交换耦合两相纳米复合永磁薄膜的理论研究
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摘要
交换耦合两相纳米复合磁体结合了软磁相的高剩磁和硬磁相的高矫顽力的优点而成为制备高性能永磁材料的热门话题。本论文运用微磁学方法,研究了交换耦合两相纳米复合磁体的成核场、钉扎场、矫顽力机制和磁滞回线,主要研究结果如下:
1.系统地研究了交换耦合硬磁/软磁/硬磁性三层膜的退磁过程,其中特别地关注了硬磁相厚度较小时的情况。得到了成核场的解析公式,并数值计算出磁性三层膜的磁滞回线以及成核与钉扎之间的角度分布。分析发现,成核场、钉扎场以及它们之间的外场跨度都随着硬磁相厚度的增加而增加。当薄膜厚度很小时,磁滞回线的形状接近方形且主导矫顽力机制为成核,只有在这种情况下,硬磁相厚度才会对磁滞回线造成非常大的影响。讨论了理论与实验中都能实现最大磁能积的薄膜厚度区域。在大多数实验条件下,硬磁相厚度可以当作足够大的,以致于硬磁相表面磁化矢量的偏转遵循简单的一致转动模型。此时只需考虑软磁相厚度的影响,从而使计算得到很大程度的简化。
2.以界面交换耦合常数和软磁相厚度为主要参变量,研究了易轴与膜面平行情况下的Nd2Fe14B/α-Fe磁性多层膜的磁矩取向随外场变化及磁滞回线。当软磁相厚度较小时,钉扎场等于成核场,随着界面交换耦合常数的减小,矫顽力机制由成核变为钉扎;当软磁相厚度较大时,矫顽力机制随界面交换耦合常数的改变情况恰好相反。钉扎场与成核场发生分离的临界厚度随着界面交换耦合常数的减小而减小。当界面交换耦合常数很小时,刚性磁体只有在软磁相厚度非常小时才会出现。退耦合作用导致软硬磁相交界面出现角度突变,使得复合多层膜由单相行为向两相行为转变,同时使得成核场减小,并且当软磁相厚度较大时导致钉扎场增加。
3.以Nd_2Fe_(14)B/α-Fe、FePt/α-Fe和SmCo_5/Co磁性三层膜为例,全面探究了界面交换耦合强度、软磁相和硬磁相厚度对成核场的影响。分析发现,当软磁相厚度约为软磁相布洛赫壁宽度的十分之一且硬磁相厚度大于硬磁相的布洛赫壁宽度时,界面交换耦合常数对成核场的影响会非常显著。计算了不同界面交换耦合常数下Nd_2Fe_(14)B/α-Fe磁性三层膜的磁滞回线。分析发现,随着软磁相厚度的增加,矫顽力逐渐减小且矫顽力机制从钉扎变为成核。当界面交换耦合常数为Jibulk (软、硬磁相对应的交换耦合常数中间值)的十分之一时, Sm_(40)Fe_(60)(63 nm)/Ni_(80)Fe_(20)(78 nm)双层膜的理论磁滞回线与实验数据复合得很好,由此说明,在实验中软、硬磁相之间的耦合作用很弱。
Exchange-coupled two-phased nanocomposite magnets, with a hard phase to provide high coercivity and a soft phase to provide high saturation, are widely regarded as the excellent candidates for permanent magnetic materials. In this thesis, the nucleation field, the pinning field, the coercivity mechanism and the hysteresis loops of the exchange-coupled two-phased nanocomposites have been investigated within a micromagnetic method.
1.The demagnetization process of a hard/soft/hard sandwich has been investigated systematically, with particular attention on the cases with small hard layer thickness. The hysteresis loops, as well as the angular distributions of the magnetization between nucleation and pinning have been obtained numerically, with the formula for the nucleation field derived. It is found that both nucleation and pinning fields, as well as the gap in between decrease as the hard layer thickness reduces. The hard layer thickness has great effect on the hysteresis loops only when the thickness is very small, where the hysteresis loop is nearly square and the dominant coercivity mechanism is the nucleation. The thickness regions at which the theoretical and practical giant energy products can be achieved have been discussed. In most cases of experiments, the hard layer can be taken as sufficiently thick so that the magnetization at its surface obeys a simple coherent rotation model. In these cases, the calculation can be simplified significantly, with only the influence of the soft layer thickness accounted.
2.Using the interface coupling constant and the soft layer thickness as the main variables, the changes of the magnetic moments with the applied field and the hysteresis loops of Nd2Fe14B/α-Fe trilayers, have been investigated. When the soft layer thickness is smaller, the pinning field equals to the nucleation field, where the coercivity mechanism transforms from nucleation to pinning as the interface coupling constant Ji decreases, whereas for large soft layer thickness this trend is reversed. The critical thickness, at which the nucleation field and pinning field detaches, decreases as Ji decreases. When the reduced exchange coupling is considered, the“Rigid Composite magnet”appears only when the soft layer thickness is very small. The reduced exchange coupling leads to a gap of angle of magnetization at the interface, which results in the changes of the behavior of the trilayers from the single-phase one to the two-phase one and that the nucleation field decreases while the nucleation field increases when the soft layer thickness is larger.
3.Taking the Nd2Fe14B/α-Fe, FePt/α-Fe and SmCo5/Co magnetic trilayers for example, the influence of the strength of interface exchange coupling as well as the soft and hard layer thickness on nucleation field has been studied systematically. Analyses show that the interface coupling constant Ji has significant effect on the nucleation field only when the soft layer thickness is about 10% of the domain wall width of the soft phase while the thickness of the hard layer is larger than its domain wall width. The hysteresis loops of the Nd_2Fe_(14)B/α-Fe/ Nd_2Fe_(14)B trilayers for various Ji have been calculated. It is found that, as the soft layer thickness increases, the coercivity decreases and the coercivity mechanism changes from pinning to nucleation. The theoretical and experimental loops of Sm40Fe60/Ni_(80)Fe_(20) bilayer agree quite well when the value of Ji is taken as10% of Jibulk(the intermediate value between the two interface coupling constants corresponding to the soft and hard layer respectively), indicating that the interface coupling in the experiment is poor.
1.系统地研究了交换耦合硬磁/软磁/硬磁性三层膜的退磁过程,其中特别地关注了硬磁相厚度较小时的情况。得到了成核场的解析公式,并数值计算出磁性三层膜的磁滞回线以及成核与钉扎之间的角度分布。分析发现,成核场、钉扎场以及它们之间的外场跨度都随着硬磁相厚度的增加而增加。当薄膜厚度很小时,磁滞回线的形状接近方形且主导矫顽力机制为成核,只有在这种情况下,硬磁相厚度才会对磁滞回线造成非常大的影响。讨论了理论与实验中都能实现最大磁能积的薄膜厚度区域。在大多数实验条件下,硬磁相厚度可以当作足够大的,以致于硬磁相表面磁化矢量的偏转遵循简单的一致转动模型。此时只需考虑软磁相厚度的影响,从而使计算得到很大程度的简化。
2.以界面交换耦合常数和软磁相厚度为主要参变量,研究了易轴与膜面平行情况下的Nd2Fe14B/α-Fe磁性多层膜的磁矩取向随外场变化及磁滞回线。当软磁相厚度较小时,钉扎场等于成核场,随着界面交换耦合常数的减小,矫顽力机制由成核变为钉扎;当软磁相厚度较大时,矫顽力机制随界面交换耦合常数的改变情况恰好相反。钉扎场与成核场发生分离的临界厚度随着界面交换耦合常数的减小而减小。当界面交换耦合常数很小时,刚性磁体只有在软磁相厚度非常小时才会出现。退耦合作用导致软硬磁相交界面出现角度突变,使得复合多层膜由单相行为向两相行为转变,同时使得成核场减小,并且当软磁相厚度较大时导致钉扎场增加。
3.以Nd_2Fe_(14)B/α-Fe、FePt/α-Fe和SmCo_5/Co磁性三层膜为例,全面探究了界面交换耦合强度、软磁相和硬磁相厚度对成核场的影响。分析发现,当软磁相厚度约为软磁相布洛赫壁宽度的十分之一且硬磁相厚度大于硬磁相的布洛赫壁宽度时,界面交换耦合常数对成核场的影响会非常显著。计算了不同界面交换耦合常数下Nd_2Fe_(14)B/α-Fe磁性三层膜的磁滞回线。分析发现,随着软磁相厚度的增加,矫顽力逐渐减小且矫顽力机制从钉扎变为成核。当界面交换耦合常数为Jibulk (软、硬磁相对应的交换耦合常数中间值)的十分之一时, Sm_(40)Fe_(60)(63 nm)/Ni_(80)Fe_(20)(78 nm)双层膜的理论磁滞回线与实验数据复合得很好,由此说明,在实验中软、硬磁相之间的耦合作用很弱。
Exchange-coupled two-phased nanocomposite magnets, with a hard phase to provide high coercivity and a soft phase to provide high saturation, are widely regarded as the excellent candidates for permanent magnetic materials. In this thesis, the nucleation field, the pinning field, the coercivity mechanism and the hysteresis loops of the exchange-coupled two-phased nanocomposites have been investigated within a micromagnetic method.
1.The demagnetization process of a hard/soft/hard sandwich has been investigated systematically, with particular attention on the cases with small hard layer thickness. The hysteresis loops, as well as the angular distributions of the magnetization between nucleation and pinning have been obtained numerically, with the formula for the nucleation field derived. It is found that both nucleation and pinning fields, as well as the gap in between decrease as the hard layer thickness reduces. The hard layer thickness has great effect on the hysteresis loops only when the thickness is very small, where the hysteresis loop is nearly square and the dominant coercivity mechanism is the nucleation. The thickness regions at which the theoretical and practical giant energy products can be achieved have been discussed. In most cases of experiments, the hard layer can be taken as sufficiently thick so that the magnetization at its surface obeys a simple coherent rotation model. In these cases, the calculation can be simplified significantly, with only the influence of the soft layer thickness accounted.
2.Using the interface coupling constant and the soft layer thickness as the main variables, the changes of the magnetic moments with the applied field and the hysteresis loops of Nd2Fe14B/α-Fe trilayers, have been investigated. When the soft layer thickness is smaller, the pinning field equals to the nucleation field, where the coercivity mechanism transforms from nucleation to pinning as the interface coupling constant Ji decreases, whereas for large soft layer thickness this trend is reversed. The critical thickness, at which the nucleation field and pinning field detaches, decreases as Ji decreases. When the reduced exchange coupling is considered, the“Rigid Composite magnet”appears only when the soft layer thickness is very small. The reduced exchange coupling leads to a gap of angle of magnetization at the interface, which results in the changes of the behavior of the trilayers from the single-phase one to the two-phase one and that the nucleation field decreases while the nucleation field increases when the soft layer thickness is larger.
3.Taking the Nd2Fe14B/α-Fe, FePt/α-Fe and SmCo5/Co magnetic trilayers for example, the influence of the strength of interface exchange coupling as well as the soft and hard layer thickness on nucleation field has been studied systematically. Analyses show that the interface coupling constant Ji has significant effect on the nucleation field only when the soft layer thickness is about 10% of the domain wall width of the soft phase while the thickness of the hard layer is larger than its domain wall width. The hysteresis loops of the Nd_2Fe_(14)B/α-Fe/ Nd_2Fe_(14)B trilayers for various Ji have been calculated. It is found that, as the soft layer thickness increases, the coercivity decreases and the coercivity mechanism changes from pinning to nucleation. The theoretical and experimental loops of Sm40Fe60/Ni_(80)Fe_(20) bilayer agree quite well when the value of Ji is taken as10% of Jibulk(the intermediate value between the two interface coupling constants corresponding to the soft and hard layer respectively), indicating that the interface coupling in the experiment is poor.
引文
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[2] A. Aharoni. Introduce to the theory of ferromagnetism. Oxford:Oxford University Press, 1996, 183-214.
[3]严密,彭晓领.磁学基础与磁性材料.杭州:浙江大学出版社,2006.
[4]田民波.磁性材料.北京:清华大学出版社,2001.
[5] U. Enz. Ferromagnetic materials. Amsterdand:Elsevier Science Public, 1988, 1-136.
[6] H. K. Kronmüller. Supermagnets, Hard Magnetic Materials. Kluwer Academic Publishers, 1991.
[7] W. F. Brown, Jr.. Virtues and weaknesses of the domain concept. Rev. Mod. Phys., 1945, 17:15-19.
[8] E. C. Stoner and E. P. Wohlfarth. A mechanism of magntic hysteresis in heterogeneous alloys. Phil. Trans. Roy. Soc., 1948, A240 : 599-642.
[9] K. H. J. Buschow. Ferromagnetic materials. Amsterdand: Elsevier Science Publications, 1988, 1-120.
[10]J. M. D. Coey. Rare-Earth Iron Permanent Magnets. Oxford:Oxford Science Public., 1996, 1-56.
[11] E. F. Kneller, R. Hawig. Exchange-spring magnet : a new materical principle for permanent magnets. IEEE Trans. Magn., 1991, 27 : 3588-3560.
[12] E. Goto, N. Hayashi, T. Miyashita, and K. Nakagawa. J. Appl. Phys., 1965, 36 : 2951.
[13] R. Skomski, J. M. D. Coey. Giant energy product in nanostractured two-phase magnets. Phys. Rev. B., 1993, 48 : 15812-15816.
[14] H. Kronmüller, R. Fischer, R. Hertel et al. Micromagnetism and the microstructure in nanocrystalline materials. J. Magn. Magn. Mater., 1997, 175 : 177-192.
[15] T. Leineweber, H. Kronmüller. Micromagnetic examination of exchange coupled ferromagnetic nanolayers. J. Magn. Magn. Mater., 1997, 176 : 145-154.
[16] T. Leineweber, H. Kronmüller. Magnetisation Reversal Modes in Inhomogeneous Magnets. Phys. Stat. Sol. 1997, 201: 291-301.
[17] E. E. Fullerton, J. S. Jiang, M. Grimsditch, C. H. Sowers, S. D. Bader. Exchange-spring behavior in epitaxial hard/soft magnetic bilayers. Phys. Rev. B, 58, 1998 : 12193-12200.
[18] S. S. Yan, M. Elkawni, D. S. Li et al. Soft/hard exchange-coupled layered structures with Modulated exchange coupling. J. Appl. Phys., 2003, 94 : 4535-4538.
[19] G. P. Zhao. X. L. Wang. Nucleation, pinning, and coercivity in magnetic nanosystem: An analytical micromagnetic approach. Phys. Rev.B, 2006, 74: 012409-1-4.
[20] G. P. Zhao, X. L. Wang, Chun Yang, L. H. Xie and G. Zhou. Self-pinning: Dominant coercivity mechanism in exchange-coupled permanent/composite magnets. J. Appl. Phys., 2007, 101: 09k1021-1-3.
[21]鲜承伟,赵国平,张庆香,徐劲松.垂直取向Nd2Fe14B/α-Fe磁性三层膜的磁化反转.物理学报,2009,58(5):3509-3514.
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