Spin-orbit torque nano-oscillator with giant magnetoresistance readout

Spin–orbit torques (SOTs), including spin Hall torque (SHT), enable current-driven auto-oscillations of ferromagnet magnetization in ferromagnet/nonmagnet bilayers, producing microwave signals via anisotropic magnetoresistance (AMR). While SOT oscillators are structurally simple and easy to fabricate compared to magnetic tunnel junction (MTJ) spin-transfer torque oscillators, their practical utility is limited by low microwave output power because AMR in thin films is small. Moreover, in conventional spin Hall oscillators (SHOs) the magnetization orientation that maximizes antidamping SHT differs from that which maximizes AMR-based microwave conversion, further reducing efficiency. This work addresses whether adding a ferromagnetic reference layer to employ current-in-plane giant magnetoresistance (CIP GMR) can substantially enhance output power without sacrificing simplicity, and whether the GMR angular dependence better matches the SHT efficiency to simultaneously minimize critical current and maximize output power.

Prior studies established SOTs and the spin Hall effect as mechanisms for driving magnetization dynamics and auto-oscillations in metallic and insulating systems, with microwave signals detected via AMR. Conventional SHOs typically yield only pW-level output due to small AMR. In contrast, MTJ-based oscillators leveraging tunneling magnetoresistance (TMR) achieve higher output power but require complex fabrication and have poor impedance matching to 50 Ω systems. Previous work also characterized bulk and edge spin-wave modes in nanowire SHOs, nonlinear frequency shifts, and linewidth broadening mechanisms including thermal effects, mode coupling, and nonlinearities. CIP GMR in spin valves offers larger magnetoresistance than AMR and a cosine angular dependence that could align better with SHT efficiency, suggesting a route to higher power SOT oscillators.

Device fabrication: GMR SHO nanowires were patterned from sapphire/Ir25Mn75(4 nm)/Co(2 nm)/Cu(4 nm)/Co(0.5 nm)/Py(Ni80Fe20, 3.5 nm)/Pt(5 nm) multilayers deposited by magnetron sputtering. A 0.5 nm Co dusting layer between Cu and Py enhanced CIP GMR. The stack was post-annealed at 523 K for 1 hour to set exchange bias along the nanowire axis. Nanowires were 65 nm wide and 40 μm long, defined by e-beam lithography and Ar ion milling. Ti(5 nm)/Au(40 nm) contact pads with a 740 nm gap formed the active region where current density and SHT are maximized. The pinned bottom Co layer is exchange-biased by IrMn, while the top free layer (Co/Py) is adjacent to Pt to receive spin Hall current. A reference AMR SHO with identical lateral dimensions was patterned from sapphire/Cu(4 nm)/Co(0.5 nm)/Py(3.5 nm)/Pt(5 nm), the Cu underlayer providing similar Oersted field conditions. Measurement conditions: All measurements were performed in a continuous-flow He cryostat at T = 4.2 K. Magnetoresistance was measured with magnetic field applied along the wire axis at a small dc probe current (0.5 mA). Microwave emission was measured by applying a dc current through a bias tee and recording spectra with a spectrum analyzer and a low-noise microwave amplifier. Background subtraction was performed by taking spectra at opposite current polarity where SHT is damping. Emission was characterized primarily at in-plane magnetic fields H = 800 Oe (angle θ to the wire) and H = 500 Oe (for angular scans). For AMR SHO emission, θ was offset by 5° from 90° to enhance AMR-based conversion. Power is reported as that available to a 50 Ω load; conversion to matched-load power follows P_matched = P_measured (R_device + R_load)/(4 R_device R_load), with R_load = 50 Ω. Data analysis included integrating power spectral density to obtain total emitted power and extracting linewidths (full-width at half-maximum).

- Magnetoresistance: GMR SHO exhibited RP = 117.8 Ω, RAP = 124.3 Ω at 0.5 mA with GMR ratio ΔRGMR/RP = 0.055. The reference AMR SHO had AMR ratio ≈ 0.004. - Auto-oscillation thresholds: AMR SHO auto-oscillations onset at Idc ≈ 3.0 mA (≈2 × 10^8 A/cm^2 in Pt) for H = 800 Oe, θ = 85°. GMR SHO exhibited onset at ≈ 4.0 mA for H = 800 Oe, θ = 90°. - Mode structure: AMR SHO showed a single auto-oscillatory mode with a small blue frequency shift with increasing current, attributed to Oersted field in the Cu layer. GMR SHO showed two modes: a higher-frequency bulk spin-wave mode similar to the AMR SHO, and a lower-frequency edge mode with a much larger blue frequency shift indicative of edge-mode nonlinearity and confinement changes. - Linewidths: AMR SHO linewidth Δf ≈ 8 MHz; GMR SHO dominant edge mode Δf ≈ 120 MHz at H = 800 Oe. The broader GMR linewidth is attributed to higher ohmic heating (higher Ic), simultaneous multi-mode excitation and mode interaction, strong nonlinear frequency shift of the edge mode, and potentially enhanced spin pumping/damping. - Output power: GMR SHO produced much higher microwave power than AMR SHO under equivalent SHT-favorable conditions. Maximum measured GMR SHO power exceeded 1 nW (deliverable to 50 Ω); power increased monotonically up to Idc = 6 mA. AMR SHO power increased then decreased at higher currents, consistent with enhanced magnon population and nonlinear scattering. - Angular dependence: For GMR SHO, maximum power occurs at φM = 90° (magnetization perpendicular to the wire), where both SHT efficiency (∝ sin θ) and CIP GMR conversion (R = RP + ΔRGMR cos φM, yielding δR ∝ −ΔRGMR sin φ0 δφ) are maximized. For AMR SHO, resistance R = RP − ΔRAMR cos^2 φM leads to δR ∝ ΔRAMR sin(2φM) δφ, so maxima are expected near 45° and 135°; experimentally, maxima occurred near 70° and 110°, reflecting the trade-off between SHT efficiency and AMR conversion and the effect of shape anisotropy and angle-dependent critical current (Ic ∝ 1/sin φM). Power drops precipitously for φH < 60° and φH > 120° at constant bias. - Physical mechanism: The dominance of the edge mode in GMR SHO is likely enhanced by stray fields from the pinned layer when the field is perpendicular to the wire, deepening edge confinement and increasing mode extent, thus boosting emitted power. - Benchmarking and impedance: Compared to three-terminal MTJ SHOs that can achieve higher reported powers but suffer from kΩ impedances and complex fabrication, the GMR SHO is simpler, more compact, and better matched to 50 Ω, improving deliverable power. Using literature MTJ resistances, deliverable power to 50 Ω can be significantly reduced versus reported values. - Improvement potential: Increasing CIP GMR ratio (e.g., to ~17%) could yield nearly an order-of-magnitude power increase (power ∝ ΔR^2), optimizing free-layer coupling and damping can increase precession cone angle and reduce critical current, and enhancing spin Hall efficiency via scattering or alloying can further lower Ic. CIP GMR readout is compatible with other SOT sources.

The study demonstrates that integrating a ferromagnetic reference layer to exploit CIP GMR substantially enhances the microwave output power of SOT nanowire oscillators relative to AMR-based designs. Crucially, the angular dependence of GMR aligns with the SHT antidamping efficiency, so that φM = 90° simultaneously minimizes the critical current and maximizes the resistance oscillation amplitude, enabling better performance without complex device architectures. The observed two-mode behavior in GMR SHOs, with a dominant edge mode exhibiting strong blue frequency shift, indicates that device-specific fields (e.g., pinned-layer stray fields) can tailor mode profiles and enhance power. The increased linewidth in GMR SHOs arises from higher bias-induced heating, mode interactions, and strong nonlinearity, highlighting a trade-off between power and spectral purity. Potential mitigation via mutual synchronization could reduce linewidth while retaining higher output power. Compared to three-terminal MTJ SHOs, the GMR SHO offers simpler fabrication, smaller footprint, and superior impedance matching to 50 Ω systems, improving useful delivered power despite somewhat lower peak intrinsic output than the best MTJ devices.

CIP GMR readout in spin Hall nano-oscillators boosts output microwave power above 1 nW while preserving structural simplicity. By matching the angular dependence of magnetoresistance to SHT efficiency, the GMR SHO can simultaneously achieve low critical current and high output power, unlike AMR SHOs that require a trade-off. These advances enhance the practicality of SOT oscillators for applications such as neuromorphic and reservoir computing and chip-to-chip wireless links. Future work should aim to increase the GMR ratio, optimize free-layer magnetic parameters to increase precession amplitude and reduce damping, enhance spin Hall efficiency through materials engineering, exploit alternative SOT sources with CIP GMR readout, and employ oscillator synchronization to reduce linewidth.

Measurements were performed at cryogenic temperature (4.2 K), leaving room-temperature performance unassessed. The GMR SHO exhibited broader linewidths than the AMR SHO due to higher operating currents, multi-mode dynamics, and nonlinearity. Only a single nanowire geometry and limited field strengths/directions were explored; generality across device sizes and operating conditions remains to be verified. The precise mechanism for edge-mode enhancement due to pinned-layer stray fields requires further theoretical and experimental validation. Potential contributions from spin torques generated in the pinned layer were argued to be small but were not quantitatively isolated.