Toward the applications to high-performance MRAMs [1, 2], spin-orbit torque (SOT)-induced magnetization switching has been widely investigated in various material systems. While 4d transition metals, such as Ru, exhibit low SOT due to the small spin-orbit coupling [3], recent works have reported the SOT generation in antiferromagnetic RuO2 film thanks to its unique band structure [4], although the detailed understanding is still lacking. Herein, we fabricate the (100)-oriented RuO2 film and evaluate SOT in a RuO2/Co-Fe-B bilayer by harmonic Hall measurement. 
RuO2 (4 nm)/Co20Fe60B20 (1.2 nm) bilayer was fabricated on a α-Al2O3 (0001) substrate by DC/RF sputtering, where (100)-oriented RuO2 film with a likely existence of a three-domain structure is expected to be grown [5]. The resistivity of the RuO2 film was estimated as 247.4 mΩcm from the sheet resistance. A Ru (4 nm)/Co20Fe60B20 (1.2 nm) bilayer was also fabricated for comparison. The harmonic Hall measurement was performed for Hall bar devices in a rotating magnetic field [Fig. 1]. A clear second harmonic signal is observed for RuO2/Co-Fe-B while it is negligible for Ru/Co-Fe-B, suggesting the SOT generation owing to the unique band structure of RuO2. For RuO2/Co-Fe-B bilayer, the effective fields with the Slonczewski-like torque (HSL) and field-like torque (HFL) symmetries were quantified by fitting [Fig. 2(a, b)]. Linear relation with the applied current density J [Fig. 2(c)] proves that these fields are induced by the current. These results would be important for further research on the current-induced magnetization switching using antiferromagnets. 
The authors acknowledge JSPS KAKENHI Grants 21K14522, 21K18189, the Core Research Cluster program, the MRAM program in CIES, Tohoku Univ.
Fig. 1: The second harmonic Hall resistance vs. field direction curves for Hall bar devices of RuO2/Co-Fe-B and Ru/Co-Fe-B.
Fig. 2: Fitting of HSL (a) and HFL (b). J dependence of μ0HSL and μ0HFL (c). 
**References:** [1] H. Honjo et al., IEDM Technical Digest 28.5, (2019). 
[2] M. Natsui et al., JSSC 56, 1116 (2020). 
[3] T. Tanaka et al., Phys. Rev. B 77, 165117 (2008) 
[4] A. Bose et al., Nature Electronics 5, 267 (2022). 
[5] H. Bai et al., Phys. Rev. Lett. 128, 197202 (2022). 

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Spin orbit torques in RuO2/Co

Current-induced spin-orbit torque (SOT) originating from the spin-Hall effect (SHE) has attracted attention due to their potential applications for SOT-MRAM, skyrmion and domain wall devices. For their applications, heavy metal with high spin Hall angle (|θSH|) and low resistivity (ρxx) is necessary for an efficient SOT operation. We have proposed Co/Pt/(Ir or Ru)/Pt/Co synthetic antiferromagnetic (AF) systems for SOT-MRAM (Fig. 1), and found that the Pt/Ir/Pt spacer layer exhibits AF interlayer exchange coupling (Jex) as well as large || and low ρxx [1, 2].
In this study, we study the current-induced SOT in perpendicularly magnetized Co/Pt/Ir/Pt/Co synthetic AF system which exhibits nearly compensated magnetization. The synthetic AF structures with various Pt, Ir and Co layer thicknesses were prepared by using UHV sputtering system. The stacks were patterned into Hall bar devices and magnitude of the was evaluated from the shift of the reversal magnetic field due to the SOT current. The results were compared with those of the ferromagnetic stack systems with Co/Pt and Co/[Pt/Ir]-multilayer structures.
Figures 2(a) and 2(b) show the typical current-induced SOT switching under various fixed external magnetic fields (Hy) in Sample A (Co/Pt structure) and Sample B (synthetic AF structure), respectively. The magnetizations of the two Co layers in the synthetic AF can be switched between two anti-parallel states simultaneously by SOT. The estimated values of current density () at Hy=0 mT for Samples A and B are 7.9×107 and 4.2×107 A/cm2, respectively [3]. The magnitude of in Sample B is about half that in Sample A. The magnitudes of estimated |θSH| for Samples A and B are 7.0% and 15.6%, respectively [3], which is consistent with SOT switching behavior (Fig. 2). In this presentation, we will also show the dependence of the magnitude of SHE on the strength of Jex in the synthetic AF system. This work was supported by the CIES Consortium, Spin-RNJ, RIEC, JST OPERA (JPMJOP1611), MEXT Next generation X-nics and JSPS KAKENHI (JP19H00844, JP21K18189).
 
**References** [1] Y. Saito, N. Tezuka, S. Ikeda and T. Endoh, Phys. Rev. B 104,064439 (2021). [2] Y. Saito, S. Ikeda and T. Endoh, Appl. Phys. Lett. 119, 142401 (2021). [3] Y. Saito, S. Ikeda and T. Endoh, Phys. Rev. B 105, 054421 (2022). 

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Enhancement of current to spin current conversion efficiency in synthetic antiferromagnetic layer system

In a recent work [1], we investigated the spin transfer toque (STT) in epitaxial magnetic tunnel junctions (MTJs) based on ferrimagnetic tetragonal Mn3Ga [2] electrodes and MgO barrier (Fig.1a,b) from first principles ballistic non-collinear spin transport (NEGF + SDFT[LSDA]), using the Smeagol code [3]. We found a long-range spatial oscillation of the in-plane STT (defined as in Ref. [4]) decaying on a length scale of a few nm. We discussed how this STT oscillation can be anticipated from the bulk electronic structure of Mn3Ga and the spin-filtering properties of the MgO barrier, and it is robust against variations in the stack geometry. This is expected to result in a net in-plane torque which oscillates as a function of the Mn3Ga thickness, but stabilizes for sufficiently thick layers (&gt;40 MLs) at larger values than the net STT in conventional Fe/MgO/Fe MTJs [1]. We also discussed a novel all-ferrimagnetic MTJ in which the Fe electrode is substituted with Mn3Ga and found similar STT properties, particularly enhanced in the case of narrow MgO barrier (of 3 MLs), due to resonant interface states. Here we focus on the TMR effect in these MTJ stacks as a function of the applied bias voltage and find a maximum value of over 300%, asymmetry and multiple sign changes in the range ± 2V (Fig.1 c) for the Fe-based junction, in a noteworthy correspondence with experimental observations for similar ferrimagnetic junctions [5]. In the case of the fully symmetric novel ferrimagnetic MTJ, we find a symmetric TMR, peaking at 0 V. This is particularly enhanced for the thin barrier (3 ML) case due to the aforementioned interface resonances. We will discuss effects of geometry optimisation and possible Hubbard U corrections to the LSDA xc-functional on the transport properties, at computed bulk parameters of Mn3Ga closer to the experimental lattice constant and magnetic moments, as well as the role of different interface terminations. In view of the stronger TMR at finite bias for the Fe-based MTJs, our calculations suggest that further efforts are justified on the Fermi-level and interface engineering of ferrimagnetic electrodes, by means of both composition and substrate-induced strain, for demanding spintronic applications.

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High bias TMR in ferrimagnetic MTJs based on Mn3Ga

Spin-orbit torque (SOT)-induced magnetization switching in heavy metal/ferromagnetic (HM/FM) heterostructures has attracted immense interest owing to its potential for the development of high-performance spintronics devices [1-4]. The SOT has been induced by two orthogonal components: the Slonczewski-like field (HSL) and the field-like field (HFL) which are equally capable of magnetization reversal [5,6]. Conventionally, HSL is considered to originate from the bulk spin Hall effect (SHE) induced by the strong spin-orbit coupling of bulk HM [1], and HFL is related to the interfacial Rashba Edelstein effect (REE) originating from the broken inversion symmetry [7]. While HFL was weaker than HSL in many HM-based SOT structures, obscuring the physics of HFL, a sizable value of HFL was reported in Ta-based SOT structures [8]. However, the detailed investigation of HFL has not been performed, and the further manipulation of HSL and HFL has not been reported. Herein, to investigate more details on the contribution of HSL and HFL to the SOT efficiency, we choose Ta/Co-Fe-B system and study the change in HSL and HFL by oxygen incorporation into Ta-layer. Furthermore, the SOT origin in these structures is also discussed. The in-plane magnetized Ta-O (tTa-O nm)/Co20Fe60B20 (1.2 nm)/MgO (1.3 nm)/Ta (1.0 nm) stacks with different Ta-O layer thickness (tTa-O) were fabricated on thermally oxidized Si substrates by RF sputtering. Ta-O underlayer (UL) was formed by in-situ natural oxidization of Ta layer at 0.3; 0.1; 0.03 Pa for 30 minutes. A stack with a 1 nm-Ta UL was also prepared as a reference sample for comparison. HSL and HFL were evaluated using an extended harmonic Hall measurement for Hall bar devices, which enables the exclusion of the thermoelectric effects. An in-plane external magnetic field with various strengths (Hext) was rotated in the film plane, whose direction was represented by the azimuthal angle θ. 10 Hz AC current was applied to the channel and the first and second harmonic Hall voltages were measured by a lock-in amplifier. Figures 1 (a) and (b) show the in-plane magnetic field angle θ dependence of the first (Rω) and second (R2ω) harmonic Hall resistance at different Hext. Rω component for the in-plane magnetized sample is dominated by the planar Hall resistance, following the equation: R1ω=RPHEsin2θ (1), where RPHE is the planar Hall resistance coefficient. From R2ω component, HSL and HFL can be obtained as follows: R2ω = - {RAHE[HSL/(Hext-Hk,eff)+RT]}×cosθ + [2RPHE(HFL+HOE)/Hext]×(2cos3θ-cosθ) (2), where RT is the second harmonic Hall resistance coefficient due to the thermoelectric voltage, HOE is the current-induced Oersted field. RAHE is the anomalous Hall effect (AHE) resistance and Hk,eff is the effective anisotropy field, evaluated from the AHE measurement in the perpendicular magnetic field direction. From the cosθ contribution of R2ω as a function of 1/(µ0Hext-µ0Hk,eff) (upper panel of Fig. 1 (c)); and the (2cos3θ - cosθ) contribution of R2ω as a function of 1/(µ0Hext) (lower panel of Fig. 1 (c)), HSL and HFL were evaluated. Figures 2 (a) and (b) show the oxygen pressure (POxygen) dependence of µ0HSL/I, µ0HFL/I, and HFL/HSL for samples with Ta-O UL of 1 nm, in comparison with the reference sample (dots at POxygen = 0 Pa). Herein, I is the total current applying to the device. The sign and magnitude of µ0HSL/I and µ0HFL/I are consistent with those in the previous work [8]. For all samples, the values of µ0HSL/I and µ0HFL/I are in the same order, implying their equal contribution to the total SOT in both Ta and Ta-O UL systems. As POxygen increases, the magnitude of µ0HSL(FL)/I decrease, which might be attributed to the low oxygen-incorporating in the system. However, HFL/HSL increases with the increase of POxygen meaning that this ratio would be enhanced by oxidization [9]. Figures 2 (c) and (d) show the tTa-O dependence of µ0HSL/I, µ0HFL/I, and HFL/HSL for stack with Ta-O UL oxidized at 0.3 Pa. The absolute values of µ0HSL/I, µ0HFL/I increase and converge to a certain value as tTa-O increases, suggesting the coexistence of bulk and interface effects originating from SHE, and REE [6]. The absolute value of µ0HFL/I is higher than that of µ0HSL/I suggesting the stronger interfacial REE-related origin than bulk SHE-related origin, which also can be observed in Fig. 2 (d) with the increase of HFL/HSL ratio as tTa-O increases. These results are of importance to understand the SOT induced magnetization switching, as well as to design the spintronic devices using oxidized HM/FM systems. The authors acknowledge the Core Research Cluster program of Tohoku University, Japan. This work was supported by JSPS KAKENHI Grant Number 19K23583, 19H00844, and by CSRN, and Cooperative Research Project from CSIS, Tohoku University, Japan. This research has been partly carried out at the Laboratory for Nanoelectronics and Spintronics, RIEC, Tohoku University. Fig. 1: Field direction dependence of Rω (a) and R2ω (b) at different magnetic field strength. The black curves are fitting data with eqs. (1), (2). (c) The cosθ contribution of R2ω as a function of 1/(μ0Hext - μ0Hk,eff) (upper); and the (2cos3θ - cosθ) contribution of R2ω as a function of 1/(μ0Hext) (lower). Fig. 2: Oxygen pressure dependence of μ0HSL(FL)/I (a) and HFL/HSL (b) for samples with 1 nm thick UL. tTa-O dependence of μ0HSL(FL)/I (c) and HFL/HSL (d) for samples with Ta-O UL oxidized at 0.3 Pa.

**References:**

[1] L. Liu et al., Science 336, 555, (2012). [2] S. Fukami et al., in Symp. VLSI Technol. Dig. Tech. Papers, pp. 60–61, (2016). [3] H. Honjo et al., IEDM Technical Digest 28.5, (2019). [4] M. Natsui et al., in Symp. VLSI Technol. Dig. Tech. Papers, CM2.2, (2020). [5] C. Zhang et al., Appl. Phys. Lett. 107, 012401, (2015). [6] Y. Du et al, Phys. Rev. Appl. 13, 054014, (2020). [7] V. M. Edelstein, Solid State Commum. 73, 233, (1990). [8] C. Zhang et al., Appl. Phys. Lett. 103, 262407, (2013). [9] H. An et al., Sci. Adv. 4, eaar2250, (2018).

**Images:**

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 Spin-orbit torque in naturally oxidized Ta-O/Co-Fe-B/Mg-O/Ta structures

Current-induced spin-orbit torque (SOT) originating from the spin-Hall effect (SHE) in heavy metal/ferromagnet (HM/FM) systems has attracted attention due to their potential application in the efficient manipulation of magnetization of nano magnets in SOT-MRAM [1], skyrmion and domain wall devices. For the application to a large-scale integration, the efficient SOT operation (high spin Hall angle |θSH|) in low resistivity (ρxx) HM is necessary [2-4]. Due to the extensive efforts, the |θSH| becomes larger day by day, however, almost all HMs with large magnitude of |θSH| have a very high ρxx. The magnitude of |θSH| for both intrinsic and extrinsic (side jump mechanism) terms is proportional to the magnitude of ρxx value (|θSH|～σSH ρxx) [5, 6], where σSH is spin Hall conductivity. Therefore, increase in the magnitude of σSH would be important from the application point of view. Recently, we observed large magnitude of σSH and enhancement of perpendicular magnetic anisotropy in MgO/CoFeB/(W/Hf)-multilayer system compared to MgO/CoFeB/β-W system [4]. In this paper, we prepared MgO/CoFeB/(W/Hf)-multilayer systems with various W/Hf thickness ratios and investigated the W/Hf thickness ratio dependence of θSH, σSH, ρxx, and (100) texture of MgO tunnel barrier in the MgO/CoFeB/(W/Hf)-multilayer systems. These systems were patterned into the microscale Hall bar by photolithography and Ar ion milling, and annealed at 573 K. SHE in MgO/CoFeB/(W/Hf)-multilayer systems by means of spin Hall magnetoresistance (SMR) was studied. The analysis based on the drift diffusion model reveals that the magnitude of σSH monotonically increases with increasing W/Hf thickness ratio (Fig. 1). Spin diffusion length (λ) for (W/Hf)-multilayer is shorter than the estimated value of β-W (Fig. 1), which would be related to the increase in the interfacial scattering of multilayer system. In this presentation, we will report the details of SMR, θSH, λ and the film structure including the (100) texture of MgO tunnel barrier. This work was supported by the JST OPERA (JPMJOP1611) and JSPS KAKENHI (19H00844).References: [1] H. Honjo et al., IEDM Tech. Dig. 28.5 (2019). [2] Y. Saito et al., Appl. Phys. Exp. 12, 053008 (2019). [3] Y. Saito et al., AIP Advances, 9, 125312 (2019). [4] Y. Saito et al., Appl. Phys. Lett. 116, 132401/1-5 (2020). [5] C. Zhang et al., Appl. Phys. Lett. 109, 192405 (2016). [6] L. Wang et al., Phys. Rev. Lett. 116, 196602 (2016).

Flat and highly (100) texture of MgO tunnel barrier and W/Hf thickness ratio dependence in MgO/CoFeB/(W/Hf)-multilayer large spin Hall effect electrode system

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Yoshiaki Saito

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Presentations

Spin orbit torques in RuO2/Co

Enhancement of current to spin current conversion efficiency in synthetic antiferromagnetic layer system

High bias TMR in ferrimagnetic MTJs based on Mn3Ga

Spin-orbit torque in naturally oxidized Ta-O/Co-Fe-B/Mg-O/Ta structures

Flat and highly (100) texture of MgO tunnel barrier and W/Hf thickness ratio dependence in MgO/CoFeB/(W/Hf)-multilayer large spin Hall effect electrode system

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