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Recent advances in spin-orbit torques: Moving towards device applications

Rajagopalan Ramaswamy, Jong Min Lee, Kaiming Cai, Hyunsoo Yang

arXiv:1808.06829v1cond-mat.mes-hall

TL;DR

Spin-orbit torque research seeks ultrafast, power-efficient magnetization manipulation suitable for practical magnetic memories. This review synthesizes SOT materials, switching dynamics, and field-free switching approaches, highlighting progress alongside unresolved device challenges.

  • Problem

    Practical SOT devices require addressing the external magnetic field needed for deterministic switching.

  • Method

    The review surveys SOT fundamentals, material choices, switching dynamics, and techniques intended to eliminate the external-field requirement.

  • Results

    The review reports that SOT switching predominantly follows domain nucleation and expansion, while field-like torque can significantly affect dynamics and cause backward switching.

  • Takeaways & Limitations

    External-field-free SOT switching is identified as a requirement for practical SOT devices, with exchange bias among the approaches discussed.

Abstract

from arXiv · show

The ability of spintronic devices to utilize an electric current for manipulating the magnetization has resulted in large-scale developments, such as, magnetic random access memories and boosted the spintronic research area. In this regard, over the last decade, magnetization manipulation using spin-orbit torque has been devoted a lot of research attention as it shows a great promise for future ultrafast and power efficient magnetic memories. In this review, we summarize the latest advancements in spin-orbit torque research and highlight some of the technical challenges for practical spin-orbit torque devices. We will first introduce the basic concepts and highlight the latest material choices for spin-orbit torque devices. Then, we will summarize the important advancements in the study of magnetization switching dynamics using spin-orbit torque, which are important from scientific as well as technological aspect. The final major section focuses on the concept of external assist field free spin-orbit torque switching which is a requirement for practical spin-orbit torque devices.

I. Introduction

Spintronics uses both charge and spin to add functions such as non-volatility and reduced power consumption. The review introduces SOT as an alternative to STT that separates read and write paths and targets ultrafast, power-efficient devices.

  • Spintronics exploits both charge and spin degrees of freedom to provide non-volatility and reduced power consumption.
  • STT electrically manipulates ferromagnets with spin-polarized current, offering scalability and lower power consumption than magnetic-field-based devices.
  • STT-MRAM uses a magnetic tunnel junction with an oxide barrier between two ferromagnetic layers, but high write current can break down the oxide.
  • Coupled read and write paths in STT-MRAM allow read current to accidentally switch the magnetic state, producing read disturbance.
  • SOT-MRAM decouples read and write paths; because the large write current bypasses the MTJ, it offers better design margins and device stability.
  • SOT uses spin-orbit interaction in an FM/NM bilayer to generate spin accumulation that can switch magnetization, move domain walls, and generate oscillations.
  • The review covers SOT physics, materials, switching dynamics, and techniques addressing the external-field requirement for practical devices.

A. Origins of spin-orbit torque

SOT spin accumulation is attributed mainly to bulk spin Hall and interfacial Rashba-Edelstein effects, but experiments indicate that both can contribute to both torque components. The section therefore emphasizes torque decomposition and the difficulty of separating microscopic contributions.

  • Origins of spin accumulation: The two main phenomena proposed to generate interfacial spin accumulation are the spin Hall effect and the interface Rashba-Edelstein effect.
  • Spin Hall effect: The bulk spin Hall effect converts an unpolarized charge current in a nonmagnet into a transverse spin current through spin-dependent asymmetric scattering.
  • Spin Hall effect: The spin Hall angle quantifies a nonmagnet’s spin-current generation efficiency, while its sign determines the direction of interfacial spin accumulation.
  • Interface Rashba-Edelstein effect: The Rashba-Edelstein effect arises from interfacial spin-orbit coupling and broken inversion symmetry, with electron spins polarized along p × E.
  • Torque decomposition of SOT: Bulk and interfacial spin-orbit coupling can both generate both SOT components, making individual contributions difficult to identify experimentally.
  • Torque decomposition of SOT: The damping-like torque tends to align magnetization with spin polarization, whereas the field-like torque drives precession around that polarization.
  • Torque characterization: Experiments commonly evaluate SOT effective fields using harmonic Hall, spin-torque ferromagnetic resonance, and magneto-optic Kerr measurements.

C. SOT induced magnetization switching

SOT electrically switches ferromagnets with perpendicular or in-plane anisotropy, but PMA is preferred for scalability and in-plane spin polarization generally requires an external assist field. The review therefore covers switching dynamics, efficiency definitions, material strategies, and approaches toward field-free operation.

  • SOT can switch ferromagnets with perpendicular or in-plane magnetic anisotropy, while current MRAM implementations favor PMA for better scalability.
  • In Ta/CoFeB/MgO with PMA, current polarity determines whether the magnetization switches up or down.
  • In-plane spin polarization generally requires an external magnetic field to break symmetry, making field dependence a major practical bottleneck.
  • Switching efficiency: SOT switching efficiency is defined using anisotropy and critical current density for macrospin switching, but domain-mediated switching in samples larger than ~50 nm requires the depinning field.
  • Material strategies: Material engineering can enhance SOT performance: Hf insertion increases surface anisotropy energy by two orders and halves Gilbert damping while maintaining SOT efficiency.
  • Material strategies: Alternative underlayers show large spin Hall responses, including Cu99.5Bi0.5 with a spin Hall angle of −0.24, Au90Ta10 with 0.5, and W(O) with −0.5.

B. Exotic non-magnetic layers

Exotic non-magnetic layers, including topological insulators, two-dimensional materials, and oxide-interface electron gases, offer unusually efficient spin-current generation and new torque symmetries. Their integration remains constrained by ferromagnet growth and compatibility with silicon processing.

  • Topological insulators: Topological insulators are estimated to have spin Hall angles 2–3 orders of magnitude larger than conventional heavy metals.
  • Topological insulators: In Bi2Se3/Py, the damping-like torque increases substantially as temperature decreases and was attributed to spin transfer from topological-surface-state-induced spins.
  • Topological insulators: Topological-insulator spin-current generation has enabled highly efficient SOT-induced magnetization switching.
  • Two-dimensional materials: WTe2 can exert an out-of-plane damping-like torque that switches a perpendicularly anisotropic magnet without an assistive magnetic field.
  • Oxide interfaces: The STO/LAO 2DEG exhibits a spin Hall angle of ~6.3 at room temperature and a reported spin diffusion length of 300 nm.
  • Integration challenges: Practical challenges include growing PMA ferromagnets on exotic layers and establishing their high-temperature stability and compatibility with silicon processing.

C. Ferromagnets and ferromagnetic multilayers

Ferromagnets and multilayers offer varied routes for SOT devices, balancing thermal stability against torque efficiency. Thick Co/Pd and Co/Ni-based structures retain perpendicular anisotropy while supporting strong torques and fast domain-wall motion.

  • Ferromagnets: Ultrathin CoFeB is commonly used for interfacial PMA and large TMR, but thickness requirements restrict lateral scaling when thermal stability must remain high.Interfacial PMA typically requires FM layers below 1.4 nm, while MRAM thermal stability depends on magnetic volume.
  • Ferromagnetic multilayers: Co/Ni, Co/Pd, and Co/Pt multilayers can be grown thick while retaining PMA, enabling alternatives to ultrathin single-layer ferromagnets.The review identifies thick ferromagnetic multilayers as a way to preserve PMA while increasing magnetic volume.
  • Ferromagnetic multilayers: ~20 nm Co/Pd multilayers retain large SOT effective fields of 1170 Oe /10^8 A cm^-2 (DL) and 5025 Oe /10^8 A cm^-2 (FL), 5–10 times larger than CoFeB.These values indicate a possible bulk origin of SOTs in ferromagnetic multilayers.
  • Ferromagnetic multilayers: SOT-driven domain-wall velocity reaches 750 m s^-1 in a Pt-underlayer Co/Ni/Co synthetic antiferromagnet.SOTs combined with exchange torques from RKKY interaction move domain walls efficiently.
  • Ferromagnetic multilayers: Synthetic antiferromagnets show field-tunable effective SOT signs, allowing different magnetization states at fixed current polarity by changing assist-field magnitude.The behavior is illustrated for [Co/Pt]n-based structures.

D. Ferrimagnets and antiferromagnets

Ferrimagnets and antiferromagnets expand SOT material choices through local spin polarization, composition tuning, and compensation effects. Near magnetic compensation, ferrimagnets can provide enhanced switching efficiency while remaining compatible with thick magnetic layers.

  • Antiferromagnets: CuMnAs can be switched between antiferromagnetic states because its two Mn sites generate opposite spin polarizations from local crystal inversion asymmetry.The switching uses local nonequilibrium spin density associated with the two Mn sites.
  • Ferrimagnets: Composition tuning in rare-earth–transition-metal ferrimagnets varies magnetization, with Co1-xGdx approaching magnetization compensation near x ≈ 25%.The review uses Co1-xGdx to illustrate composition-controlled magnetization.
  • Ferrimagnets: The enhanced ferrimagnetic SOT near compensation is attributed to additional torque from negative exchange coupling.Negative exchange coupling is enhanced close to the compensation point.
  • Ferrimagnets: Ferrimagnetic SOTs can operate in thick layers: GdFeCo has been switched at 30 nm thickness, while CoTb efficiency also increases near compensation.In CoTb, Dzyaloshinskii–Moriya energy increases with Tb concentration.
  • Ferrimagnets: Damping-like SOT increases near magnetic compensation temperature, whereas field-like SOT changes little with temperature in GdFeCo and CoTb.The two torque components therefore exhibit different temperature dependences.

III. SOT switching dynamics

SOT switching dynamics depend on device size, pulse duration, thermal fluctuations, and multiple torque components. Experiments show that larger nanodots switch through domain nucleation and propagation, while smaller devices approach coherent behavior but still expose quantitative limits of macrospin models.

  • Switching dynamics: Macrospin models qualitatively explain assist-field requirements, but experimentally observed switching current densities are substantially smaller than their predictions.The discrepancy indicates that switching in large magnets proceeds through domain nucleation and expansion rather than uniform rotation.
  • Torque contributions: The field-like torque and Oersted field both contribute to quantitative short-pulse switching dynamics, beyond models considering only the damping-like torque.The Oersted field can speed switching in a ferromagnet with in-plane anisotropy.
  • Short pulse current injection: For pulses shorter than 10 ns, critical current increases significantly as duration decreases; above 10 ns, thermal fluctuations dominate and current depends weakly on duration.The two regimes separate torque-dominated short-pulse switching from thermally influenced long-pulse switching.
  • Short pulse current injection: A 0.58 mA intrinsic critical current for a Pt/Co nanodot is below the 2.05 mA macrospin prediction, supporting domain nucleation and propagation in the ~90 nm device.The incubation time is negligibly small at ~10–20 ± 2 s.
  • Device-size dependence: Reducing Ta/CoFeB/MgO nanodot diameter from 80 to 30 nm increases critical current density and indicates a transition from incoherent to coherent switching.For a 40 nm device, sub-10 ns critical current density changes only slightly with decreasing pulse duration, as predicted by a macrospin model.
  • Device-size dependence: The 40 nm macrospin description agrees qualitatively but fails to quantitatively reproduce the extracted spin Hall angle and effective anisotropy.The discrepancy reflects limits of the model’s quantitative agreement for that nanodot.

B. Spatial and time resolved measurements

Spatially and temporally resolved studies show that SOT switching in large structures proceeds through domain nucleation and wall propagation rather than a single macrospin rotation. DMI, assist fields, field-like torque, heating, and device size shape where switching begins and how domains move.

  • Spatially resolved dynamics: The observed dynamics demonstrate that switching behavior depends on microscopic nucleation and propagation processes, limiting a single-site macrospin description for large magnetic structures.Spatially resolved MOKE and X-ray magnetic circular dichroism were used to investigate these processes.
  • Microscopic switching mechanism: Without an assistive field, DMI-stabilized chiral Néel walls experience opposite effective fields at opposite domain edges, displacing rather than expanding the reversed domain.A sufficiently strong assistive field aligns the effective fields and enables reversed-domain expansion.
  • Microscopic switching mechanism: Time-resolved imaging of a 500 nm Pt/Co/AlO_X dot tracked switching every 100 ps during a 2 ns current pulse.The measurements provided 25 nm spatial and 100 ps temporal resolution.
  • Spatially resolved dynamics: In one study, nucleation consistently occurred at the left or right edge, with the site set by DMI, assist field, and damping-like torque.Field-like torque produced top-bottom asymmetry, enhanced switching efficiency, and influenced domain-wall tilt.
  • Spatially resolved dynamics: Other measurements found random nucleation of small magnetic bubbles when sample heating was considered, while larger W/CoFeB/MgO devices showed random nucleation at diameters of 700 nm and above.Simulations linked the combined DMI and field-like torque to long switching times.

C. Oscillatory switching behavior

Strong field-like torque can alter domain-wall dynamics enough to produce oscillatory SOT switching. Longer pulses may therefore drive switching back, reducing the final switching probability after an initially successful reversal.

  • Switching probability: Switching probability in a 1 μm Ta/Co40Fe40B20/MgO dot oscillated between 0% and 100% as pulse duration increased at a given current density.This behavior was observed under short-pulse current injection.
  • Switching probability: A reduction in switching probability after 100% switching indicates a switching-back phenomenon for longer pulses.Time-resolved MOKE experiments observed a similar reduction.
  • Switching-back mechanism: Micromagnetic simulations associate shorter and longer pulses with switching and switching back, respectively, through domain-wall reflection at the sample edges.The compared pulse widths were 1.7 ns and 1.8 ns.
  • Oscillatory dynamics: Strong field-like torque can stabilize reflected domain walls and generate oscillatory switching behavior.The reflected walls can return the magnetization toward its initial state.
  • Device implications: Oscillatory switching was used to demonstrate a unipolar SOT switching scheme that could replace driving transistors with diodes to improve scalability.The application depends on the oscillatory switching behavior described above.

A. Wedged structural engineering

Wedged structures and related anisotropy engineering break magnetic symmetry to enable deterministic SOT switching without an external field. Their microscopic mechanisms differ, and wafer-level uniformity remains a practical concern.

  • Lateral oxide gradients: Under-oxidation in lateral oxide structures produced zero-external-field SOT switching because only one magnetic state remained stable for a given current direction.The sign and strength of the perpendicular field-like field depend on oxidation level.
  • Lateral oxide gradients: A wedge-shaped oxide creates oxidation and perpendicular magnetic anisotropy gradients that generate an additional perpendicular field-like effective field.This field breaks the symmetry between the two magnetic states for a fixed current direction.
  • Microscopic mechanism: Microscopic studies found zero-field switching in lateral oxidation-gradient structures was dominated by the perpendicular field-like torque, whereas large-assist-field switching was dominated by the conventional damping-like field.This distinguishes the mechanisms across assist-field regimes.
  • Material extensions: Wedging the ferromagnetic layer in a Ta/CoFeB/MgO/TaO_x stack retained qualitatively similar field-free switching, and the approach extended to Hf/CoFeB/MgO, Hf/CoFeB/TaO_x, and Pt/Co/MgO.The CoFeB layer, rather than the earlier TaO_x layer, was wedged in the later stack.
  • Anisotropy tilting: An alternative wedge without MgO tilts the nanomagnet easy axis, enabling zero-field deterministic switching attributed to damping-like torque alone rather than generation of a perpendicular field-like field.Micromagnetic simulations supported this interpretation.
  • Practical considerations: Wedging may obstruct mass production because wafer-level homogeneity of magnetic and electrical properties is desired.This is a practical boundary on otherwise promising symmetry-engineering approaches.

B. Exchange coupling based techniques

Exchange-bias and interlayer-coupling schemes incorporate an effective assist field within the SOT stack to achieve external-field-free switching. Their integration, heating, stability, and microscopic-material constraints remain important for memory applications.

  • Scheme classes: Exchange-coupling approaches use antiferromagnets or additional ferromagnets to provide an internal assist field, while some antiferromagnets also serve as SOT sources.The reviewed schemes include exchange bias and interlayer exchange coupling.
  • Exchange-bias schemes: PtMn-based stacks can combine exchange bias and SOT generation, while stack engineering can enhance perpendicular anisotropy, exchange bias, and reduce switching current density.Examples include replacing Pt with Ru as a seed layer and inserting a thin Pt layer.
  • Exchange-bias schemes: In an IrMn-based structure, bottom IPA CoFeB enhanced exchange coupling sufficiently to achieve complete field-free switching.IrMn served as both an antiferromagnetic SOT source and exchange-bias material in the described scheme.
  • Material constraints: Another IrMn exchange-bias stack showed incomplete switching, attributed to IrMn’s polycrystalline grain structure, indicating that local antiferromagnet crystallinity matters.The exact microscopic origin of SOT in IrMn remained unclear in one system and required further exploration.
  • Interlayer coupling: Interlayer exchange coupling through Ru required 2 nm antiferromagnetic or 2.5 nm ferromagnetic coupling configurations to retain PMA while demonstrating field-free SOT switching.Smaller Ru thicknesses produced coupling strong enough to overwhelm the bottom free layer’s PMA.
  • Integration and reliability: Structures with buried PMA free layers are difficult to integrate with a tunnel barrier, whereas other stack arrangements allow an MgO barrier to contact the free layer more directly.Joule heating and repeated switching can weaken exchange bias and degrade long-term memory stability.
  • Integration and reliability: Heating can decrease exchange bias, and repeated switching can degrade exchange bias between antiferromagnetic and ferromagnetic layers, affecting long-term device stability.These effects constrain exchange-based field-free switching schemes for memory applications.

C. Electric field controlled SOT switching

External-field-free SOT switching is pursued through electric control and engineered spin-polarization structures. Demonstrated approaches include ferroelectric programming and dual-heavy-metal stacks that generate an out-of-plane effective field.

  • Electric-field control: Ferroelectric programming demonstrated field-free SOT switching by electrically setting the direction of the effective magnetic field.A Pt/Co/Ni/Co/Pt stack on PMN-PT used substrate voltage to set ferroelectric polarization before switching measurements.
  • Electric-field control: Changing the substrate-voltage polarity reversed the direction of the switching loops.Switching loops were measured after applying +500 V and -500 V to the PMN-PT substrate.
  • OOP spin generation: Out-of-plane spin polarization can switch a ferromagnet without an assist field, unlike conventionally generated in-plane spins.The review describes crystal symmetry, FM/NM bilayers, and dual-heavy-metal bilayers as routes to OOP spin generation.
  • OOP spin generation: A Ti/FM interface was identified as producing OOP spin polarization, supported by hysteresis measurements and numerical simulations.The cited stack structures include NiFe or CoFeB interfaced with Ti.
  • OOP spin generation: Opposite-spin-Hall-angle W/Pt layers generated an OOP effective field that switched perpendicular CoFeB without an external magnetic field.The effect required appropriate W and Pt thicknesses and remains beyond current SOT understanding.
  • Device integration: These field-free stacks can interface the perpendicular ferromagnet with MgO and therefore pose fewer engineering challenges for three-terminal memories.The review presents this compatibility as relevant to incorporating the structures into magnetic-memory devices.

E. Geometrical domain-wall pinning

Geometrical domain-wall pinning enables deterministic field-free switching by moving a single domain wall between engineered pinning sites. The review places this approach alongside broader SOT materials, dynamics, and device-engineering challenges.

  • Geometrical domain-wall pinning: A new field-free SOT concept combines SOT-driven domain-wall motion with geometrical domain-wall pinning.The approach uses anti-notched structures at both ends of a magnetic wire.
  • Geometrical domain-wall pinning: SOT can drive domain walls in perpendicular ferromagnets without an external field, although conventional in-plane-spin SOT does not directly switch the magnet.The review distinguishes domain-wall motion from magnetization switching in this context.
  • Geometrical domain-wall pinning: Opposite current polarities move the domain wall back and forth, while anti-notches contain one wall within the strip.The center magnetization state serves as the memory state and can be read out.
  • Device implications: SOT-MRAM decouples read and write paths, providing greater design flexibility and minimizing tunnel-breakdown concerns relative to STT-MRAM.The SOT scheme instead places the high write current in the normal metal, increasing electromigration risk.
  • Challenges and outlook: Practical deployment remains limited by unresolved reliability, switching-error, current-density, and materials-integration challenges.The review highlights endurance testing, electromigration, annealing stability, and compatibility with existing silicon platforms.
  • Switching dynamics: SOT switching predominantly follows domain nucleation and expansion, while field-like torque can contribute strongly and even cause backward switching.The incubation delay at switching onset was measured to be negligible.
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