Highly sensitive nanoscale spin-torque diode

Journal name:
Nature Materials
Volume:
13,
Pages:
50–56
Year published:
DOI:
doi:10.1038/nmat3778
Received
Accepted
Published online

Abstract

Highly sensitive microwave devices that are operational at room temperature are important for high-speed multiplex telecommunications. Quantum devices such as superconducting bolometers possess high performance but work only at low temperature. On the other hand, semiconductor devices, although enabling high-speed operation at room temperature, have poor signal-to-noise ratios. In this regard, the demonstration of a diode based on spin-torque-induced ferromagnetic resonance between nanomagnets represented a promising development, even though the rectification output was too small for applications (1.4 mV mW−1). Here we show that by applying d.c. bias currents to nanomagnets while precisely controlling their magnetization-potential profiles, a much greater radiofrequency detection sensitivity of 12,000 mV mW−1 is achievable at room temperature, exceeding that of semiconductor diode detectors (3,800 mV mW−1). Theoretical analysis reveals essential roles for nonlinear ferromagnetic resonance, which enhances the signal-to-noise ratio even at room temperature as the size of the magnets decreases.

At a glance

Figures

  1. Nonlinear effect in nanomagnets and the spin-torque diode device.
    Figure 1: Nonlinear effect in nanomagnets and the spin-torque diode device.

    a, Schematic image of a nonlinear effect in nanomagnets. Under FMR with an asymmetrical potential, the orbital centre of the free-layer magnetization (PRF≠0, orange) rotates away from the initial state (PRF = 0, blue), causing a change in the average resistance; the application of a d.c. bias converts the average resistance change into a large RF detection voltage. b, Spin-torque diode device and measurement set-up. The device is based on a MTJ with a MgO tunnel barrier and FeB magnetic free layer. The RF detection output of the MTJ is measured by a low-frequency (10 kHz) modulation method, using a lock-in amplifier.

  2. Basic device characteristics.
    Figure 2: Basic device characteristics.

    a, The device resistance as a function of the applied magnetic field. The right axis represents the relative angle between the magnetization directions of the free (FeB) and pinned (CoFeB) layers, which is calculated from equation (1). The orange curve in the inset shows the schematic path of the free-layer magnetization direction (Mfree). The black (Mpin) and white (H) arrows represent the direction of the pinned-layer magnetization and the applied magnetic field, respectively. RF detection measurements were conducted under the magnetic field condition of H = 1.1 kOe, where a large relative angle between Mfree and Mpin is realized. b, The ferromagnetic resonant frequency of the free layer as a function of the applied magnetic field. From the fits (grey solid curves), the perpendicular anisotropy (hard axis is z) and the in-plane anisotropy (hard axis is y) fields were estimated to be 0.9 and 0.035 kOe, respectively. Both of these values are small compared with those in previous studies, and a condition in which the free-layer magnetization easily oscillates under a shallow magnetization-potential was obtained. From the analysis, magnetization potential in the free layer is determined (see Fig. 1a). c,d, The RF detection voltage (V detect) as a function of the RF input frequency under various d.c. bias currents (Id.c.) The RF input power (PRF) is 0.01 μW (4.8 μA). A large output voltage was obtained at the resonant frequency, and d.c. bias was found to enhance the detection voltage.

  3. d.c. bias and RF input power dependence.
    Figure 3: d.c. bias and RF input power dependence.

    a, The RF detection voltage (V detect) as a function of the d.c. bias (Id.c.). The solid orange curve and the grey dotted curve represent the theoretical values obtained from equation (4) and macro-spin simulation, respectively. The enhancement of the detection voltage under d.c. bias was well explained by a nonlinear effect which is described in Fig. 1a. b, The RF detection sensitivity (V detect/PRF) as a function of the RF input power (PRF). The maximum sensitivity is 12,000 mV mW−1, greater than that of the semiconductor Schottky diode detectors (3,800 mV mW−1). c, Spectral linewidths of the FMR in the free-layer magnetizations as a function of the d.c. bias current (Id.c.). By extrapolating the fitting line (grey solid line), the critical current for magnetization stability in the free layer is estimated to be Ic = −0.42 mA.

  4. Noise characteristics.
    Figure 4: Noise characteristics.

    a, Noise voltages around 0 Hz as a function of the RF input power (PRF) estimated by theory. For a small RF input, the dominant contribution is the nonlinear magnetic noise. b, d.c. bias (Id.c.) dependence of the nonlinear magnetic noise around 0 Hz, which is well reproduced by theory (Supplementary equation (18)). c, The NEP. The white star represents the MTJs in this study. It is possible to obtain an NEP as small as the Schottky diode detectors (black dotted curve) by modifying the junction size (black arrow). The white circle represents the value for the MTJ with the highest reported unit-area conductance40, 41. As the junction size decreases, the nonlinear FMR (Fig. 1a) enhances signals more than noise. Therefore, increasing the unit-area conductance and lowering the junction size is effective for obtaining superior NEP values.

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Author information

  1. These authors contributed equally to this work

    • S. Miwa &
    • S. Ishibashi

Affiliations

  1. Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan

    • S. Miwa,
    • S. Ishibashi,
    • H. Tomita,
    • T. Nozaki,
    • E. Tamura,
    • K. Ando,
    • N. Mizuochi &
    • Y. Suzuki
  2. National Institute of Advanced Industrial Science and Technology (AIST), Spintronics Research Center, Tsukuba, Ibaraki 305-8568, Japan

    • S. Ishibashi,
    • T. Nozaki,
    • T. Saruya,
    • H. Kubota,
    • K. Yakushiji,
    • T. Taniguchi,
    • H. Imamura,
    • A. Fukushima,
    • S. Yuasa &
    • Y. Suzuki
  3. Present address: Process Development Center, Canon ANELVA Corporation, Kawasaki, Kanagawa 215-8550, Japan

    • T. Saruya

Contributions

S.M. and S.I. performed the experiments and the analysis; they wrote the paper with T.N., N.M. and Y.S.’s appraisals and inputs. H.T. and S.M. conducted the simulations. S.I., T.S., H.K., K.Y., A.F. and S.Y. prepared the samples. E.T. and K.A. helped with the development of the theory and the measurements, respectively. T.T. and H.I conducted the theoretical analysis about the spin motive force. Y.S. conceived and designed the experiment and developed the theory. All authors contributed to the general discussion.

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The authors declare no competing financial interests.

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