CoFeB Magnetic ... - IOPscience

Applied Physics Express 3 (2010) 073001

Large Diode Sensitivity of CoFeB/MgO/CoFeB Magnetic Tunnel Junctions Shota Ishibashi1 , Takeshi Seki1 , Takayuki Nozaki1 , Hitoshi Kubota2 , Satoshi Yakata2 , Akio Fukushima2 , Shinji Yuasa2 , Hiroki Maehara3 , Koji Tsunekawa3 , David D. Djayaprawira3 , and Yoshishige Suzuki1;2 1

Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan National Institute of Advanced Industrial Science and Technology (AIST), Spintronics Research Center, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan 3 Canon ANELVA Corporation, Magnetic Thin Film Development Division Process Development Center General Business Headquarters, 2-5-1 Kurigi, Asao, Kawasaki, Kanagawa 215-8550, Japan 2

Received April 14, 2010; accepted May 29, 2010; published online June 18, 2010 We report on rf current-induced excitation of the ferromagnetic resonance in CoFeB/MgO/CoFeB magnetic tunnel junctions under a perpendicular magnetic field. By choosing an appropriate external field and using an Fe-rich CoFeB free layer, the effective precession of the free layer could be excited. In a measurement of homodyne detection, a large dc output voltage of 180 V was obtained when an rf signal power of 25 dBm was applied. The sensitivity of this junction, as an rf rectifier, reaches about 170 mV/mW (280 mV/mW after impedance matching correction), which is the same order compared with that of a Schottky diode operated at room temperature. # 2010 The Japan Society of Applied Physics DOI: 10.1143/APEX.3.073001

hen a spin-polarized electric current is injected to a thin ferromagnetic layer, the spin angular momentum of conduction electrons is transferred to the local spin system (d-electron spin) in the ferromagnetic layer. The change in the local spin moment per unit time, as a consequence of the spin transfer, gives rise to a torque exerted on the local spin system. This torque is called the ‘‘spin torque’’ and is used to manipulate the magnetization direction.1–4) Spin torque provides us with many useful phenomena such as magnetization switching,5–7) the spontaneous oscillation of the magnetization8,9) and the spin torque diode effect.10,11) The spin torque diode effect is a quadratic rectification effect of the rf current in a magnetoresistive junction because the resonant oscillation of the junction resistance is synchronized with the injected rf current. Recently, the spin torque diode effect has attracted much attention as a technique for quantitatively measuring spin torque.12–14) However, the rectification efficiency in the spin torque diode is still insufficient for practical device application as a diode detector. According to a previous paper,10) the rectified dc voltage output is inversely proportional to the critical voltage required for spin torque switching. It has been reported that an increase in Fe concentration in the CoFeB free layer results in a reduction in the critical switching current in a magnetic tunnel junction (MTJ) comprising a CoFeB free layer/MgO/CoFeB pinned layer.15) Consequently, it is expected that a larger spin torque diode voltage can be generated with this MTJ. Moreover, application of a magnetic field perpendicular to the junction may lead to the larger relative angle between the free and pinned layer magnetizations,16) resulting in a large diode voltage. In this paper, we performed spin torque diode measurements under a perpendicular magnetic field on a CoFeB/ MgO/CoFeB MTJ with an Fe-rich CoFeB free layer. The rf sensitivity of the present spin torque diode was compared with that of a conventional semiconductor diode. We prepared 100 150 nm2 (the design size) MTJs with the following structure: SiO2 wafer/buffer layer/PtMn (15 nm)/CoFe (2.5 nm)/Ru (0.85 nm)/Co60 Fe20 B20 (3 nm)/ MgO (1.2 nm)/Co16 Fe64 B20 (2 nm)/capping layer. The numbers are designed thicknesses. The etching of the element was controlled by the secondary ion mass spec-

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troscopy (SIMS) and stopped at the surface of the PtMn layer, i.e., the synthetic antiferromagnetic (SAF) pinned layer is also patterned. In this experiment, the free layer was enriched with Fe, the effect of which was to reduce the critical current density (Jco ) of the sample (9:1 106 A/cm2 ) compared to that reported in previous works (Co60 Fe20 B20 free layer, 2:1 107 A/cm2 ).16) From a magnetoresistance (MR) measurement under an in-plane magnetic field, the device resistance in the parallel magnetic configuration and the MR ratio were evaluated to be 98 and 105%, respectively. The resistance area product (RA) was 2.1 m2 in the parallel magnetic configuration. Due to the orange peel coupling, hysteresis shift of 60 Oe (100 Oe) was observed in the patterned (continuous) samples. Because of this, the parallel magnetic configuration is stable at zero external magnetic field (Hext ). The spin torque diode measurement (see the ref. 12 for the detailed technique) was performed under a magnetic field applied perpendicular to the film plane. Application of an rf current to the MTJ exerts an oscillating spin torque on the magnetization of the free layer, leading to excitation of the ferromagnetic resonance (FMR) mode. The dynamics of the free layer causes oscillation of the tunnel magnetoresistance (TMR). Since the oscillating resistance partially rectifies the rf current, this is a kind of homodyne detection. All measurements were carried out at room temperature (RT) without dc bias current. Figure 1 shows a typical TMR curve measured under a perpendicular magnetic field. Since the Fe-rich CoFeB film exhibits perpendicular magnetic anisotropy,16) the magnetization of the free layer is easily directed to the out-of-plane by the relatively low external perpendicular field. Therefore, the relative angle () between the free and pinned layers continues to increase gradually in magnetic fields of less than 5 kOe, resulting in an increase in resistance. Around Hext ¼ 5 kOe, which is close to the demagnetization field (Hd ) of the free layer, the resistance reaches a maximum. Increasing Hext beyond this causes reduction in the resistance, because the pinned layer also rotates toward the perpendicular direction due to the large Hext . Figure 2 shows spin torque diode spectra measured under various Hext . In this measurement, the perpendicular magnetic field was slightly tilted (5 degree) from the normal

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# 2010 The Japan Society of Applied Physics

Appl. Phys. Express 3 (2010) 073001

S. Ishibashi et al.

(a)

(b) Fig. 1. R–H curve measured under a perpendicular magnetic field. The inset shows the direction of the applied magnetic field. In this figure, the gray and blue arrows indicate the magnetizations of the free and pinned layers, respectively.

(c)

(d)

Fig. 2. Spin torque diode spectra measured under various external magnetic fields. The field direction was slightly tilted (5 degree) from the normal to the film plane toward the in-plane hard axis direction, as schematically shown in the inset.

axis toward the in-plane hard axis direction (see the inset in Fig. 2). Due to the perpendicular field application, single peak structure could be obtained in wide magnetic field ranges. The Hext dependence of the (a) , estimated from the resistance, (b) the peak frequency, (c) the linewidth () and, (d) the diode voltage (Vdiode ) are summarized in Fig. 3. The Hext was applied perpendicular to the film plane (red marks) or slightly tilted from the normal axis by 5 degree (blue marks). The dashed lines in Figs. 3(b) and 3(c) show the results of fitting based on the Kittel’s equation and the equation described below [eq. (3)].15,16) From the fit, Hd , the coercitivity field (Hc ) and the Gilbert damping factor () of the free layer were evaluated to be 4.9 kOe, 90 Oe, and 0.012, respectively. The obtained Hd is much lower than that in a conventional Co60 Fe20 B20 free layer (11 kOe),16) indicating the existence of a high perpendicular magnetic anisotropy in the Fe-rich samples. Around Hext ¼ Hd , the peak frequency and the linewidth show minima, whereas and Vdiode show maxima in the case of the perpendicular magnetic field. For Hext > Hd , since the free layer magnetization is truly perpendicular, indicates the angle of magnetization in the pinned layer relative to the normal axis. Figure 3(d) shows the magnetic field dependence of

Fig. 3. Applied external magnetic field (Hext ) dependence of (a) the relative angle between the free and pinned layers, (b) the resonant frequency, (c) the linewidth and (d) the amplitude of the spin torque diode voltage. Hext was applied perpendicular to the film plane (red points) or slightly tilted from the normal axis by 5 degrees (blue points). Hd indicates the demagnetization field of the free layer.

the diode voltage. The observed strong dependence on the external magnetic field originates from the external magnetic field dependence of the , resistance RðÞ and . Under the magnetic field of slightly tilted condition, dc voltage of Vdiode ¼ 180 V was obtained at Hext ¼ 3:5 kOe when an rf power (Prf ) of 25 dBm in the transmission line was applied. The diode sensitivity is defined as

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diode sensitivity ¼

Vdiode : Prf

ð1Þ


Appl. Phys. Express 3 (2010) 073001

S. Ishibashi et al.

In the experiment reported here, the maximum diode sensitivity was 170 mV/mW. This value is 3 times larger than that obtained in the previous work, which was performed under an in-plane magnetic field.17) After calibrating the impedance mismatch of the device, the inherent diode sensitivity is evaluated to be 280 mV/mW. This is the same order of the sensitivity of a conventional Schottky diode, which is theoretically estimated as 500 mV/mW at RT. Analytical expression of the rectification voltage for the spin torque diode under the magnetic field perpendicular to the film plane can be easily derived from eqs. (10), (11), and (13) in ref. 18 as follows,19) jVdc j ¼

Prf,sample ðHext ¼ 0Þ 1 2 RðÞ2 sin ; ð2Þ MR P j 4 ðHext Hd Þ RAP RP jIc0

P where RðÞ, RPðAPÞ , and Ic0 are the resistance at , the resistance in the parallel(anti-parallel) magnetic configuration, the critical current required for the spin torque switching in the parallel magnetic configuration, respectively. Prf,sample is the power consumed in the MTJ sample. ðHext ¼ 0Þ and ðHext Hd Þ are the linewidths without the external field and under the perpendicular magnetic field whose magnitude is comparable to the demagnetization field and are expressed as,18) ðHext ¼ 0Þ ¼ ðÞðHd þ Hc Þ ; ð3Þ ðHext Hd Þ ¼ ðÞð2jHd Hext j þ Hc Þ

where (¼ 1:76 1011 T1 s1 ) is the gyromagnetic ratio. One of the reasons for the enhancement of the diode sensitivity is the increase in , which could be realized by the application of the perpendicular magnetic field and the small Hd of the Fe-rich free layer. A second reason is the reduction P in Ic0 , which is also originated from the small Hd .15) A third reason is the reduction in ðHext Hd Þ, which is originated from the perpendicular field. Using the parameters of ¼ 60 deg, RðÞ ¼ 115 , RP ¼ P 98 , RAP ¼ 200 , MR ¼ 1:05, Ic0 ¼ 2:5 mA, ðHext Hd Þ ¼ 80 MHz and ðHext ¼ 0Þ ¼ 170 MHz, we can estimate the theoretical expectation value of the diode sensitivity as 110 mV/mW from the eq. (2) under the perpendicular magnetic field. Here, ðHext Hd Þ is the experimentally observed value as seen in the Fig. 2(c). On the other hand, ðHext ¼ 0Þ is obtained by substituting (¼ 0:012), (¼ 1:76 1011 T1 s1 ), Hd (¼ 4:9 kOe) and Hc (¼ 90 Oe) into the eq. (3), because the diode voltage cannot be observed under the completely zero magnetic field in principle. However, it should be noticed that the obtained value is almost the same as the linewidth (¼ 190 MHz) observed under the near zero field [see Fig. 2(c)]. The theoretical expectation value agrees well with the observed value (96 mV/mW) in this experiment. However, if we use ðHext Hd Þ as the theoretical expression of the linewidth (¼ 3 MHz) [eq. (3)], we may expect the diode efficiency as high as 3000 mV/mW. This means that there are possibilities to make the effect much larger as that was realized here by an application of external magnetic field with tilted angle.

Main origin of the observed wide linewidth may come from incoherent precession in a cell [see Fig. 3(c)]. By making the linewidth narrower, we may expect much larger diode sensitivity. Other effects like dynamic coupling between free and pinned layers14) and the non-linear precession should also be clarified. In summary, spin torque diode measurements were performed in a CoFeB/MgO/CoFeB magnetic tunnel junction with an Fe-rich CoFeB free layer under a perpendicular magnetic field. Under an application of an optimized external magnetic field, a large dc voltage of 180 V was obtained by an rf power application of 25 dBm. The maximum observed sensitivity is about 170 mV/mW (280 mV/mW after impedance matching correction), which is of the same order compared with that of a semiconductor diode. Acknowledgments This work was partly supported by Strategic Information and Communications R&D Promotion Programme from the Ministry of Internal Affairs and Communications and a Grant-in-Aid (A19206002) from the Ministry of Education, Culture, Sports, Science and Technology.

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