Fe/Ni多层膜的结构与磁性研究
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摘要
随着信息存储需求的不断提高,磁存储技术作为最主要的存储方式,其研究与发展受到学术界与工业界的广泛关注。磁记录密度已从1957年的2 Kbit/in2提高到了如今的329 Gbit/in2。当硬盘记录颗粒尺寸降低至纳米量级时,为了防止超顺磁效应(即保证记录的热稳定性),硬盘介质的矫顽力很大。一般而言,磁头的写入场要达到记录介质矫顽力的两倍,才能有效写入。因此,记录密度的继续提高,很大程度上受限于磁头的写入能力。
写磁头材料应具有良好的软磁性能,包括高磁化饱和强度、低矫顽力、高磁导率等。通常,铁基材料的饱和磁化强度很高；然而,它的磁晶各向异性能也很高,按照传统方法难以降低其矫顽力,因此无法作为理想的磁头材料投入应用。但是,纳米结构介质的发展开创了磁头软磁材料研究的新时期。通过减小晶粒尺寸,可使有效各向异性常数相比局部的磁晶各向异性常数K1小几个数量级,有利于提高磁导率。多层膜溅射是一种方便有效的制备纳米结构软磁材料的方法,IBM和Hitachi都曾采用Fe基多层膜来制备高效能的磁头。依照不同类型的中间层,铁基软磁多层膜大体可分为两类：磁性中间层与非磁性中间层。后者的性能往往更好,磁性中间层能贡献磁矩,提高材料的饱和磁化强度。
在本文中,我们采用Fe/Ni多层膜结构来制备软磁纳米结构材料介质。Fe,Ni单层厚度远小于畴壁厚度时,由于层间耦合作用,磁矩的翻转不再受局部高磁晶各向异性的控制,而是被平均化后大为降低的各向异性所影响,因此材料的矫顽力可显著降低。在论文的第一部分,我们分析了Fe/Ni的结构,以及在此结构下的多层膜的磁性,主要讨论了Fe(或Ni)层厚度变化对矫顽力的影响及其原因,还通过掺Cu层进一步验证了层间耦合作用在Fe/Ni多层膜体系中的重要性。在论文的第二部分,鉴于以往的研究对软磁多层膜的粗糙度关注有限,而随着极薄薄膜的发展,界面情况对多层膜的结构与磁性的影响愈发重要,我们研究了粗糙度对Fe/Ni多层膜的结构与磁性的影响。我们使用两种方法调控粗糙度,一是改变样品生长的温度,二是在MgO基板预先溅射Ag作为底层。我们探讨了Fe/Ni多层膜的磁化过程的影响因素,最终解释了界面粗糙度对矫顽力的影响的原因,提出改善Fe/Ni多层膜软磁性能的方法。
As the demand for information storage continuously increases, magnetic recording has been widely studied and developed as the chief data-storage technique, gaining intense interest both from academe and industry. The areal recording density has increased from2 Kbit/in2 in 1957 to 329 Gbit/in2 recently. When the grain size of recording media is reduced to several nanometers, the magneto-crystalline anisotropy needs to be large enough to avoid superparamagnetic effect, in other words, to stabilize the recorded bit. However, high coercivity due to large anisotropy gives rise to the difficulty of magnetic writing, for the writing field has to be approximately twice the coercivity of the recording media for effective writing. Therefore, further increase of magnetic recording density is largely confined by the writing ability of the magnetic head.
The material for magnetic writing head needs to be reasonably soft, its permeability must be sufficiently high and its coercivity sufficiently small so that the field intensity within the head pole and yoke becomes negligible. In early research, Fe-based material has high saturation magnetization, but also a high magneto-crystalline anisotropy, its coercivity then can hardly be lowered by traditional methods. However, the development of nanocrystallite materials ushered a new era for the study of soft magnetic materials. By reducing the grain size, the effective anisotropy can be decreased to extremely low value, usually several orders less than the local anisotropy. Multilayers preparation by sputtering is an effective way to fabricate nanocrystalline soft materials; IBM and Hitachi both have adopted Fe-based multilayers in the production of magnetic heads. In general, two kinds of multilayers are classified based on different spacer materials in between:one is nonmagnetic and the other ferromagnetic. The latter kind is more favorable, for ferromagnetic spacers can help to increase the saturation magnetization of multilayers.
In our research, we prepare Fe/Ni multilayers to study their structural and magnetic properties. As the Fe (or Ni) layer thickness is far less than the domain wall width, due to exchange coupling, the magnetization of Fe and Ni layers cannot follow each easy axis, and then the anisotropies are averaged out, the coercivity of multilayers is largely decreased. In the first part of this paper, the structure and orientation of Fe/Ni multilayers is analyzed, and the influence of layer thickness on the coercivity is discussed in detail. Cu insertion confirms the dominant role of exchange coupling in the multilayer system. In the second part, the effects of interface roughness on the structural and magnetic properties of Fe/Ni multilayers are investigated. Interface roughness is introduced in two ways, one is to tune the growth temperature, and the other is to prefabricate Ag on MgO substrate as underlayer. The mechanism of Fe/Ni multilayers'magnetization is discussed, smoother interfaces are more advantageous to optimize the exchange coupling, and the coercivity can be decreased more effectively.
写磁头材料应具有良好的软磁性能,包括高磁化饱和强度、低矫顽力、高磁导率等。通常,铁基材料的饱和磁化强度很高；然而,它的磁晶各向异性能也很高,按照传统方法难以降低其矫顽力,因此无法作为理想的磁头材料投入应用。但是,纳米结构介质的发展开创了磁头软磁材料研究的新时期。通过减小晶粒尺寸,可使有效各向异性常数
在本文中,我们采用Fe/Ni多层膜结构来制备软磁纳米结构材料介质。Fe,Ni单层厚度远小于畴壁厚度时,由于层间耦合作用,磁矩的翻转不再受局部高磁晶各向异性的控制,而是被平均化后大为降低的各向异性所影响,因此材料的矫顽力可显著降低。在论文的第一部分,我们分析了Fe/Ni的结构,以及在此结构下的多层膜的磁性,主要讨论了Fe(或Ni)层厚度变化对矫顽力的影响及其原因,还通过掺Cu层进一步验证了层间耦合作用在Fe/Ni多层膜体系中的重要性。在论文的第二部分,鉴于以往的研究对软磁多层膜的粗糙度关注有限,而随着极薄薄膜的发展,界面情况对多层膜的结构与磁性的影响愈发重要,我们研究了粗糙度对Fe/Ni多层膜的结构与磁性的影响。我们使用两种方法调控粗糙度,一是改变样品生长的温度,二是在MgO基板预先溅射Ag作为底层。我们探讨了Fe/Ni多层膜的磁化过程的影响因素,最终解释了界面粗糙度对矫顽力的影响的原因,提出改善Fe/Ni多层膜软磁性能的方法。
As the demand for information storage continuously increases, magnetic recording has been widely studied and developed as the chief data-storage technique, gaining intense interest both from academe and industry. The areal recording density has increased from2 Kbit/in2 in 1957 to 329 Gbit/in2 recently. When the grain size of recording media is reduced to several nanometers, the magneto-crystalline anisotropy needs to be large enough to avoid superparamagnetic effect, in other words, to stabilize the recorded bit. However, high coercivity due to large anisotropy gives rise to the difficulty of magnetic writing, for the writing field has to be approximately twice the coercivity of the recording media for effective writing. Therefore, further increase of magnetic recording density is largely confined by the writing ability of the magnetic head.
The material for magnetic writing head needs to be reasonably soft, its permeability must be sufficiently high and its coercivity sufficiently small so that the field intensity within the head pole and yoke becomes negligible. In early research, Fe-based material has high saturation magnetization, but also a high magneto-crystalline anisotropy, its coercivity then can hardly be lowered by traditional methods. However, the development of nanocrystallite materials ushered a new era for the study of soft magnetic materials. By reducing the grain size, the effective anisotropy can be decreased to extremely low value, usually several orders less than the local anisotropy. Multilayers preparation by sputtering is an effective way to fabricate nanocrystalline soft materials; IBM and Hitachi both have adopted Fe-based multilayers in the production of magnetic heads. In general, two kinds of multilayers are classified based on different spacer materials in between:one is nonmagnetic and the other ferromagnetic. The latter kind is more favorable, for ferromagnetic spacers can help to increase the saturation magnetization of multilayers.
In our research, we prepare Fe/Ni multilayers to study their structural and magnetic properties. As the Fe (or Ni) layer thickness is far less than the domain wall width, due to exchange coupling, the magnetization of Fe and Ni layers cannot follow each easy axis, and then the anisotropies are averaged out, the coercivity of multilayers is largely decreased. In the first part of this paper, the structure and orientation of Fe/Ni multilayers is analyzed, and the influence of layer thickness on the coercivity is discussed in detail. Cu insertion confirms the dominant role of exchange coupling in the multilayer system. In the second part, the effects of interface roughness on the structural and magnetic properties of Fe/Ni multilayers are investigated. Interface roughness is introduced in two ways, one is to tune the growth temperature, and the other is to prefabricate Ag on MgO substrate as underlayer. The mechanism of Fe/Ni multilayers'magnetization is discussed, smoother interfaces are more advantageous to optimize the exchange coupling, and the coercivity can be decreased more effectively.
引文
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[31]刘岁林,田云飞,陈红,吉晓江,《现代仪器》,第六期,9(2006)
[32]周上祺,《X射线衍射分析原理方法应用》[M],工业技术图书馆(1991)
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[36]M.A. Willard, M.Q. Huang, D.E. Laughlin, M.E. Mchenry, J.O. Cross, V.G. Harris and C. Franchetti, Magnetic properties of HITPERM (Fe,Co)88Zr7B4Cu1 magnets [J], J. Appl. Phys.85.4421 (1999)
[37]Suzuki K, Makino A, Inoue A, et al. Soft magnetic properties of nanocrystalline bcc FeZrB and FeMBCu (M=transition metal) alloys with high saturation magnetization [J], J. Appl. Phys,70,6232 (1991)
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[39]H. B. Nie, C. K. Ong and J. P. Wang, Evolution of in-plane magnetic a nisotropy in sputtered FeTaN/TaN/FeTaN sandwich films [J], J. Appl. Phys.93 7252 (2002)
[40]H. J. de Wit, soft magntic multilayers [J], Rep. Prog. Phys.55 113 (1992)
[41]H. J. de Wit, Ferromagnetism and small grains [J], J. Magn. Magn. Mater.79 167 (1989)
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[2]Lse K et al, High Writing-sensitivity single-pole head with cups-field coils [J], IEEE Trans. Magn.36,2520 (2000)
[3]Yamakawa K et al, A new single-pole head structure for high writability [J], IEEE Trans. Magn.38,163(2002)
[4]T. Kobayashi, R. Nakatani, S. Ootomo, and N. Kumasaka, Soft magnetic properties of Fe-C/Ni-Fe multilayered films [J], J. Appl. Phys.,64,3157(1988)
[5]N. Kumasaka, N. Saito, Y. Shiroishi, K. Shiiki, H. Fujiwara, and M. Kudo, Magnetic properties of multilayered Fe-Si films [J], J. Appl. Phys.,55,2238 (1984)
[6]Y. Nagai, M. Senda, and T. Toshima, Properties of ion-beam-sputtered Ni/Fe artificial lattice film [J], J. Appl. Phys.,63,1136 (1988)
[7]K. Sin, S. X. Wang, FeN/AIN multilayer films for high moment thin film recording heads[J], IEEE Trans. Magn.,32,3509 (1996)
[8]M. H. Kryder, S. Wang, and K. Rook, FeAlN/SiO2 and FeAlN/A12O3 multilayers for thin-film recording heads[J], J. Appl. Phys.,73,6212 (1993)
[9]Kanu G. Ashar, Magnetic Disk Drive Technology [M], IEEE Press, (1996)
[10]W. K. Shen, Exchange Coupled Composite Media for Perpendicular Magnetic Recording [M], University of Minnesota, (2006)
[11]S.Iwasaki, Guiding principle for research on perpendicular magnetic recording [J], IEEE Trans. Magn.,41,683 (2005).
[12]G. Herzer, Magnetization process in nanocrystalline ferromagnets [J], Mater. Sci. & Eng.,A1331(1991)
[13]G. Herzer, Soft magnetic nanocrystalline materials [J], Scripta Metall Mater,33, 1741 (1995)
[14]Inoue A, Makino A, Chapt.9 in Nanostructured Materials-Processing, Properties and Potential Applications [A], Koch C C (edi.), Noyes Publications, Norwich, 355-395 (2002)
[15]G. Herzer, Magnetic Hysteresis in Novel Magnetic Materials [M], Hadjipanaysis G C(ed.), Kluwer Academic Publishers, Dordrecht,711-730 (1996)
[16]McHenry M E, Willard M A, Laughlin D E., Amorphous and nanocrystalline materials for applications as soft magnets [J]. Prog. Mater. Sci.,44,291 (1999)
[17]Hono K, Hiraga K, Wang Q, et al., The microstructure evolution of a Fe73.5Si13. 5B9Nb3Cu1 nanocrystalline soft magnetic material [J], Acta. Metall. Mater.,40,2137 (1992)
[18]Yamaguchi K, Yoshizawa Y, Recent development of nanocrystalline soft magnetic alloys [J], Nanostructured Materials,6,247 (1995)
[19]杨邦朝,王文生,《薄膜物理与技术》[M],电子科技大学出版社,(1994)
[20]田民波,刘德令,《薄膜科学与技术手册》[M],机械工业出版社,(1991)
[2l]郑伟涛,《薄膜材料与薄膜技术》[M],化学工业出版社,(2008)
[22]埃克托瓦著,王广阳等译,《薄膜物理学》[M],科学出版社,(1986)
[23]曲喜新,《薄膜物理》[M],上海科学技术出版社,(1986)
[24]薛增泉,《薄膜物理》[M],电子工业出版社,(1991)
[25]吴自勤,王兵,《薄膜生长》[M],科学出版社,(2001)
[26]唐伟忠,《薄膜材料与制备原理、技术及应用》[M],冶金工业出版社,(1998)
[27]C.V. Thompson, Structure evolution during processing of polycrystalline films [J], Annu. Rev. Matter. Sci.30,159, (2000)
[28]Carl.V.Thompson, Roland Carel, Texture development in polycrystalline thin films [J], Materials Science and Engineering, B32,211, (1995)
[29]J.E.Sanchez and E.Arzt, Effects of grain orientation on hillock formation and grain growth in aluminum films on silicon substrates [J], Scripta Metall.Mater.27, 285,(1992)
[30]R.Venkatraman and J.C.Bravman, Separation of film thickness and grain boundary strengthening effects in Al thin films on Si [J], J.Mater.Res.7,2040 (1992)
[31]刘岁林,田云飞,陈红,吉晓江,《现代仪器》,第六期,9(2006)
[32]周上祺,《X射线衍射分析原理方法应用》[M],工业技术图书馆(1991)
[33]Wilson A J, Mathematical Theory of X-ray Powder Diffractotry[M], London Cleaver-home,1963
[34]杨传铮,薄膜、多层膜和一维超点阵材料的X射线分析新进展[J],物理学进展19,183-216(1999)
[35]E. Chason and T:M. Mayer, Thin Film and Surface Characterization by Specular X-Ray Reflectivity [J], Critical Reviews in Solid State and Materials Sciences,22, 1-61 (1997)
[36]M.A. Willard, M.Q. Huang, D.E. Laughlin, M.E. Mchenry, J.O. Cross, V.G. Harris and C. Franchetti, Magnetic properties of HITPERM (Fe,Co)88Zr7B4Cu1 magnets [J], J. Appl. Phys.85.4421 (1999)
[37]Suzuki K, Makino A, Inoue A, et al. Soft magnetic properties of nanocrystalline bcc FeZrB and FeMBCu (M=transition metal) alloys with high saturation magnetization [J], J. Appl. Phys,70,6232 (1991)
[38]Willard M A, Laughlin D E, Mchenry M E, et al., Structure and magnetic properties of (Fe0.5Co0.5)88Zr7B4Cu1 nanocrystalline alloys [J], J. Appl. Phys.,84,6773 (1998)
[39]H. B. Nie, C. K. Ong and J. P. Wang, Evolution of in-plane magnetic a nisotropy in sputtered FeTaN/TaN/FeTaN sandwich films [J], J. Appl. Phys.93 7252 (2002)
[40]H. J. de Wit, soft magntic multilayers [J], Rep. Prog. Phys.55 113 (1992)
[41]H. J. de Wit, Ferromagnetism and small grains [J], J. Magn. Magn. Mater.79 167 (1989)
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