Pt/h-LuFeO3 Bilayers: Spintronic Heterostructures
- Pt/h-LuFeO3 bilayers are heterostructures combining a metallic Pt layer with a hexagonal LuFeO3 film that exhibits multiferroic behavior and topologically nontrivial magnetic states.
- Interfacial phenomena, including the topological Hall effect and magnetic proximity, are engineered through controlled growth, defect tuning, and strain effects.
- These bilayers enable electrically controllable spintronic responses with promising applications in nonvolatile memories, quantum devices, and phase-tunable Josephson junctions.
Pt/h-LuFeO₃ bilayers are heterostructures composed of a metallic platinum (Pt) layer adjacent to a hexagonal lutetium ferrite (h-LuFeO₃) film. The h-LuFeO₃ provides a nontrivial multiferroic and topological magnetic insulator platform, while Pt acts as a heavy-metal channel with strong spin–orbit coupling and excellent carrier mobility. These bilayers are recognized for hosting robust interfacial phenomena arising from magnetic proximity, strain engineering, improper ferroelectricity, and topologically nontrivial spin arrangements, yielding pronounced and electrically controllable spintronic responses, including the interfacial topological Hall effect (ITHE) and tunable magnetoelectric coupling.
1. Structural and Magnetic Properties of h-LuFeO₃
The h-LuFeO₃ layer is an insulating ferrite with Fe³⁺ moments that order in a 120° triangular antiferromagnetic lattice in the ab plane. Each spin trimer exhibits a solid angle Ω due to a small out-of-plane canting, imparting noncoplanarity and finite scalar spin chirality (Ω ∝ S₁·(S₂×S₃)). Despite a minimal net magnetization (∼0.025 μ_B/Fe), the crystal hosts a robust topological spin texture. The emergent field associated with this topology can reach approximately 50 T as determined by the solid angle and geometric area per trimer. The crystalline structure also supports "improper" ferroelectricity induced by a K₃ ferrodistortion: a trimerization of the lattice that couples to a spontaneous out-of-plane polarization, characterized by an amplitude Q, where the atomic displacement d = 1.5Q. This multiferroic behavior can be modulated by epitaxial strain, interface engineering, and growth protocol (Li et al., 8 Mar 2025, Sinha et al., 2016).
2. Growth Mechanisms and Interface Engineering
Growth of LuFeO₃ and related phases (e.g., LuFe₂O₄) by pulsed laser deposition relies on a delicate balance between nucleation kinetics and adatom desorption. The adatom residence time τ_ad = (1/ν)exp(E_des/(kT)), where Fe atoms desorb more rapidly than Lu, leads to sensitivity in Lu:Fe stoichiometry. Nucleation rate follows J_nuc ∝ (Δμ/T)1/2exp(−κ/(Δμ kT)), with supersaturation Δμ_O(ad) = Δμ_0(T) + (3/4)N_A kT ln(P_O₂). This thermochemistry establishes a narrow process window for phase-pure h-LuFeO₃ formation. Minute admixtures of h-LuFeO₃ arise as impurity phases and play a critical role in segmenting magnetic domains and modifying interfacial magnetism; defect engineering further tunes domain sizes and coercivity (Wang et al., 2012). For ultrathin ferroelectric h-LuFeO₃, interfacial structural matching (e.g., insertion of a Lu₂⁄₃Fe₁⁄₃O₇⁄₆ monolayer) preserves primary order parameters even at the monolayer limit, enabling device miniaturization with high functional stability (Li et al., 8 Mar 2025).
3. Elastic Strain Effects and Multiferroic Tuning
Application of biaxial in-plane compressive strain to h-LuFeO₃ films enhances the K₃ ferrodistortion, increasing polarization: P(Δa) ≈ P₀ + k (∂Q₍K₃₎/∂a) Δa, for strain Δa < 0. However, the same strain reduces the Fe–Fe interatomic vector |r₍Fe–Fe₎|, thereby suppressing the Dzyaloshinskii–Moriya interaction D ~ r₍Fe–Fe₎×δ_z and lowering the canting angle of the weak ferromagnetic moment: M_FM ∝ sin(θ) ~ D/J. Consequently, strain engineering enables simultaneous enhancement of electric polarization and control over the magnetic ground state, a duality exploitable in Pt/h-LuFeO₃ bilayers for optimized spintronic and magnetoelectric device coupling (Sinha et al., 2016).
4. Interfacial Topological Hall Effect (ITHE)
Pt/h-LuFeO₃ bilayers exhibit a persistent and giant interfacial topological Hall effect arising from the magnetic proximity effect (MPE), whereby the topological spin texture of h-LuFeO₃ is imprinted onto the adjacent Pt film. The 120° noncoplanar ordering yields an emergent topological field that induces a positive Hall resistivity ρ_xy, reaching up to 0.5% of the longitudinal resistivity in Pt and a Hall-conductivity/magnetization ratio exceeding 2 V⁻¹—substantially larger than values typical of the anomalous Hall effect. Unlike narrow peak–and–dip THE associated with conducting magnets, ITHE in this insulating bilayer is broad, stable against magnetic fields up to 14 T, and persists below the Néel temperature (T_N ≈ 130 K). The temperature–field dependence and the electrical readout mechanism make ITHE a powerful probe for topological magnetism in insulators and provide a direct electrical interface to complex magnetic order (Li et al., 16 Sep 2025).
| Phenomenon | Key Formula/Metric | Observed Behavior/Role |
|---|---|---|
| Emergent topology | Ω ∝ S₁·(S₂×S₃); B_e ∝ (1/A)·Ω | Topological Hall effect in Pt |
| ITHE signal strength | ρ_xy,ITHE up to 0.5% ρ_xx; ratio > 2 V⁻¹ | Dominates over AHE, robust to B |
| Superparamagnetic model | M ∝ p [coth(μB/(k_BT)) − 1/x] | Models cluster contributions |
5. Electrical Control and Superconducting Functionality
In advanced bilayer stacks, PT-symmetric antiferromagnetic order and interlayer exchange fields in h-LuFeO₃-based bilayers enable the realization of spin-layer locked Cooper pairs via superconducting proximity. A perpendicular electric displacement field V_d actuates field-induced finite-momentum Cooper pairing, Q ≈ V_d/√(μλ), allowing electrical modulation of the Josephson current’s phase: transitions between 0- and π-junction regimes (I_s = I_c sin φ vs. I_s = I_c sin(φ+π)), with suppressed first harmonics near the transition. The tunneling of Cooper pairs depends sensitively on the Néel order alignment, resulting in a Josephson giant magnetoresistor (GMR) effect, with η approaching unity for small interlayer coupling (g ≪ Jₑₓ). These architectures are prime candidates for phase-controllable Josephson junctions, electrically addressable MRAM, and qubit platforms requiring minimal stray field interference and ultralow-power operation (Hu et al., 10 Sep 2025).
6. Spintronic and Device Implications
Pt/h-LuFeO₃ bilayers offer robust electrical readout of topological magnetic states in insulating films. The combination of giant ITHE, tunable polarization, and strain-engineered magnetoelectric coupling provides a versatile platform for next-generation nonvolatile memories, topological sensors, and low-energy spin logic where device scaling and stability are critical. The ability to electrically induce and read out topological spin configurations at the ultrathin limit—down to individual unit cells—positions these bilayers as foundational elements in advanced spintronic circuits and quantum information architectures. Persistent ITHE and electrically controlled Josephson phenomena in these systems establish a new class of devices where functional behavior is derived directly from nontrivial spin topology rather than net magnetization, broadening design principles for future oxide electronics and quantum devices (Li et al., 16 Sep 2025, Hu et al., 10 Sep 2025).
7. Prospective Directions and Considerations
Continued advances in interface engineering—such as the use of structurally matched insertion layers (e.g., Lu₂⁄₃Fe₁⁄₃O₇⁄₆)—are expected to further stabilize improper ferroelectricity and foster new modes of interfacial coupling. The precise tuning of strain, defect density, and film composition remains central for optimizing both topological and functional properties. Systematic exploration of additional heavy metal partners, gate-controlled displacement fields, and integration with superconductors will extend the operational domain of Pt/h-LuFeO₃ bilayers, with an emphasis on scalable, energy-efficient, and topologically protected device functionalities. These heterostructures exemplify the growing paradigm wherein engineered interfacial phenomena, rather than bulk order, drive emergent quantum and spintronic behaviors.
