Giant spin-torque diode sensitivity in the absence of bias magnetic field

Abstract

Microwave detectors based on the spin-torque diode effect are among the key emerging spintronic devices. By utilizing the spin of electrons in addition to charge, they have the potential to overcome the theoretical performance limits of their semiconductor (Schottky) counterparts. However, so far, practical implementations of spin-diode microwave detectors have been limited by the necessity to apply a magnetic field. Here, we demonstrate nanoscale magnetic tunnel junction microwave detectors, exhibiting high-detection sensitivity of 75,400 mV mW^-1 at room temperature without any external bias fields, and for low-input power (micro-Watts or lower). This sensitivity is significantly larger than both state-of-the-art Schottky diode detectors and existing spintronic diodes. Micromagnetic simulations and measurements reveal the essential role of injection locking to achieve this sensitivity performance. This mechanism may provide a pathway to enable further performance improvement of spin-torque diode microwave detectors.

ARTICLE

Received 15 Oct 2015 | Accepted 7 Mar 2016 | Published 7 Apr 2016

Bin Fang1, Mario Carpentieri2, Xiaojie Hao3, Hongwen Jiang3, Jordan A. Katine4, Ilya N. Krivorotov5,Berthold Ocker6, Juergen Langer6, Kang L. Wang7, Baoshun Zhang1, Bruno Azzerboni8, Pedram Khalili Amiri7, Giovanni Finocchio9 & Zhongming Zeng1

Microwave detectors based on the spin-torque diode effect are among the key emerging spintronic devices. By utilizing the spin of electrons in addition to charge, they have the potential to overcome the theoretical performance limits of their semiconductor (Schottky) counterparts. However, so far, practical implementations of spin-diode microwave detectors have been limited by the necessity to apply a magnetic eld. Here, we demonstrate nanoscale magnetic tunnel junction microwave detectors, exhibiting high-detection sensitivity of 75,400 mVmW 1 at room temperature without any external bias elds, and for low-input power (micro-Watts or lower). This sensitivity is signicantly larger than both state-of-theart Schottky diode detectors and existing spintronic diodes. Micromagnetic simulations and measurements reveal the essential role of injection locking to achieve this sensitivity performance. This mechanism may provide a pathway to enable further performance improvement of spin-torque diode microwave detectors.

1 Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Ruoshui Road 398, Suzhou 215123, China. 2 Department of Electrical and Information Engineering, Polytechnic of Bari, Bari 70125, Italy. 3 Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA. 4 HGST Inc, 3403 Yerba Buena Road, San Jose, California 95135, USA. 5 Department of Physics and Astronomy, University of California, Irvine, California 92697, USA. 6 Singulus Technologies, Kahl am Main 63796, Germany. 7 Department of Electrical Engineering, University of California, Los Angeles, California 90095, USA. 8 Department of Engineering, University of Messina, Messina 98166, Italy.

9 Department of Mathematical and Computer Sciences, Physical Sciences and Earth Sciences, University of Messina, Messina 98166, Italy. Correspondence and requests for materials should be addressed to P.K.A. (email: mailto:[email protected]

Web End [email protected] ) or to G.F. (mailto:[email protected]

Web End [email protected]) or to Z.Z. (mailto:[email protected]

Web End =zmzeng2012@sinano. ac.cn).

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DOI: 10.1038/ncomms11259 OPEN

Giant spin-torque diode sensitivity in the absence of bias magnetic eld

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11259

The continuous progress in the development of magnetic materials and nanostructures has enabled spintronic devices with performance superior to semiconductor-based

electronics, offering promising solutions for a range of future high-speed and energy-saving electronic systems17. In particular, the spin-transfer torque8,9 induced by d.c. spin-polarized current can switch the magnetization10,11, or excite self-oscillations12,13 giving rise to applications such as memories and nanoscale oscillators1416. On the other hand, microwave detectors (rectiers) on the basis of the spin-torque diode effect can be realized when the d.c. input is replaced by a microwave current17,18. The spin-torque diode effect is the result of spin-torque-induced ferromagnetic resonance (FMR), which leads to a rectication effect (that is, d.c. voltage, Vdc) in magneto-resistive nanoscale devices. Since its discovery17, this effect has been used for quantitative measurements of magnetic torques in magnetic tunnel junctions (MTJs) (for example, Slonczewski, eld-like, and voltage-controlled torques)1922.

For practical applications in microwave detectors, the ability to obtain a device with high-detection sensitivity without using external magnetic elds, at room temperature and at low-input microwave powers is crucial. However, there are currently no practical (spintronic or conventional semiconductor) solutions that achieve all of these requirements simultaneously. In spin-torque diodes studied so far, the application of an additional external magnetic eld (often canted at an angle with respect to the device plane) is required to achieve large microwave detection sensitivity21,2328. Although this external eld may in principle be integrated into the device, for example, by engineering the material stack, or by using coils or permanent magnets, it is undesirable from a practical point of view due to increased size and cost of the device. On the other hand, semiconductor-based Schottky diode detectors, while not requiring a magnetic bias, fail to offer sufcient sensitivity for low input microwave powers due to the thermodynamic limit (the theoretical limit being B4,000 mV mW 1)28.

In this work, we present a spin-torque diode microwave detector meeting all of the above-mentioned requirements. This is achieved by incorporating three elements into the device. First, a perpendicularly magnetized free layer6,7,29, which allows for device operation in the absence of external magnetic elds30. Second, an MgO-based MTJ material stack exhibiting high -tunnel magnetoresistance (TMR). Third, the injection-locking

mechanism31 due to the simultaneous application of d.c. and microwave currents. The operation mechanism and the fundamental role of the injection locking are discussed on the basis of measurements combined with micromagnetic simulations.

ResultsSpin-torque diode device. The devices studied in this work have an MTJ structure consisting of a synthetic antiferromagnetic Co70Fe30 (2.3 nm)/Ru (0.85 nm)/Co40Fe40B20 (2.4 nm) reference layer, exchange biased by a PtMn lm, designed to have an in-plane easy axis, and a Co20Fe60B20 perpendicularly magnetized free layer, separated from the reference layer by a 0.8 nm MgO tunnel barrier. A schematic of the device is shown in Fig. 1a. The free layer has an out-of-plane easy axis at zero bias eld, which is realized by controlling the perpendicular magnetic anisotropy at the interface of the CoFeB layer with the MgO tunnel barrier6,7,29,30. This magnetization conguration enables the device to excite large-amplitude magnetization precession under a small spin-torque30. In addition, the CoFeBMgO CoFeB material combination ensures high TMR47. These factors are key ingredients to enhance the sensitivity of the spin-torque diode. Electron-beam lithography and ion milling were used to fabricate the pillar-shaped devices with elliptical cross-section. All data in the main text are from one 150 nm 60 nm device. Other

measured samples exhibited similar behaviour.

Figure 1b shows the resistance as a function of the in-plane magnetic eld applied parallel to the ellipse major axis (H8), at a bias current of Idc 10 mA. As the eld increases from 1,000 to 1,000 Oe, the resistance increases gradually as the free-layer magnetization aligns anti-parallel to the reference layer magnetization. The resistance curve scan as a function of the out-of-plane eld (H>) (inset of Fig. 1b) clearly indicates the perpendicular free layer. At zero eld, a small tilting angle (y 76) from the out-of-plane conguration is measured due to

the coupling between the free and reference layers. We estimated y, the angle between the magnetization vectors of the free layer and the reference layer, from the MTJ resistance28

R 1y

R 1P R 1AP

2

R 1P R 1AP

2 cosy; 1 where the resistances in the anti-parallel (RAP) and parallel (RP)

congurations are 1,200 and 640 O, respectively. In addition, the

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Figure 1 | Spin-torque diode device. (a) Spin-torque diode device and schematic of circuit used for FMR measurements. The device is based on an MTJ with an in-plane magnetized reference layer and a perpendicularly magnetized free layer, separated by an MgO tunnel barrier. The detected voltage (Vdc) is

measured by a low-frequency (10 kHz) modulation method using a lock-in amplier. (b) The magnetoresistance curve of the MTJ device under in-plane magnetic eld (H8) and perpendicular magnetic eld (H>) for d.c. current (Idc) of 10 mA. The resistance scan as a function of the out-of-plane eld (inset of b) clearly indicates the perpendicular free layer. The black (red) arrow denotes the magnetization direction of the reference (free) layer.

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free layer exhibits a voltage-controlled interfacial perpendicular magnetic anisotropy (VCMA) estimated to be 34 fJV 1m 1 (see Supplementary Fig. 1 and Supplementary Note 1), in good agreement with previous reports for similar material structures21,22,32,33. All measurements reported below were carried out at room temperature and zero bias magnetic eld.

Detection properties. We studied the spin-torque diode response by using FMR measurements as shown in Fig. 1a (ref. 18). A weak microwave current Iac sin (2pfact) and a d.c. current Idc were applied to the device through a bias Tee using a signal generator (E8257D, Agilent Technologies) and a source metre (2400, Keithley). At Idc 0 mA, when a microwave current at a

frequency fac is applied, the free layer magnetization starts to precess at the same frequency, resulting in a time-dependent resistance oscillation due to the TMR effect. As a result, a rectied voltage is generated across the MTJ17. To improve the signal-to-noise ratio, the microwave input was modulated at a low frequency (10 kHz), and the resulting rectied voltage Vdc was

measured with a lock-in amplier (SR830, Standard Research Systems). Figure 2a shows the measured rectied voltage as a function of the microwave frequency at Idc 0 mA, for an input

microwave power ranging from 3.2 to 100 nW. The maximum voltage is measured at the resonant frequency f0 1.2 GHz.

The FMR spectra in Fig. 2a are well tted by a sum of symmetric and antisymmetric Lorentzian functions with identical resonant frequency f0. The origin of the asymmetric line shape is related to the VCMA effect21. The detection sensitivity, dened as the rectied voltage divided by the incident microwave power (PRF), that is, Vdc/PRF, was obtained to be 970 mV mW 1

(see Supplementary Fig. 2 for similar data from an additional device). This is on the same level as previously reported values (440 and 630 mV mW 1)21,28 for MTJ-based diode detectors, and at a comparable level to unbiased commercial Schottky diode detectors (5001,000 mV mW 1, Herotek, Inc.).

We next conducted spin-torque diode response studies under different d.c. bias currents. Figure 2b,c shows the detected voltage curves as a function of the microwave frequency, at a low-input microwave power of 10 nW, for the range of d.c. bias currents from 0.34 to 0.25 mA. Positive d.c. currents were found to

suppress the detection voltage, because in this case spin torque increases the damping of the magnetization precession, while VCMA increases the perpendicular anisotropy in the free layer. For a range of negative currents ( 0.32oIdc o 0.22 mA),

the detection voltages were signicantly enhanced and the spectra shifted to a lower frequency. Figure 2d summarizes the maximum rectied voltage (Vmax) as a function of d.c. bias current, while the inset of Fig. 2d shows the spectral linewidth of the spin-torque FMR data, with the indication of the region where the larger detection voltage is measured. As discussed later in detail, this corresponds to the current region where the injection locking is achieved.

Figure 3a shows the detection voltage as a function of PRF for

various d.c. bias currents. The device shows a quadratic detection dependence on the d.c. bias, consistent with a previous study28. Moreover, a large detection voltage (BmV order) is achieved for small input power. For example, the maximum detected voltage reaches 754 mV at Idc 0.25 mA and PRF 0.01 mW, which is

about 80 larger than the one measured at zero bias current.

The corresponding detection sensitivity of 75,400 mV mW 1 is substantially larger than state-of-the-art biased Schottky diode

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ldc (mA)

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Figure 2 | Voltage detection characteristics of spin-torque diode device. (a) Detection voltage (Vdc) as a function of microwave frequency, for various input microwave powers (PRF) at zero d.c. bias current. (b,c) Vdc as a function of microwave frequency under various d.c. bias currents (Idc). The d.c. bias was found to signicantly affect Vdc. (d) Maximum Vdc as a function of the d.c. bias. The inset in d shows the spectral linewidth of the free-layer FMR as a function of Idc. By extrapolating the tting line (grey solid line), the critical current (Ic) for magnetization stability in the free layer is estimated to be 0.2 mA.

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a b

This study No need for external magnetic field

Low temperature (4K)

Linear fitting

Out-of-plane free-layer/in-plane reference layer Room temperature

Tilted magnetic field from the film normal

Ref. 27

Ref. 28 tilted magnetic field from the film normal

Ref. 21 tilted magnetic field from the film normal

Ref. 23

optimized in-plane magnetic field direction

Ref. 24

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0.10 mA

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0 500

Bias magnetic field (Oe)

1,000 3,000 3,500

PRF (W)

Figure 3 | RF power dependence and the comparison of detection sensitivity with the indication of the required bias eld. (a) The dependence of output voltage on the RF input power under different d.c. bias current (Idc) values. (b) The detection sensitivity values reported previously requirean external magnetic eld with a particular direction and amplitude, while in this study the giant sensitivity is achieved in the absence of any bias magnetic eld.

detectors (3,800 mV mW 1, Herotek, Inc.) and the best existing spintronic diodes (12,000 mV mW 1), which additionally require large magnetic bias elds for their operation28.

Figure 3b compares our results with previously reported detection sensitivities for different spin-torque diodes, with the indication of the required external magnetic eld. It can be observed that the devices reported in this work represent the combination of high-detection sensitivity and bias-eld-free operation. In addition, the device exhibits signal-to-noise ratio, which is of the same order as those of previous reports28 (see Supplementary Note 2 and Supplementary Figs. 3,4,5). It is worth noting that the giant detection sensitivities observed here are not only substantially larger than those of Schottky diodes, but they are also achieved at low microwave input power (o100 nW). By comparison, Schottky diodes typically do not provide satisfying microwave-to-d.c. conversion efciency for sub-mW input microwave power34. Furthermore, the spin-torque diode devices can be scaled down to nano-metre size (0.07 mm2 in this study), which makes them potentially suitable for compact on-chip microwave detectors.

Discussion of physical mechanism. We next focus on the mechanisms responsible for the large-voltage detection sensitivity. Under d.c. bias, two previous experiments demonstrated that the detection voltage can be enhanced due to the nonlinear FMR27,28. In thermally assisted nonlinear FMR27 (or stochastic resonance), an applied radiofrequency (RF) current excites a large-amplitude precession with the assistance of thermal energy, and the detection voltage shows an exponential dependence on the RF input power. Another type of nonlinear FMR (called nonlinear FMR with asymmetric potential), in which the large detection voltage is linked to a rotation of the orbital centre of the free-layer magnetization, has been also observed28. The maximum detection voltage is achieved near Idc Ic, where Ic is the critical current at which the

equilibrium conguration of the magnetization is destabilized by the spin-transfer torque28. By comparison, the device discussed in this article, exhibits quadratic detection properties for small RF input power (see Fig. 3a), and enhanced detection voltage in the specic range from 0.22 to 0.32 mA (see Fig. 2d), which is

above Ic. Here, Ic is estimated to be 0.2 mA, from the

extrapolation of the tted line in the d.c. current dependence of the FMR spectral linewidths, as shown in the inset of Fig. 2d. Hence, the underlying physics in this study is different from that of nonlinear FMR observed in previous experiments.

An alternative mechanism is the theory related to the out-of-plane precession35, in which the device operates as a non-resonant broadband microwave detector, and the output voltage is virtually independent of the input microwave power. The device discussed here exhibits a resonant character (see Fig. 2b,c), and the output voltage depends on PRF

(see Fig. 3a). Hence, the mechanism driving the large detection sensitivity in this study is different from this picture as well.

Role of injection locking on detection behaviour. To gain a deeper understanding of the ultrahigh sensitivity mechanism, we performed micromagnetic simulations (see Methods). Our computational results show persistent magnetization dynamics (self-oscillations) (at Iac 0 mA), as well as injection locking

(at Iac40 mA)31,3638. Figure 4a shows an example of the detection voltage as a function of frequency (fac), as computed by means of micromagnetic simulations for Idc 0.26 mA and

Iac 7 mA (PRF 0.15 mW). The large detection voltage is

observed in the frequency range corresponding to the locking region (between 510 and 540 MHz), as shown in Fig. 4b, where it can be clearly observed that the microwave emission frequency fp is locked to the frequency fac of the input microwave current. Within the locking region, an intrinsic phase shift Fs (ref. 39) exists between the oscillation resistance and the microwave current. The origin of Fs can be understood from the theory developed in ref. 40, in which it is directly linked to the coupling between the amplitude and the frequency of the self-oscillation (see equation (56) in ref. 40). It is noted that the value of Fs inside the locking bandwidth depends on the microwave frequency (Fig. 4c), similar to that in a previous study41. Figure 4d shows a comparison between the experimental and theoretical values for the frequency f0 of the maximum rectication voltage, as a function of the d.c. bias current. The simulation data (open circles) are quantitatively consistent with the experimental data (olive colour), namely, a red shift is observed in the frequency f0 with increasing the amplitude of the negative current, along with a frequency jump of about 300 MHz. This analysis points to a scenario where the injection locking is responsible for the observed giant detection sensitivity.

To further verify the role of the injection locking, we conducted additional experiments to study the current-induced microwave emission12 with and without a RF input in our samples, recorded using a 9 kHz26.5 GHz spectrum analyzer (see Supplementary Fig. 6). The output of microwave emissions in the absence of PRF

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a b

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fac (MHz)

Figure 4 | Micromagnetic simulations. (a) Rectied voltage and (b) Oscillation frequency (fp) of the microwave emission, as a function of the input microwave frequency (fac). (c) Intrinsic phase shift between the microwave current and the oscillating TMR signal. The results of micromagnetic simulations are obtained for d.c. bias currents Idc 0.26 mA and microwave current Iac 7 mA. (d) A comparison between the experimental and

simulated resonant frequency (f0) of the maximum detection voltage as a function of Idc.

as a function of Idc is shown in Fig. 5a. The oscillation frequency (fp) exhibits a red shift with increasing the bias current. The frequency jump in the fp versus Idc response is related to the sign change of the x-component of the oscillation axis from negative to positive, as also indicated from the change in the resistance curve (see Fig. 5d). Similar behaviour was also observed in spin-torque oscillators at low out-of-plane bias elds42. Near Idc 0.34 mA, the steady magnetization dynamics are switched

off, analogous to the disappearance of the detection spectrum in the spin-torque diode measurements (see Fig. 2c). Figure 5b shows the analogous data when an RF drive PRF 1 mW at

480 MHz is added to Idc. At Idc 0.25 mA the oscillation

frequency locks to the drive frequency, and the locking continues up to Idc 0.29 mA. At larger bias currents the oscillation

frequency is unlocked from the drive frequency. In the locking range, the linewidth of the microwave emission is signicantly narrowed by a factor of 8 (to 15 MHz), as can be observed in

Fig. 5c, but is still much larger than the linewidth of the external microwave signal (on the order of several Hertz). These results indicate that the frequency-locking does not always result in a full phase-locking of the spin-torque nano-oscillator to the external microwave signal, consistent with previous reports37,38. This can be attributed to the inuence of noise37,38,43, that is, the external microwave signal has to compete with noise, which results in uctuations of the oscillator phase.

Despite full phase-locking not being reached, the detection voltage Vdc is drastically increased as shown in Fig. 5e.

Quantitatively, for a xed frequency, the detection voltage is given by Vdc 12 IacDRs cos Fs

h i IdcDRdc (refs 27,28), where

DRdc(Iac) Rdc(Iac) Rdc(0) is the difference between the average

resistance in the presence and in the absence of Iac, while hFsi is

the time-average intrinsic phase shift, and DRs is the amplitude of the oscillating resistance in the presence of both Idc and Iac. It is worth pointing out that the value of DRs can be signicantly larger in the presence of d.c. bias, given that it results from the amplitude of the self-oscillation of the magnetization, rather than conventional FMR. As an example, for Idc 0.26 mA

and Iac 18 mA (PRF 1 mW, fac 480 MHz), from Fig. 5d

DRdc(Iac) 60 O, while from the output power (Fig. 5b) it can

be estimated that the oscillation is due to 30% of the total TMR signal, which corresponds to DRs 170 O. Considering hFsiE0,

the calculated detection voltage of B17.13 mV is consistent with the measurements in Fig. 5e. From a theoretical point of view, according to the above analysis the detection voltage is limited by the maximum TMR signal. In Fig. 5f, the locking range is more explicitly shown as a function of the RF drive amplitude. The locking range is deduced from the dependence of the oscillation frequency fp on the drive frequency fac, where the microwave emission is locked to the injected signal (see Fig. 4b for simulation results, or Supplementary Note 3 and Supplementary Fig. 7 for additional experimental data). The locking range increases with the drive amplitude, which is consistent with the simulation data and in agreement with the analytical theory40.

DiscussionIn summary, we have demonstrated giant sensitivity of nanoscale spintronic diodes in the absence of any external magnetic eld, at room temperature and for low-input microwave power. The analysis of microwave emission measurements with and

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a b c

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Figure 5 | Injection locking of spin-torque microwave emission to an external microwave signal. (a) Plot of spin-torque microwave emission spectra as a function of d.c. bias currents Idc, with amplitude shown in a linear colour scale from 0 (blue) to 40 nW GHz 1 (red). (b) Same as a but with a RF drive at 480 MHz and RF power PRF 1 mW. (c) Spectral linewidth of microwave emission as a function of Idc, with/without a RF drive deduced from a and b.

(d) Device resistance as a function of Idc with and without the RF drive. The precession axis of the free-layer magnetization (red arrows) and reference layer polarization (black arrows) are also indicated. (e) The detected voltage Vdc as a function of Idc. Vertical lines indicate the region of locking. (f) A comparison between experimental values (olive dots) and micromagnetic calculations (blue circles) of the locking range as a function of RF drive amplitude, atIdc 0.26 mA.

without RF input, as well as micromagnetic simulations, reveal that injection locking is the key mechanism to achieve this large sensitivity. It is anticipated that this mechanism may provide a pathway for development of high-performance spintronic microwave detectors.

Methods

Sample preparation. The continuous multilayer thin lms with stacks of composition PtMn (15)/Co70Fe30 (2.3)/Ru (0.85)/Co40Fe40B20 (2.4)/MgO (0.8)/

Co20Fe60B20 (1.63) (thickness in nm) were deposited using a Singulus TIMARIS physical vapour deposition system and annealed at 300 C for 2 h in a magnetic eld of 1 T. The lms were subsequently patterned into ellipse-shaped pillars using optical and electron-beam lithography combined with ion milling. The resistance-area product in the parallel magnetization conguration was 4.5 O mm2, and the in-plane TMR ratio, dened as (RAP RP)/RP, was 87.5%.

Micromagnetic simulations. We numerically solve the LandauLifshitzGilbert Slonczewski equation which includes the eld-like torque TOP (refs 44,45), and the voltage dependence of the anisotropy, that is, VCMA21. The TOP is considered to be dependent on the square of the bias voltage up to a maximum value of 10% of the in-plane torque, computed for a current density |J| 4.0 106A cm 2.

(ref. 19). However, our simulations show that the eld-like torque term does not qualitatively inuence the detection voltage, as already observed in the scenario of ref. 21. The total torque, including also the in-plane component TIP is given by

TIP TOP

g mB

References

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j jJm; mp e

j j g0 M2st

gTm; mp m m mp qVm mp

h i; 2

where g is the gyromagnetic splitting factor, g0 is the gyromagnetic ratio, mB is the Bohr magneton, q(V) is the voltage-dependent parameter for the perpendicular torque, J(m,mp) is the spatially nonuniform current density, V is the voltage (computed from Idc R curves, see Fig. 5d), t is the thickness of the free layer, and

e is the electron charge. The effective eld takes into account the standard micromagnetic contributions (exchange, self-magnetostatic) as well as the Oersted eld due to both microwave and d.c. currents. The presence of the VCMA has been modelled as an additional contribution to the effective eld. The parameters used

for the CoFeB are saturation magnetization Ms 9.5 105 A m 1, exchange

constant A 2.0 10 11 J m 1, and damping parameter a 0.02. The zero bias

anisotropy constant ku 5.52 105 J m 3 has been estimated from the tting of

the FMR frequency f0 1.245 GHz from Fig. 2a, while the VCMA coefcient is

34 fJV 1m 1 (see Supplementary Note 1). The minimum value of the ku achieved at Idc 0.35 mA is 5.45 105 J m 3. The polarization function

gT(m, mp) 2ZT(1 2ZT2 m mp) 1, where m and mp are the normalized

magnetizations of the free and pinned layers, has been computed by Slonczewski46,47. We have used for the spin-polarization ZT the value 0.6 (ref. 19).

6 NATURE COMMUNICATIONS | 7:11259 | DOI: 10.1038/ncomms11259 | http://www.nature.com/naturecommunications

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11259 ARTICLE

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Acknowledgements

This work was supported by the National Science Foundation of China (11274343,

11474311) and the National key foundation for exploring scientic instrument of China

(2011YQ120053). The work at UCLA was partially supported by the DARPA STT-RAM

and Non-Volatile Logic programs, and by the NSF Nanosystems Engineering Research

Centre for Translational Applications of Nanoscale Multiferroic Systems (TANMS). This

work was also in part supported by the project PRIN2010ECA8P3 from Italian MIUR

and NSF support through grants DMR-1210850, DMR-1124601 and ECCS-1309416.

Author contributions

B.F., Z.M.Z., G.F. and H.W.J. initiated this work. P.K.A. and I.N.K. designed the MTJ

devices. J.L. and B.O. deposited the lms, and J.A.K. fabricated the devices. B.F. and X.J.H.

performed the measurements, M.C. and G.F. performed micromagnetic simulations.

Z.M.Z. and G.F. analysed the data and wrote the paper with contributions from P.K. A. All

authors contributed to the discussion and commented on the manuscript.

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How to cite this article: Fang, B. et al. Giant spin-torque diode sensitivity in the absence

of bias magnetic eld. Nat. Commun. 7:11259 doi: 10.1038/ncomms11259 (2016).

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