Efficient perpendicular magnetization switching by a magnetic spin Hall effect in a noncollinear antiferromagnet

The study addresses the inefficiency and nondeterministic nature of conventional spin-orbit torque (SOT) approaches for switching perpendicular magnetic anisotropy (PMA) magnets, where spin Hall effect (SHE) or Rashba-Edelstein effect generate in-plane spin polarizations that yield in-plane anti-damping torques ill-suited for PMA switching. Typical solutions require external magnetic fields or high current densities, hindering low-power applications. The authors propose exploiting the magnetic spin Hall effect (MSHE) in noncollinear antiferromagnets (AFMs) such as Mn3Sn to generate a z-polarized spin current that produces an out-of-plane anti-damping torque directly aligned with the PMA easy axis, enabling deterministic, field-free switching at reduced current densities. Because MSHE is odd under time-reversal, the torque’s sign can be reversed via AFM order reversal, offering additional control.

Prior work has attempted deterministic PMA switching by: (i) applying exchange bias from adjacent AFMs (IrMn, PtMn), stray fields from neighboring ferromagnets, or structural engineering (growth direction gradients) to break symmetry; (ii) lowering symmetry of the spin source or introducing magnetic order to allow z-polarized spin currents; (iii) interface engineering (spin-orbit filtering/precession/scattering) to produce z spin polarization; and (iv) using FM/NM/FM trilayers or low-symmetry growth directions in (anti)ferromagnets to realize out-of-plane anti-damping torques. These approaches show promise but often rely on interfacial effects, additional layers, or nonstandard growth directions. The MSHE in noncollinear AFMs has been theoretically and experimentally identified as a bulk effect capable of generating unconventional spin currents with z polarization, reversible by magnetic order, offering a pathway to efficient, field-free PMA switching.

- Materials and device stack: Epitaxial Mn3Sn(7 nm)/Cu(1 nm)/[Ni(0.4 nm)/Co(0.2 nm)]3/Cu(1 nm)/SiO2(2 nm) grown on MgO(111) by demagnetron sputtering; Hall bars fabricated by standard lithography and etching for transport measurements. A Cu spacer magnetically decouples Mn3Sn and the ferromagnet (FM) to isolate SOT effects. - Structural and magnetic characterization: XRD and high-resolution TEM confirmed Mn3Sn (0001) orientation and sharp interfaces with negligible interdiffusion; transport measurements verified film quality comparable to bulk/epitaxial Mn3Sn; anomalous Hall effect (AHE) confirmed strong perpendicular magnetic anisotropy in [Ni/Co]3. Resistivities: Mn3Sn ~367.5 μΩ·cm; Cu/[Ni/Co]3/Cu ~45.0 μΩ·cm. Saturation magnetization of [Ni/Co]3 ~496 emu/cm3. - MSHE prediction: First-principles calculations evaluated magnetic spin Hall conductivities (MSHC) in Mn3Sn for two inverse triangular AFM orders (AFM1, AFM2), showing finite σxz and σyz components that reverse with magnetic order, enabling z-polarized spin currents from in-plane charge currents. - Electrical measurements: AHE hysteresis R_AHE vs out-of-plane field Hz recorded under dc bias current I along x = [0110] to detect loop center shift ΔHz indicating out-of-plane anti-damping torque. Threshold current identified by onset of ΔHz. Measurements with varying in-plane field Hx separated contributions from y- and z-polarized spins to extract effective SOT fields and spin current conductivities σyz and σxz. - Field-free switching tests: Pulsed current along [0110] direction measured R_AHE vs I at zero external field to demonstrate deterministic switching. Switching ratio estimated from R_AHE relative to saturation. - Domain control: In-plane magnetic field Hx applied to align Mn3Sn AFM domains (owing to weak in-plane net magnetizations in AFM1/AFM2) and study its effect on switching ratio and polarity. - Thermal considerations: Estimated Joule heating raised device temperature to ~360 K during switching, below Mn3Sn Néel temperature (~420 K), minimizing thermal disruption of AFM order. - Simulations: Macro-spin simulations explored switching dynamics as a function of σ_yx^z/σ_yx^y, showing that for ratios ~30.5% (as measured) dynamics is characteristic of out-of-plane anti-damping torque; also indicated lower critical currents for MSHE-driven compared to SHE-driven switching. - Benchmark comparison: Fabricated conventional β-Ta(7 nm)/Cu(2 nm)/FM(1.8 nm) device with identical geometry to compare SOT efficiency. Required in-plane field for deterministic switching due to lack of z spin component; switching performance compared in terms of switching ratio and current density.

- Finite z-polarized spin current detected: AHE hysteresis loop center shift ΔHz appears above threshold current ~10 mA; ΔHz scales approximately linearly with I thereafter. Such ΔHz without applied Hx is incompatible with conventional SHE-only devices. - Spin current conductivities: Extracted σyz ≈ 6.02 × 10^4 (in units of spin current conductivity) and σxz ≈ 1.83 × 10^4, yielding σxz/σyz ≈ 30.5%, consistent with Mn3Sn single-crystal reports. These indicate efficient generation of z-polarized spin current via MSHE in Mn3Sn. - Deterministic field-free switching: Current-induced switching of the [Ni/Co]3 PMA layer observed with a hysteretic R_AHE(I) at zero external magnetic field, with switching ratio ~60% of the FM volume attributed to the out-of-plane anti-damping torque from the z-polarized spin current. - Domain effects and tunability: Due to multi-domain Mn3Sn, effective σ_yx^z is reduced, limiting switching ratio. Applying small in-plane fields Hx aligns domains, increasing switching ratio up to ~80% at ~30 Oe and reversing switching polarity when Hx is reversed. Polarity inversion occurs near Hx ≈ -8 Oe, suggesting a small intrinsic bias from interfacial/defect-induced domain preference. - Thermal robustness: Device temperature during switching (~360 K) remains below Mn3Sn TN (~420 K), ruling out Joule heating as the origin of switching behavior. - Mechanism validated by simulation: For σ_yx^z/σ_yx^y ≈ 30.5%, macro-spin modeling reproduces out-of-plane anti-damping torque-driven dynamics; lower critical current predicted for MSHE-driven switching versus SHE-driven switching. - Efficiency advantage over conventional SHE device: β-Ta-based SHE device achieves only ~17% switching ratio at high current density ~1.3 × 10^7 A/cm2 and requires Hx = 300 Oe, whereas Mn3Sn-based MSHE device achieves ~60% switching ratio field-free at much lower current densities (~10^5 A/cm2 scale).

The results confirm that the magnetic spin Hall effect in noncollinear AFM Mn3Sn generates a z-polarized spin current from an in-plane charge current, producing an out-of-plane anti-damping torque that deterministically switches a perpendicular magnet without external magnetic fields. This directly addresses the key limitation of conventional SOT approaches for PMA switching, offering a bulk-material-based solution with standard growth direction. The torque’s odd time-reversal character enables control via AFM domain orientation, as evidenced by field-tunable switching polarity and efficiency. The reduced critical current and elimination of assist fields demonstrate significant potential for low-power, scalable spintronic memory technologies. The partial switching observed is linked to multi-domain AFM textures; modest in-plane fields improve domain alignment and switching ratio, indicating that domain engineering can further enhance performance.

This work demonstrates efficient, deterministic, field-free switching of a perpendicular magnet by leveraging the magnetic spin Hall effect in a noncollinear antiferromagnet (Mn3Sn). Epitaxial Mn3Sn thin films generate sizable z-polarized spin currents (σxz/σyz ≈ 30.5%), driving out-of-plane anti-damping torques that switch an adjacent [Ni/Co]3 PMA layer at substantially reduced current densities compared to conventional SHE devices. Switching polarity and efficiency are controllable via the AFM domain orientation using small in-plane fields, reflecting the MSHE’s odd time-reversal character. Compared against a β-Ta SHE device, the MSHE device achieves significantly higher switching ratios at lower currents without external fields. Future research directions include engineering single-domain or exchange-coupled AFM states to achieve near-100% switching, optimizing interfaces and layer thicknesses for enhanced spin transparency, exploring other noncollinear AFMs with larger MSHE, and integrating such spin sources into device-relevant memory architectures.

- Partial switching (~60% at zero field) due to multi-domain Mn3Sn reduces effective z-polarized spin current; aligning domains with small in-plane fields is required to approach ~80% switching. - Presence of a small effective bias field (~-8 Oe) likely from interfacial or defect-induced domain preferences indicates sensitivity to growth-induced asymmetries. - The exact unit-normalized values of spin current conductivity depend on modeling assumptions and separation of y- and z-polarized contributions; interfacial spin transparency and shunting may influence quantitative estimates. - While Joule heating is below TN and unlikely to drive switching, elevated operating temperatures could affect domain configurations in other conditions. - Demonstrations focused on Ni/Co multilayers; generality to other PMA systems and long-term reliability/endurance were not addressed.