping-like torque (DLT) arising from the spin Hall effect (SHE) in heavy metals and their alloys has been widely explored for applications in spin–orbit torque MRAM, auto-oscillations, spin waves, and domain wall motion. In conventional materials, the DLT efficiency is limited by intrinsic properties, while attaining strong spin–orbit coupling and higher spin-charge interconversion, with no compromise to electric properties, is the need of the hour. In this Letter, we report more than 3.5 times increase in DLT efficiency, θDL, of modified Pt-oxide by employing a better approach of low energy 20 keV O+ ion implantation. The highest fluence of O+ implantation (1 × 1017 ions cm−2) in Pt enhanced the DLT efficiency from 0.064 to 0.230 and improved the spin transmission for a smaller trade-off in the longitudinal resistivity (ρPt to ρPt−Oxide) from 55.4 to 159.5 μΩ cm, respectively. The transverse spin Hall resistivity, ρSH, is found to be proportional to the square of the longitudinal resistivity, i.e., ρimpSH∝ρ2imp, implying that the enhanced SHE in O-implanted Pt is due to a side-jumping mechanism. Further, no break in the twofold as well as mirror symmetry of torques from the O-implanted Pt allows the use of spin-torque ferromagnetic resonance-based line shape analysis to quantify such torques.

1. Introduction and theory||2. Experimental methods||3. Enhanced spin Hall effect in S-implanted Pt||4. Dose dependent spin Hall effect in O-implanted Pt and Ox sputtered Pt||5. Disentanglement of intrinsic and extrinsic side-jump scattering induced spin Hall effect in O and N-implanted Pt||6. Giant spin Hall effect in P-implanted Pt layers||7. Beyond Ion-implanted Pt: Room temperature charge-to-spin conversion from q-2DEG-based interfaces, Spin Hall effect （SHE）, initially predicted by Dyakonov and Perel in 1971 and later revisited by Hirsch in 1999 [1], is the generation of spin current density js from a charge current density jc. The conversion of jc to js via SHE, can exert a damping-like spin-orbit torque （SOT） tDL on the magnetization of a ferromagnetic （FM） layer attached to the heavy metal （HM）. The charge-to-spin conversion is quantified by the damping-like torque efficiency theta DL, which is given by the relation js = （ℏ/2e）theta DL（jc×sigma）, where ℏ is the reduced Planck constant, e is electronic charge and sigma is polarization of spin current. Typically, 4d and 5d transition metals （such as Pt, Ta and W） are used as the HM, and their corresponding theta DL are found to be ≈0.1. This intrinsic mechanism depends on the Berry curvature of the material, in which an anomalous velocity arises from a momentum-space Berry phase [2]. An increase in theta DL has been found by alloying the HM with Au, Pd and also by incorporating non-metallic elements （with smaller atomic number Z） into the HM. However, in these materials, a complete understanding of the mechanism responsible for the increase in sigma DL remains elusive. Another phenomenon, the Rashba-Edelstein effect （REE）, leads to the generation of nonequilibrium spins at the interface of an inversion asymmetry, in a metal oxide heterostructure [3]. The oxygen vacancies created at the interface lead to mobile conduction electrons forming a quasi-two dimensional electron gas （q-2DEG）. An electron having momentum p, in the presence of an electric field E, experiences an effective magnetic field along （p×E）. This field leads to spin polarization of the electrons and subsequently a spin current flows from the metal oxide heterostructure to the FM and exerts a tDL on the magnetization of the FM [4]. Similar to SHE, we can define a charge-to-spin conversion efficiency theta DL. So far, a relation between the number of oxygen vacancies and theta DL is yet to be found. Overall, a high theta DL as well as an investigation of its mechanism for the origin of SHE and REE is important in switching the magnetization for writing operation in next generation based magnetic random access memory （MRAM）. In this thesis, we perform ion implantation in Pt, using different nonmetallic elements, namely sulfur （S）, oxygen （O）, nitrogen （N）, and phosphorus （P） and study the theta DL via spin-torque ferromagnetic resonance （ST-FMR） measurements [5]. Additionally, we incorporate O in Pt via sputtering, PtOx to compare the sputtering and ion implantation methods. We also compare the q-2DEG created at the interface of SrTiO3/AlN and SrTiO3/Al2O3 in terms of theta DL and a detailed investigation of the angular symmetry of spin-orbit torque （SOT） [6], as well as the calculation of theta DL as a function of temperature and implantation dose （oxygen vacancy） is carried out, revealing the mechanism responsible for enhancement of the SHE （REE）. We use a low energy of 12 keV to implant S in Pt, Pt（S） at a dose of 01016-51016 ions/cm2. Next, we use an energy of 20 keV to implant O and N in Pt, Pt（O） and Pt（N）, respectively at a varying dose of 01016-11017 ions/cm2. Finally, we move to a moderate energy of 30 keV to implant P in Pt in a wide range of 01016-91016 ions/cm2. For Pt（S）, we achieve a high theta DL≈0.50 at 10 K, which is 8-times higher than the theta DL or pure Pt at 10 K. We observe a highly monotonic dose dependence in theta DL of Pt（O） from 0.064 to 0.230 at 293 K, with a smaller trade-off in beta xx from 55.4 to 159.5 mu Ω cm, respectively as we increase the dose from 01016-11017 ions/cm2. For Pt（N）, we attain a high theta DL≈at 293 K for a particular dose of 51016 ions/cm2. Finally, for the Pt（P）, we obtain a giant theta DL≈ at 293 K, which makes it an ideal candidate of spintronics based MRAM. For PtOx, we also report a high theta DL≈ 0.40 attesting that O in Pt enhances the SHE via both ion implantation and sputtering method. Finally, we explore REE of the q-2DEG created at the interface of SrTiO3/amorphous oxides, namely the SrTiO3/AlN and SrTiO3/Al2O3. We observe a very high theta DL ≈ 2.44 for SrTiO3/AlN than theta DL ≈1.01 for SrTiO3/Al2O3 due to a higher oxygen vacancy enabled REE for the former. The high theta DL is an order higher than the theta DL of 5d transition metal such as Pt. For the dominant underlying mechanism for the ion implanted samples, we confirm a linear dependence of spin Hall resistivity from impurities, beta SH imp with the square of resistivity from impurities, beta 2 imp, i.e., beta SH imp ∝ beta imp [7], implying an extrinsic origin of side-jump scattering as the dominant origin of SHE for the Pt（S）, Pt（O）, Pt（N） samples studied as a function of implantation and temperature. On the other hand for the PtOx, we do not get a linear beta SH imp ∝ beta 2 mp especially for the lower concentration （n %） of O in Pt, suggesting an intrinsic mechanism while we obtain an extrinsic side-jump for higher n %. In order to disentangle the dominant extrinsic side-jump from the intrinsic SHE, we express the spin Hall conductivity, sigma xy SH as the sum of intrinsic and extrinsic SHE （skew scattering/side-jump） and study it as a function of square of conductivity, sigma 2 xx [8]. We exclude skew scattering as a possible extrinsic mechanism due to the ion-implanted samples not lying in higher of sigma xx of super clean metals （10 6 < sigma xx < 10 8 Ω-1cm-1） [2]. Then, we show that a sudden decrease in intrinsic spin Hall conductivity, sigma int SH is counterbalanced by the increase in side-jump induced SHE, theta sj SH due to the increase in residual resistivity from impurities, beta xx,0. Hence, for all ion implanted samples, Pt（S）, Pt（O）, Pt（N）, Pt（P）, we find that higher the beta xx,0, higher is the theta sj SH, and lower is the sigma int SH. We obtain a simple model that theta DL via SHE can be enhanced by simply increasing beta xx,0 in a 5d transition metal. Moreover, we find a crossover of intrinsic to extrinsic side-jump SHE as we increase the dose of S, O, N, P ions in Pt from ≈ 01016-11017 in a wide energy range of 12-30 KeV. Overall, the contribution of the extrinsic side jump induced SHE to the increase in theta DL is clearly identified through our studies on the ion implanted samples. To gain insight into the overlooked contribution of oxygen vacancy to the theta DL via REE in q-2DEG, we find that the higher oxygen vacancy created at SrTiO3/AlN in comparison to SrTiO3/Al2O3, not only plays a role in enhancing the electronic transport, but may also lead to a higher theta DL. To confirm the high theta DL in SrTiO3/AlN, we observe a large direct current modulation of resonance linewidth via direct current biased ST-FMR measurements [5] which is 1-2 order higher than Pt. For understanding the angular symmetry of SOT, which is crucial in estimation of theta DL, via ST-FMR lineshape analysis, we study the ST-FMR spectrum as a function of phi （where phi is the angle between microwave current in device and external magnetic field）. In our Pt（S）, Pt（O）, Pt（N）, Pt（P）, and PtOx, we observe no breaking of the two-fold and mirror symmetry of SOT due to the 100 % sin2 phi cos phi dependence of symmetric（S） and antisymmetric （A） component of ST-FMR spectrum leading to simple estimation of theta DL. Whereas for q-2DEG, we observe a broken symmetry of SOT and develop an analysis protocol by filtering out the sin2 phi cos phi from S and A arising from tDL and reliably estimate the thata DL. In conclusion, we report that theta DL can be enhanced in host Pt by incorporating non-metallic impurity such as S, O, N, and P in Pt due to the extrinsic side-jump scattering induced SHE from impurities. We obtain a high theta DL for Pt（S） at 10 K and Pt（P） at 293 K. For the q-2DEG, we observe a high theta DL ≈2.44 for SrTiO3/AlN than theta DL ≈1.01 for SrTiO3/Al2O3, which is an order higher than Pt. We explicitly probe the overlooked contribution of oxygen vacancy to the theta DL enabled by REE. These results suggest that the SHE and REE materials could help us in the development of not only MRAM but also in various spintronic based memory devices utilizing the high charge-to-spin conversion in future., 九州工業大学博士学位論文 学位記番号: 情工博甲第374号 学位授与年月日: 令和5年3月24日, 令和4年度