Spin-orbit torque (SOT) is an effective method for manipulating magnetization in spintronic devices. Efficient magnetization switching ideally requires the spin polarization (p) generated by an in-plane charge current (J) to be parallel to the magnetization's (m) easy axis. This allows the anti-damping torque −m × (m × p) to directly alter effective damping and switch the ferromagnet (FM). However, in conventional spin Hall effect (SHE) or Rashba-Edelstein effect (REE), the induced spin polarization is always in-plane, making perpendicular magnetization switching inefficient and non-deterministic. This requires an external magnetic field and high current density, hindering applications in high-density, low-power devices. To overcome this, an out-of-plane anti-damping torque −m × (m × z) is desirable, directly opposing magnetization damping. Generating a z-polarized spin current is crucial for this. Previous attempts involved low crystal symmetry materials, additional magnetic order, or interface engineering, but a bulk spin source with conventional epitaxial growth is preferred for practical applications. This paper demonstrates out-of-plane anti-damping torque switching using the noncollinear antiferromagnet Mn3Sn via the magnetic spin Hall effect (MSHE), achieving deterministic, field-free switching with lower critical current density than conventional SHE.

Extensive research has focused on achieving efficient spin-orbit torque (SOT) switching of perpendicular magnetization. Conventional SHE-based devices suffer from inefficiencies due to in-plane spin polarization requiring external magnetic fields and high current densities for deterministic switching. Several strategies to overcome these limitations have been explored including using an exchange bias from an adjacent antiferromagnet, stray fields from neighboring ferromagnetic layers, structural engineering, and the interplay between spin-orbit and spin-transfer torques. However, generating a z-polarized spin current to enable a direct out-of-plane anti-damping torque remains a significant challenge. Previous works demonstrated out-of-plane anti-damping torque switching using low crystal symmetry materials, additional magnetic order, or interface engineering, but these methods often have limitations in terms of efficiency and scalability. The use of noncollinear antiferromagnets as a source of z-polarized spin currents has been recently proposed as a promising alternative.

The study utilizes Mn3Sn, a noncollinear antiferromagnet with a hexagonal crystal structure, as a spin source material. The chiral alignment of Mn moments within the Kagome-type lattice planes leads to a noncollinear AFM order and an MSHE. Unlike conventional SHE, MSHE is odd under time reversal symmetry and reversible by flipping magnetic orders. First-principles calculations were performed to confirm the finite and reversible σxz and σyz components of the magnetic spin Hall conductivity (MSHC) in Mn3Sn. An MSHE SOT device was fabricated consisting of a Mn3Sn(7)/Cu(1)/[Ni(0.4)/Co(0.2)]3/Cu(1)/SiO2(2) stack epitaxially grown on MgO(111). The device's structural and magnetic properties were characterized using X-ray diffraction, high-resolution transmission electron microscopy, and magneto-transport measurements. The presence of a z-polarized spin current was verified by measuring the hysteresis loop of the anomalous Hall resistance (RAHE) as a function of out-of-plane magnetic field (Hz) under various bias currents (I). The effective SOT fields from y- and z-polarized spin currents were estimated, yielding spin current conductivities. Deterministic field-free magnetization switching was demonstrated by measuring the AHE resistance as a function of pulsed current. The switching ratio was investigated under various in-plane magnetic fields (Hx) to control the AFM domain orientation. Macro-spin simulations were conducted to analyze the role of the z-polarized spin current in the switching mechanism. Finally, a comparison of the SOT switching efficiency was performed between the Mn3Sn-based MSHE device and a conventional β-Ta-based SHE device.

The study successfully demonstrated efficient perpendicular magnetization switching using a noncollinear antiferromagnet (Mn3Sn) as a spin source. The key findings include: (1) Confirmation of a significant z-polarized spin current component generated by the magnetic spin Hall effect (MSHE) in Mn3Sn thin films. This was evidenced by the observed shift in the anomalous Hall resistance hysteresis loops under application of bias current. The measured spin current conductivities (σxz and σyz) were comparable to those reported in previous studies for similar systems. (2) Achievement of deterministic field-free magnetization switching in the absence of any external magnetic field. This field-free switching was attributed to the out-of-plane anti-damping torque generated by the z-polarized spin current. A switching ratio of approximately 60% was achieved. (3) Control of switching polarity and enhancement of switching ratio by applying an in-plane magnetic field. The in-plane field was shown to control the orientation of the AFM domains in Mn3Sn, affecting the overall z-polarized spin current and hence the switching behavior. The switching ratio was increased to ~80% with the application of a small in-plane field. (4) Confirmation through macro-spin simulations that the switching mechanism is indeed driven by the z-polarized spin current from the MSHE rather than the y-polarized spin current from SHE. The simulations also indicated that the critical current for MSHE-driven switching was significantly smaller than that for SHE-driven switching. (5) Superior switching efficiency of the Mn3Sn-based MSHE device compared to a conventional β-Ta based SHE device. The Mn3Sn device demonstrated a much larger switching ratio (~60%) at significantly lower current densities compared to the β-Ta device (~17% at 1.3 × 107 A cm−2). This makes the MSHE-based approach more favorable for low power spintronics applications.

The results demonstrate a highly efficient and deterministic method for perpendicular magnetization switching based on the magnetic spin Hall effect in the noncollinear antiferromagnet Mn3Sn. The use of Mn3Sn eliminates the need for an external magnetic field, which is a significant advantage for energy-efficient spintronic devices. The observed field-free switching is directly attributed to the out-of-plane anti-damping torque originating from the z-polarized spin current generated by the MSHE. The ability to control the switching polarity and improve the switching ratio using a small in-plane magnetic field further strengthens the practical potential of this approach. The superior performance compared to conventional SHE-based devices highlights the advantages of exploiting the MSHE for spintronic applications. The findings open up new avenues for designing low-power, high-density magnetic memory and storage devices.

This research successfully demonstrated highly efficient and deterministic perpendicular magnetization switching using a Mn3Sn-based MSHE device. The field-free operation and low current density requirements highlight the significant advantages of this approach over conventional SHE-based methods. The ability to control switching polarity and enhance switching ratio with a small magnetic field further increases its potential for practical applications. Future research could focus on optimizing the Mn3Sn film quality and device design to achieve even higher switching ratios and lower critical current densities, paving the way for advanced spintronic devices.

The study achieved a maximum switching ratio of approximately 80%, indicating that not all domains in the Mn3Sn film contribute effectively to the switching. This is likely due to the presence of multiple AFM domains with varying orientations and a resulting reduced effective MSHC. Further investigation into techniques for controlling domain alignment could improve the overall switching efficiency. The influence of potential interface effects or defects on the MSHE and switching mechanism warrants further exploration. While Joule heating effects were considered and deemed insignificant, other potential device limitations should be further investigated for robust application.