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Stress-enhanced interlayer exchange coupling and optical mode FMR frequency in self-bias FeCoB/Ru/FeCoB trilayers Shandong Li, Guo-Xing Miao, Derang Cao, Qiang Li, Jie Xu, Zheng Wen, Youyong Dai, Shi-shen Yan, and Yueguang Lü ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19684 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018
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ACS Applied Materials & Interfaces
Stress-enhanced interlayer exchange coupling and optical mode FMR frequency in self-bias FeCoB/Ru/FeCoB trilayers Shandong Li, †* Guo-Xing Miao,‡ Derang Cao,† Qiang Li,† Jie Xu,† Zheng Wen,† Youyong Dai,§ Shishen Yan,§ and Yueguang Lüǁ †
College of Physics, Qingdao University, Qingdao 266071 China
‡
Institute for Quantum Computing, Department of Electrical and Computer
Engineering, University of Waterloo, Waterloo, N2L 3G1, Canada §
ǁ
School of Physics, Shandong University, Jinan 250100, China
Department of Physics, School of Science, Harbin Institute of Technology, Harbin,
150001, China. ABSTRACT: Nowadays, the most popular method to increase ferromagnetic resonance (FMR) frequency ( ) in self-bias soft magnetic films is to improve the anisotropy field HK. However, to push to higher frequencies only via raising becomes increasingly challenging since is already higher than 10 GHz by now. In this study, we fabricated a series of magnetically anisotropic FeCoB/Ru/FeCoB sandwich films possessing antiferromagnetic-like coupling and gradually increased uniaxial stress in the FeCoB sublayers from 52 to 110 MPa. It is quite remarkable that the acoustic mode of FMR gradually disappears while the optical mode is enhanced in these structures. We observed simultaneous enhancement of and interlayer coupling field ( ) with the uniaxial stress, which leads to a very pronounced optical mode frequency increase from 8.67 to 11.62 GHz with a very sensitive stress response of 51 Hz/Pa. In contrast, the in a FeCoB single layer (acoustic mode) only varies 1 ACS Paragon Plus Environment
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from 3.47 to 5.05 GHz under similar stress. We believe the strain-induced electron density variation of the Ru spacer’s Fermi surface in the out-of-plane direction is responsible for the enhancement of . This study demonstrates that the antiferromagnetic coupling is a new route to achieve higher and provides the possibility of engineering and manipulating optical mode resonance simply by controlling the interlayer coupling strength via stress. KEYWORDS: ferromagnetic resonance, optical mode FMR, interlayer exchange coupling, antiferromagnetic coupling, uniaxial stress
■ INTRODUCTION As modern electronics progresses towards faster and faster operation speeds, higher and higher FMR frequency in magnetic monolithic microwave integrated circuit (MMIC) devices is desperately required.1,2 As described in Kittel’s formula,3
=
( )( )
(1)
where γ, , 4π , and stand for the gyromagnetic ratio, the internal anisotropy field, the saturation magnetization of the soft magnetic films (SMFs), and the external magnetic field, respectively. The self-bias (with zero external bias field) in SMFs with large 4π ! (e.g. FeCo-based alloys) is dominated by the anisotropy field . Therefore, the majority of research work on self-bias microwave SMFs is focused on the enhancement of . Recently, rapid developments were made in enhancing , which brings the self-bias of FeCo-based SMFs from radio frequency to microwave X-bands (around 10 GHz).4-15 However, to further increase only via 2 ACS Paragon Plus Environment
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increasing becomes increasingly challenging. So finding other new approaches is imminent. The optical mode FMR frequency " along the easy axis (EA) in exchange coupled ferromagnetic sandwich films can be expressed as,16,17
" =
( #$%& )∙( #$%& )
(2)
From Eq. (1), we see that the interlayer exchange coupling (IEC) cannot influence the acoustic mode resonance since the magnetizations of upper and lower ferromagnetic sublayers are opposite and in-phase. So the dispersion relations of acoustic mode are the same for single- and trilayer films. However, from Eq. (2), an additional was added into the optical mode’s dispersion relation since the magnetization vectors are out-of-phase therefore strengthen each other.18 Thus, the optical mode " can be significantly larger than the acoustic mode ( in a coupled multilayer structure.19 It has been reported that very strong antiferromagnetic (AFM) coupling with as high as several kOe is present in some sandwich films,20-23 which may lead to an ultrahigh up to 20-50 GHz. It’s well known that the intensity and oscillation period of IEC in nonmagnetic (NM) spacer separated sandwich films are dominated by the conduction electron wave vectors at the Fermi surface of NM spacers,24,25 which is sensitive to the stress (or strain) exerted on NM spacers.26 J. H. Wong et al. reported the influence of strain on graphene’s electronic structures.27 They found that the uniaxial strain can increase the band overlap over 100 times in bilayer graphene, and even trigger a semimetal−insulator transition when a compressive uniaxial strain is applied on the bilayer graphene along the zigzag direction. The influence of strain 3 ACS Paragon Plus Environment
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on the reactivity of metal surfaces was ab initio simulated by M. Mavrikakis, et al.,28 showing that the lattice expansion leads to an increase of surface reactivity. R. Otero, et al. reported a substantial upward shift of Cu surface states in the Cu/Ru (0001) bilayer with a lateral tensile strain in Cu layers.29 J. L. Leal et al. revealed that the oscillation period in spin valves with Cu spacers is evidently enhanced by strain.30 Therefore, stress (or strain) is an effective way to change the electronic structure of the spacers, and to tailor the intensity of IEC in sandwich structures. Recently, the influence of boron on the properties of metallic borides was studied thoroughly,34,35 therefore, in this study, FeCoB alloys were chosen to study their high-frequency soft magnetic properties. We had found a strong optical mode in AFM coupled FeCoB/Ru/FeCoB sandwich films,31 which gives a new approach to improve high-frequency microwave performances in SMFs via optical mode engineering. In this study, we deposited a series of FeCoB/Ru/FeCoB sandwich films by composition gradient sputtering (CGS) method.9,10,12 The trilayers have the same Ru thickness of 3 Å, but a B-doping induced stress gradually increases in them along the length direction of the substrate. The stress modifies the Ru spacer’s Fermi surface, and stress enhanced IEC was achieved. As a result, a dramatically increased optical mode FMR frequency up to 11.62 GHz at zero bias magnetic field with an ultrahigh frequency to stress ratio ( /σ) of 51 Hz/Pa was achieved. In the CGS method, the ferromagnetic target (e.g. Fe0.7Co0.3) is set at normal incidence and the doping target (for instance B in this study) at oblique incidence to produce the doping element gradient. A uniaxial anisotropy field is caused by the 4 ACS Paragon Plus Environment
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intrinsic stress in FeCoB films due to the gradient B-doping from the CGS method. Under stress, the magnetostriction contribution to the magentic energy ) is expressed as, ) = −+ ,-./ 0
(3)
where 1! , 2, 3 refer to the saturation magnetostriction coefficient, the stress, and the angle between stress and magnetization directions. From Eq. (3), a compressive stress prefers to orient magnetic moments perpendicular to it in films with positive 1! (as the case in this study), and vice versa.32,33 Therefore, the uniaxial intrinsic stress orients the magnetic moments in FeCoB layers along the direction perpendicular to the stress direction. As a result, an endogenetic uniaxial field is present in the films in the form of an effective field, which leads to a very high self-bias FMR frequency even without external magnetic field. It is the CGS method that makes sure the good high-frequency performance at zero field. In this study, we designed an AFM coupled FeCoB/Ru/FeCoB sandwich structure to further explore the influence of uniaxial stress on the IEC and optical mode FMR. For this series of samples, the ferromagnetic FeCoB sublayers have a gradient B composition and gradually increasing compressive stress, which also modifies the Ru spacer layer’s electronic structures.
■ EXPERIMENTAL DETAILS A Si single crystal substrate with the dimension of 50 mmLength ×10 mmWidth ×0.5 mmThickness was loaded on the turntable of the magnetron sputtering chamber with the
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sample’s length (L) direction along the turntable’s radial (R) direction. The upper and lower 25 nm FeCoB sublayers were prepared by CGS method, while the 3 Å (slightly larger than one monolayer) Ru spacer was sputtered homogeneously. The trilayer films are labeled TL. The detailed preparation procedure was described in Ref. 31. The resultant FeCoB/Ru/FeCoB sandwich film shows two important characteristics, that the FeCoB sublayer is uniaxial magnetic anisotropic with the same EA orientation and that an AFM-like interlayer coupling occurs between the ferromagnetic sublayers. At the same time, a 50 nm FeCoB single layer was also prepared as a reference sample (labeled SL). The single layer and trilayer films were cut equally into 10 segments along the L direction for measuring the magnetic and microwave performances. The sample segments were successively numbered as n = 1‒10 along the B element increasing direction. For example, the 5th segment was named as sample position n=5. It should be mentioned here that the effects of FeCoB and Ru thicknesses on the high-frequency performance of the trilayer films have been systematically studied. A periodic oscillation in the strength of the interlayer exchange coupling occurs with the thickness of Ru spacer. Thus, both ferro- and antiferro-magnetic coupling between the ferromagnetic FeCoB layers may occur depending on the Ru thickness. The optimized Ru spacer thickness is 3 Å to provide a strong antiferromagnetic interlayer coupling without the two layers directly coupled to each other. On the other hand, the interlayer exchange coupling only takes place below a limited FeCoB thickness. For practical microwave applications, we would like the total FM thickness to be fairly thick. 6 ACS Paragon Plus Environment
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However, for CoFeB thicker than 30 nm, partial ferromagnetic coupling appears likely due to appearance of ferromagnetic Neel coupling with increased film roughness. We’ve checked a wide range of FM layer thicknesses, and the experimental results revealed that the optimized thickness of FeCoB is around 25 nm in the case of 3 Å Ru spacer. The composition along the R direction (also the L direction) of the turnplate was detected by a field emission electron probe microanalyzer (FE-EPMA, JXA-8500F). The static magnetic measurement was carried out by a physical properties measurement system with a vibrating sample magnetometer accesory (PPMS-VSM, Quantum Design Co. EverColl II). The microstructure of the samples was characterized by high-resolution transmission electron microscopy (TEM) and X-ray diffractometer (XRD). We use an Agilent N5224A vector network analyzer (VNA) to measure the microwave performance through a coplanar waveguide transmission line fixture. The film was set on the fixture, and the VNA transmits and receives the microwave signals. When the FMR happens, the microwave signals are strongly absorbed by the magnetic films, leading to an absorption peak of the scattering parameter S21 around the FMR frequency. The S11 and S21 parameters were recorded by the VNA, and the complex permeability of the films was extracted using the Landau-Liftshitz-Gilbert (L-L-G) equation: 4 ′ = 1 + 478 9 ∙ :;47 + (1 + ? 9 )@AB9 (1 + ? 9 ) − A9 C + ;47 + 9 KL
2 ∙ (?A9 )E × G@AB9 (1 + ? 9 ) − A9 C9 + H?A8 ∙ ;47 + 2I J 7 ACS Paragon Plus Environment
(4)
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9
4 ′′ = 478 9 ∙ G?A8 M;47 + × (1 + ? 9 ) + A9 NJ × :@AB9 (1 + ? 9 ) − A9 C9 + H?A8 ∙ ;47 + 2IE
KL
P
AB = 8HO ∙ ;47 + I
(5) (6)
where AB , α, and ω stand for the FMR frequency, the damping parameter, and the operation frequency, respectively. The real and imaginary parts of the complex permeability can be extracted from Eq. (4) and (5), respectively. Thus, the magnetic parameters of 8, α, AB , 47 ! ·and
