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Ultralow-current-density and bias-field-free spin-transfer nano-oscillator
Zhongming Zeng, Giovanni Finocchio, Baoshun Zhang, Pedram Khalili Amiri, Jordan A. Katine, Ilya N. Krivorotov, Yiming Huai, Juergen Langer, Bruno Azzerboni, Kang L. Wang, Hongwen Jiang
TL;DR
STNO microwave emission has generally required large drive currents and external magnetic fields, limiting practical integration. This study experimentally measures MgO-based STNOs with a planar polarizer and perpendicular free layer, observing large-power emission without bias fields at ultralow current densities. The devices achieve output power at least one order of magnitude larger than previous field-free STNOs, with critical current densities below 6×10^5 A/cm^2.
Problem
Existing STNO microwave emission mainly relies on large drive currents and external magnetic fields, motivating evidence for field-free operation at lower current density.
Method
The study experimentally measures MgO-based STNOs combining a planar polarizer with a perpendicular free layer.
Results
At least one order of magnitude larger output power and critical current densities below 6×10^5 A/cm^2 were achieved without a bias field.
Takeaways & Limitations
The results suggest improved STNO integration with complementary metal-oxide-semiconductor technology and a route toward on-chip oscillators.
Takeaways & Limitations
The higher frequency tunability comes at the cost of a larger linewidth than magnetic vortex self-oscillations.
Abstract
from arXiv · showhide
The spin-transfer nano-oscillator (STNO) offers the possibility of using the transfer of spin angular momentum via spin-polarized currents to generate microwave signals. However, at present STNO microwave emission mainly relies on both large drive currents and external magnetic fields. These issues hinder the implementation of STNOs for practical applications in terms of power dissipation and size. Here, we report microwave measurements on STNOs built with MgO-based magnetic tunnel junctions having a planar polarizer and a perpendicular free layer, where microwave emission with large output power, excited at ultralow current densities, and in the absence of any bias magnetic fields is observed. The measured critical current density is over one order of magnitude smaller than previously reported. These results suggest the possibility of improved integration of STNOs with complementary metal-oxide-semiconductor technology, and could represent a new route for the development of the next-generation of on-chip oscillators.
Results
At zero bias magnetic field, the devices produced GHz microwave emission in an intermediate-resistance state at negative currents, with ultralow critical current densities and low dissipation. Measurements and simulations identified an out-of-plane, quasi-uniform oscillation mode whose frequency decreases with increasing current.
- Similar results were achieved for more than 5 devices for each free layer thickness.
- Spin-transfer torque dynamics: 28 MHz was the minimum linewidth, while oscillation frequency red-shifted with current at 1.75 GHz mA-1.For |I| ≤ 0.3 mA, spectra showed a single Lorentzian GHz-range peak from an out-of-plane oscillation mode.
- Spin-transfer torque dynamics: 18 nW measured output power (> 60 nW delivered to a matched load) was obtained at I = -0.3 mA, with 850 MHz frequency and 73 MHz linewidth.At this condition, dissipated power was of the order of 50-70 µW for t = 1.60 nm.
Discussion
The work demonstrates large-power, field-free STNO oscillations at an ultralow critical current density, outperforming previous field-free approaches in output power and excitation current. The authors identify linewidth as a limitation while highlighting prospects for phase-locked arrays, CMOS integration, and magnonic devices.
- Key result: This work experimentally demonstrates large oscillation power without bias field at an ultralow critical current density Jc below 6×105A/cm2.The authors describe this as the first experimental demonstration of these combined properties.
- Performance comparison: Compared with previous field-free STNOs, the devices provide output power at least one order of magnitude larger and critical current densities one order of magnitude smaller.Other field-free solutions produced microwave power smaller (<1 nW) with excitation current higher (> 106A/cm2).
- Dynamic properties: The oscillation frequency is of the same order as in other STNOs, while tunability reaches ~ 1.75 GHz mA-1.This exceeds the tunability measured for spin-transfer-driven vortex oscillations: 0.03 GHz mA-1 and 0.08 GHz mA-1.
- Limitations: The enhanced tunability comes at the cost of a larger linewidth than in magnetic vortex self-oscillations.The linewidth is attributed primarily to thermal fluctuations and coupling between oscillator phase and power.
- Mitigation strategies: Phase-locked arrays could significantly decrease linewidth and increase output power to over the tens of µW.Low-frequency current modulation is presented as an alternative solution.
- Applications: The results could facilitate direct CMOS integration, reduced power consumption, portable and wireless applications, and ultralow-critical-current zero-field spin-wave sources for magnonic logic.Suggested applications include embedded communications and power-efficient local clock generation in digital systems.
Methods
The methods combine sputtered MgO-based magnetic stacks, thickness-dependent device characterization, and micromagnetic simulations incorporating perpendicular and field-like torques. Simulations identify the excited STNO state as a quasi-uniform out-of-plane mode.
- Sample preparation: The magnetic stacks were deposited by sputtering and comprised PtMn/Co70Fe30/Ru/Co40Fe40B20/MgO/Co20Fe60B20 layers.Layer thicknesses were specified in nanometers, with the free-layer thickness varied from 1.5 to 2.0 nm.
- Micromagnetic simulation: Micromagnetic simulations solved the Landau-Lifshitz-Gilbert-Slonczewski equation while including field-like torque and standard effective-field contributions.The effective field included exchange, self-magnetostatic, free-layer–polarizer coupling, Oersted-field, and perpendicular-anisotropy terms.
- Micromagnetic simulation: The field-like torque was modeled as voltage-dependent up to 25% of the in-plane torque at |J| = 6.0×106A/cm2.Its voltage dependence was taken proportional to the square of the bias voltage.
- Micromagnetic simulation: Simulations indicate an out-of-plane mode with a quasi-uniform spatial distribution of magnetization in the STNO with perpendicular anisotropy.The polarization function was computed using the Slonczewski formulation, with spin polarization η = 0.5730 for fitting the critical current density.
Additional information
The paper provides supplementary information online, declares no competing financial interests, and is distributed under a Creative Commons license. It is cited as a 2013 Scientific Reports article.
- Supplementary information: Supplementary information accompanies the paper and is available through the Scientific Reports website.The supplied passage directs readers to the online supplementary materials.
- Competing financial interests: The authors declare no competing financial interests.
- License: The work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.The license passage also provides a link for viewing a copy of the license.
Figure captions
The figure captions describe the sample structure, zero-field transport and microwave measurements, micromagnetic simulations, and comparisons of STNO performance parameters. They emphasize uniform simulated dynamics and results for free-layer thicknesses of 1.60 nm and 1.62 nm.
- Figure 1: The sample combines an in-plane magnetized fixed (polarizer) layer with an out-of-plane magnetized free layer.
- Figure 1: The captions track the STNO sample 1 linewidth and f0 as functions of current, while comparing magnetic-field behavior for 1.60 nm and 1.62 nm samples.The sample-1 thickness is t = 1.60 nm.
- Figure 1: Zero-field measurements include resistance–current characteristics, parallel and antiparallel configurations, and microwave spectra versus d.c. current bias.The spectra are vertically offset by approximately 20 nW GHz-1 for clarity.
- Figure 1: -82, -164 and -288 µA are the currents used to compute magnetization-vector trajectories in micromagnetic simulations.The simulations also show snapshots of the spatial magnetization distribution indicating uniform dynamics.
- Table 1: The performance comparison covers external field, critical current density, oscillation power, oscillation frequency, and minimum linewidth for different STNO solutions and this work.This work is compared at zero field for thicknesses t = 1.60 nm and 1.62 nm; Pmes denotes measured integrated peak power and Pmax the matched-load power.