Abstract
Spin orbit torque (SOT) magnetization switching of ferromagnets with large perpendicular magnetic anisotropy has a great potential for the next generation non-volatile magnetoresistive random-access memory (MRAM). It requires a high performance pure spin current source with a large spin Hall angle and high electrical conductivity, which can be fabricated by a mass production technique. In this work, we demonstrate ultrahigh efficient and robust SOT magnetization switching in fully sputtered BiSb topological insulator and perpendicularly magnetized Co/Pt multilayers. Despite fabricated by the magnetron sputtering instead of the laboratory molecular beam epitaxy, the topological insulator layer, BiSb, shows a large spin Hall angle of θSH = 10.7 and high electrical conductivity of σ = 1.5 × 105 Ω−1 m−1. Our results demonstrate the feasibility of BiSb topological insulator for implementation of ultralow power SOT-MRAM and other SOT-based spintronic devices.
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Introduction
Embedded non-volatile memories have great impacts on energy-efficient electronics, including Internet-of-Thing, Artificially Intelligent (AI), among others. To be successful, non-volatile memories have to satisfy several requirements, such as high writing endurance, high capacity, high speed, and low fabrication cost. Among many emerging non-volatile memory technologies, magnetoresistive random-access memory (MRAM) is one of the most promising that have gained development commitment from several leading semiconductor companies. The latest MRAM technology using the sophisticated spin–transfer–torque (STT) writing technique has just been commercially available very recently, but already found various important applications, such as highly efficient AI chips. However, in STT-MRAM, a large writing current has to be injected directly to magnetic tunneling junctions (MTJs), which leads to reliability issues such as accelerated aging of the oxide tunnel barrier1. In addition, large writing currents require large driving transistors, making it difficult to increase the bit density of STT-MRAM beyond the 1 Gbit capacity. Recently, the spin–orbit–torque (SOT) technique has emerged as a promising writing method for the next generation MRAM2. In SOT-MRAM, a charge current flowing in a non-magnetic layer with large spin–orbit interaction can generate a pure spin current by the spin Hall effect. The pure spin current is then injected to the magnetic free layer for magnetization switching. The relationships between the spin current IS and the charge current IC is given by IS = (ℏ/2e)(L/t)θSHIC, where L is the MTJ size, and t the thickness of the spin Hall layer, and θSH is the spin Hall angle. Theoretically, the charge-to-spin conversion efficiency (L/t)θSH in SOT-MRAM can be larger than unity, meaning that lower driving currents can be expected. Furthermore, since there is no current flowing into the MTJs, reliability can be significantly improved. Finally, since the spin-polarization of the pure spin current is perpendicular to the magnetization direction of the free magnetic layer, the spin torque is maximized and the magnetization can switch very fast (< ns) in SOT-MRAM with perpendicular magnetic anisotropy (PMA)3. Because of those merits of SOT-MRAM comparing with STT-MRAM, there have been huge efforts to find spin Hall materials with large θSH and high electrical conductivity for SOT-MRAM implementation. Heavy metals, such as Pt4,5, Ta2, and W Next, we performed the second harmonic Hall measurements to evaluate the spin Hall angle30. An alternating current (AC) J = J0sin ωt (ω = 259.68 Hz) was applied to the Hall bar under a swee** external field along the x direction. We measured the 2nd harmonic Hall resistance \(R_{\text{H}}^{{2{\upomega }}}\), which is originated from the oscillation of the net magnetic moment under the spin–orbit effective magnetic fields31,32,33. Figure 2a shows a representative \(R_{\text{H}}^{{2{\upomega }}}\)- Hx curve measured at JBiSb = 3.6 × 105 Acm−2. The second harmonic Hall resistance \(R_{\text{H}}^{{2{\upomega }}}\) can be expressed as33 where HAD is the antidam**-like effective field, HFL+Oe is the sum of the fieldlike and Oesterd field, RPHE is the planar Hall resistance, and Rthermal is the contribution from the anomalous Nernst (ANE) and spin Seebeck (SSE) effects. Note that the contribution of the fieldlike and Oesterd field to \(R_{\text{H}}^{{2{\upomega }}}\) is much smaller than that of the antidam**like field in BiSb34,35. Fitting Eq. (1) to the high field data in the \(R_{\text{H}}^{{2{\upomega }}}\)- Hx curve yields HAD (red curves in Fig. 2a). Figure 2b shows HAD as a function of JBiSb. From the HAD / JBiSb gradient, we can calculate the effective spin Hall angle \({\theta }_{\mathrm{SH}}^{\mathrm{eff}}\) = \(\frac{2e}{\hbar }M_{{{\text{CoPt}}}} t_{{{\text{CoPt}}}} \frac{{H_{{{\text{AD}}}} }}{{J^{{{\text{BiSb}}}} }}\) = 12.3, where e is the electron charge and \(\hbar\) is the reduced Plank constant. Recently, it was reported that CoPt multilayers can generate a “self” spin–orbit torque36. We found that the “self” spin–orbit torque in CoPt can contribute to 13% of the total spin–orbit torque (see Suppl. Info. Section 6). Furthermore, we confirmed that there is no artifact contribution from the asymmetric magnon scattering mechanism (see Suppl. Info. Section 7), as observed in the case of the magnetic topological insulator (CrBiSb)2Te337. Subtracting the contribution from CoPt, we obtain the intrinsic spin Hall angle of BiSb θSH = 10.7, which demonstrates the feasibility of BiSb for ultralow power SOT-MRAM. Evaluation of the spin Hall angle by the second harmonic measurements. (a) 2nd harmonic Hall resistance as a function of the in-plane external magnetic field H applied along the x direction. The red curves are the theoretical fitting using Eq. (1). (b) HAD as a function of JBiSb. Next, we demonstrate ultrahigh efficient and robust SOT magnetization switching in the CoPt/BiSb multilayers. Figure 3 shows the SOT magnetization switching by DC currents with an applied external field along the x direction. We achieved Hall resistance switching whose amplitude is consistent with that of the Hall resistance loop shown in Fig. 1d, indicating full magnetization switching. The switching direction is reversed when the external magnetic field direction is reversed, which is consistent with the characteristic of SOT. Typical DC threshold switching current density \(J_{{{\text{th}}}}^{{{\text{BiSb}}}}\) is 1.5 × 106 Acm−2 at the bias field of 2.75 kOe. Note that thanks to the high electrical conductivity σ = 1.5 × 105 Ω−1 m−1 of BiSb, the total current density including the shutting current in the CoPt is kept at 2.6 × 106 Acm−2. Next, we performed SOT magnetization switching by pulse currents. Figure 4a,b show representative SOT switching loops by 0.1 ms pulse currents at + 1.83 kOe and − 1.83 kOe, respectively. Figure 4c plots \(J_{{{\text{th}}}}^{{{\text{BiSb}}}}\) at various pulse width tpulse, and the theoretical fitting using the thermal activation model \(J_{{{\text{th}}}}^{{{\text{BiSb}}}}\) = \(J_{{0}}^{{{\text{BiSb}}}}\)×\(\left[ {1 - \frac{1}{\Delta }\ln \left( {\frac{{\tau_{{{\text{pulse}}}} }}{{\tau_{0} }}} \right)} \right]\)38, where \(J_{{0}}^{{{\text{BiSb}}}}\) is the zero-kelvin threshold switching current density, ∆ is the thermal stability factor, and 1/τ0 = 1 GHz (τ0 = 1 ns) is the attempt switching frequency. The fitting yields \(J_{{0}}^{{{\text{BiSb}}}}\) = 4.6 × 106 Acm−2 and Δ = 38. Because magnetization switching occurs by domain wall nucleation and domain wall motion, Δ reflects the energy barrier of the volume with size equal to the domain wall width, i.e. Δ should be considered as the energy barrier to nucleate a domain wall, rather than the energy barrier for coherently switching of the whole volume of the magnetic layer39. Therefore, Δ evaluated by this way is smaller than that should be expected for switching the whole volume of the magnetic layer. Nevertheless, the obtained Δ of CoPt is large enough to ensure that the total Δ in ferromagnetically (antiferromagnetically) coupled CoFeB/Ta(Ru)/CoPt free layer can exceeds 60 for 10 years thermal stability, while the switching current density remains the same40. Finally, we demonstrate robust SOT switching in the CoPt/BiSb junction. For this purpose, we applied a sequence of 150 pulses (\(J_{{{\text{th}}}}^{{{\text{BiSb}}}}\) = ± 4.4 × 106 Acm−2, tpulse = 0.1 ms) as shown in the top panel of Fig. 4c. The Hall resistance data recorded for a total of 150 pulses under ± 1.83 kOe are shown in the bottom panel in Fig. 4c. We observed a robust SOT switching with no change in the device characteristics, indicating that the BiSb topological insulator deposited by the sputtering technique has great potential for realistic SOT-MRAM. SOT magnetization switching by pulse currents. (a,b) Switching loop by 0.1 ms pulse currents under an in-plane magnetic field of H = + 1.83 kOe and − 1.83 kOe, respectively. (c) Threshold current density \(J_{{{\text{th}}}}^{{{\text{BiSb}}}}\) as a function of tpulse. (d) Robust SOT magnetization switching by 0.1 ms pulse current. Table 1 summarizes θSH, σ, the spin Hall conductivity σSH = (ħ/2e)σθSH, and the SOT normalized power consumption Pn at room temperature of several heavy metals and TIs. Here, θSH of TIs are their best values reported in literature. For the calculation of the Pn, we assumed bilayers of spin Hall material (thickness t = 6 nm for heavy metals and t = 10 nm for TIs) and CoFeB (thickness tFM = 1.5 nm, conductivity σFM = 6 × 105 Ω−1 m−1). Considering the shunting current in the ferromagnetic layer, the SOT power consumption is proportional to (σt + σFMtFM)/(σtθSH)2. One can see that not only θSH but also σ affect the SOT power consumption, a fact usually overlooked in literature. For example, while the sputtered BixSe1-x has a much larger spin Hall angle (θSH = 18.6) than that (θSH = 3.5) of MBE-grown Bi2Se3, their power consumption is nearly the same, because BixSe1-x has poorer crystal quality than Bi2Se3 and thus very low conductivity. Meanwhile, the sputtered BiSb thin film in this work shows both high σ = 1.5 × 105 Ω−1 m−1 and large θSH = 10.7, which are optimal for both small switching current density and small switching power consumption41. Indeed, the switching power consumption for sputtered BiSb is 50 times smaller than that for sputtered BixSe1-x, and over 300 times smaller than that for W, which is the most used heavy metal in SOT-MRAM development. The small switching current density and switching power also help suppress failure of the spin Hall layer due to electromigration and Joule heating42. Our results demonstrate the feasibility of BiSb for not only ultralow power SOT-MRAM but also other SOT-based spintronic devices, such as race-track memories43 and spin Hall oscillators44,45. Finally, we discuss about the remaining challenges for realization of ultralow power BiSb-based SOT-MRAM. One of material challenges is the large surface roughness of BiSb due to the unusually large crystal grain size comparing with the layer thickness. Furthermore, atomic interdiffusion between BiSb and the free magnetic layer during annealing process of the MTJ is also a challenge to be resolved. Thereby, a realistic pathway to integrate BiSb to SOT-MRAM is to fabricate full stack MTJs with PMA (p-MTJ) first, then deposit the BiSb layer on top of the MTJs at the last step after the annealing process, i.e. BiSb-on-MTJ structure. This will help avoid the interdiffusion and the BiSb surface roughness problems in the MTJ-on-BiSb structure. We deposited multilayers of (0.4 nm Co / 0.4 nm Pt)2 / 10 nm Bi0.85Sb0.15 / 1 nm MgO / 1 nm Pt on c-plane sapphire substrates by DC (for Co, Pt, BiSb) and RF (for MgO) magnetron sputtering in a multi-cathode chamber. All layers are deposited by sputtering from their single targets using Ar plasma without breaking the vacuum at room temperature. The samples were patterned into 90 μm-long × 25 μm-wide Hall bars by optical lithography and lift-off. A 45 nm-thick Pt were deposited as electrodes by DC magnetron sputtering, which reduces the effective length of the devices to 50 μm. The samples were mounted inside a vacuumed cryostat equipped with an electromagnet. For the second harmonic measurements, a NF LI5650 lock-in amplifier was employed to detect the first and the second harmonic Hall voltages under sine wave excitation generated by a Keithley 6221 AC/DC current source. For the DC (pulse) current-induced SOT magnetization switching, a Keithley 2400 SourceMeter (6221 AC/DC current source) was used, and the Hall signal was measured by a Keithley 2182A NanoVoltmeter.Evaluation of the spin Hall angle by the second harmonic Hall measurements
Ultrahigh efficient spin–orbit torque magnetization switching by DC and pulse currents
SOT performance benchmarking and future prospective
Method
Material growth
Device fabrication
SOT characterization
Data availability
The data that support this study results are available from the corresponding author upon reasonable request.
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Acknowledgements
This work is supported by the CREST program of the Japan Science and Technology Agency (No. JPMJCR18T5). We sincerely thank N. Hatakeyama and H. Iida at the Material Analysis Division of Tokyo Institute of Technology for their help in TEM and XRD measurements.
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T.F. fabricated the samples with contribution from N.H.D.K and S.N. T.F. performed spin-torque measurements. P.N.H supervised the project. T.F. and P.N.H wrote the paper with input from N.H.D.K and S.N.
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Fan, T., Khang, N.H.D., Nakano, S. et al. Ultrahigh efficient spin orbit torque magnetization switching in fully sputtered topological insulator and ferromagnet multilayers. Sci Rep 12, 2998 (2022). https://doi.org/10.1038/s41598-022-06779-3
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DOI: https://doi.org/10.1038/s41598-022-06779-3
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