Ferrimagnetic Co/Gd Multilayers in Spintronics

• Ferrimagnetic Co/Gd multilayers are synthetic heterostructures with alternating Co and Gd layers exhibiting robust antiferromagnetic coupling and tunable compensation points.
• Their design leverages precise deposition and interface engineering to achieve ultrafast all-optical switching, controllable perpendicular anisotropy, and high-speed domain wall motion.
• Applications in spintronics include energy-efficient memory, racetracks, and skyrmion stabilization, enhanced by optimized capping layers and annealing treatments.

Ferrimagnetic Co/Gd-based multilayers are a class of synthetic ferrimagnetic heterostructures comprising alternating layers of cobalt (Co, a 3d transition ferromagnet) and gadolinium (Gd, a 4f rare earth with large localized moment), often interfaced with heavy metals such as Pt. These multilayers exhibit antiferromagnetic interfacial exchange coupling between Co and Gd, resulting in rich static and dynamic magnetic phenomena, including perpendicular magnetic anisotropy (PMA), tunable compensation points, ultrafast single-pulse all-optical switching (AOS), efficient spin-orbit-torque manipulation, and high-velocity current-driven domain wall motion. Their properties are controlled by layer thickness, interfacial quality, capping materials, external fields, and thermal treatments, making them prime candidates for next-generation photonic-spintronics, memory, and logic devices.

1. Structural Design, Growth, and Proximity Effects

Co/Gd-based multilayers are realized via DC magnetron sputtering or related physical vapor deposition, with base pressures in the 10⁻⁹ mbar range to ensure ultraclean interfaces. A representative architecture is Ta(4 nm)/Pt(4 nm)/Co(0.6–1 nm)/Gd(0.3–3 nm)/[repeats]/TaN or Pt cap, with optional wedge-shaped Gd thickness to enable spatial mapping of compensation and switching properties (Kools et al., 2023, Li et al., 2022).

Interfacial atomic structure determines magnetic exchange. The presence of sharp, continuous layers is confirmed by STEM and EDX, while atomically sharp interfaces ensure robust proximity-induced magnetization in Gd. X-ray magnetic circular dichroism (XMCD) and density functional theory (DFT) reveal a complex "flipped spin state" (FSS) in Gd: layers adjacent to Co acquire an antiparallel (negative) induced moment (~–0.54 μ_B/atom at 300 K), followed by oscillatory parallel alignment deeper into Gd (Brandão et al., 6 Apr 2024). Proximity effects also transfer a small induced moment to Pt at the interfaces, but this is negligible at the Gd/Pt boundary. Intermixing and roughness at the Co/Gd interface reduce Gd's effective moment and are strongly influenced by capping layer choice.

2. Static Magnetic Properties and Compensation Mechanisms

The magnetic state of Co/Gd multilayers is governed by competition between exchange coupling, demagnetization, and interfacial anisotropy. The system exhibits:

• Antiferromagnetic Co-Gd exchange of order J_ex ≈ –0.5×10⁻³ J/m² (from DFT and micromagnetic fits (Brandão et al., 6 Apr 2024)), leading to antiparallel net moments in Co and Gd sublattices.
• Proximity-induced Gd magnetization: Gd is paramagnetic in bulk above T_C, but adjacent to Co develops a strong magnetic moment decaying exponentially over λ_Gd ≈ 0.3–1.5 nm (Kools et al., 2022). This decay profile is modeled as M_Gd(z) = M₀,Gd e^{–z/λ_Gd}, and the effective Gd moment is highly sensitive to interface diffusion and capping-layer choice.
• Compensation point tuning: Magnetization compensation (M_tot = 0) arises when M_s^Co·t_Co = M_s^Gd,eff·t_Gd. For a [Co(0.6)/Gd(1.5)/Co(0.7)/Gd(wedge)] structure, compensation occurs at t_Gd ≈ 0.97–1.25 nm, shifting with intermixing or altered capping (Kools et al., 2022, Li et al., 2022).
• Perpendicular anisotropy: Arises predominantly from Co/Pt interfaces (K_S ≈ 1.2–1.7 mJ/m²), decaying into Gd over λ_K ≈ 0.7 nm. PMA persists up to Co or Gd thickness limits set by spin-reorientation transitions (Kools et al., 2022, Masciocchi et al., 2023).
• Magnetic field and temperature gradients: The H–T phase diagram includes ferrimagnetic, spin-flop (canted), and paramagnetic phases, with spin-flop transitions set by the competition between K_eff and inter-sublattice exchange (Buzdakov et al., 16 Oct 2024). The compensation temperature T_comp (where net moment vanishes) is widely tunable (120–200 K at room temperature) by growth parameters (Kools et al., 2023, Wang et al., 2020).

The robustness of these properties under strain is notable: in-plane tensile strain (ε ≈ 0.1%) increases PMA marginally (Δμ₀H_k ≈ 1 mT), with the SRT boundary shifting by Δt_Co ≈ 0.1 nm; the compensation point remains unchanged, confirming resilience for device integration (Masciocchi et al., 2023).

3. Ultrafast Dynamics and All-Optical Switching

Co/Gd-based multilayers support single-pulse, helicity-independent AOS on femtosecond to picosecond scales. Experimental and theoretical analyses based on microscopic three-temperature models (M3TM), multisublattice Landau-Lifshitz-Bloch (LLB) equations, and time-resolved MOKE reveal the following:

• Laser-induced magnetization reversal is governed by differential demagnetization: Co demagnetizes within ≈200 fs, Gd over 1–2 ps (Hintermayr et al., 2023, Beens et al., 2019).
• Exchange-driven front propagation: A reversed Co magnetization front nucleates at Co/Gd interfaces via rapid exchange scattering, propagating through the Co layer even for thicknesses far from compensation—contrasting with rare-earth–transition-metal alloys, which require strict compensation for AOS (Beens et al., 2019).
• Switching thresholds: The single-pulse switching fluence is fluence-independent of compensation for Co/Gd: typical F_th ≈ 0.9–4.4 mJ/cm² for multilayer stacks; insertion of Tb enhances PMA without altering F_th or slowing dynamics (Hintermayr et al., 2023, Li et al., 2022).
• Switching times: All-optical switching occurs with zero-crossing of magnetization in ≈1 ps (Co/Gd/Tb/Co stacks) or ≈700–800 fs in FeCo/Gd theory/experiment (Buzdakov et al., 16 Oct 2024).
• No strict requirement for compensation: Deterministic AOS persists even for thick ferromagnetic layers and away from net-moment compensation (Beens et al., 2019, Li et al., 2022).
• Effect of annealing: Annealing to 300 °C reduces F_0 by 28%, attributed to intermixing-induced enhancement of the exchange-scattering channel and increased proximity-magnetized Gd volume (Wang et al., 2020).

4. Spin-topological and Dynamic Phenomena: Domain Walls, Spin Spirals, Skyrmions

Co/Gd multilayers display a wide array of topological and domain phenomena relevant for high-speed memory and logic:

• Current-induced domain wall motion: Near compensation, domain wall velocities v_DW > 2000 m/s are achievable under current pulses (J ≈ 3.6 TA/m²), enabled by SOT from heavy-metal underlayers (Pt/Co interface, θ_SH ≈ 0.08), highly efficient exchange-coupling torques, and minimal net moment (Li et al., 2022).
• Dynamic pinning and annealing: DW velocities rise up to 30× with annealing at 300 °C due to reduction in pinning energy scales U_cH_dep^μ by ∼40% (μ = 1/4, universal for 2D interfaces); smoother interfacial alloying enhances creep flow (Wang et al., 2020).
• Labyrinthine spin textures: Mumax³ simulations and scanning transmission X-ray microscopy confirm the existence of labyrinthine spin spiral domains (period ≈100 nm) stabilized by sizable Dzyaloshinskii-Moriya interaction (D ≈ 1.2×10⁻³ J/m²), with a balance between A, K_eff, and J_ex essential for topological stability (Brandão et al., 6 Apr 2024).
• Flipped spin states and compensation: Atomistic spin dynamics shows Gd layers relax into a layered FSS with compensation near remanence, enabling low-stray-field operation and potential for skyrmion stabilization (Brandão et al., 6 Apr 2024).

The interplay between proximity-induced magnetization, DMI, and capping layer selection enables the stabilization and manipulation of 2D/3D domain structures, crucial for racetrack and skyrmion-based memory concepts.

5. Interface Engineering and Long-Term Stability

The temporal stability of Co/Gd magnetic properties is governed by capping layer chemistry and interface control:

• Quenching mechanisms: Metallic caps (Pt, Ta) strongly intermix with Gd, rapidly quenching the proximity-induced Gd moment and shifting T_comp by up to 40 K in 90 days; the effect is more severe for Pt due to Gd–Pt intermetallic formation (Kools et al., 2023, Kools et al., 2022).
• Protection strategies: Reactive-sputtered TaN caps impede intermixing/oxidation, offering stable net moment and T_comp for ~1 month, after which ambient oxidation (TaN→TaO_x) allows Gd oxidation and resumed quenching. Bilayer TaN/Pt caps offer the most robust long-term passivation (Δ\tilde m ≈ –3%, ΔT_comp ≈ +2 K after 90 days) (Kools et al., 2023).
• Implication: Interface/capping layer design directly affects device reproducibility and lifetime; quantifying diffusion coefficients and activation energies (e.g., via future Arrhenius modeling of M(t)) remains an open route for predictive lifetime control (Kools et al., 2023, Kools et al., 2022).

6. Device Applications and Outlook

Ferrimagnetic Co/Gd-based multilayers provide a unique combination of properties that directly address central challenges in spintronics and opto-spintronic integration:

• All-optical memory: Single-shot AOS over a wide thickness range at room temperature allows for robust, composition-tolerant memory bits with write energies <25 fJ/50×50 nm², scalable to dense photonic memory arrays (Li et al., 2022).
• Ultrafast racetracks: [Co/Gd]₂ quadlayers show record DW velocities for current-induced shifting, with independent thickness tuning enabling optimal positioning at angular momentum compensation and minimal AOS threshold (Li et al., 2022).
• Skyrmionic and ripple-based spin textures: The balance of J_ex, DMI, and proximity-induced Gd magnetization stabilizes zero-field skyrmions and stripe domains, with FSS and ripple characteristics relevant for advanced logic or neuromorphic computing (Brandão et al., 6 Apr 2024, Hermosa-Muñoz et al., 2021).
• Integration and process compatibility: The resilience to moderate in-plane strain, low pinning after annealing, and capping-layer passivation make these multilayers compatible with MTJ fabrication and wafer-scale integration (Masciocchi et al., 2023, Wang et al., 2020, Kools et al., 2023).
• Design recommendations include: sub-nm thickness control of Gd and Co, nitride/metal bilayer capping, annealing at ≤300 °C, and operation slightly Gd-rich of compensation to account for shifts induced by current or thermal transient effects.

Ferrimagnetic Co/Gd-based multilayers have thus emerged as a paradigm material platform enabling ultrafast, energy-efficient, scalable, and tunable magnetic control for both photon- and electron-driven spintronic technologies. Future research priorities include full mapping of interfacial diffusion kinetics, further exploration of complex spin textures, and quantitative modeling of device shelf life and failure mechanisms, as well as system integration for memory, logic, and topological-spin platforms (Kools et al., 2023, Brandão et al., 6 Apr 2024, Li et al., 2022, Kools et al., 2022).