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Weak-field precession of nano-pillar spin-torque oscillators using MgO-based perpendicular magnetic tunnel junction
Journal of Magnetism and Magnetic Materials 452 (2018) 188–192
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Weak-field precession of nano-pillar spin-torque oscillators using MgObased perpendicular magnetic tunnel junction Changxin Zhang a,b, Bin Fang b, Bochong Wang c,⇑, Zhongming Zeng a,b,⇑ a
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China c Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University, Qinhuangdao 066004, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 4 November 2017 Received in revised form 15 December 2017 Accepted 20 December 2017 Available online 20 December 2017 Keywords: Spin-torque oscillator Perpendicular magnetic tunnel junction Weak magnetic field
a b s t r a c t This paper presents a steady auto-oscillation in a spin-torque oscillator using MgO-based magnetic tunnel junction (MTJ) with a perpendicular polarizer and a perpendicular free layer. As the injected d.c. current varied from 1.5 to 3.0 mA under a weak magnetic field of 290 Oe, the oscillation frequency decreased from 1.85 to 1.3 GHz, and the integrated power increased from 0.1 to 74 pW. A narrow linewidth down to 7 MHz corresponding to a high Q factor of 220 was achieved at 2.7 mA, which was ascribed to the spatial coherent procession of the free layer magnetization. Moreover, the oscillation frequency was quite sensitive to the applied field, about 3.07 MHz/Oe, indicating the potential applications as a weak magnetic field detector. These results suggested that the MgO-based MTJ with perpendicular magnetic easy axis could be helpful for developing spin-torque oscillators with narrow-linewidth and high sensitive. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Spin transfer torque (STT), induced by a spin-polarized current [1,2], can excite a steady magnetization precession of the soft magnetic layer when it fully compensates the damping. Because of the magneto-resistance (MR) effect, the magnetization precession leads to a high-frequency electrical signal emitted from the STT device referred as spin-torque oscillators (STOs) [2,3]. STOs have been proposed as a new type of microwave generators [4] and magnetic field sensors [5], due to the exceptional features of large frequency tunability, nanoscale size, board working temperature and simple structure, compared with the traditional voltage controlled oscillator (VCO) [6]. In the past years, many efforts have been made to achieve large emission power and narrow linewidth (high Q-factor), which are two critical parameters of STOs for the practical application, through geometry design [7], material development [8] and technique optimization [9]. Apart from the progress of external factors, the researches on the magnetization states and dynamics are more fundamental and important to improve the STO properties. ⇑ Corresponding authors at: Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China (Zhongming Zeng) and Yanshan University, Qinhuangdao 066004, People’s Republic of China (Bochong Wang). E-mail addresses: [email protected] (B. Wang), [email protected] (Z. Zeng). https://doi.org/10.1016/j.jmmm.2017.12.071 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
In the early stage, a STO contains an in-plane magnetized freelayer (FL) and an in-plane magnetized reference layer as a polarizer. Under a spin-polarized current, the magnetization of FL behaves dynamically while the one of reference layer remained fixed. In this scheme, the FL processes in the film of plane, yielding a low output power around several pW. Recent works suggest that a d.c. electrical current could yield a sustained stable magnetization procession in spin valve metallic structure with perpendicular easy axis theoretically [10,11] and in MgO-based magnetic tunnel junction (MTJ) with perpendicular free layer and in-plane reference layer experimentally [12]. In comparison with in-plane magnetized scheme, perpendicular MTJs (p-MTJs) usually provide a better thermal stability [13], smaller size [14] and low dissipation power [15]. Currently, most investigations on p-MTJs focus on the STT-induced magnetization switching for magnetic random access memory [16,17], while there is no report related to the STTinduced microwave emission in the MTJs with both perpendicular free and reference layer. Theoretically, the perpendicular magnetic anisotropy fields in free and reference layer are benefit for realizing the uniform precession and stable oscillation, which can enhance the Q-factor and reduce the requirement of an external magnetic field. In this paper, we reported a steady auto-oscillation under weak applied magnetic field in a spin-torque oscillator using MgO-based p-MTJ. The microwave spectra under different injected electrical current and external magnetic field were investigated systematically.
C. Zhang et al. / Journal of Magnetism and Magnetic Materials 452 (2018) 188–192
2. Experimental details The p-MTJ films with a layered structure of bottom contact/ Co20Fe60B20 (1.2)/MgO (1)/Co40Fe40B20 (1.5)/[Co/Pd]4 (5)/Ru (0.85)/[Co/Pd]10 (10)/top contact (numbers in parentheses are nominal thickness in nanometers), as shown in Fig. 1(a), were prepared by UHV magnetron sputtering system. The free layer Co20Fe60B20 (1.2 nm) and the reference layer Co40Fe40B20 (1.5 nm) were designed having a perpendicular easy axis to the film plane due to the interfacial anisotropy of the CoFeB/MgO interface, which has been reported in previous works [18,19]. The reference layer was exchange biased by a [Co/Pd]4 (5 nm)/Ru (0.85 nm)/[Co/Pd]10 (10 nm) multilayer that was also shown perpendicular magnetic anisotropy. The films were annealed at 300 °C for 2.0 h under a magnetic field of 1 T and then patterned into circular shaped pillars with a diameter of 200 nm by electron beam lithography combined with Ar ion milling technique. The setup and measurement technique were described in Refs. [15,20]. The d.c. bias currents were applied to the device through a bias Tee. The positive current was defined as the electrons flowing from the free layer to the polarizer. The microwave voltages generated by STT were recorded by a spectrum analyzer after an amplification of 40 dB and a baseline taken at Idc = 0 mA was subtracted from the measured spectral data. All measurements were carried out at room temperature.
3. Results and discussion We firstly measured the static transport properties before dynamic characterization. Fig. 1(b) displays a magnetoresistance curve for one representative device at a bias current Idc = 10 lA under out-of-plane (HOP) magnetic field, where the applied magnetic field direction is normal to the film plane. As HOP decreases from 600 Oe to 600 Oe, the resistance changes which corresponds to the switching of the magnetization directions between the parallel state with low-resistance (RP) and the antiparallel alignment with high-resistance (RAP). The steps in R-H loop around ±50 Oe suggest that the multiple-domain may exist in the free layer due to the large size [21]. The tunnel magnetoresistance (TMR) ratio (defined as 100% (RAPRP)/RP) and the resistance-area (RA) product are 35% and 5 Xlm2, respectively. The low RA value allows
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the d.c. electrical current sufficiently passing through the MgO barrier to excite the spin polarized current. Next, we focus on the STT induced magnetization dynamics. As demonstrated in our previous work [22], the magnetization precession amplitude changes with the direction of applied magnetic field because of the angle dependence of STT. For circular devices presented here, the linewidth is found to be the narrowest when the magnetic field is applied at an angle of approximately 10° off the film normal. Fig. 2(a) shows the power spectrum density (PSD) measured under different d.c. current at 290 Oe. The spin torque can counteract the damping of the free layer and leads to a stable magnetization precession. Below 1.5 mA, no obvious microwave signals are observed. By increasing the current above 1.8 mA, corresponding to a current density of 5.7 MA/cm2, one dominant single oscillation peak appears obviously, suggesting the onset of a uniform steady magnetization precession. Fig. 2(b) depicts an example of spectrum at Idc = 2.7 mA which is well fitting with a Lorentian function. The central peak frequency is 1.47 GHz and a narrow linewidth (Df) of 7 MHz are obtained, corresponding to a high Q factor (Q = f/Df) of 220. Above 2.8 mA, a second precession mode appears, such as the second peak around 1.7 GHz, whose power is much weaker than the fundamental one. Note that the fundamental frequency peak may be attributed to a centre precession mode while the higher-frequency peak probably arises from a mode at the sides of the nanopillar [23] or the nonuniform precession component in the STO owing to the large current density. Quantitative data are gathered in Fig. 3, where the dependence of frequency, linewidth (Df), and integrated output power (Pout) of the STOs as a function of Idc are summarized. These parameters are extracted using a Lorentian fitting to the power spectra. It can be seen from the Fig. 3(a) that the frequency decreases gradually with increasing Idc, resulting in a current modulation capability of 0.35 GHzmA1, which is similar to the previous reports in the system with in-plane magnetized system [24]. The integrated power of the centre mode increases gradually with increasing the d.c. current and then reaches rapidly 74 pW at Idc = 3 mA as shown in Fig. 3(b). The power value is still too low to be used for practical applications. This is due to low dynamic resistance change during oscillation and the large resistance which causes a low electrical current under the same bias voltage. The emission power can be
Fig. 1. (a) Schematic illustration of the p-MTJ STO device; (b) Magnetoresistance curve of the p-MTJ STO device measured by applying magnetic field perpendicular to the film plane. The bias current is 10 mA. The arrows represent the sweeping field direction. The hollow arrows represent the directions of the free (lower) and reference (upper) layer magnetizations, respectively.
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C. Zhang et al. / Journal of Magnetism and Magnetic Materials 452 (2018) 188–192
Fig. 2. (a) The power spectrum density of a typical STO as a function of current under the out-of-plane magnetic field of 290 Oe; (b) A typical spectrum of STO and the Lorentian fitting curve for HOP = 290 Oe and Idc = 2.7 mA.
Fig. 3. Microwave emission properties of a typical STO. (a) Frequency, (b) Integrated power, (c) Linewidth of the centre mode as a function of applied current under out-of-plane magnetic field of 290 Oe, respectively.
improved by several strategies [25]: 1) reducing the device resistance by increasing the diameter of pillar samples or optimizing the tunnel barrier, thus improving transmission efficiency; 2) preparing high quality MTJ with larger MR ratio, which may lead to an enhanced power. 3) optimizing the pillar shape and boundary conditions to achieve large-amplitude precession of the magnetic moment; 4) increasing the amplitude of the injected DC current since the output power is proportional to the square of DC current.
Fig. 3(c) depicts the linewidth of the fundamental peak as a function of d.c. current. The linewidth decreases as the bias current increases from 1.5 mA to 2.1 mA, and almost keeps constant with further increase the current. The narrowest linewidth (Df) of 7 MHz is obtained at Idc = 2.7 mA with a high Q factor (Q = f/Df) of 220, which is also shown in Fig. 2(b). The linewidth value is significantly smaller than that in our previous works [22]. According to the nonlinear theories [26,27], the linewidth Df above threshold regime depends not only on the frequency nonlinearity N = df/dIdc but also the oscillation energy that is proportional to the output power Pout, that is, Df / N/Pout. It can be obtained that the current dependence of Df calculated from Fig. 3(a) is within the trend of observed Dfexp in experiment as shown in Fig. 3(c). Furthermore, two main features can be identified for the observed narrow linewidth. First, the free layer has a relatively large perpendicular anisotropy field, which leads to a uniform demagnetizing field in nanosized pillars [20]. As a result, a homogeneous demagnetization field may contribute to the coherent precession, which can avoid arising from the jagged side wall of the free layer caused by etching damage in the nanopillar-patterning process. Second, the precession amplitude plays an important role in determining the linewidth. At low current regime (1.5 mA Idc 2.1 mA), STTinduced dynamics are largely influenced by the enhanced effect of thermal fluctuations for small-amplitude magnetization precession. So this small-amplitude motion of the magnetization produces the low emitted power and relatively large linewidth [28] (Fig. 3(c)). At high current regime (Idc 2.1 mA), the dynamics become relatively large-amplitude precession and the influence of thermal fluctuations becomes smaller, so the dynamics become coherent leading to a reduced linewidth. In general, the spectral linewidth in MgO-based MTJ system is overly large due to the chaotic and non-linearity behavior [29]. In our device, the linewidth is suppressed below 10 MHz for a small out-of-plane magnetic field. These properties make the p-MTJ STOs as an attractive candidate to design novel types of magnetic sensors and future magnetic recording applications. At last, the dependence of the dynamical properties on the different external magnetic fields (HOP) was studied. The external magnetic field dependence of the microwave emission spectra, peak frequency, integrated power and resistance for a bias current of 2.5 mA are shown in Fig. 4(a–d), respectively. It can be seen that when HOP changes gradually from 100 to 500 Oe, the precession
C. Zhang et al. / Journal of Magnetism and Magnetic Materials 452 (2018) 188–192
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Fig. 4. The external magnetic field dependence of the (a) microwave emission spectra, (b) peak frequency, (c) integrated power and (d) resistance for a bias current Idc = 2.5 mA.
frequency exhibits an approximately linear dependence on field with a slope of 3.07 MHz/Oe. This result suggests that the microwave frequency of p-MTJ based STO is quite sensitive to the magnetic field and it can be used as a field sensor to detect the weak magnetic field [5]. Based on the Kittel’s formula, f = c (Hop + HK 4pMs)/2p, where HK is the perpendicular anisotropy field and the 4pMs is the saturation magnetization of Co20Fe60B20 free layer. Using linear fitting, the HK 4pMs value of 238 Oe is obtained. The experimental value of saturation magnetization 4pMs is 14.7 kOe [30]. Then, the perpendicular anisotropy field can be estimated as 14.94 kOe which is a little smaller than that in Ref. [30]. The difference stems from the number of interfaces in two systems. In the sample presented in this study the perpendicular anisotropy is due to the MgO/CoFeB interface MgO barrier, while there are two interfaces with a sandwich structure of MgO in Ref. [30]. In Fig. 4(c), the integrated power decreases with increasing magnetic field. According to our previous study [25], the emission power of STO is related to the precession cone angle of the free layer magnetization Dh, average angle between free and reference layer magnetization h0, transmission efficiency g, MR ratio, resistance R(h0) and electrical current Idc. In this study, the electrical current is fixed at 2.5 mA. The resistance slightly decreases from 141.3 to 140.9 X with the magnetic field (seen in Fig. 4(d)), then we can consider that the resistance R(h0), MR ratio and transmission efficiency g are constant. By increasing the magnetic field, the magnetization direction of free layer is tilting parallel to the one of reference layer, thus the value of h0 becomes smaller. And the precession cone angle Dh also becomes smaller, proved by the increase of peak frequency. As a result, the decrease of the emission power can be attributed to the decrease of Dh and h0.
4. Conclusions In conclusion, we have experimentally observed the steady auto-oscillation in nanoscale STOs with MgO-based magnetic tunnel junctions having perpendicular polarizer and perpendicular free layer under a weak external magnetic field. The microwave spectrum possesses a relatively narrow linewidth (7 MHz) and high Q factor (2 2 0) at Idc = 2.7 mA and HOP = 290 Oe. The precession performances at different bias currents and external magnetic fields are qualitative analyzed. These results not only give us a better understanding of dynamical magnetization phenomena of practical nanoscale spin-torque oscillators but also suggest the advantageous features of STOs with perpendicular magnetic easy axis as the next RF generator and weak magnetic field sensor. Acknowledgments This work was supported by the National Science Foundation of China (11474311, 51761145025) and the executive programme of Scientific and technological cooperation between Italy and China (2016YFE0104100). References [1] [2] [3] [4]
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Weak-field precession of nano-pillar spin-torque oscillators using MgObased perpendicular magnetic tunnel junction Changxin Zhang a,b, Bin Fang b, Bochong Wang c,⇑, Zhongming Zeng a,b,⇑ a
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China c Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University, Qinhuangdao 066004, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 4 November 2017 Received in revised form 15 December 2017 Accepted 20 December 2017 Available online 20 December 2017 Keywords: Spin-torque oscillator Perpendicular magnetic tunnel junction Weak magnetic field
a b s t r a c t This paper presents a steady auto-oscillation in a spin-torque oscillator using MgO-based magnetic tunnel junction (MTJ) with a perpendicular polarizer and a perpendicular free layer. As the injected d.c. current varied from 1.5 to 3.0 mA under a weak magnetic field of 290 Oe, the oscillation frequency decreased from 1.85 to 1.3 GHz, and the integrated power increased from 0.1 to 74 pW. A narrow linewidth down to 7 MHz corresponding to a high Q factor of 220 was achieved at 2.7 mA, which was ascribed to the spatial coherent procession of the free layer magnetization. Moreover, the oscillation frequency was quite sensitive to the applied field, about 3.07 MHz/Oe, indicating the potential applications as a weak magnetic field detector. These results suggested that the MgO-based MTJ with perpendicular magnetic easy axis could be helpful for developing spin-torque oscillators with narrow-linewidth and high sensitive. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Spin transfer torque (STT), induced by a spin-polarized current [1,2], can excite a steady magnetization precession of the soft magnetic layer when it fully compensates the damping. Because of the magneto-resistance (MR) effect, the magnetization precession leads to a high-frequency electrical signal emitted from the STT device referred as spin-torque oscillators (STOs) [2,3]. STOs have been proposed as a new type of microwave generators [4] and magnetic field sensors [5], due to the exceptional features of large frequency tunability, nanoscale size, board working temperature and simple structure, compared with the traditional voltage controlled oscillator (VCO) [6]. In the past years, many efforts have been made to achieve large emission power and narrow linewidth (high Q-factor), which are two critical parameters of STOs for the practical application, through geometry design [7], material development [8] and technique optimization [9]. Apart from the progress of external factors, the researches on the magnetization states and dynamics are more fundamental and important to improve the STO properties. ⇑ Corresponding authors at: Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China (Zhongming Zeng) and Yanshan University, Qinhuangdao 066004, People’s Republic of China (Bochong Wang). E-mail addresses: [email protected] (B. Wang), [email protected] (Z. Zeng). https://doi.org/10.1016/j.jmmm.2017.12.071 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
In the early stage, a STO contains an in-plane magnetized freelayer (FL) and an in-plane magnetized reference layer as a polarizer. Under a spin-polarized current, the magnetization of FL behaves dynamically while the one of reference layer remained fixed. In this scheme, the FL processes in the film of plane, yielding a low output power around several pW. Recent works suggest that a d.c. electrical current could yield a sustained stable magnetization procession in spin valve metallic structure with perpendicular easy axis theoretically [10,11] and in MgO-based magnetic tunnel junction (MTJ) with perpendicular free layer and in-plane reference layer experimentally [12]. In comparison with in-plane magnetized scheme, perpendicular MTJs (p-MTJs) usually provide a better thermal stability [13], smaller size [14] and low dissipation power [15]. Currently, most investigations on p-MTJs focus on the STT-induced magnetization switching for magnetic random access memory [16,17], while there is no report related to the STTinduced microwave emission in the MTJs with both perpendicular free and reference layer. Theoretically, the perpendicular magnetic anisotropy fields in free and reference layer are benefit for realizing the uniform precession and stable oscillation, which can enhance the Q-factor and reduce the requirement of an external magnetic field. In this paper, we reported a steady auto-oscillation under weak applied magnetic field in a spin-torque oscillator using MgO-based p-MTJ. The microwave spectra under different injected electrical current and external magnetic field were investigated systematically.
C. Zhang et al. / Journal of Magnetism and Magnetic Materials 452 (2018) 188–192
2. Experimental details The p-MTJ films with a layered structure of bottom contact/ Co20Fe60B20 (1.2)/MgO (1)/Co40Fe40B20 (1.5)/[Co/Pd]4 (5)/Ru (0.85)/[Co/Pd]10 (10)/top contact (numbers in parentheses are nominal thickness in nanometers), as shown in Fig. 1(a), were prepared by UHV magnetron sputtering system. The free layer Co20Fe60B20 (1.2 nm) and the reference layer Co40Fe40B20 (1.5 nm) were designed having a perpendicular easy axis to the film plane due to the interfacial anisotropy of the CoFeB/MgO interface, which has been reported in previous works [18,19]. The reference layer was exchange biased by a [Co/Pd]4 (5 nm)/Ru (0.85 nm)/[Co/Pd]10 (10 nm) multilayer that was also shown perpendicular magnetic anisotropy. The films were annealed at 300 °C for 2.0 h under a magnetic field of 1 T and then patterned into circular shaped pillars with a diameter of 200 nm by electron beam lithography combined with Ar ion milling technique. The setup and measurement technique were described in Refs. [15,20]. The d.c. bias currents were applied to the device through a bias Tee. The positive current was defined as the electrons flowing from the free layer to the polarizer. The microwave voltages generated by STT were recorded by a spectrum analyzer after an amplification of 40 dB and a baseline taken at Idc = 0 mA was subtracted from the measured spectral data. All measurements were carried out at room temperature.
3. Results and discussion We firstly measured the static transport properties before dynamic characterization. Fig. 1(b) displays a magnetoresistance curve for one representative device at a bias current Idc = 10 lA under out-of-plane (HOP) magnetic field, where the applied magnetic field direction is normal to the film plane. As HOP decreases from 600 Oe to 600 Oe, the resistance changes which corresponds to the switching of the magnetization directions between the parallel state with low-resistance (RP) and the antiparallel alignment with high-resistance (RAP). The steps in R-H loop around ±50 Oe suggest that the multiple-domain may exist in the free layer due to the large size [21]. The tunnel magnetoresistance (TMR) ratio (defined as 100% (RAPRP)/RP) and the resistance-area (RA) product are 35% and 5 Xlm2, respectively. The low RA value allows
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the d.c. electrical current sufficiently passing through the MgO barrier to excite the spin polarized current. Next, we focus on the STT induced magnetization dynamics. As demonstrated in our previous work [22], the magnetization precession amplitude changes with the direction of applied magnetic field because of the angle dependence of STT. For circular devices presented here, the linewidth is found to be the narrowest when the magnetic field is applied at an angle of approximately 10° off the film normal. Fig. 2(a) shows the power spectrum density (PSD) measured under different d.c. current at 290 Oe. The spin torque can counteract the damping of the free layer and leads to a stable magnetization precession. Below 1.5 mA, no obvious microwave signals are observed. By increasing the current above 1.8 mA, corresponding to a current density of 5.7 MA/cm2, one dominant single oscillation peak appears obviously, suggesting the onset of a uniform steady magnetization precession. Fig. 2(b) depicts an example of spectrum at Idc = 2.7 mA which is well fitting with a Lorentian function. The central peak frequency is 1.47 GHz and a narrow linewidth (Df) of 7 MHz are obtained, corresponding to a high Q factor (Q = f/Df) of 220. Above 2.8 mA, a second precession mode appears, such as the second peak around 1.7 GHz, whose power is much weaker than the fundamental one. Note that the fundamental frequency peak may be attributed to a centre precession mode while the higher-frequency peak probably arises from a mode at the sides of the nanopillar [23] or the nonuniform precession component in the STO owing to the large current density. Quantitative data are gathered in Fig. 3, where the dependence of frequency, linewidth (Df), and integrated output power (Pout) of the STOs as a function of Idc are summarized. These parameters are extracted using a Lorentian fitting to the power spectra. It can be seen from the Fig. 3(a) that the frequency decreases gradually with increasing Idc, resulting in a current modulation capability of 0.35 GHzmA1, which is similar to the previous reports in the system with in-plane magnetized system [24]. The integrated power of the centre mode increases gradually with increasing the d.c. current and then reaches rapidly 74 pW at Idc = 3 mA as shown in Fig. 3(b). The power value is still too low to be used for practical applications. This is due to low dynamic resistance change during oscillation and the large resistance which causes a low electrical current under the same bias voltage. The emission power can be
Fig. 1. (a) Schematic illustration of the p-MTJ STO device; (b) Magnetoresistance curve of the p-MTJ STO device measured by applying magnetic field perpendicular to the film plane. The bias current is 10 mA. The arrows represent the sweeping field direction. The hollow arrows represent the directions of the free (lower) and reference (upper) layer magnetizations, respectively.
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Fig. 2. (a) The power spectrum density of a typical STO as a function of current under the out-of-plane magnetic field of 290 Oe; (b) A typical spectrum of STO and the Lorentian fitting curve for HOP = 290 Oe and Idc = 2.7 mA.
Fig. 3. Microwave emission properties of a typical STO. (a) Frequency, (b) Integrated power, (c) Linewidth of the centre mode as a function of applied current under out-of-plane magnetic field of 290 Oe, respectively.
improved by several strategies [25]: 1) reducing the device resistance by increasing the diameter of pillar samples or optimizing the tunnel barrier, thus improving transmission efficiency; 2) preparing high quality MTJ with larger MR ratio, which may lead to an enhanced power. 3) optimizing the pillar shape and boundary conditions to achieve large-amplitude precession of the magnetic moment; 4) increasing the amplitude of the injected DC current since the output power is proportional to the square of DC current.
Fig. 3(c) depicts the linewidth of the fundamental peak as a function of d.c. current. The linewidth decreases as the bias current increases from 1.5 mA to 2.1 mA, and almost keeps constant with further increase the current. The narrowest linewidth (Df) of 7 MHz is obtained at Idc = 2.7 mA with a high Q factor (Q = f/Df) of 220, which is also shown in Fig. 2(b). The linewidth value is significantly smaller than that in our previous works [22]. According to the nonlinear theories [26,27], the linewidth Df above threshold regime depends not only on the frequency nonlinearity N = df/dIdc but also the oscillation energy that is proportional to the output power Pout, that is, Df / N/Pout. It can be obtained that the current dependence of Df calculated from Fig. 3(a) is within the trend of observed Dfexp in experiment as shown in Fig. 3(c). Furthermore, two main features can be identified for the observed narrow linewidth. First, the free layer has a relatively large perpendicular anisotropy field, which leads to a uniform demagnetizing field in nanosized pillars [20]. As a result, a homogeneous demagnetization field may contribute to the coherent precession, which can avoid arising from the jagged side wall of the free layer caused by etching damage in the nanopillar-patterning process. Second, the precession amplitude plays an important role in determining the linewidth. At low current regime (1.5 mA Idc 2.1 mA), STTinduced dynamics are largely influenced by the enhanced effect of thermal fluctuations for small-amplitude magnetization precession. So this small-amplitude motion of the magnetization produces the low emitted power and relatively large linewidth [28] (Fig. 3(c)). At high current regime (Idc 2.1 mA), the dynamics become relatively large-amplitude precession and the influence of thermal fluctuations becomes smaller, so the dynamics become coherent leading to a reduced linewidth. In general, the spectral linewidth in MgO-based MTJ system is overly large due to the chaotic and non-linearity behavior [29]. In our device, the linewidth is suppressed below 10 MHz for a small out-of-plane magnetic field. These properties make the p-MTJ STOs as an attractive candidate to design novel types of magnetic sensors and future magnetic recording applications. At last, the dependence of the dynamical properties on the different external magnetic fields (HOP) was studied. The external magnetic field dependence of the microwave emission spectra, peak frequency, integrated power and resistance for a bias current of 2.5 mA are shown in Fig. 4(a–d), respectively. It can be seen that when HOP changes gradually from 100 to 500 Oe, the precession
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Fig. 4. The external magnetic field dependence of the (a) microwave emission spectra, (b) peak frequency, (c) integrated power and (d) resistance for a bias current Idc = 2.5 mA.
frequency exhibits an approximately linear dependence on field with a slope of 3.07 MHz/Oe. This result suggests that the microwave frequency of p-MTJ based STO is quite sensitive to the magnetic field and it can be used as a field sensor to detect the weak magnetic field [5]. Based on the Kittel’s formula, f = c (Hop + HK 4pMs)/2p, where HK is the perpendicular anisotropy field and the 4pMs is the saturation magnetization of Co20Fe60B20 free layer. Using linear fitting, the HK 4pMs value of 238 Oe is obtained. The experimental value of saturation magnetization 4pMs is 14.7 kOe [30]. Then, the perpendicular anisotropy field can be estimated as 14.94 kOe which is a little smaller than that in Ref. [30]. The difference stems from the number of interfaces in two systems. In the sample presented in this study the perpendicular anisotropy is due to the MgO/CoFeB interface MgO barrier, while there are two interfaces with a sandwich structure of MgO in Ref. [30]. In Fig. 4(c), the integrated power decreases with increasing magnetic field. According to our previous study [25], the emission power of STO is related to the precession cone angle of the free layer magnetization Dh, average angle between free and reference layer magnetization h0, transmission efficiency g, MR ratio, resistance R(h0) and electrical current Idc. In this study, the electrical current is fixed at 2.5 mA. The resistance slightly decreases from 141.3 to 140.9 X with the magnetic field (seen in Fig. 4(d)), then we can consider that the resistance R(h0), MR ratio and transmission efficiency g are constant. By increasing the magnetic field, the magnetization direction of free layer is tilting parallel to the one of reference layer, thus the value of h0 becomes smaller. And the precession cone angle Dh also becomes smaller, proved by the increase of peak frequency. As a result, the decrease of the emission power can be attributed to the decrease of Dh and h0.
4. Conclusions In conclusion, we have experimentally observed the steady auto-oscillation in nanoscale STOs with MgO-based magnetic tunnel junctions having perpendicular polarizer and perpendicular free layer under a weak external magnetic field. The microwave spectrum possesses a relatively narrow linewidth (7 MHz) and high Q factor (2 2 0) at Idc = 2.7 mA and HOP = 290 Oe. The precession performances at different bias currents and external magnetic fields are qualitative analyzed. These results not only give us a better understanding of dynamical magnetization phenomena of practical nanoscale spin-torque oscillators but also suggest the advantageous features of STOs with perpendicular magnetic easy axis as the next RF generator and weak magnetic field sensor. Acknowledgments This work was supported by the National Science Foundation of China (11474311, 51761145025) and the executive programme of Scientific and technological cooperation between Italy and China (2016YFE0104100). References [1] [2] [3] [4]
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