Magnetic memory driven by topological insulators

Non-volatile magnetic memory, particularly magnetic random-access memory (MRAM), is a promising next-generation memory technology. MRAM boasts ultralow energy consumption, ultrafast speed, and high endurance. Information is stored in the magnetic tunnel junction (MTJ) – a ferromagnetic electrode/insulator/ferromagnetic electrode (FM/I/FM) structure – where tunneling resistance depends on the magnetization orientation of the FM electrodes. Current-induced spin torques, such as spin-transfer torque (STT) and spin-orbit torque (SOT), offer efficient magnetization switching mechanisms. SOT-driven switching, demonstrated in heavy metal/ferromagnet (HM/FM) structures, utilizes spin current generated by strong spin-orbit coupling in HMs to switch magnetization. Three-terminal SOT-MRAM, where the writing current flows transversely, minimizes damage to the tunneling barrier, improving endurance. A major challenge is reducing energy consumption. Quantum materials like topological insulators (TIs) offer a potential solution by increasing the charge-spin conversion efficiency (θ_SH = J_s/J_e), which is typically less than 1 in classical materials. In TIs, topological surface states, with their spin-momentum locking, lead to a large θ_SH. While TI/FM bilayers show significantly higher θ_SH than HMs, integrating TIs with MTJs for SOT-MRAM applications presents challenges: TI layers require epitaxial growth on specific substrates, necessitating solutions for interface control, element diffusion during annealing, and chemical degradation during photolithography to preserve the topological surface states and achieve high TMR ratios. This research directly addresses these challenges.

Extensive research has explored SOT-MRAM based on heavy metals, demonstrating the feasibility of current-induced magnetization switching. However, the relatively low charge-spin conversion efficiency in these materials limits their energy efficiency. The exploration of topological insulators as a replacement for heavy metals has gained significant traction due to the potential for significantly enhanced spin-orbit torques. Previous studies have reported observations of large spin-orbit torques in TI/FM heterostructures, but challenges in integrating these materials with MTJs for practical device applications remain. This paper builds upon this existing body of work, addressing the critical limitations of integrating TIs into a functional MRAM device.

This study fabricated a TI-driven SOT-MRAM cell using a (BiSb)₂Te₃ topological insulator epitaxially grown on an Al₂O₃(0001) substrate via molecular beam epitaxy (MBE). A magnetron sputtering system was then used to deposit the MTJ stack: Ru(5)/CoFeB(2.5)/MgO(1.9)/CoFeB(5)/Ta(8)/Ru(7) (thickness in nm). The Ru interlayer is crucial for decoupling the exchange interaction and preventing element diffusion during the annealing process, which could damage the topological surface states. Photolithography and electron-beam lithography, along with ion milling, were used for device patterning. A 300 °C annealing process, with an in-plane magnetic field, created an in-plane magnetic easy axis for the CoFeB electrodes. The device structure was characterized using cross-sectional scanning transmission electron microscopy (STEM). Magnetic hysteresis (M-H) loops were measured to determine the coercive fields of the CoFeB layers. Tunneling resistance and TMR ratio were measured as a function of magnetic field. Current-driven SOT switching was evaluated in two configurations: collinear and orthogonal spin polarization and easy axis (EA). A writing current pulse was applied to induce SOT, followed by a small reading current to detect the magnetization state. The SOT-induced effective field was quantified by measuring the shift in the magnetic switching field of the free CoFeB layer under opposite bias currents. The charge-spin conversion efficiency was also determined using SOT-induced ferromagnetic resonance (ST-FMR) measurements, which involve exciting magnetic resonance with a microwave current while scanning an in-plane magnetic field. To demonstrate industry-level compatibility, an all-sputtered TI-MTJ device was fabricated and characterized.

The researchers successfully fabricated and characterized a TI-MTJ device exhibiting a state-of-the-art TMR ratio exceeding 100% and an ultralow switching current density of 1.2 × 10⁵ A cm⁻² at room temperature. This represents a significant improvement compared to conventional heavy metal-based SOT-MRAM devices. Two methods were used to independently quantify the charge-spin conversion efficiency (θ_SH): 1) SOT-induced shift of the magnetic switching field yielded θ_SH = 1.59; 2) SOT-induced ferromagnetic resonance (ST-FMR) yielded θ_SH = 1.02. Both values are an order of magnitude larger than in conventional heavy metals. The study demonstrated both field-free switching (collinear configuration) with a switching speed of 7.5 ns and faster switching (orthogonal configuration, 1.0 ns) with an external magnetic field. The authors observed that the successful full SOT switching required scaling down the MTJ size to dimensions comparable to the (BiSb)₂Te₃ crystal grain size (200–300 nm). Finally, the researchers demonstrated SOT switching in an all-sputtered TI-MTJ device, showcasing the potential for industrial-scale manufacturing. The all-sputtered device had a TMR ratio of around 90% and a switching current density of 1.4 × 10⁶ A cm⁻², suggesting a viable path for scalable production.

This work addresses the critical challenge of integrating topological insulators with magnetic tunnel junctions for energy-efficient SOT-MRAM. The achievement of a high TMR ratio exceeding 100% and an ultralow switching current density demonstrates the feasibility of using TIs to dramatically improve the energy efficiency of spintronic memory devices. The consistent values of θ_SH obtained through two independent measurement techniques—SOT-induced switching field shift and ST-FMR—strongly support the efficacy of the TI layer in enhancing the charge-spin conversion efficiency. The demonstration of both fast and field-free switching modes provides design flexibility for future SOT-MRAM applications. The successful integration of all-sputtered components is a crucial step towards enabling industrial-scale production. This research suggests that TIs could replace heavy metals in next-generation SOT-MRAM, significantly impacting the field of spintronics.

This study successfully demonstrated a functional TI-MTJ device for SOT-MRAM with superior performance compared to conventional heavy metal-based devices. The high TMR ratio and ultralow switching current density achieved highlight the significant potential of TIs for energy-efficient magnetic memory. The successful demonstration of the all-sputtered device indicates a path towards industry-compatible manufacturing processes. Future research could focus on further optimization of the TI-MTJ interface, exploring different TI materials and heterostructures, and investigating the long-term reliability of TI-based SOT-MRAM devices.

The study observed that only partial SOT switching was achieved in larger MTJs due to magnetic domain pinning. Scaling down the MTJ size to the order of the TI grain size was necessary to achieve full SOT switching. The switching current density of the all-sputtered TI-MTJ device was an order of magnitude higher than that of the MBE-grown device, suggesting that further optimization of the sputtering process is needed. Long-term reliability and endurance studies are needed to assess the viability of TI-based SOT-MRAM for practical applications.