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1. Magnetic memory driven by topological insulators

   https://doi.org/10.1038/s41467-021-26478-3  

   Wu, Hao; Chen, Aitian; Zhang, Peng; He, Haoran; Nance, John; Guo, Chenyang; Sasaki, Julian; Shirokura, Takanori; Hai, Pham Nam; Fang, Bin; et al (December 2021, Nature Communications)

   Abstract Giant spin-orbit torque (SOT) from topological insulators (TIs) provides an energy efficient writing method for magnetic memory, which, however, is still premature for practical applications due to the challenge of the integration with magnetic tunnel junctions (MTJs). Here, we demonstrate a functional TI-MTJ device that could become the core element of the future energy-efficient spintronic devices, such as SOT-based magnetic random-access memory (SOT-MRAM). The state-of-the-art tunneling magnetoresistance (TMR) ratio of 102% and the ultralow switching current density of 1.2 × 10 5  A cm −2 have been simultaneously achieved in the TI-MTJ device at room temperature, laying down the foundation for TI-driven SOT-MRAM. The charge-spin conversion efficiency θ SH in TIs is quantified by both the SOT-induced shift of the magnetic switching field ( θ SH  = 1.59) and the SOT-induced ferromagnetic resonance (ST-FMR) ( θ SH  = 1.02), which is one order of magnitude larger than that in conventional heavy metals. These results inspire a revolution of SOT-MRAM from classical to quantum materials, with great potential to further reduce the energy consumption. 

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2. Dramatic Effects of Electrode Metal on Tunnel Junction-Based Molecular Spintronic Devices

   Pawan Tyagi; Eva Mutunga, Hayden Brown (January 2022, MRS 2022, Hawaii)

   Magnetic tunnel junction (MTJ) can serve as an excellent testbed for connecting Molecule between two ferromagnetic electrodes. A paramagnetic molecule covalently bonded to two ferromagnetic electrodes with two thiol functional groups can produce intriguing transport and magnetic properties. We have chemically bonded paramagnetic molecules between two ferromagnetic electrodes of a MTJ along the exposed side edges. In this paper we discussed the observation of Molecule induced dramatic changes in the magnetic and transport properties of the conventional magnetic tunnel junctions. Paramagnetic molecules were chemically bonded to ferromagnetic electrodes to bridge them across the insulating spacer along the exposed edges. Paramagnetic molecular channels along the tunnel junction edges decreased the overall current, through tunnel barrier and molecular channels, > 5 orders of magnitude below the leakage current of the bare tunnel junction at room temperature. These molecules caused significant changes in the spin density of states due to potential spin filtering effect. Also, paramagnetic molecules produced antiferromagnetic coupling between the affected magnetic electrodes. In this state spin transport in the magnetic tunnel junction based molecular devices plummeted by several orders. It is also noteworthy that our experimental studies provide a platform to connect a vast variety of ferromagnetic leads to the even broader array of high potential molecules such as single molecular magnets, porphyrin, and single ion molecules. The strength of exchange coupling between ferromagnetic electrodes and molecules can be tailored by utilizing different tethers and terminal functional groups. The MTJMSD can provide an advanced form of logic and memory devices, including a testbed for the Molecule based quantum computation devices. Future study about the interaction between molecular magnets and ferromagnets and interaction of thiol ended alkanes with ferromagnets will be of very valuable. This study indicates the potential of magnetic molecules as a mean to transforming conventional magnetic tunnel junctions and producing unprecedented magnetic and transport properties. 

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3. Cubic molecule anisotropy impacting long-range magnetic ordering on magnetic tunnel junction-based molecular spintronic devices (MTJMSD)

   https://doi.org/10.1039/D5CP04692K  

   Peigney, E; D’Angelo, C; Tyagi, P (July 2026, Physical Chemistry Chemical Physics)

   This study investigates how molecular anisotropy influences the switchability of magnetic tunnel junction molecular spintronic devices (MTJMSDs). This theoretical study provides insights about experimentally observed 4900% conductance increase at room temperature on Co/NiFe/AlOx/NiFe magnetic tunnel junction(MTJ) with Octametallic Molecular Clusters(OMC). Via thiol surface attachment to two ferromagnetic metal leads of MTJ, an array of paramagnetic, cyanide-bridged octametal complexes, [(pzTp)FeIII(CN)3]4[NiII(L)]4[O3SCF3]4 (1) [(pzTp) = tetra(pyrazol-1-yl)borate; L = 1-S(acetyl)tris(pyrazolyl)- decane], were covalently linked onto the electrodes, forming a dominant spin conduction channel. Using extensive Monte Carlo simulations based on the Heisenberg model, we analyzed how anisotropy along the cubic molecular channels affects spin coupling between ferromagnetic electrodes and paramagnetic molecules. Our results identify a critical anisotropy threshold (Am = 0.4) that fundamentally alters device behavior. Below this threshold, the molecules stabilize an antiparallel coupling between the ferromagnetic electrodes, maintaining well-aligned molecule spins. Above Ay = 0.4, the magnetic moment of the two ferromagnetic electrodes stabilizes in different directions with respect to the direction of molecular anisotropy. More importantly, around Am = 0.4, two ferromagnetic electrodes stabilize in a superposition state; there is a significant probability of switching between the magnetic moments of both FM electrodes, switching between +1 and 1. These findings provide valuable insights for designing gate-controlled three-terminal molecular spintronic devices, as disclosed in a recent patent (P. Tyagi, 2023, US Pat. 11,621,345), enabling the variation in molecular anisotropy due to the gate voltage via the voltage-controlled magnetic anisotropy phenomenon, potentially enabling advancements in spin logic, magnetoresistive memory, and quantum computing. 

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4. Magnetic force microscopy revealing long-range room temperature stable molecule bridge-induced magnetic ordering on magnetic tunnel junction (MTJ) pillars

   https://doi.org/10.1063/9.0000937  

   Tyagi, Pawan (March 2025, AIP Advances)

   Magnetic tunnel junctions (MTJs) can integrate novel single molecular device elements to overcome long-standing fabrication challenges, thus unlocking their novel potential. This study employs magnetic force microscopy (MFM) to demonstrate that organometallic molecules, when placed between two ferromagnetic electrodes along cross-junction shaped MTJ edges, dramatically altered the magnetic properties of the electrodes, affecting areas several hundred microns in size around the molecular junction vicinity at room temperature. These findings are supported by magnetic resonance and magnetometer studies on ∼7000 MTJ pillars. MFM on the pillar sample showed an almost complete disappearance of the magnetic contrast. The spatial magnetic image suggests that molecular channels significantly impacted the spin density of states in the ferromagnetic electrodes. This advancement in MTJ-based molecular devices paves the way for a new generation of commercially viable logic and memory devices controlled by molecular quantum states at near-room temperatures. 

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5. Perspectives on field-free spin–orbit torque devices for memory and computing applications

   https://doi.org/10.1063/5.0135185  

   Lopez-Dominguez, Victor; Shao, Yixin; Khalili Amiri, Pedram (January 2023, Journal of Applied Physics)

   The emergence of embedded magnetic random-access memory (MRAM) and its integration in mainstream semiconductor manufacturing technology have created an unprecedented opportunity for engineering computing systems with improved performance, energy efficiency, lower cost, and unconventional computing capabilities. While the initial interest in the existing generation of MRAM—which is based on the spin-transfer torque (STT) effect in ferromagnetic tunnel junctions—was driven by its nonvolatile data retention and lower cost of integration compared to embedded Flash (eFlash), the focus of MRAM research and development efforts is increasingly shifting toward alternative write mechanisms (beyond STT) and new materials (beyond ferromagnets) in recent years. This has been driven by the need for better speed vs density and speed vs endurance trade-offs to make MRAM applicable to a wider range of memory markets, as well as to utilize the potential of MRAM in various unconventional computing architectures that utilize the physics of nanoscale magnets. In this Perspective, we offer an overview of spin–orbit torque (SOT) as one of these beyond-STT write mechanisms for the MRAM devices. We discuss, specifically, the progress in developing SOT-MRAM devices with perpendicular magnetization. Starting from basic symmetry considerations, we discuss the requirement for an in-plane bias magnetic field which has hindered progress in developing practical SOT-MRAM devices. We then discuss several approaches based on structural, magnetic, and chiral symmetry-breaking that have been explored to overcome this limitation and realize bias-field-free SOT-MRAM devices with perpendicular magnetization. We also review the corresponding material- and device-level challenges in each case. We then present a perspective of the potential of these devices for computing and security applications beyond their use in the conventional memory hierarchy. 

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