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Recovering in-plane six-fold magnetic symmetry of epitaxial Fe films by N+ implantation

Jiang Xing-Dong Guan Xing-Yin Huang Juan-Juan Fan Xiao-Long Xue De-Sheng

Recovering in-plane six-fold magnetic symmetry of epitaxial Fe films by N+ implantation

Jiang Xing-Dong, Guan Xing-Yin, Huang Juan-Juan, Fan Xiao-Long, Xue De-Sheng
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  • In order to study the effect of ion implantation on the in-plane magnetic anisotropy of epitaxial magnetic films, a 3-nm Al buffer layer is epitaxially grown on an Si (111) substrate with a miscut angle, and then 25-nm Fe is grown on the buffer layer. High-resolution X-ray diffraction reveals that the epitaxial Fe film has a (111)-oriented bcc structure. The epitaxial Fe films are implanted by 10 keV N+ ions with dose up to 5 × 1016 ions/cm2. The change and mechanism of the in-plane magnetic anisotropy of the epitaxial Fe film are studied systematically. It is found that the in-plane magnetic anisotropy of the epitaxial Fe film is gradually changed from two-fold to six-fold symmetry with the increase of N+ implantation dose. It is confirmed by transmission electron microscopy and etching experiments that ion implantation changes the surface and interface state of Fe film. This result is consistent with the result from the SRIM software simulation. The in-plane magnetic uniaxial anisotropy of epitaxial Fe film comes from atomic steps at the surface and the interface of the Fe film. These steps result from Si (111) substrate with a miscut angle. Ion implantation has effects on sputtering and atomic diffusion. The sputtering effect causes the step at the surface of the Fe film to be erased, and the diffusion of the atom leads the step at the interface of the Fe film to disappear. The in-plane uniaxial anisotropy induced by the atomic step is weakened, and the magnetocrystalline anisotropy induced by the Fe (111) plane is dominant. Therefore, the epitaxial Fe film exhibits Fe (111) plane induced six-fold magnetic symmetry after high-dose N+ implantation. This work indicates that the in-plane magnetic anisotropy of Fe films epitaxially grown on Si (111) substrate with miscut angle can be modified and precisely controlled by ion implantation. This work may be of practical significance for improving the density of in-plane magnetic recording material.
      Corresponding author: Jiang Xing-Dong, jiangxd@lzu.edu.cn ; Xue De-Sheng, xueds@lzu.edu.cn
    [1]

    杨丽 2010 博士学位论文 (哈尔滨: 哈尔滨工业大学)

    Yang L 2010 Ph. D. Dissertation (Harbin: Harbin Institute of Technology) (in Chinese)

    [2]

    Chen C H, Talnagi J W, Liu L F, Vora P, Higgins A, Liu S 2005 IEEE Trans. Magn. 41 3832

    [3]

    李哲夫, 贾彦彦, 刘仁多, 徐玉海, 王光宏, 夏晓彬, 沈卫祖 2018 物理学报 67 016104

    Li Z F, Jia Y Y, Liu R D, Xu Y H, Wang G H, Xia X B, Shen W Z 2018 Acta Phys. Sin. 67 016104

    [4]

    Maziewski A, Mazalski P, Kurant Z, Liedke M O, Mccord J, Fassbender J, Ferré J, Mougin A, Wawro A, Baczewski L T 2012 Phys. Rev. B 85 054427

    [5]

    丁斌峰, 相凤华, 王立明, 王洪涛 2012 物理学报 61 046105

    Ding B F, Xiang F H, Wang L M, Wang H T 2012 Acta Phys. Sin. 61 046105

    [6]

    Bali R, Wintz S, Meutzner F, Hübner R, Boucher R, Ünal A A, Valencia S, Neudert A, Potzger K, Bauch J 2014 Nano Lett. 14 435

    [7]

    Jaafar M, Sanz R, Mccord J, Jensen J, Schäfer R, Vázquez M, Asenjo A 2011 Phys. Rev. B 83 094422

    [8]

    McCord J, Schultz L, Fassbender J 2008 Adv. Mater. 20 2090

    [9]

    Kasiuk J, Fedotova J, Przewoźnik J, Kapusta C, Skuratov V, Svito I, Bondariev V, Kołtunowicz T 2017 Acta Phys. Pol. 132 206

    [10]

    Sakamaki M, Amemiya K, Liedke M, Fassbender J, Mazalski P, Sveklo I, Maziewski A 2012 Phys. Rev. B 86 024418

    [11]

    Shin S C, Kim S, Han J, Hong J, Kang S 2011 Appl. Phys. Express 4 116501

    [12]

    Beaujour J M, Kent A D, Ravelosona D, Tudosa I, Fullerton E E 2011 J. Appl. Phys. 109 033917

    [13]

    Mccord J, Gemming T, Schultz L, Fassbender J, Liedke M O, Frommberger M, Quandt E 2005 Appl. Phys. Lett. 86 162502

    [14]

    Woods S, Ingvarsson S, Kirtley J, Hamann H, Koch R 2002 Appl. Phys. Lett. 81 1267

    [15]

    Fassbender J, von Borany J, Mücklich A, Potzger K, Möller W, McCord J, Schultz L, Mattheis R 2006 Phys. Rev. B 73 184410

    [16]

    Jaworowicz J, Maziewski A, Mazalski P, Kisielewski M, Sveklo I, Tekielak M, Zablotskii V, Ferré J, Vernier N, Mougin A 2009 Appl. Phys. Lett. 95 022502

    [17]

    Wei Y P, Gao C X, Dong C H, Ma Z K, Li J G, Xue D S 2014 Appl. Surf. Sci. 293 71

    [18]

    Ziegler J F, Ziegler M D, Biersack J P 2010 Nucl. Instrum. Meth. Phys. Res. B 268 1818

    [19]

    Ye J, He W, Wu Q, Liu H L, Zhang X Q, Chen Z Y, Cheng Z H 2013 Sci. Rep. 3 2148

    [20]

    Liu H L, He W, Wu Q, Zhang X Q, Yang H T, Cheng Z H 2012 J. Appl. Phys. 112 093916

    [21]

    Rezende S M, Moura J, de Aguiar F, Schreiner W H 1994 Phys. Res. B 49 15105

    [22]

    Men F K, Liu F, Wang P J, Chen C H, Cheng D L, Lin J L, Himpsel F J 2002 Phys. Rev. Lett. 88 096105

    [23]

    Viernow J, Lin J L, Petrovykh D, Leibsle F, Men F, Himpsel F 1998 Appl. Phys. Lett. 72 948

    [24]

    Kirakosian A, Bennewitz R, Crain J N, Fauster T, Lin J L, Petrovykh D Y, Himpsel F J 2001 Appl. Phys. Lett. 79 1608

    [25]

    Wu Q, He W, Liu H L, Ye J, Zhang X Q, Yang H T, Chen Z Y, Cheng Z H 2013 Sci. Rep. 3 1547

    [26]

    黎振, 徐超辉, 王群, 付翔 2013 电子工业专用设备 42 4

    Li Z, Xu C H, Wang Q, Fu X 2013 Equipment for Electronic Products Manufacturing 42 4

    [27]

    Dos S M C, Geshev J, Schmidt J E, Teixeira S R, Pereira L G 2000 Phys. Res. B 61 1311

    [28]

    李华, 郭党委 2015 实验技术与管理 32 51

    Li H, Guo D W 2015 Experimental Technology and Management 32 51

  • 图 1  ω-2θ扫描得到的外延Fe膜(110)面的HRXRD图谱

    Figure 1.  The ω-2θ scan of the (110) plane.

    图 2  室温下不同剂量离子注入的外延Fe膜的归一化面内剩磁极图

    Figure 2.  Azimuthal dependence of the normalized in-plane remanence for epitaxial Fe films with different dose implantation at room temperature.

    图 3  室温下不同剂量离子注入的外延Fe膜的归一化面内剩磁曲线

    Figure 3.  Normalized in-plane remanence curves for the epitaxial Fe films with different doses of ion implantation at room temperature.

    图 4  不同剂量离子注入样品的切面高分辨TEM (a) 未注入样品; (b) 辐照剂量为5 × 1015 ions/cm2; (c) 辐照剂量为5 × 1016 ions/cm2

    Figure 4.  Cross-sectional TEM images for the as-deposited and implanted samples with a series of different N+ dose: (a) The as-deposited samples; (b) the irradiated samples dose of 5 × 1015 ions/cm2; (c) the irradiated samples dose of 5 × 1016 ions/cm2.

    图 5  室温下未注入Fe膜和刻蚀后的Fe膜的归一化面内剩磁极图

    Figure 5.  Azimuthal dependence of the normalized in-plane remanence for the as-deposited and ion beam etched samples at room temperature.

    图 6  室温下未注入Fe膜和刻蚀后的Fe膜的归一化剩磁曲线

    Figure 6.  Normalized in-plane remanence curves for the as-deposited and ion beam etched samples at room temperature.

  • [1]

    杨丽 2010 博士学位论文 (哈尔滨: 哈尔滨工业大学)

    Yang L 2010 Ph. D. Dissertation (Harbin: Harbin Institute of Technology) (in Chinese)

    [2]

    Chen C H, Talnagi J W, Liu L F, Vora P, Higgins A, Liu S 2005 IEEE Trans. Magn. 41 3832

    [3]

    李哲夫, 贾彦彦, 刘仁多, 徐玉海, 王光宏, 夏晓彬, 沈卫祖 2018 物理学报 67 016104

    Li Z F, Jia Y Y, Liu R D, Xu Y H, Wang G H, Xia X B, Shen W Z 2018 Acta Phys. Sin. 67 016104

    [4]

    Maziewski A, Mazalski P, Kurant Z, Liedke M O, Mccord J, Fassbender J, Ferré J, Mougin A, Wawro A, Baczewski L T 2012 Phys. Rev. B 85 054427

    [5]

    丁斌峰, 相凤华, 王立明, 王洪涛 2012 物理学报 61 046105

    Ding B F, Xiang F H, Wang L M, Wang H T 2012 Acta Phys. Sin. 61 046105

    [6]

    Bali R, Wintz S, Meutzner F, Hübner R, Boucher R, Ünal A A, Valencia S, Neudert A, Potzger K, Bauch J 2014 Nano Lett. 14 435

    [7]

    Jaafar M, Sanz R, Mccord J, Jensen J, Schäfer R, Vázquez M, Asenjo A 2011 Phys. Rev. B 83 094422

    [8]

    McCord J, Schultz L, Fassbender J 2008 Adv. Mater. 20 2090

    [9]

    Kasiuk J, Fedotova J, Przewoźnik J, Kapusta C, Skuratov V, Svito I, Bondariev V, Kołtunowicz T 2017 Acta Phys. Pol. 132 206

    [10]

    Sakamaki M, Amemiya K, Liedke M, Fassbender J, Mazalski P, Sveklo I, Maziewski A 2012 Phys. Rev. B 86 024418

    [11]

    Shin S C, Kim S, Han J, Hong J, Kang S 2011 Appl. Phys. Express 4 116501

    [12]

    Beaujour J M, Kent A D, Ravelosona D, Tudosa I, Fullerton E E 2011 J. Appl. Phys. 109 033917

    [13]

    Mccord J, Gemming T, Schultz L, Fassbender J, Liedke M O, Frommberger M, Quandt E 2005 Appl. Phys. Lett. 86 162502

    [14]

    Woods S, Ingvarsson S, Kirtley J, Hamann H, Koch R 2002 Appl. Phys. Lett. 81 1267

    [15]

    Fassbender J, von Borany J, Mücklich A, Potzger K, Möller W, McCord J, Schultz L, Mattheis R 2006 Phys. Rev. B 73 184410

    [16]

    Jaworowicz J, Maziewski A, Mazalski P, Kisielewski M, Sveklo I, Tekielak M, Zablotskii V, Ferré J, Vernier N, Mougin A 2009 Appl. Phys. Lett. 95 022502

    [17]

    Wei Y P, Gao C X, Dong C H, Ma Z K, Li J G, Xue D S 2014 Appl. Surf. Sci. 293 71

    [18]

    Ziegler J F, Ziegler M D, Biersack J P 2010 Nucl. Instrum. Meth. Phys. Res. B 268 1818

    [19]

    Ye J, He W, Wu Q, Liu H L, Zhang X Q, Chen Z Y, Cheng Z H 2013 Sci. Rep. 3 2148

    [20]

    Liu H L, He W, Wu Q, Zhang X Q, Yang H T, Cheng Z H 2012 J. Appl. Phys. 112 093916

    [21]

    Rezende S M, Moura J, de Aguiar F, Schreiner W H 1994 Phys. Res. B 49 15105

    [22]

    Men F K, Liu F, Wang P J, Chen C H, Cheng D L, Lin J L, Himpsel F J 2002 Phys. Rev. Lett. 88 096105

    [23]

    Viernow J, Lin J L, Petrovykh D, Leibsle F, Men F, Himpsel F 1998 Appl. Phys. Lett. 72 948

    [24]

    Kirakosian A, Bennewitz R, Crain J N, Fauster T, Lin J L, Petrovykh D Y, Himpsel F J 2001 Appl. Phys. Lett. 79 1608

    [25]

    Wu Q, He W, Liu H L, Ye J, Zhang X Q, Yang H T, Chen Z Y, Cheng Z H 2013 Sci. Rep. 3 1547

    [26]

    黎振, 徐超辉, 王群, 付翔 2013 电子工业专用设备 42 4

    Li Z, Xu C H, Wang Q, Fu X 2013 Equipment for Electronic Products Manufacturing 42 4

    [27]

    Dos S M C, Geshev J, Schmidt J E, Teixeira S R, Pereira L G 2000 Phys. Res. B 61 1311

    [28]

    李华, 郭党委 2015 实验技术与管理 32 51

    Li H, Guo D W 2015 Experimental Technology and Management 32 51

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  • Received Date:  23 January 2019
  • Accepted Date:  02 April 2019
  • Available Online:  16 August 2019
  • Published Online:  01 June 2019

Recovering in-plane six-fold magnetic symmetry of epitaxial Fe films by N+ implantation

    Corresponding author: Jiang Xing-Dong, jiangxd@lzu.edu.cn
    Corresponding author: Xue De-Sheng, xueds@lzu.edu.cn
  • 1. Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China
  • 2. Northwest Institute of Nuclear Technology, Xi’an 710024, China

Abstract: In order to study the effect of ion implantation on the in-plane magnetic anisotropy of epitaxial magnetic films, a 3-nm Al buffer layer is epitaxially grown on an Si (111) substrate with a miscut angle, and then 25-nm Fe is grown on the buffer layer. High-resolution X-ray diffraction reveals that the epitaxial Fe film has a (111)-oriented bcc structure. The epitaxial Fe films are implanted by 10 keV N+ ions with dose up to 5 × 1016 ions/cm2. The change and mechanism of the in-plane magnetic anisotropy of the epitaxial Fe film are studied systematically. It is found that the in-plane magnetic anisotropy of the epitaxial Fe film is gradually changed from two-fold to six-fold symmetry with the increase of N+ implantation dose. It is confirmed by transmission electron microscopy and etching experiments that ion implantation changes the surface and interface state of Fe film. This result is consistent with the result from the SRIM software simulation. The in-plane magnetic uniaxial anisotropy of epitaxial Fe film comes from atomic steps at the surface and the interface of the Fe film. These steps result from Si (111) substrate with a miscut angle. Ion implantation has effects on sputtering and atomic diffusion. The sputtering effect causes the step at the surface of the Fe film to be erased, and the diffusion of the atom leads the step at the interface of the Fe film to disappear. The in-plane uniaxial anisotropy induced by the atomic step is weakened, and the magnetocrystalline anisotropy induced by the Fe (111) plane is dominant. Therefore, the epitaxial Fe film exhibits Fe (111) plane induced six-fold magnetic symmetry after high-dose N+ implantation. This work indicates that the in-plane magnetic anisotropy of Fe films epitaxially grown on Si (111) substrate with miscut angle can be modified and precisely controlled by ion implantation. This work may be of practical significance for improving the density of in-plane magnetic recording material.

    • 高能粒子对材料辐照效应的研究背景主要是核能和空间技术的开发及应用. 由于磁性材料在能量和信息的传递、转换和存储等方面具有重要的应用价值, 在航空航天、粒子加速器、同步辐射装置等领域的应用也日益广泛, 为此开展高能粒子对磁性材料的辐射效应研究非常必要[1-3]. 另外, 材料的磁性与材料微结构密切相关, 离子辐照能够在材料中产生可控的缺陷、结构无序、应力和相变以及掺杂原子[4,5], 因此利用离子辐照技术调控材料磁性已经引起了研究者极大的兴趣, 而磁性薄膜材料由于广阔的应用更是得到了关注[6-8]. 磁性薄膜材料的磁各向异性对于自旋电子学应用非常重要, 利用离子辐照调控磁性薄膜材料的磁各向异性因此成为了研究的焦点[7,9-14]. 例如, 用Cr离子注入坡莫合金薄膜, 随着注入剂量的增加, 薄膜的饱和磁化强度降低、各向异性场减小[15]. 用Ga离子辐照Pt/Co/Pt薄膜, 磁各向异性随着辐照剂量的增加从面内转变到垂直又转变为面内[16]. 另外离子辐照时利用掩模等技术可以产生磁图形从而影响薄膜的各向异性[7]. 这些研究主要是通过离子辐照改变材料的结晶度、破坏界面、形成有序相、掺杂其他原子等方式改变磁微结构, 从而影响薄膜材料的磁各向异性. 然而这些研究对象主要是多晶颗粒薄膜, 并且磁各向异性的变化主要是面内方向与垂直方向间的转变. 外延单晶铁磁薄膜的辐照效应及离子辐照对其磁各向异性在面内的影响还鲜有报道.

      本文利用有错切角的单晶Si(111)基底外延生长了具有面内磁单轴各向异性的单晶Fe膜, 用N+注入外延Fe膜. 发现通过离子注入改变外延Fe膜的表面和界面状态可以精确控制面内磁各向异性从两重对称到六重对称转变. 这项工作对于提高面内磁记录密度有潜在的应用.

    2.   实 验
    • 室温下, 通过分子束外延技术在Si(111)基底上生长外延薄膜. 分子束外延生长设备是德国SPECS公司生产的AnaGrowth-STM-2型分子束外延低维功能材料制备系统, 使用的高纯度的Fe, Al材料是Alfa Aesar公司生产的. 生长前, Si基片通过三氯甲烷、丙酮、甲醇进行了超声清洗并且最后在5%的HF溶液中浸洗2 min. 将Si基片装入生长室后, 对Si基片在800 ℃进行热处理. 然后冷却Si基片至室温, 生长了3 nm的Al缓冲层, 再在缓冲层上生长了25 nm的Fe. 生长过程中通过反射高能电子衍射原位监测成核和生长[17].

      我们用10 keV的N+对外延制备的薄膜样品注入, 离子束入射方向与膜面垂直, 注入剂量分别为5 × 1012, 5 × 1013, 5 × 1014, 5 × 1015和5 × 1016 ions/cm2. 外延生长的Fe薄膜密度设定为7.866 g/cm3, Fe原子的离位能为45 eV, 通过用SRIM-2008软件模拟[18], 得到10 keV的N+对样品的射程是12 nm (歧离10 nm). 离子辐照实验在中国科学院兰州近代物理研究所320 kV高压平台进行, 温度为室温, 真空度为10–5 Pa. 用panalytical X’Pert3 MRD高分辨X射线衍射仪(HRXRD)表征了外延单晶Fe膜的晶体结构和外延取向. 用FEI 200C透射电子显微镜(TEM)分析薄膜切面的微结构和薄膜界面状态. 振动样品磁强计(VSM, microsense EV7 system)用来测量薄膜面内的等温剩磁曲线.

    3.   结果与讨论
    • 外延生长Fe膜的晶体结构和外延取向关系用HRXRD测量获得. 图1给出了当χ = 35.35°时, 未辐照的外延单晶铁膜用非对称的ω-2θ模式扫描得到HRXRD图谱. 可以发现只有Fe(110)和(220)的两个衍射峰出现, 说明外延得到的Fe膜是(111)取向的单晶结构. (110)衍射峰对应的2θ角度为44.709°, 对应的晶格常数为2.8645 Å, 与PDF卡片06-0696体心立方结构Fe的晶格常数2.8664非常接近, 说明外延Fe膜是体心立方结构.

      Figure 1.  The ω-2θ scan of the (110) plane.

      室温下我们对注入前后的样品用VSM在薄膜面内每隔5°进行等温剩磁测量. 从归一化的极图(图2)可以看出, 未注入的外延Fe膜(图2(a))主要表现为面内磁单轴各向异性(面内2重磁对称). 外延Fe膜的面内磁各向异性主要来自于台阶诱导的界面各向异性、立方磁晶各向异性和应力各向异性[19]. 据报道[20], 外延Fe膜的厚度大于临界厚度后, 应力就逐渐被释放. 本文样品厚度远大于临界厚度, 因此应力对磁各向异性的影响很小, 可以忽略[19]. 对于在Si(111)基底上外延生长的bcc结构的Fe膜, 当磁化严格限制在Fe(111)面上时, 即只有Fe(111)面诱导形成的磁晶各向异性时, 薄膜面内表现为6重磁对称[19,21]. 当Si(111)晶面有错切角时, 在Si(111)晶面上会形成原子台阶[22,23], 于是外延生长的Fe膜界面和表面也会出现这些台阶, 这些台阶会诱导形成平行于台阶的单轴磁各向异性[19,21,24], 文献[25]报道, 即使当错切角仅为0.1°时, 外延Fe膜在面内也表现为单轴磁各向异性. 通常由于硅晶圆加工工艺问题, 实验所用的单晶Si片有可能存在错切角[26,27]. 我们外延生长使用的单晶硅片误差为0.3°, 因此可以推断未辐照样品的面内磁单轴各向异性(两重磁对称)是由于Si(111)基底的错切角诱导所产生的. 另外, 从图2(a)还可以看到在60°和240°附近也显示了较弱的各向异性, 这是表面和界面处原子台阶诱导形成的面内单轴磁各向异性和bcc结构的Fe膜的磁晶各向异性相互作用的结果[25]. 如图2(b)所示, 用剂量为5 × 1012 ions/cm2 N+注入后, 样品在面内的磁各向异性并没有发生明显的改变. 随着注入剂量增加, 可发现磁各向异性在逐渐发生改变, 当注入剂量为5 × 1015 ions/cm2时(图2(e)), 明显观察到4重对称. 当注入剂量为5 × 1016 ions/cm2时(图2(f)), 表现为完美的六重对称. 从图3所示剩磁曲线可以更清晰地看出, 当注入剂量为5 × 1014 ions/cm2时, 对比未注入样品, 剩磁曲线表现为两个展宽了的峰. 注入剂量继续增加到5 × 1015 ions/cm2时, 发现剩磁曲线表现为独立的4个峰. 最大剂量(5 × 1016 ions/cm2)注入后, 剩磁曲线表现为6个峰(面内6重对称), 即在外延Fe膜面内同时出现了3个难易轴. 总体来看, 随着N+注入剂量的增加, Fe膜面内磁各向异性经历了从2重对称到4重对称再到6重对称的转变, 即外延Fe膜的面内磁各向异性随着离子注入剂量的增加, 从由台阶诱导形成的2重磁对称占主导转变为由磁晶各向异性诱导形成的6重磁对称占主导.

      Figure 2.  Azimuthal dependence of the normalized in-plane remanence for epitaxial Fe films with different dose implantation at room temperature.

      Figure 3.  Normalized in-plane remanence curves for the epitaxial Fe films with different doses of ion implantation at room temperature.

      为了理解外延Fe膜面内磁各向异性的转变机理, 需要讨论离子注入对外延Fe膜的辐照效应. 首先, 高能离子在晶体内的位移可能导致Fe膜和缓冲层相互扩散, 从而可能导致Fe膜与缓冲层的界面处的原子台阶消失, 界面处台阶诱导的面内单轴各向异性减弱. 为了证实离子注入对外延Fe膜界面的影响, 对未注入样品和高剂量注入后样品的切面进行高分辨TEM观察(图4). 可以发现未注入的样品(图4(a)) Si与Al层, Al与Fe层界面清晰, Fe膜厚度约为25 nm, Al缓冲层厚度约为3 nm. 当注入剂量为5 × 1015 ions/cm2时(图4(b)), 可以看到Fe向Al缓冲层明显扩散, 而且有向Si层扩散的趋势. 最大剂量(5 × 1016 ions/cm2)注入后(图4(c)), Al缓冲层和Fe层之间的界面已经完全不能区分, 扩散层厚度增加到7.3 nm, Fe膜厚度明显减少, 剩余约20 nm. 高分辨TEM照片证实了Fe膜与缓冲层间的界面由于离子注入而逐渐消失, 说明Fe膜在界面外的原子台阶因为离子注入被消除, 界面台阶诱导形成的单轴磁各向异性消失.

      Figure 4.  Cross-sectional TEM images for the as-deposited and implanted samples with a series of different N+ dose: (a) The as-deposited samples; (b) the irradiated samples dose of 5 × 1015 ions/cm2; (c) the irradiated samples dose of 5 × 1016 ions/cm2.

      另外, 由于外延薄膜表面处原子台阶也对面内单轴各向异性有贡献, 所以需要探讨离子注入对外延薄膜表面的影响. 离子注入过程中离子束对铁磁薄膜表面有溅射作用. 通过用SRIM-2008 full-cascade模式模拟, 取Fe的表面结合能为4.34 eV, 可以得到平均溅射效率为1.53 atom/ion, 则在最大剂量时(5 × 1016 ions/cm2), 溅射厚度为8.5 nm. 除了模拟计算, 从图4所示的TEM照片也可以明显看出, Fe膜厚度明显减少, 在最大剂量(5 × 1016 ions/cm2)注入后, 剩余约20 nm (与SRIM模拟的规律符合). 这个结果说明在离子辐照后, 外延Fe膜表面的原子台阶消失, 由其诱导形成的磁单轴各向异性消失. 为了进一步说明离子注入的溅射作用对外延Fe膜面内磁各向异性的影响, 对未注入外延Fe膜进行了离子刻蚀实验. 刻蚀实验采用北京埃德万斯LKJ-1D-150离子束刻蚀系统用400 eV的Ar+离子入射角度呈50°刻蚀7 s, 理论上溅射厚度为2 nm[28]. 通过用SRIM软件计算, 400 eV Ar+的射程是0.7 nm (歧离0.7 nm), 刻蚀参数的设置保证了刻蚀后Fe膜表面能够被破坏, 而Al缓冲层和Fe层之间的界面不受影响. 从图5所示的剩磁极图可以看出, 刻蚀后的样品并没有出现6重磁对称. 对比未注入样品, 剩磁曲线(图6)表现为两个展宽了的峰, 这与注入剂量为5 ×1014 ions/cm2时剩磁曲线的趋势相似(峰展宽, 磁对称性向4重对称发展). 这个结果验证了离子注入过程中, 外延Fe膜表面被溅射, 表面处的原子台阶消失, 其诱导形成的单轴各向异性也被擦除.

      Figure 5.  Azimuthal dependence of the normalized in-plane remanence for the as-deposited and ion beam etched samples at room temperature.

      Figure 6.  Normalized in-plane remanence curves for the as-deposited and ion beam etched samples at room temperature.

      综上, 离子注入外延Fe膜时, 注入的扩散作用使Fe膜在界面外的原子台阶消失, 表面处的原子台阶因为注入的溅射作用也被擦除, 从而表面和界面处台阶诱导形成的单轴磁各向异性消失, 使得外延铁膜面内的磁各向异性表现为Fe(111)面诱导形成的磁晶各向异性, 即薄膜面内表现为6重磁对称.

    4.   结 论
    • 本文利用10 keV的N+注入Si (111)基底上外延生长的单晶Fe膜样品, 注入剂量最大至5 × 1016 ions/cm2. 系统研究了外延Fe膜的面内磁各向异性随注入剂量的变化规律和机理. 发现在Si (111)基底上外延生长制备的Fe膜面内磁各向异性以两重对称为主, 而随着离子注入剂量的增加, Fe膜面内磁各向异性从2对称到6重对称的转变. 由于外延Fe膜面内磁单轴各向异性来自于因为Si(111)面错切从而在外延铁膜表面和界面处形成的原子台阶, 而离子注入有溅射效应和原子扩散作用, 溅射效应使Fe膜表面处原子台阶被擦除, 原子的扩散导致Fe膜界面处原子台阶消失. 因此原子台阶诱导的面内单轴各向异性减弱, 使Fe(111)面诱导形成的磁晶各向异性占主导, 从而在大剂量离子辐照后外延Fe膜面内表现为磁6重对称. 本文研究表明, 离子注入有错切角的Si(111)基底上外延生长的Fe膜, 可修正和精确调控由于基底错切造成的面内磁各向异性. 这项工作有助于提高面内磁各向异性的自由度, 可能对提高面内磁记录材料的密度有实际应用价值.

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