Ir bilayer

Journal Pre-proofs Determination of spin pumping effect in CoFeB/Ir bilayer Rui Sun, Yan Li, Z.K. Xie, Yang Li, Xiao-Tian Zhao, Wei Liu, Z.D. Zhang, T. Zhu, Zhao-Hua Cheng, Wei He PII: DOI: Reference:

S0304-8853(19)33033-1 https://doi.org/10.1016/j.jmmm.2019.165971 MAGMA 165971

To appear in:

Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

30 August 2019 5 October 2019 9 October 2019

Please cite this article as: R. Sun, Y. Li, Z.K. Xie, Y. Li, X-T. Zhao, W. Liu, Z.D. Zhang, T. Zhu, Z-H. Cheng, W. He, Determination of spin pumping effect in CoFeB/Ir bilayer, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm.2019.165971

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© 2019 Published by Elsevier B.V.

Determination of spin pumping effect in CoFeB/Ir bilayer Rui Sun

1,2,

Yan Li1,2, Z. K. Xie1,2, Yang Li1,2, Xiao-Tian Zhao3, a), Wei Liu3, Z. D.

Zhang3, T. Zhu1,2, Zhao-Hua Cheng1,2, and Wei He1,2,4, b) 1

State Key Laboratory of Magnetism and Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China 2

School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, PR China 3

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, PR China 4

Fujian Institute of Innovation, Chinese Academy of Sciences, Fuzhou, Fujian 350108, PR China Abstract The spin pumping effect of CoFeB/Ir bilayers has been investigated via the broadband ferromagnetic resonance measurement. With increasing the thickness of heavy metal Ir layer, the Gilbert damping factor of 8 nm CoFeB film increases from 7 10-3 to 1.1  10-2, and the additional damping factors induced by the spin pumping effect quickly reaches its saturation value. According to the spin pumping theory, the spin mixing conductance of CoFeB/Ir and the spin diffusion length of Ir were obtained. The spin mixing conductance is 2.141019 m-2. Furtherly, the spin diffusion length is only 1.34nm, which is quite shorter compared with the heavy metal Pt. Our results reveal the existence of the strong spin-orbit coupling in the heavy metal Ir, which can be a potential material for the application in spintronic devices. a) Electronic

mail: [email protected]

b) Electronic

mail: [email protected].

Key words: spin pumping; spin diffusion length; spintronics; heavy metal; thin film;

1

1. Introduction For last two decades, a lot of attentions have been paid to the heterostructures FM/HM (FM is ferromagnet and HM is heavy metal) for their advances of both fundamental researches in spin current transport and many implications for the development of spintronic devices such as spin-obit torque magnetoresistive random access memory (SOT-MRAM) [1][2] and magneto-electric spin-orbit(MESO)[3][4] device. In this bilayers, one of key probes to unravel the spin dependent transport properties is spin current. Among the spin-current-generating phenomena, the spin pumping effect has been intensely explored due to its facilities of generation of spin current in ferromagnets and injection to its adjacent normal metals via microwave excitation. In the process, the precession of magnetization serving as a spin battery emits spin current across the HM/FM interface, and the adjacent HM absorbs the spin current acting as a spin sink for the energy and angular momentum dissipation channels [5-7]. Usually, the magnetization’s precession and its relaxation can be phenomenologically descripted by the Landau-Lifshitz-Gilbert (LLG) equation with a Gilbert-type damping parameter [8,9]. The so-called spin pumping effect will cause an additional non-local Gilbert-type damping, which can be used to assess the transmission and reflection of spin currents at ↓ the HM/FM interface in term of the spin mixing conductance 𝑮 ↑𝒎𝒊𝒙 . In addition, the spin

diffusion length, through which spin-information can be transported, could be also characterized by varying the thickness of heavy metal layer, which reflects spin-flip scattering induced by strong spin-orbital coupling (SOC) [6]. The heavy metal with strong spin-orbit coupling is highly desired for their large spin-orbital torque (SOT)

2

generator. Many spin-pumping related experimental works have been reported in Pt, W, Ta and so on. Among them, Pt is considered as excellent of converter between spin and charger and its spin diffusion length is 2.2nm[7]. The spin mixing conductance of the interface between Pt and insulator YIG[12,13] or metal Ni80Fe20[14] is 1.2 1018 m 2 or

3.0 1019 m 2 , respectively. Recently, 5d metal Ir and its alloys have attracted increasing attention for their advance of the spin-orbit properties [14-21]. The emergence of DMI (Dzyaloshinskii-Moriya Interaction) in Co/Ir and Fe/Ir interfaces causes the magnetic chirality and the formation of skyrmion[14-16]. On the other hand, the large spin-orbit coupling in Ir atoms can be coupled to another atoms in the alloy [18]. Chen et al., predicted a large anomalous Hall conductivity in the antiferromagnetic alloy IrMn3. Zhang et al., explored the stronger intrinsic SHE in IrMn and determined the spin diffusion length as 0.7±0.2 nm, which is slightly longer than that of PtMn but shorter than that of PdMn[19]. Hence not only in element Ir but also in its alloys, the spin-orbit coupling from the contribution of Ir atom is considerably dominated. However, some fundamental spin-dependent parameters for Ir thin film, like spin diffusion length and spin mixing conductance, are still of lack. Only recently, Ishikuro et al., measured the spin-orbit torque caused by Ir and concluded that Ir has a small spin Hall angle of ~0.01 and a short spin diffusion length less than 1nm[17]. It reveals that Ir is a good spin sink. Consequently, the values of spin diffusion length and spin mixing conductance for element Ir obtained from other spin phenomena, like spin pumping effect, are feasibly and highly desired for revealing its spin properties in Ir-related interface or alloys. In this letter, the broadband

3

ferromagnetic resonance (FMR) was employed to explore the spin pumping effect in CoFeB/Ir bilayers. After analyzing the linewidth of FMR, the Ir thickness-dependent damping factor of CoFeB, and consequently, the spin diffusion length of Ir and the spin mixing conductance of CoFeB/Ir interface were obtained. It unequivocally demonstrates that Ir, as the 5d element, possesses a strong spin-orbit coupling and is quite suitable for Ir-based spintronic materials engineering. 2. Sample fabrication Ta/CoFeB/Ir(t) films were deposited on thermally oxidized silicon wafer by dc magnetron sputtering at an Ar pressure of 0.5 Pa. The thickness of buffer layer Ta is 4 nm and magnetic layer CoFeB is 8 nm. For the Ir layer, thickness t is varied from 0.4 nm to 12 nm. The base pressure of the sputtering system was less than 4×10-5 Pa. Sputtering rates for CoFeB, Ir and Ta were about 0.37, 0.18 and 0.27 Å/s, respectively. Then another sample CoFeB/Cu was also fabricated as the reference and marked as the sample with thickness Ir (t=0 nm) since the spin-flip scattering of Cu is small and cause a negligible spin pumping effect in here. The static hysteresis loops of CoFeB were measured by magneto-optical Kerr effect (MOKE) and the dynamical properties were explored by the broadband ferromagnetic resonance (FMR). 3. Results and discussions The longitudinal Kerr loops was measured and plotted in Fig 1(a) for CoFeB(8)/Cu and CoFeB/Ir(2.4), respectively. The magnetic hysteresis loops at different angles were measured. The remanence ratios Mr/Ms are obtained and plotted in Fig 1(b).

4

Both of them show uniaxial magnetic anisotropy due to the unintentional small angle oblique deposition through the magnetron sputtering. Considering the small coercivity of CoFeB and the remanence ratios at hard axis 0.7, the uniaxial magnetic anisotropy is quite small. It will be discussed below in accord with the FMR results. To investigate the spin pumping effect of CoFeB thin film influenced by heavy metal Ir, the samples were characterized by the broadband ferromagnetic resonance at room temperature. The samples were mounted on the top of the coplanar waveguide (CPW) and the external magnetic field was applying along the easy axis of these samples. Fig. 2 (a) shows the typical FMR spectra of CoFeB/Cu and CoFeB/Ir(2.8 nm) heterostructure at 8 GHz. As shown in Fig. 2(a), the peaks of two spectra were marked by the lines and the shift between two peaks clearly indicate that FMR linewidth in CoFeB(8)/Ir(2.8) is broadened comparing to the CoFeB(8)/Cu sample. It is a sign that the 5d metal Ir may have stronger spin-orbit coupling than copper. Usually the FMR spectra line is a Lorentz line superimposed by an anti-Lorentz line. The resulting FMR spectra line can be fitted with derivative line-shape equations: 𝒅𝑷 𝒅𝑯

= 𝒂𝟏 ∗

∆𝑯 ∗ (𝑯 ― 𝑯𝒓𝒆𝒔) 𝟒 ∗ ((𝑯 ― 𝑯𝒓𝒆𝒔)𝟐 + ∆𝑯𝟐)

𝟐

― 𝒂𝟐 ∗

∆𝑯𝟐 ― 𝟒 ∗ (𝑯 ― 𝑯𝒓𝒆𝒔)𝟐 𝟏𝟔 ∗ ((𝑯 ― 𝑯𝒓𝒆𝒔)𝟐 + ∆𝑯𝟐)

𝟐

,

(1)

where H is applied field, 𝝁𝟎𝑯𝒓𝒆𝒔 is the magnetic resonance field, and 𝝁𝟎∆𝑯 is full-widthhalf-maximum linewidths(FWHM) of FMR, respectively. a1 and a2 are the coefficient of the Lorentz and anti-Lorentz line terms. According to this equation, 𝝁𝟎∆𝑯 and 𝝁𝟎𝑯𝒓𝒆𝒔 were obtained. The value of linewidth 𝝁𝟎∆𝑯 = 𝟒.𝟑𝟑𝒎𝑻 for CoFeB/Ir(2.8) is quite larger than 𝝁𝟎∆𝑯 = 𝟐.𝟑𝟒𝒎𝑻 for the CoFeB/Cu(3), indicating the enhanced damping in

5

CoFeB/Ir bilayers. As we know the strong spin-orbit coupling materials can greatly modify the magnetization dynamic of ferromagnetic layer, such as the precession angle, damping factor and so on. Fig. 2(b) demonstrates this process which is the so called spin pumping. Within the framework of spin pumping, the CoFeB film under ferromagnetic resonance emits spin current into the Ir layer along with a spin accumulation at the CoFeB/Ir interface. Due to spin-flip scattering caused by the spin-orbit coupling of conduction electrons in Ir, the spin angular momentum dissipations within the thickness of spin diffusion length. So spin angular momentum transfers from source CoFeB layer and losses in spin sink Ir layer, which results in the linewidth broadening or enhancement of Gilbert damping. Therefore, the enhanced Gilbert damping factor is a key parameter which determines the spin pumping effect. The FMR linewidth is related to the damping factor. However, the FMR linewidth can be caused not only by the intrinsic damping including the nonlocal term resulted by spin pumping effect, but also by the extrinsic damping, like inhomogeneous broadening term. In order to distinguish them and extract the intrinsic damping factor, the FMR was carried out at varying frequency from 8 GHz to 17 GHz. The FMR spectra of CoFeB/Ir(2.8) were plotted in Fig. 2(c). The resonance field and linewidth were varied with the microwave frequency and counted by fitting Eq. (1). The fitted lines at different frequency were also plotted in Fig. 2(c). Furthermore, the relationship of frequency and resonance field Hres was depicted in the Fig. 3(a), which can be descried by the Kittle equation as the following formula: 𝒇=

𝝁𝟎𝒈𝝁𝑩 ℏ

(𝑯𝒖 + 𝑯𝒓𝒆𝒔)(𝑯𝒖 + 𝑯𝒓𝒆𝒔 + 𝑴𝒆𝒇𝒇)

(2) 6

where g is the spectroscopic splitting factor and for CoFeB g=2.13 is used, 𝝁𝑩 is Bohr magneton, ℏ is reduced Planck constant, Meff is the effective magnetization, 𝑯𝒖 is the inplane uniaxial anisotropy field, respectively. The values of effective magnetization of all the sample are found to be almost same about 1.3T, and 𝝁𝟎𝑯𝒖 obtained from Kittle equation was under a negligible value about 0.1mT, which indicates the inappreciable uniaxial anisotropy. Fig. 3(b) shows the frequency dependence of 𝝁𝟎∆𝑯. Via the linear fitting of equation: 𝝁𝟎∆𝑯 =

𝟐𝝅𝜶 𝜸

𝒇 + 𝝁𝟎∆𝑯𝟎 ,

(3)

the Gilbert damping factor 𝜶 is proportional to the slope of linewidth and was obtained as the varying thickness of Ir layers. In addition, 𝝁𝟎∆𝑯𝟎 describes inhomogeneous broadening term caused by the roughness, defects. It presents the quality of films. As depicted in Fig. 2(c), the intercept 𝝁𝟎∆𝑯𝟎 of fitted curves are below 0.5mT, indicating the high quality of CoFeB thin films through magnetron sputtering. In our samples, the high quality of magnetic film minimizes the inhomogeneous broadening as well as the two-magnon scattering. The slope of curves with Ir layers in Fig. 3(b) are larger than that of CoFeB/Cu reference sample obviously, which confirms that the enhanced Gilbert damping factor origins from addition spin dissipation of Ir via the spin pumping effect. The Gilbert damping factors 𝜶 were extracted and plotted as the function of the thickness of Ir layer in Fig. 3(c). The Gilbert damping begins saturated above 4 nm around 𝟏 × 𝟏𝟎 ―𝟐. The absorption efficiency of spin current in Ir is dependent on as well the interface property, usually described by the spin mixing conductance parameter as the

7

thickness of spin sink. In order to quantify the spin diffusion length and spin mixing conductance of Ir, the Gilbert damping as the function of Ir thickness was fitted by

𝜶𝒆𝒇𝒇(𝒕𝑰𝒓) = 𝑹𝒆

[

↓ 𝑮 ↑𝒎𝒊𝒙 𝝁𝑩

𝟐𝒕 𝑰𝒓

(𝟏 ― 𝒆 )] + 𝜶

𝟒𝝅𝑴𝒔𝒕𝑭

―

𝝀𝒅

𝑪𝒐𝑭𝒆𝑩

(4)

where  B is the Bohr magneton, g is 2.13, 𝜆𝑑 is the spin diffusion length of Ir, and 𝜶𝑪𝒐𝑭𝒆𝑩 ↓ is the Gilbert-type damping of the film Cu/CoFeB. 𝑮 ↑𝒎𝒊𝒙 is the spin mixing conductance. ↓ The fitting yields 𝑮 ↑𝒎𝒊𝒙 = 𝟐.𝟏𝟒 × 𝟏𝟎𝟏𝟗𝒎 ―𝟐 and 𝜆𝑑 = 1.3 𝑛𝑚. The spin mixing

conductance of CoFeB/Ir is larger than Py/W (0.7 × 1019m ―2)[22], Py/Ta (0.9 × 1019 m ―2)[22,23], but smaller than Py/Pt (3.0 × 1019m ―2[14],or 3.2 × 1019m ―2 [24]). Anyhow, more than a bulk property, the spin mixing conductance is also a property of the interface. It is not determined as simply by the atomic number and is related to the band matching across the FM/HM interface[22]. The spin mixing conductance is not only HMdependent but also FM-dependent. It has been reported that the enhancement of Gilbert damping in CoFeB/Pt is slightly 30% higher than it in Py/Pt[25]. Therefore we can predicate that Ir contributes to damping roughly about half as strongly as Pt for same FM in FM/HM interface. However, the spin diffusion length of Ir is in consistence with the previous result [17] and is much smaller than that of Pt (2.7nm). It reveals that Ir is a good spin sink with strong spin-orbit coupling. As demonstrated by our experimental data the Ir layer is an efficient spin sink. The short spin diffusion length indicates the spin-flip scattering caused by strong spin-orbit interaction. In previous reports, some alloys such as IrMn, IrMn3 predicted Ir usually owns much short spin diffusion length. As well in some doping system, for example the

8

copper doped with Ir, large enhancement of damping factor and spin Hall Effect were found[26,27]. Although the doping Ir atoms serve as the skew scattering center to enhance this spin current dissipation from spin current into charge current, the essence is that the strong spin-orbit coupling strength dominate the skew scattering process physically[28], which validates our results. In addition, a recently predicted spin Hall material Cr3Ir[29] possesses a large intrinsic spin Hall conductivity. While the heavy metal Ir with strong spin-orbit coupling plays an essential role in inducing the energy band anti-crossings. Even more works are necessary in the need of exploring the spinorbit coupling, 5d metal Ir and its alloy have revealed their novel potential for designing modern spintronic devices. 4. Conclusions In summary, we investigated the spin pumping effect of CoFeB/Ir bilayers via the broadband ferromagnetic resonance. With increasing the thickness of heavy metal Ir layer, the Gilbert damping factor of 8nm CoFeB film enhances from 7 10-3 to 1.1  10-2, and the additional damping factors induced by the spin pumping effect quickly reaches its saturated value. The obtained spin mixing conductance of CoFeB/Ir was about 2.141019 m-2. And the spin diffusion length of Ir is only 1.34nm compared to 2.2nm of Pt, indicating a stronger spin-orbit coupling than Pt. Our results provide helpful information for the potential application of 5d metal Ir in spintronic devices. ACKNOWLEDGEMENTS This work is supported by the National Natural Sciences Foundation of China (Grant Nos. 51871235, 51671212, and 51801212), the National Key Research Program of China

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(Grant Nos. 2015CB921403, 2016YFA0300701, and 2017YFB0702702), and the Key Research Program of Frontier Sciences, CAS (Grant Nos. QYZDJ-SSW-JSC023, KJZDSW-M01 and ZDYZ2012-2).

10

1.0

CoFeB/Ir(2.4) CoFeB/Cu(3)

0.6

0.8

0.2

0.7

-0.2 -0.6

(a)

-1.0 -1

CoFeB/Cu CoFeB/Ir

0

1

2

30

150

0

0.6 180 0.7 0.8

-2

60

120

0.9

Mr/Ms

Kerr Signal (a.u.)

90

1.0

210

330

(b)

0.9 1.0

240

300 270

Applied Field (mT)

Fig.1 In-plane M-H loop measured by MOKE along hard axis (a), and the angledependent remanence ratios of CoFeB(8)/Cu(3) and CoFeB(8)/Ir(2.4) (b) , respectively.

11

0 2.8 Fitted curves

(b)

dP/dH

(a)

-20

-10

0

10

20

0(H-Hres) (mT) 8 GHz 9.5 GHz 11 GHz 12.5 GHz 14 GHz 14.5 GHz 16 GHz Fitting

dP/dH

(c)

40

60

80

100

120

140

160

180

200

220

Applied Field (mT) Fig.2 (a) The FMR spectra observed for CoFeB/Cu(3)(black rectangular) and CoFeB/Ir(2.8)(red rectangular). The linewidth shows a considerable enhancement. (b) The schematic illustration of the spin current transport through Ir layer by spin pumping effect. (c) The in-plane FMR spectra in the form of derivative absorption for the CoFeB/Ir(2.8) film from 8GHz to 17GHz. Through equation (2), the linewidth and resonance field can be obtained.

12

Ir(0)/CoFeB Ir(0.8)/CoFeB Ir(1.6)/CoFeB Ir(2.4)/CoFeB Ir(4)/CoFeB Ir(6)/CoFeB Ir(12)/CoFeB fitted curves

5

10

(b)

Ir(0)/CoFeB Ir(0.8)/CoFeB Ir(1.6)/CoFeB Ir(2.4)/CoFeB Ir(4)/CoFeB Ir(6)/CoFeB Ir(12)/CoFeB fitted curves

6

0H (mT)

f (GHz)

15

7

(a)

4 3 2

5

1

12

damping factor ( x 10-3)

20

(c)

CoFeB (8nm)/Ir(t) spin diffusion length and mixing conductance

11 10 9

=1.34nm 19 -2 g mix=2.1410 m

8 7

0

0

50

100

150

0Hres(mT)

200

250

0

0

5

10

f (GHz)

15

0

2

4

6

8

10

12

Ir thickness (t nm)

Fig.3 (a) The resonance frequency f vs resonance field Hres for samples CoFeB/Ir(t=0,0.8, 1.6, 2.4, 4, 6,12 nm, t is the thickness of Ir layer) and their linewidth vs frequency (b). (c) The obtained damping factor for all samples, the red dashed line is the fitted curve according to eq.(4).

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Highlights for “MAGMA_2019_2837” Recently a lot of reports have pointed out that Ir and its alloys have strong spin-orbit coupling. Our results give some particular spin transport parameters of 5d Ir as follows: 1, The spin diffusion length of Ir and the spin mixing conductance of CoFeB/Ir interface were obtained through charactering the spin pumping effect in CoFeB/Ir bilayers. 2,The spin mixing conductance is 2.141019 m-2 and the spin diffusion length is only 1.34nm, which is quite shorter than that of Pt. 3, Ir possesses a strong spin-orbit coupling and is quite suitable for Ir-based spintronic materials engineering. We are hopeful that these results improve the recognition of element Ir in spintronics.

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