Enhanced Broad-band Radio Frequency Detection in Nanoscale

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Surfaces, Interfaces, and Applications

Enhanced broadband RF detection in nanoscale magnetic tunnel junction by interface engineering Like Zhang, Bin Fang, Jialin Cai, Weican Wu, Baoshun Zhang, Bochong Wang, Pedram Khalili Amiri, Giovanni Finocchio, and Zhongming Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06706 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 27, 2019

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ACS Applied Materials & Interfaces

Enhanced broadband RF detection in nanoscale magnetic tunnel junction by interface engineering Like Zhang,†, ‡Bin Fang,† Jialin Cai,† ,‡ Weican Wu,† Baoshun Zhang,†, ‡ Bochong Wang, # Pedram Khalili Amiri, ƭ Giovanni Finocchio,§ and Zhongming Zeng*,†, ‡

†Key

Laboratory of Multifunctional Nanomaterials and Smart Systems, Suzhou Institute of

Nano-Tech and Nano-Bionics, CAS, Suzhou, Jiangsu 215123, People’s Republic of China School of Nano Technology and Nano Bionics, University of Science and Technology of China,

‡

Hefei, Anhui 230026, People’s Republic of China Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science,

#

Yanshan University, Qinhuangdao 066004, People’s Republic of China ƭ

Department of Electrical and Computer Engineering, Northwestern University, Evanston,

Illinois 60208, USA Department of Mathematical and Computer Sciences, Physical Sciences and Earth Sciences,

§

University of Messina, Messina 98166, Italy

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Broadband RF detection is of great interest for its potential applications in wireless charging and energy harvesting. Here, we demonstrate that the bandwidth of broadband RF detection in spin torque diodes based on magnetic tunnel junctions (MTJs) can be enhanced through engineering the interface perpendicular magnetic anisotropy (PMA) between the CoFeB free layer and MgO tunnel barrier. An ultra-wide RF detection bandwidth of over 3 GHz is observed in the MTJs, and the broadband RF detection behavior can be modulated by tuning the free layer PMA. Furthermore, a wide RF detection bandwidth (about 1.8 GHz) can be realized even without any external bias fields, for free layers with a thickness of about 1.65 nm. Finally, the dependence of broadband RF detection bandwidth on external magnetic field and RF power are discussed. Our results pave the way for RF energy harvesting for future portable nanoelectronics.

KEYWORDS: Magnetic tunnel junction, perpendicular magnetic anisotropy, spin torque diode, energy harvesting, radiofrequency detection, broadband detection


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1. INTRODUCTION Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) have attracted tremendous attention in recent years, becoming the materials of choice for the development of high-density, high thermal stability, and low-power consumption spin torque magnetic random access memory (ST-MRAM)1-7 which is now entering volume production across the semiconductor industry, as well as other emerging devices including high emissionpower microwave spin-torque oscillators (STOs)8-10 and highly sensitive spin-torque microwave detectors (STMDs).11-13 The physical principle of STMDs is the spin-torque diode (STD) effect, which was first experimentally reported in 2005 by Tulapurkar et al.14 In the earlier work, the detection sensitivity (< 10 mV/mW) was relatively low compared with semiconductor Schottky diode detectors (1000 ~ 3800 mV/mW).15 Then, much research was dedicated to improving the detection sensitivity11-13,16. In our recent work, a giant sensitivity of 2.1×105 mV/mW was realized by using an injection locking mechanism in an MTJ featuring interfacial PMA.13 In most spin-torque diodes reported to date, they exhibit a resonant peak at a certain frequency. Recently, a broadband detector, which can convert RF in a range of frequencies into



DC voltage, based on an MTJ with a large enough out of-plane field was proposed theoretically, with potential application in wireless charging and microwave energy harvesting.17 This concept was experimentally realized by making use of MTJs with a PMA free layer.18,19 In such system, the PMA originates from both CoFeB/MgO1,2,8,9 and CoFeB/heavy metal interfaces.20-23 The interfacial nature of the PMA between the oxide and ferromagnetic metal (CoFeB/MgO) is attributed to hybridization of Fe 3dz and O 2pz orbitals, allowing to control the PMA field through varying the free layer CoFeB thickness.1,2 In our latest work,19 the bandwidth of broadband detectors based on MTJs with PMA was shown to approach 1.2 GHz, and the generated DC voltage via RF-to-DC conversion was sufficiently high to power a low-power nano-device, i.e. a black phosphorus photo-sensor. However, in the ambient conditions there exist several high-frequency RF energy sources – e.g. Wi-Fi (2.4 and 5.0 GHz) and 3G/4G mobile signals (700-850 MHz and 1.7-2.1 GHz) – a broadband signal detection covering at least a ~2.4 GHz bandwidth is essential for energy harvesting applications. In this work, we report on an enhanced broadband RF detection by tuning the PMA of a CoFeB free layer that is dependent on its thickness. A RF detection bandwidth of more than 3 GHz


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can be achieved with the assistance of an external bias magnetic field, when the CoFeB thickness is 1.61 nm. Even without external bias fields, a bandwidth of 1.8 GHz is realized under the thickness of 1.65 nm, which covers the most commonly used 3G/4G communication bands. Finally, the dependence of broadband RF detection bandwidth on external magnetic field and RF power are systematically investigated. 2. MATERIALS AND METHODS The MTJ multilayer stacks, which consist of a wedge Co20Fe60B20 free layer (tCoFeB = 1.60 ~ 1.72 nm), with a layer structure of the bottom electrode /Ta (5)/CoFeB (t)/MgO (0.9)/CoFeB (3)/Ru (0.85)/ CoFe (2.3)/PtMn (20)/top electrode (thicknesses in nanometers) were deposited on a thermally oxidized silicon substrate using a magnetron sputtering system. The complete stack was annealed at 300 °C for 2 hours in a magnetic field of 1 T. Figure 1a schematically displays the device’s core structure. Electron-beam lithography and ion beam milling were used to fabricate the multilayer stacks to 70 nm × 175 nm nanopillars, with the two ground contacts forming a ground-signal-ground (GSG) coplanar waveguide and long axis parallel to the annealing magnetic field direction, as shown in the Figure 1b. It is worth noting that, i) the free layer is designed in the



bottom in order to precisely control the PMA, which is different from that in our previous work where the free layer is in the top19; ii) for all devices, only varying the CoFeB free layer thickness while fixing the thickness of heavy metal Ta layer, hence, the PMA change is dominant by the CoFeB/MgO interface. The measurement setup is similar to that described in previous work.11 In the experiments, the RF current generated by a Signal Generator (N5183B) was applied to the device through a bias Tee, and the rectified voltage induced by the microwave signal was measured by a nano-voltmeter. The direction of magnetic field was along the easy magnetization direction of the reference layer. All measurements were carried out at room temperature. 3. RESULTS AND DISCUSSION Firstly, we studied the perpendicular magnetic anisotropy field which can be tuned by the CoFeB free layer thickness. The effective first- (Hk1eff) and second-order (Hk2) perpendicular magnetic anisotropy field of the free layer were obtained by ferromagnetic resonance (FMR) measurements19,24 (See the Supporting information for the method used to obtain the values of Hk1eff and Hk2). Figure 2a shows Hk1eff and Hk2 as a function of position along the CoFeB free layer gradient. The value of Hk1eff decreases linearly with the increasing of CoFeB thickness, whereas


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the magnitude of Hk2 is virtually independent of the CoFeB thickness. These results are consistent with previous reports.25 Figure 2b shows the magnetoresistance as a function of applied magnetic field for typical devices with different CoFeB thickness (tCoFeB). Magnetoresistance loops are measured by applying a magnetic field along the easy direction of the reference layer and passing a 10 μA current through the MTJs. The tCoFeB dependence of the MR loops is consistent with previous reports.26-28 For the device with tCoFeB equal to 1.61 nm, the PMA field is about 1630 Oe and overcomes the demagnetizing field to have net out-of-plane anisotropy. Therefore, a linear MR curve is observed, which indicates that the magnetization of the CoFeB free layer is out of plane. When tCoFeB increases, the PMA value in the free layer decreases. The MR curves become square-shaped, confirming the transition of the CoFeB free layer magnetization from out-of-plane to in plane with increasing tCoFeB.28,29 In contrast to previous resonant STD measurements, the broadband RF detection in our devices is highlighted by the fact that the rectified voltage does not change obviously, or decrease slowly, before it reaches a drop-off RF frequency point, which signifies its bandwidth. In order to investigate the influence of tCoFeB on spin-torque-induced dynamics of MTJs, we chose three



typical devices (A, B, C) with CoFeB thickness of tCoFeB = 1.61 nm, 1.65 nm and 1.72 nm, corresponding to the estimated Hk1eff = 1630 Oe, 1246 Oe and 370 Oe, respectively. The microwave response spectra under external magnetic field and frequency for different devices are shown in Figure 3a-c. The RF response is quite sensitive to the CoFeB thickness, which corresponds to the PMA, and the external magnetic field, which influences the equilibrium magnetization direction of the CoFeB free layer. Under a constant thickness, the RF response curve is different from the traditional resonant spin-torque ferromagnetic resonance (ST-FMR) curve when the external magnetic field is positive.14 In order to exhibit the FMR property in detail, we plot three typical microwave response curves at in-plane external magnetic field of Hext = 400 Oe, 0 Oe and -400 Oe, as shown in Figure 3d-f and the dashed lines in the color maps. For device A in Figure 3d that has strong PMA, the microwave response exhibits a traditional ST-FMR curve under a negative magnetic field of -400 Oe. Without the magnetic field, the rectified voltage decreases quickly in the whole frequency range. Under Hext = 400 Oe, the rectified voltage decreases slowly until the frequency reaches 3 GHz. This is an ultrawide broadband RF response characteristic. By increasing the CoFeB


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thickness, the device B possesses moderate Hk1eff of 1246 Oe. The rectified voltage shows similar broadband RF response behavior as device A under positive Hext, and keeps the same features even as Hext decreases to zero, which is of importance for the practical applications. The bandwidth of RF detection of device B is about 1.8 GHz. For device C with weak PMA, the rectified voltage decreases rapidly from 0 to 3 GHz in spite of the Hext. This means that no broadband RF response behavior presents in this type of devices. Now let us discuss the possible mechanism underlying to govern the phenomena observed above. As predicted in theoretical work,17 the broadband RF detection originates from the largeamplitude out-of-plane (OOP) precession. For device A, the strong PMA leads to a perpendicular magnetization direction of CoFeB free layer, which we refer to as perpendicular type spin-torque diode (P-type STD) in the following discussion. In this case, an external in-plane magnetic field is required to induce a tilted magnetization and a large-amplitude OPP. For device B, the moderate PMA almost cancels the out-of-plane demagnetizing field (4πMs, Ms is the saturated magnetizations), resulting in a tilted magnetization direction of CoFeB free layer, referred to this device configuration as a canted type spin-torque diode (C-type STD). In this case, the large-



amplitude OOP procession is realized in the absence of external magnetic field, resulting in a broadband detection. Note that, at a given condition, the threshold current Ith for large-amplitude precession in the STDs can be given as9,17 𝐼th ∝ 𝑀eff = 𝐻𝑘 +(4π𝑀𝑠 ― 𝐻𝑘 ⊥ )/2, where Meff is the effective demagnetizing field, Hk ( ≪ 4π𝑀𝑠) is the in-plane shape induced anisotropy, 𝐻𝑘 ⊥ is the PMA field in the free layer. It can be seen that the 𝐼th value is larger in the device A than that in the device B because of larger Meff value arising from the strong PMA field. This may be the reason why the detection voltage in the device A is smaller than that in the device B at the same condition. While for device C with weak PMA, because of the large Meff value and the magnetization of free layer aligns in the film plane, it requires an external magnetic field and a large bias current to achieve large-amplitude procession. At the measured condition, it fails to meet the driving condition for large-amplitude precession (if the bias current is too large, the tunnel barrier may be damaged), thus no broadband detection is observed. Next, we investigate the magnetic field dependence of broadband detection. Figure 4a and Figure 4b show the microwave response spectra under the different external magnetic fields at RF input power PRF = 63 μW for P-type STD and C-type STD (More data for other RF powers are


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shown in Figure S2). Normally, the bandwidth is defined as the full width at half maximum of a peak. However, the left part of resonance peak changes slowly and extends to 0 GHz for the broadband RF response. Hence, we calculate the bandwidth from 0 GHz (left) to the half maximum of the resonance peak (right), as shown in the shadow area in Figure 4a. The dependence of bandwidth on external magnetic field is presented in Figure 4c. For the C-type STD, the bandwidth increases with Hext from 0 to 300 Oe and then saturates, while for the P-type STD, due to the strong PMA, the OOP precession is insensitive to the weak external magnetic field, and the precession frequency is larger than the one of C-type STD.30 As a result, the bandwidth of P-type STD barely changes and is larger than the C-type STD. Last but not least, we investigate the influence of input RF power on the broadband microwave response. Figure 5a and b show the examples of broadband microwave response as a function of input RF frequency for various incident RF powers at Hext = 0 Oe and 400 Oe, respectively (the microwave response spectra under other external magnetic fields are shown in Figure S3). It can be seen that each spectrum exhibits broadband RF detection behavior. The bandwidth of broadband RF detection as a function of RF power is shown in Figure 5c. when Hext



= 0 Oe and 100 Oe, the frequency bandwidth remains constant with the increase of the RF input power, while for the larger magnetic field (Hext = 200 Oe, 300 Oe and 400 Oe), the bandwidth remains unchanged in the low RF power and then begins to decrease as RF power increases. Figure 5d shows the RF-to-DC conversion efficiency (defined as Vdc/(IRF×R), one important factor for the characterization of device performance in energy harvesting applications) as a function of RF input power at Hext = 0 Oe and 400 Oe for device A and B, according to the calculating method in previous work.18 The conversion efficiency η increases as RF input power increases. The maximum measured conversion efficiency is estimated to be 1% and 2% for device A and B, respectively. It is important to note that the generated DC voltage in this work is a small fraction of the maximum value, thus the conversion efficiency can be enhanced by increasing the resistance oscillation through exciting a larger-amplitude precession oscillation. In addition, in the presented diodes, the TMR ratio of the device A and B are 36% and 48%, respectively. This promises a high potential for improvement in the future since the TMR ratio can be up to 200%.31,32 4. CONCLUSIONS In conclusion, we have experimentally investigated the broadband RF detection in MTJs, and


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demonstrated its interfacial engineering to achieve record-wide detection bandwidth for energy harvesting applications. In particular, we describe in detail the dependence of broadband RF detection on interfacial magnetic anisotropy field in MgO-based MTJ with the wedge free layer CoFeB. The bandwidth of broadband RF detection can reach over 3 GHz with the assistance of external magnetic field, when the CoFeB free layer thickness is 1.61 nm. In addition, a wide RF detection about 1.8 GHz without any external bias fields is realized when the CoFeB free layer thickness is 1.65 nm. Finally, the dependence of broadband RF detection bandwidth on external magnetic field and RF power were discussed. These results indicate that bandwidth of broadband RF detection can be enhanced by tuning the CoFeB free layer thickness, which provides a potential pathway for MTJs in wireless sensing and energy harvesting for future portable nanoelectronics. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: (i) The effective first- (Hk1eff) and second-order (Hk2) perpendicular magnetic anisotropy PMA)



field of free layer. (ii) The broadband microwave response spectrum of device A and B at different magnetic field and RF power (0.63, 6.3 and 20 μW). (iii) Broadband detection PRF dependence of device B at the other two magnetic fields (100 Oe, 300 Oe). AUTHOR INFORMATION Corresponding Author * [email protected]. Author Contributions Z.M.Z., P.K.A. and G.F. managed the project. L.K.Z., B.F. and J.L.C. performed experiments. Z.M.Z. analysed the data and wrote the paper with contributions from P.K.A. and G.F. All authors contributed to the discussion and commented on the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Executive Programme of Scientific and Technological Cooperation Between Italy and China for the years 2016–2018 (code CN16GR09,


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2016YFE0104100). This work was supported in part by the National Science Foundation of China (Nos. 51761145025 and 11804370) and the National Postdoctoral Program for Innovative Talents (No. BX201700275) and China Postdoctoral Science Foundation Funded Project (No. 2017M621858).



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(32) Almasi, H.; Xu, M.; Xu, Y.; Newhouse-Illige, T.; Wang, W. G. Effect of Mo Insertion Layers on the Magnetoresistance and Perpendicular Magnetic Anisotropy in Ta/CoFeB/MgO Junctions. Appl. Phys. Lett. 2016, 109,032401.



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Figures & Captions

Figure 1. Device geometry and measurement configuration. (a) Schematic of device structure. The device core structure from bottom to top is Co20Fe60B20 (t)/MgO (0.9)/Co40Fe40B20 (3) (thickness in nanometers). The Co20Fe60B20 free layer on bottom of the MgO film was deposited with varying thickness across the wafer, resulting in a wedge shape. (b) Structure of one device in the array (scale bar, 50 μm) and the measurement configuration.



Figure 2. Magnetic properties. (a) Perpendicular anisotropy field Hk1eff and Hk2 as a function of position/thickness of the CoFeB free layer at room temperature. (b) Magnetic resistance dependence of CoFeB free layer thickness.


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Figure 3. Broadband microwave response characteristics. (a-c) Microwave response spectrum of devices with different thicknesses of CoFeB free layer at various magnetic field and specific microwave power (PRF = 63 μW). (d-f) Spectrum corresponding to the dark cyan, red and green vertical line in a-c) at 400, 0 and -400 Oe.



Figure 4. Broadband detection magnetic field dependence. (a)-(b) Broadband detection as a function of RF frequency at different magnetic field at PRF = 63 μW for the device A with the 1.61 nm thickness free layer and the device B with the 1.65 nm thickness free layer, respectively. (c) The dependence of frequency broadband on the Hext for the device A and B with varying PMA field: P-type spin torque diode and C-type spin torque diode.


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Figure 5. Broadband detection PRF dependence. (a-b) RF frequency dependence of detection voltages of device B with the 1.65 nm thickness free layer for six microwave power, PRF = 63, 20, 6.3, 2, 0.63 and 0.2 μW at an external magnetic field of 0 and 400 Oe. Lines in the inset are the microwave response spectra for microwave power, PRF = 2, 0.63 and 0.2 μW. (c) PRF dependence of bandwidth for different magnetic field. (d) RF-to-DC conversion efficiency as a function of PRF for device A and B at an external magnetic field of 0 and 400 Oe.


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