Research ArticleSPINTRONICS

Giant nonreciprocal emission of spin waves in Ta/Py bilayers

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Science Advances  01 Jul 2016:
Vol. 2, no. 7, e1501892
DOI: 10.1126/sciadv.1501892
  • Fig. 1 Device structure and spin wave signals with out-of-plane bias fields.

    (A) The device consists of Si/SiO2 substrate/Ta (tTa nm)/Py (20 nm)/Ru (3 nm)/SiO2 (35 nm). The thickness of the Ta underlayer ranges from 1.2 to 8.9 nm. ACPS are used to apply local excitation fields and inductively detect magnetization precessions using a sampling oscilloscope after amplification with a low-noise amplifier (LNA). (B) Contour plot of spin waves in the time domain with various Hz for tTa = 6.1 nm. The fields (|Hz| < 490 mT) are applied perpendicularly to the film plane and are smaller than the demagnetization field, and thus cannot excite MSFV mode. (C) Spin wave packet at Hz = 25.8 mT (corresponding to the red dotted line in Fig. 1B). The amplitude (amp.) is defined as the difference between the maximum and minimum of the wave packet. (D) Contour plot of FFT of time domain signals. The spin wave resonance frequency (fR) increases with increasing |Hz|. (E) The range of fR corresponding to the external fields differs from that of the conventional spin wave modes, such as MSSW, MSBV, and MSFV. The fR plots for MSSW, MSBV, MSFV, and out-of-plane (sim.) are obtained from the simulations, in which the material parameters of Py are used. The experimental data of fR for the out-of-plane (exp.) are obtained from the sample of tTa = 6.1 nm.

  • Fig. 2 Change in spin wave amplitudes with different tTa.

    (A) Contour plot of spin wave (SW) amplitudes as a function of tTa and Hz. The red and blue arrows on the right side indicate Hz = ±25.8 mT. (B) Spin wave packets at Hz = ±25.8 mT. The amplitude at −Hz is higher than that at +Hz in the device for 1.2 < tTa < 3.3 nm. The amplitude at +Hz is higher than that at −Hz for 4.7 < tTa < 8.9 nm. (C) Amplitudes of the wave packets from Fig. 2B are plotted with different tTa, and the nonreciprocity (κ) is shown in the inset. κ is found to be ~0.2 for tTa = 0 nm. (D) Simulation results are shown for comparison with experimental results. Hz sweeps from negative to positive fields. The simulations show good agreement with measurements. The experimental data are obtained from a thin case (tTa = 1.9 nm) and a thick case (tTa = 8.2 nm). arb., arbitrary units.

  • Fig. 3 Anisotropic interface magnetoresistance.

    ΔROP is plotted with different tTa and shows a polarity change at tTa ~3 nm and an increase with increasing tTa. It indicates that different tTa changes the interfacial anisotropy directions.

  • Fig. 4 Spin pumping measurements by spin waves and ferromagnetic resonance data with out-of-plane Hz.

    (A) The spin pumping voltage is measured at the end of both electrodes. The applied 1.3-GHz microwave continuously excites local magnetization oscillations. (B) The amplitude of spin pumping signals increases with thicker tTa. It is expected that the amplitude shows the maximum for tTa ~ spin diffusion length (7 nm) in Ta. The spin pumping signal with tTa = 1.2 nm is used as a background signal and subtracted from that of all other devices. (C) A sudden increase in Vsp is observed at tTa = 6.8 nm. It is inferred that the spin pumping effect plays a significant role for tTa ≥ 6.8 nm. The plotted damping enhancement (Δα) as a function of the Ta thickness shows good agreement with spin pumping signals.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/7/e1501892/DC1

    section S1. Nonreciprocal ratios of surface spin wave mode with in-plane fields

    section S2. Nonreciprocity ratios from spin wave amplitudes in the time domain

    section S3. Nonreciprocity from spin wave intensity in the frequency domain

    section S4. Anisotropic magnetoresistance measurements

    section S5. VSM measurements

    section S6. Simulations: Out-of-plane hysteresis and nonreciprocity

    section S7. Antenna–spin wave coupling

    section S8. Spin wave amplitude–dependent damping

    section S9. Ta on the top of the Py layer

    section S10. Dependence of nonreciprocity on different directions of field sweep

    section S11. Dependence of nonreciprocity on the sign of the wave vector

    section S12. Effect of antenna-to-antenna distance on nonreciprocity

    fig. S1. Nonreciprocal spin wave emission in MSSW mode.

    fig. S2. Nonreciprocity ratios in the time domain.

    fig. S3. FFT of spin wave signals.

    fig. S4. AMR measurements.

    fig. S5. VSM measurements.

    fig. S6. Out-of-plane hysteresis and nonreciprocity from simulations.

    fig. S7. Spatial and temporal magnetization excited by the antenna.

    fig. S8. Ta thickness dependence of spin wave resonance frequencies.

    fig. S9. Nonreciprocity in devices with an Ta overlayer.

    fig. S10. Nonreciprocity from different directions of field sweep.

    fig. S11. Nonreciprocity from opposite signs of the wave vector.

    fig. S12. Spin wave packets in devices with a longer antenna-to-antenna distance (20 μm).

    References (5658)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Nonreciprocal ratios of surface spin wave mode with in-plane fields
    • section S2. Nonreciprocity ratios from spin wave amplitudes in the time domain
    • section S3. Nonreciprocity from spin wave intensity in the frequency domain
    • section S4. Anisotropic magnetoresistance measurements
    • section S5. VSM measurements
    • section S6. Simulations: Out-of-plane hysteresis and nonreciprocity
    • section S7. Antenna–spin wave coupling
    • section S8. Spin wave amplitude–dependent damping
    • section S9. Ta on the top of the Py layer
    • section S10. Dependence of nonreciprocity on different directions of field sweep
    • section S11. Dependence of nonreciprocity on the sign of the wave vector
    • section S12. Effect of antenna-to-antenna distance on nonreciprocity
    • fig. S1. Nonreciprocal spin wave emission in MSSW mode.
    • fig. S2. Nonreciprocity ratios in the time domain.
    • fig. S3. FFT of spin wave signals.
    • fig. S4. AMR measurements.
    • fig. S5. VSM measurements.
    • fig. S6. Out-of-plane hysteresis and nonreciprocity from simulations.
    • fig. S7. Spatial and temporal magnetization excited by the antenna.
    • fig. S8. Ta thickness dependence of spin wave resonance frequencies.
    • fig. S9. Nonreciprocity in devices with an Ta overlayer.
    • fig. S10. Nonreciprocity from different directions of field sweep.
    • fig. S11. Nonreciprocity from opposite signs of the wave vector.
    • fig. S12. Spin wave packets in devices with a longer antenna-to-antenna distance (20 μm).
    • References (5658)

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