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Acta Metall Sin  2018, Vol. 54 Issue (9): 1281-1288    DOI: 10.11900/0412.1961.2017.00492
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Influence of Oblique Sputtering on Stripe Magnetic Domain Structure and Magnetic Anisotropy of CoFeB Thin Films
Xiaoqin MA1,2, Qingfeng ZHAN2,3(), Jincai LI2, Qingfang LIU1, Baomin WANG2, Runwei LI2
1 Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, School of Physical Sciences and Technology, Lanzhou University, Lanzhou 730000, China
2 Key Laboratory of Magnetic Materials and Devices, Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China
3 State Key Laboratory of Precision Spectroscopy, School of Physics and Materials Science, East China Normal University, Shanghai 200241, China
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Abstract  

Magnetic anisotropy is one of the most important fundamental properties of magnetic thin film. The strength of magnetic anisotropy determines the ferromagnetic resonance frequency of magnetic films in the high-frequency applications. Because of the directionality of conventional static magnetic anisotropy in magnetic film, the high-frequency device usually shows an obvious directionality. When the microwave magnetic ?eld deviates from the perpendicular direction of magnetic anisotropy, the devices cannot reveal their best performance. The magnetic ?lm with a stripe magnetic domain structure displays an in-plane rotatable magnetic anisotropy, which can be an important strategy to solve the problem of magnetic ?eld orientation dependent performance in high-frequency device. Therefore, the magnetic domain, the magnetic anisotropy, and the high-frequency behaviors for magnetic ?lms with a stripe magnetic domain structure have received extensive attention. Previously, most of the studies focused on the stripe magnetic domain structure of polycrystalline thin films. However, less attention was paid on amorphous magnetic thin films. Since the amorphous magnetic films have no long-range ordered crystal structure, no magnetocrystalline anisotropy, no grain boundary defects resistance hindering the domain wall displacement, they usually show excellent soft magnetic properties and have been widely applied in high-frequency devices. CoFeB alloy is one of the most important amorphous magnetic materials and has been extensively applied in various spintronic devices. In this work, amorphous CoFeB magnetic thin films were prepared by using a method of oblique sputtering technique at room temperature. The influences of oblique sputtering on the stripe magnetic domain structure, the in-plane static magnetic anisotropy, the in-plane rotational magnetic anisotropy, and the perpendicular magnetic anisotropy of the amorphous CoFeB films were studied by scanning probe microscope, vibrating sample magnetometer, ferromagnetic resonance. It is found that the method of oblique sputtering could effectively reduce the critical thickness for the appearance of stripe magnetic domain in amorphous CoFeB films. For a non-oblique sputtered CoFeB film, the critical thickness for the appearance of the stripe magnetic domain is above 240 nm. In contrast, after been subjected to the oblique sputtering, the critical thickness becomes below 240 nm. The different magnetic characterizations indicate that for the growth of CoFeB films with stripe magnetic domain structure, the oblique sputtering could not only enhance the strength of in-plane static magnetic anisotropy, but also improve the in-plane rotational magnetic anisotropy and the perpendicular magnetic anisotropy. All of the magnetic anisotropies are increased with the angle of oblique sputtering. The observation results of XRD and TEM prove that the prepared CoFeB thin films tend to amorphous structure. The characterization of SEM observation indicates that although the amorphous CoFeB films do not possess long-range ordered crystalline structure, they still could form a kind of columnar structure. The slanted columnar structure of CoFeB films could significantly increase the perpendicular magnetic anisotropy, thus lead to the appearance of stripe magnetic domain structure.

Key words:  CoFeB thin film      stripe magnetic domain      ferromagnetic resonance      oblique sputtering      rotational magnetic anisotropy     
Received:  27 November 2017     
ZTFLH:  O469  
Fund: Supported by National Natural Science Foundation of China (Nos.11674336, 51522105, 51525103 and 11627801), National Key Research and Development Program of China (No.2016YFA0201102) and Ningbo Science and Technology Innovation Team (No.2015B11001)

Cite this article: 

Xiaoqin MA, Qingfeng ZHAN, Jincai LI, Qingfang LIU, Baomin WANG, Runwei LI. Influence of Oblique Sputtering on Stripe Magnetic Domain Structure and Magnetic Anisotropy of CoFeB Thin Films. Acta Metall Sin, 2018, 54(9): 1281-1288.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00492     OR     https://www.ams.org.cn/EN/Y2018/V54/I9/1281

Fig.1  Schematic of fabricating amorphous CoFeB magnetic films at oblique incidence (β is the oblique sputtering angle; HA and EA represent the hard and easy axes caused by oblique sputtering, respectively)
Fig.2  XRD spectra for CoFeB thin films fabricated at different oblique angles (β)
Fig.3  Cross-section TEM image (a) and SAED pattern (b) of CoFeB thin films grown at β=0°
Fig.4  Normalized hysteresis loops of CoFeB films grown at β=0° (a), β=30° (b), β=45°(c), β=60° (d), and the corresponding magnetic domains in a scanning area of 10 μm×10 μm (insets) (M/Ms—normalized magnetization, H—magnetic field intensity)
Fig.5  Angular dependence of the remanence ratio for CoFeB films grown at different oblique angles (Mr/Ms—remanence ratio, θ—angle between the magnetic field and the in-plane static magnetic anisotropy)
Fig.6  Schematic of ferromagnetic resonance measurement in different orientations of magnetic field (Hp—microwave magnetic field)
Fig.7  Angular dependence of the ferromagnetic resonance fields for CoFeB films grown at β=0o (a), β=30o (b), β=45o (c) and β=60o (d) (Hr—ferromagnetic resonance field intensity)
Fig.8  Angular dependence of the ferromagnetic resonance field intensity for CoFeB films grown at different oblique angles (M—magnetization, θH—angle between the magnetic field and the normal direction of film surface, θM—angle between the magnetization and the normal direction of the film surface) (a) and cross-section SEM image of the CoFeB film prepared at β=60o (b)
Fig.9  Stripe magnetic domains of CoFeB thin films with different thicknesses of 180 nm (a), 200 nm (b), 240 nm (c) and 300 nm (d) grown at β=45o
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