- Email: [email protected]
Origin of perpendicular magnetic anisotropy of SmCo5 thin films with Cu underlayer
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 301 (2006) 271–278 www.elsevier.com/locate/jmmm
Origin of perpendicular magnetic anisotropy of SmCo5 thin films with Cu underlayer Junichi Sayamaa, Kazuki Mizutania, Toru Asahia,b, Jun Ariakec, Kazuhiro Ouchic, Tetsuya Osakaa,b, a Graduate School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Consolidated Research Institute for Advanced Science and Medical Care, Waseda University, 513 Wasedatsurumaki-cho, Shinjuku-ku, Tokyo 162-0041, Japan c Akita Research Institute of Advance Technology (AIT), 4-21 Sanuki, Araya, Akita 010-1623, Japan
b
Received 4 February 2005; received in revised form 14 June 2005 Available online 8 August 2005
Abstract Effects of the Cu underlayer thickness and the addition of Cu to a Sm–Co layer on magnetic properties and microstructure of SmCo5 thin films exhibiting perpendicular magnetic anisotropy were studied. The origin of the perpendicular magnetic anisotropy was discussed from these experimental results. A thick Cu underlayer of more than 100 nm brought about high perpendicular magnetic anisotropy leading to the squareness ratio equal to unity. The Cu addition enhanced the perpendicular magnetic anisotropy and reduced the Cu underlayer thickness required to obtain the squareness ratio of unity. X-ray diffractometry showed that the crystalline orientation of the Sm–Co layer did not correlate with that of the Cu underlayer. Auger electron spectroscopy revealed that Cu atoms were diffused up to the Sm–Co layer from the Cu underlayer. From the results, Cu atoms existing in the Sm–Co layer were suggested to be strongly related with an appearance of the perpendicular magnetic anisotropy by introducing the Cu underlayer. r 2005 Elsevier B.V. All rights reserved. PACS: 75.30.Gw; 75.50.Ww; 75.70.Ak Keywords: SmCo5 thin film; Perpendicular magnetic anisotropy; Cu underlayer; Cu addition; Sm–Co–Cu intermetallic phase
1. Introduction
Corresponding author. Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 1698555, Japan. Tel.: +81 3 5286 3202; fax: +81 3 3205 2074. E-mail address: [email protected] (T. Osaka).
A SmCo5 alloy is a representative permanent magnet and has been extensively studied since it was developed in the late 1960s [1]. A major characteristic of the SmCo5 alloy is an extremely strong uniaxial magnetocrystalline anisotropy,
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.07.002
ARTICLE IN PRESS 272
J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
whose anisotropy constant, Ku, is more than 1.1 108 erg/cm3 [2]. The strong magnetic anisotropy brings about very large coercivity. In addition, the SmCo5 alloy is characterized by the high Curie temperature [2]. The addition of other elements such as Fe, Cu, Zr, Ti, and Hf to Sm–Co permanent magnets with various compositions also has been widely studied [3–6]. Among the additives, Cu is a key element for controlling microstructure and magnetic properties of the SmCo5 alloy. It is well known that the additive Cu stabilizes the SmCo5 phase and makes the Sm2Co17 phase unstable [7–9]. This effect of the additive Cu is more conspicuous in the higher Cu composition of the Sm–Co-Cu compounds. That is, Cu is much more soluble in the SmCo5 phase than in the Sm2Co17 phase. In the Sm–Co– Cu ternary system, the Sm(Co, Cu)5 single phase is formed along the SmCo5–SmCu5 quasi-binary line [8,9]. These experimental results were supported by first principles calculations [10]. It was also reported that the addition of Cu to the SmCo5 phase is helpful in promoting its crystallization [11] and increasing its anisotropy field [12]. These days, thin films composed of permanent magnet materials attract much attention in the field of magnetic devices and are expected to be applied to a magnetic recording medium [13] and a minute magnet for micro-electro-mechanical systems (MEMS). For a practical use in such high performance devices, it is a great advantage for the thin films to have perpendicular magnetic anisotropy. SmCo5 thin films with in-plane magnetic anisotropy have already been reported by many researchers [14–16]. However, a SmCo5 thin film exhibiting perpendicular magnetic anisotropy has not yet been prepared. Recently, we successfully afforded distinct perpendicular magnetic anisotropy to a sputterdeposited SmCo5 thin film by introducing a relatively thick Cu underlayer whose thickness was more than 100 nm [17]. Takei et al. also obtained a similar result [18]. Moreover, the perpendicular magnetic anisotropy was markedly enhanced by using a Cu/Ti dual underlayer which produced a smooth film surface and a high preferred crystalline orientation [19]. These results contribute significantly to the progress in applica-
tions of SmCo5 thin films to various magnetic devices. In this paper, effects of the thickness of Cu underlayer and the addition of Cu to a Sm–Co layer on magnetic properties and microstructure of the SmCo5 thin films were investigated. Furthermore, based on the experimental results, the origin of the perpendicular magnetic anisotropy of the SmCo5 thin films with Cu underlayer was discussed.
2. Experimental The layer structure of the films studied in this paper was Sm–Co(–Cu) (25 nm)/Cu (10–500 nm)/ Ti (25 nm)/glass disk or Sm–Co(–Cu) (25 nm)/Ti (50 nm)/glass disk. Based on our previous study [20], the Sm–Co layer was formed by laminating Sm (0.31 nm) and Co (0.41 nm) sublayers alternately 35 times. This method was useful in promoting the crystallization of SmCo5 and the preferred orientation of its c-axis (axis of easy magnetization). However, judging from an X-ray diffractometry, a multilayered structure of Co and Sm sublayers did not remain in the films and the sublayers diffused each other. The thickness ratio of Sm and Co sublayers produced the Sm–Co layer with an atomic composition equal to Sm24Co76, which was measured with inductively coupled plasma atomic emission spectroscopy. The Cu atoms were added to the Sm–Co layer by substituting Cu layers for part of the Co constituent layers, namely, laminating Sm, Co, and Cu sublayers. The composition of the Sm–Co–Cu layer was expressed as Sm24Co76 xCux. Here, the composition is defined as that in the deposit of Sm–Co(–Cu) layer only. Thus, the diffusion of Cu from the Cu underlayer to the Sm–Co(–Cu) layer, which is mentioned later, was not taken into consideration. In other words, the actual Sm content was lower than 24 at%. The films were prepared using a DC magnetron sputtering system. The base pressure in the sputtering chamber was 2 10 7 Torr or lower. The Ar gas pressure during the deposition of Cu and Ti underlayers was adjusted at 5 mTorr and that of Sm–Co(–Cu) layer at 20 mTorr. The substrate temperature was set at 20 1C for the
ARTICLE IN PRESS J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
273
deposition of underlayers, followed by the deposition of the Sm–Co(–Cu) layer at 325 1C. Magnetic properties of the films were measured using a vibrating sample magnetometer with the maximum applied field of 15 kOe. The crystalline structure was analyzed with an X-ray diffractometer (XRD) using Cu-Ka radiation at an acceleration voltage of 50 kV and an emission current of 200 mA. Compositional distributions of the films along the growth direction were evaluated with an Auger electron spectrometer (AES).
3. Results 3.1. Appearance of perpendicular magnetic anisotropy by introducing Cu underlayer As mentioned above, we found that the introduction of relatively thick Cu underlayer imparted high perpendicular magnetic anisotropy to Sm–Co thin films. Figs. 1(a) and (b) show dependencies of coercivity, Hc, and squareness ratio, SQR, for the films consisting of Sm–Co (25 nm)/Cu (10– 500 nm)/Ti (25 nm)/glass disk on the Cu underlayer thickness, respectively. The values of Hc and SQR in the direction perpendicular to the film plane markedly became large with an increase of the Cu underlayer thickness up to 100 nm, whereas those in the direction of the film plane showed drastic decrease with an increase of Cu underlayer thickness. The films with a Cu underlayer thickness more than 100 nm exhibited the SQR equal to unity within the experimental error in the perpendicular direction. Typical M–H hysteresis loops for the films containing Cu underlayer with a different thickness are shown in Figs. 2(a)–(c), and the values of Hc and SQR in the perpendicular direction are listed in Table 1. The thicker the Cu underlayer, the smaller the hysteresis loss in the film plane. The value of Ku for the film with a 100 nm-thick Cu underlayer reached as large as 4.0 107 erg/cm3 [19]. Fig. 3 shows the 2y–y scan patterns of XRD for the films in Figs. 2(a)–(c). It was confirmed that (0 0 l) reflections of the hexagonal SmCo5 phase were clearly observed in the films exhibiting distinct perpendicular magnetic anisotropy, namely, its c-axis was highly
Fig. 1. Dependencies of coercivity (a) and squareness ratio (b) for the films consisting of Sm–Co (25 nm)/Cu (10–500 nm)/Ti (25 nm)/glass disk on the thickness of Cu underlayer.
oriented in the perpendicular direction. On the other hand, reflections of the other Sm–Co intermetallic phases were not observed [19]. Therefore, it can be stated that the introduction of thick Cu underlayer produces SmCo5 thin films exhibiting high perpendicular magnetic anisotropy. In other words, the thick Cu underlayer is required in order to impart high perpendicular magnetic anisotropy to Sm–Co thin films. 3.2. Effect of Cu addition to Sm– Co layer on magnetic properties The perpendicular magnetic anisotropy, which was brought about by the Cu underlayer, was
ARTICLE IN PRESS 274
J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
Fig. 2. M–H hysteresis loops for the films consisting of (a, b, c) Sm–Co (25 nm)/Cu (25, 50, 100 nm)/Ti (25 nm)/glass disk, respectively, together with the loops for (d) Sm–Co–Cu (25 nm)/Cu (50 nm)/Ti (25 nm)/glass disk. Thick and thin lines represent the loops in the directions perpendicular and parallel to the film plane, respectively. Table 1 Values of coercivity and squareness ratio in the direction perpendicular to the film plane for the films shown in Fig. 2 Film
Hc (kOe)
SQR
(a) (b) (c) (d)
7.9 10.4 12.0 10.6
0.67 0.92 1 1
further enhanced by adding Cu to the Sm–Co layer. Figs. 4(a) and (b) show dependencies of Hc and SQR on the Cu composition in the Sm–Co– Cu layer for the films consisting of Sm–Co–Cu (25 nm)/Cu (25 nm)/Ti (25 nm)/glass disk, respectively. The Cu addition increases the values of Hc and SQR in the perpendicular direction. On the other hand, saturation magnetization, Ms, was markedly decreased in the films with additive Cu in excess of 10 at%. Then, the Cu composition was fixed at 7 at% in adding Cu to the Sm–Co layer. The values of Ms for the films fabricated without Cu addition and with the Cu addition of a 7 at% were almost 600 and 500 emu/cm3, respectively.
Fig. 3. 2y–y scan patterns of XRD for the films consisting of (a, b, c) Sm–Co (25 nm)/Cu (25, 50, 100 nm)/Ti (25 nm)/glass disk, respectively.
Figs. 5(a) and (b) show dependencies of Hc and SQR for the films of Sm–Co–Cu (25 nm)/Cu (10–500 nm)/Ti (25 nm)/glass disk on the Cu underlayer thickness, respectively. Compared with the films fabricated without the Cu addition shown in Fig. 1, the films with the Cu addition exhibited large SQR in the perpendicular direction even in small thickness region of the Cu underlayer. For example, M–H loops and the values of Hc and SQR in the perpendicular direction for the film with the Cu addition, whose Cu underlayer thickness is 50 nm, are also shown in Fig. 2(d) and in the last row of Table 1, respectively. Judging from these results and their comparison, the Cu addition to the Sm–Co layer was certainly effective to reduce the Cu underlayer thickness necessary for imparting high perpendicular magnetic anisotropy to Sm–Co thin films. This is a useful technique for applying SmCo5 thin films to double-layered perpendicular magnetic recording media because the underlayer of the films called as an intermediate layer in the media is desired to be as thin as possible. The above-mentioned effect of Cu addition to the Sm–Co layer was confirmed in the films without Cu underlayer as well. Figs. 6(a) and (b) show M–H loops for the films consisting of
ARTICLE IN PRESS J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
275
Fig. 4. Dependencies of coercivity (a) and squareness ratio (b) on Cu composition in Sm–Co layer for the films consisting of Sm–Co–Cu (25 nm)/Cu (25 nm)/Ti (25 nm)/glass disk.
Fig. 5. Dependencies of coercivity (a) and squareness ratio (b) for the films consisting of Sm–Co–Cu (25 nm)/Cu (10–500 nm)/ Ti (25 nm)/glass disk on the thickness of Cu underlayer.
Sm–Co(–Cu) (25 nm)/Ti (50 nm)/glass disk fabricated without and with the Cu addition, respectively. The values of Hc and SQR in the perpendicular direction are listed in Table 2. The film without Cu addition exhibited weak in-plane magnetic anisotropy and low Hc. On the contrary, by adding Cu to the Sm–Co layer, Hc in the perpendicular direction markedly increased and became much larger than that in the film plane. However, we should note that the perpendicular magnetic anisotropy of the film with the Ti underlayer only was evidently weaker than that of the film with the Cu/Ti dual underlayer, and
that the Cu underlayer was indispensable for obtaining the SQR equal to unity at this stage.
4. Discussions 4.1. Effect of Cu underlayer on crystalline orientation of SmCo5 thin films Mechanism of an appearance of the perpendicular magnetic anisotropy in the SmCo5 thin films by introducing Cu underlayer is discussed in detail below. First, the most likely possibility is a
ARTICLE IN PRESS 276
J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
Fig. 6. M–H hysteresis loops for the films consisting of (a) Sm–Co (25 nm)/Ti (50 nm)/glass disk and (b) Sm–Co–Cu (25 nm)/Ti (50 nm)/glass disk. Thick and thin lines represent the loops in the directions perpendicular and parallel to the film plane, respectively.
Table 2 Values of coercivity and squareness ratio in the direction perpendicular to the film plane for the films shown in Fig. 6 Film
Hc (kOe)
SQR
(a) (b)
2.3 10.1
0.07 0.72
hetero epitaxial crystal growth from the Cu underlayer, which was brought about by a good lattice matching between SmCo5(0 0 1) and Cu(1 1 1) whose misfit is no more than 2.4% [17,18]. Accordingly, crystalline orientations of the Sm–Co layer and the Cu underlayer were investigated. Fig. 7 shows dependencies of the values of Dy50 for SmCo5(0 0 1) and Cu(1 1 1), which is defined as the full-width at half maximum in the rocking curve of each reflection, on the thickness of the Cu underlayer for the films shown in Fig. 1. Fig. 7 also includes the dependency of the peak intensity of SmCo5(0 0 1) reflection. As to the SmCo5(0 0 1) reflection, the peak intensity increased and the values of Dy50 markedly decreased in thickness of Cu underlayer up to 100 nm. That is, the thick Cu underlayer promoted the formation of SmCo5 phase and the preferred orientation of its c-axis in the perpendicular direction. This microstructure change in the Sm–Co layer is consistent with magnetic properties shown in Fig. 1. On the other hand, the values of Dy50 for Cu(1 1 1) only slightly decreased even when the Cu
Fig. 7. Dependencies of the values of Dy50 for SmCo5(0 0 1) and Cu(1 1 1) and the peak intensity of the SmCo5(0 0 1) reflection on the thickness of Cu underlayer for the films shown in Fig. 1.
underlayer thickness increased. These results indicate that the crystalline orientations of the Sm–Co layer and the Cu underlayer are not correlative so much. 4.2. Compositional distributions along the film growth direction We direct our attention to the microstructure in the cross-sectional direction of the SmCo5 thin films with Cu underlayer, especially to compositional distributions along the film growth direction. Figs. 8(a)–(c) show AES depth profiles of three different films shown in Figs. 2(a)–(c). Here, the profiles of Sm are omitted because of their very weak intensities. Their Auger electron intensities represent relative values of the atomic composition, not absolute values of the composition. The values of surface roughness, Ra, for the three films with Cu underlayer thickness of 25, 50, and 100 nm were 3.5, 3.0, and 3.4 nm, respectively, and were almost equal to each other. Thus, there is no significant difference in the influence of the surface roughness on the AES profiles. The profiles have two features. One is the noticeable interdiffusion between Cu and Ti underlayers. We have already pointed out that a Cu–Ti intermetallic phase, i.e., Cu3Ti, was formed in the interdiffusion regions [19], which was also
ARTICLE IN PRESS J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
277
thickness more than 50 nm. On the other hand, Ti atoms were diffused up to the surface of Cu underlayer, but did not reach the Sm–Co layer.
4.3. Origin of the perpendicular magnetic anisotropy of SmCo5 thin films
Fig. 8. AES depth profiles for the films consisting of (a, b, c) Sm–Co (25 nm)/Cu (25, 50, 100 nm)/Ti (25 nm)/glass disk, respectively.
confirmed in this study as shown in Fig. 3. The other, which is much more important, is interdiffusion between the Sm–Co layer and the Cu underlayer. Co and Cu atoms penetrated each other. The intensities of Auger electron for Cu suggested that the amount of Cu atoms in the Sm–Co layer is the greater in the films with the thicker Cu underlayer. It was also clearly observed that the Cu atoms were diffused up to the surface of Sm–Co layer in the cases of the Cu underlayer
From the above-mentioned results, the Cu atoms diffusing up to the Sm–Co layer seems to be related with the origin of the perpendicular magnetic anisotropy. Referring to the previous studies on the Sm–Co permanent magnets mentioned in the introductory section, we consider that the Cu atoms in the Sm–Co layer stabilize the SmCo5 phase and promote its crystallization. Based on this consideration, it is reasonable that the film with the thicker Cu underlayer exhibited the stronger SmCo5(0 0 1) reflection because the increase of Cu underlayer thickness produced the Sm–Co layer containing a large amount of Cu atoms. Thus, the Cu underlayer is conjectured to be useful in the respect of the formation of a Sm–Co–Cu intermetallic phase, i.e., Sm(Co, Cu)5 phase. In the XRD measurement shown in Fig. 3, SmCo5(0 0 l) reflections were observed at lower 2y angles than those of literature values. This is an evidence of the formation of Sm(Co, Cu)5 phase because substitutionally dissolved Cu atoms in Co sites of SmCo5 elongate the crystal lattice. Moreover, it should be emphasized that the Cu underlayer was indispensable for high perpendicular magnetic anisotropy even in the case of the films fabricated with the Cu addition to the Sm–Co layer. This suggests that a large amount of Cu atoms in the initial growth region of the Sm–Co layer is essential to obtain a high perpendicular magnetic anisotropy. The Cu-rich initial growth region forms the well-crystallized Sm(Co, Cu)5 phase. The well-crystallized Sm(Co, Cu)5 phase is considered to work as the seed promoting the crystallization and the preferred orientation of the Sm–Co layer deposited upon the initial region. The direct addition of Cu to the Sm–Co layer assists the formation of Sm(Co, Cu)5 phase and enhance the perpendicular magnetic anisotropy, but it does not generate a high perpendicular magnetic anisotropy by itself.
ARTICLE IN PRESS 278
J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
To confirm our considerations and elucidate the origin of the perpendicular magnetic anisotropy completely, the atomic scale microstructure of the SmCo5 thin films exhibiting perpendicular magnetic anisotropy should be analyzed, and the microstructural study by means of a high-resolution transmission electron microscope is now in progress.
for COE Research ‘‘Establishment of Molecular Nano-Engineering by Utilizing Nanostructure Arrays and its Development into Micro-System’’ and the Special Coordination Funds for Promoting Science and Technology ‘‘Creation of Novel Magnetic Recording Materials Using Nano-Interface Technology’’ and ‘‘Establishment of Consolidated Research Institute for Advanced Science and Medical Care’’ from MEXT.
5. Conclusions References Effects of the Cu underlayer thickness and the addition of Cu to a Sm–Co layer on magnetic properties of the SmCo5 thin films exhibiting perpendicular magnetic anisotropy were investigated. A thick Cu underlayer of more than 100 nm brought about high perpendicular magnetic anisotropy leading to the squareness ratio equal to unity. The Cu addition to the Sm–Co layer enhanced the perpendicular magnetic anisotropy and reduced the Cu underlayer thickness required to obtain the squareness ratio of unity. A hetero epitaxial crystal growth from the Cu underlayer does not seem a sufficient condition to generate the perpendicular magnetic anisotropy. AES measurements revealed that the larger amount of Cu atoms were diffused up to the Sm–Co layer from the thicker Cu underlayer and suggested that the Cu atoms in the Sm–Co layer stabilized the SmCo5 phase and promoted its crystallization. Therefore, the Cu underlayer is conjectured to be useful for the appearance of the perpendicular magnetic anisotropy as a supplier of Cu atoms to the Sm–Co layer.
Acknowledgements This work was carried out at the ‘‘Center for Practical Nano-Chemistry’’ for the 21C-COE Programme of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. It was also partly supported by the Grant-in-Aid
[1] D.K. Das, IEEE Trans. Magn. 5 (1969) 214. [2] K.J. Strnat, in: E.P. Wohlfarth, K.H.J. Buschow (Eds.), Ferromagnetic Materials, vol. 4, North-Holland, Amsterdam, 1988, pp. 131–209 and references cited therein. [3] H. Senno, Y. Tawara, Jpn. J. Appl. Phys. 14 (1975) 1619. [4] J.D. Livingston, D.L. Martin, J. Appl. Phys. 48 (1977) 1350. [5] T. Ojima, S. Tomizawa, T. Yoneyama, T. Hori, IEEE Trans. Magn. 13 (1977) 1317. [6] R.K. Mishra, G. Thomas, T. Yoneyama, A. Fukuno, T. Ojima, J. Appl. Phys. 52 (1981) 2517. [7] F. Hofer, IEEE Trans. Magn. 6 (1970) 221. [8] K. Kamino, Y. Kimura, T. Suzuki, Y. Itayama, Trans. Jpn. Inst. Met. 14 (1973) 135. [9] A.J. Perry, J. Less-Common Met. 51 (1977) 153. [10] R.F. Sabirianov, A. Kashyap, R. Skomski, S.S. Jaswal, D.J. Sellmyer, Appl. Phys. Lett. 85 (2004) 2286. [11] W.Y. Zhang, X.D. Zhang, Y.C. Yang, B.G. Shen, J. Alloy. Compd. 353 (2003) 274. [12] E. Lectard, C.H. Allibert, R. Ballou, J. Appl. Phys. 75 (1994) 6277 and references cited therein. [13] E.M.T. Velu, D.N. Lambeth, IEEE Trans. Magn. 28 (1992) 3249. [14] R. Rani, F.J. Cadieu, X.R. Qian, W.A. Mendoza, S.A. Shaheen, J. Appl. Phys. 81 (1997) 5634. [15] Y. Liu, R.A. Thomas, S.S. Malhotra, Z.S. Shan, S.H. Liou, D.J. Sellmyer, J. Appl. Phys. 83 (1998) 6244. [16] V. Neu, S.A. Shaheen, J. Appl. Phys. 86 (1999) 7006. [17] J. Sayama, T. Asahi, K. Mizatani, T. Osaka, J. Phys. D: Appl. Phys. 37 (2004) L1. [18] S. Takei, A. Morisako, M. Matsumoto, J. Magn. Magn. Mater. 272–276 (2004) 1703. [19] J. Sayama, K. Mizutani, T. Asahi, T. Osaka, Appl. Phys. Lett. 85 (2004) 5640. [20] J. Sayama, K. Mizutani, T. Asahi, J. Ariake, K. Ouchi, S. Matsunuma, T. Osaka, J. Magn. Magn. Mater. 287 (2005) 239.
Journal of Magnetism and Magnetic Materials 301 (2006) 271–278 www.elsevier.com/locate/jmmm
Origin of perpendicular magnetic anisotropy of SmCo5 thin films with Cu underlayer Junichi Sayamaa, Kazuki Mizutania, Toru Asahia,b, Jun Ariakec, Kazuhiro Ouchic, Tetsuya Osakaa,b, a Graduate School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Consolidated Research Institute for Advanced Science and Medical Care, Waseda University, 513 Wasedatsurumaki-cho, Shinjuku-ku, Tokyo 162-0041, Japan c Akita Research Institute of Advance Technology (AIT), 4-21 Sanuki, Araya, Akita 010-1623, Japan
b
Received 4 February 2005; received in revised form 14 June 2005 Available online 8 August 2005
Abstract Effects of the Cu underlayer thickness and the addition of Cu to a Sm–Co layer on magnetic properties and microstructure of SmCo5 thin films exhibiting perpendicular magnetic anisotropy were studied. The origin of the perpendicular magnetic anisotropy was discussed from these experimental results. A thick Cu underlayer of more than 100 nm brought about high perpendicular magnetic anisotropy leading to the squareness ratio equal to unity. The Cu addition enhanced the perpendicular magnetic anisotropy and reduced the Cu underlayer thickness required to obtain the squareness ratio of unity. X-ray diffractometry showed that the crystalline orientation of the Sm–Co layer did not correlate with that of the Cu underlayer. Auger electron spectroscopy revealed that Cu atoms were diffused up to the Sm–Co layer from the Cu underlayer. From the results, Cu atoms existing in the Sm–Co layer were suggested to be strongly related with an appearance of the perpendicular magnetic anisotropy by introducing the Cu underlayer. r 2005 Elsevier B.V. All rights reserved. PACS: 75.30.Gw; 75.50.Ww; 75.70.Ak Keywords: SmCo5 thin film; Perpendicular magnetic anisotropy; Cu underlayer; Cu addition; Sm–Co–Cu intermetallic phase
1. Introduction
Corresponding author. Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 1698555, Japan. Tel.: +81 3 5286 3202; fax: +81 3 3205 2074. E-mail address: [email protected] (T. Osaka).
A SmCo5 alloy is a representative permanent magnet and has been extensively studied since it was developed in the late 1960s [1]. A major characteristic of the SmCo5 alloy is an extremely strong uniaxial magnetocrystalline anisotropy,
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.07.002
ARTICLE IN PRESS 272
J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
whose anisotropy constant, Ku, is more than 1.1 108 erg/cm3 [2]. The strong magnetic anisotropy brings about very large coercivity. In addition, the SmCo5 alloy is characterized by the high Curie temperature [2]. The addition of other elements such as Fe, Cu, Zr, Ti, and Hf to Sm–Co permanent magnets with various compositions also has been widely studied [3–6]. Among the additives, Cu is a key element for controlling microstructure and magnetic properties of the SmCo5 alloy. It is well known that the additive Cu stabilizes the SmCo5 phase and makes the Sm2Co17 phase unstable [7–9]. This effect of the additive Cu is more conspicuous in the higher Cu composition of the Sm–Co-Cu compounds. That is, Cu is much more soluble in the SmCo5 phase than in the Sm2Co17 phase. In the Sm–Co– Cu ternary system, the Sm(Co, Cu)5 single phase is formed along the SmCo5–SmCu5 quasi-binary line [8,9]. These experimental results were supported by first principles calculations [10]. It was also reported that the addition of Cu to the SmCo5 phase is helpful in promoting its crystallization [11] and increasing its anisotropy field [12]. These days, thin films composed of permanent magnet materials attract much attention in the field of magnetic devices and are expected to be applied to a magnetic recording medium [13] and a minute magnet for micro-electro-mechanical systems (MEMS). For a practical use in such high performance devices, it is a great advantage for the thin films to have perpendicular magnetic anisotropy. SmCo5 thin films with in-plane magnetic anisotropy have already been reported by many researchers [14–16]. However, a SmCo5 thin film exhibiting perpendicular magnetic anisotropy has not yet been prepared. Recently, we successfully afforded distinct perpendicular magnetic anisotropy to a sputterdeposited SmCo5 thin film by introducing a relatively thick Cu underlayer whose thickness was more than 100 nm [17]. Takei et al. also obtained a similar result [18]. Moreover, the perpendicular magnetic anisotropy was markedly enhanced by using a Cu/Ti dual underlayer which produced a smooth film surface and a high preferred crystalline orientation [19]. These results contribute significantly to the progress in applica-
tions of SmCo5 thin films to various magnetic devices. In this paper, effects of the thickness of Cu underlayer and the addition of Cu to a Sm–Co layer on magnetic properties and microstructure of the SmCo5 thin films were investigated. Furthermore, based on the experimental results, the origin of the perpendicular magnetic anisotropy of the SmCo5 thin films with Cu underlayer was discussed.
2. Experimental The layer structure of the films studied in this paper was Sm–Co(–Cu) (25 nm)/Cu (10–500 nm)/ Ti (25 nm)/glass disk or Sm–Co(–Cu) (25 nm)/Ti (50 nm)/glass disk. Based on our previous study [20], the Sm–Co layer was formed by laminating Sm (0.31 nm) and Co (0.41 nm) sublayers alternately 35 times. This method was useful in promoting the crystallization of SmCo5 and the preferred orientation of its c-axis (axis of easy magnetization). However, judging from an X-ray diffractometry, a multilayered structure of Co and Sm sublayers did not remain in the films and the sublayers diffused each other. The thickness ratio of Sm and Co sublayers produced the Sm–Co layer with an atomic composition equal to Sm24Co76, which was measured with inductively coupled plasma atomic emission spectroscopy. The Cu atoms were added to the Sm–Co layer by substituting Cu layers for part of the Co constituent layers, namely, laminating Sm, Co, and Cu sublayers. The composition of the Sm–Co–Cu layer was expressed as Sm24Co76 xCux. Here, the composition is defined as that in the deposit of Sm–Co(–Cu) layer only. Thus, the diffusion of Cu from the Cu underlayer to the Sm–Co(–Cu) layer, which is mentioned later, was not taken into consideration. In other words, the actual Sm content was lower than 24 at%. The films were prepared using a DC magnetron sputtering system. The base pressure in the sputtering chamber was 2 10 7 Torr or lower. The Ar gas pressure during the deposition of Cu and Ti underlayers was adjusted at 5 mTorr and that of Sm–Co(–Cu) layer at 20 mTorr. The substrate temperature was set at 20 1C for the
ARTICLE IN PRESS J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
273
deposition of underlayers, followed by the deposition of the Sm–Co(–Cu) layer at 325 1C. Magnetic properties of the films were measured using a vibrating sample magnetometer with the maximum applied field of 15 kOe. The crystalline structure was analyzed with an X-ray diffractometer (XRD) using Cu-Ka radiation at an acceleration voltage of 50 kV and an emission current of 200 mA. Compositional distributions of the films along the growth direction were evaluated with an Auger electron spectrometer (AES).
3. Results 3.1. Appearance of perpendicular magnetic anisotropy by introducing Cu underlayer As mentioned above, we found that the introduction of relatively thick Cu underlayer imparted high perpendicular magnetic anisotropy to Sm–Co thin films. Figs. 1(a) and (b) show dependencies of coercivity, Hc, and squareness ratio, SQR, for the films consisting of Sm–Co (25 nm)/Cu (10– 500 nm)/Ti (25 nm)/glass disk on the Cu underlayer thickness, respectively. The values of Hc and SQR in the direction perpendicular to the film plane markedly became large with an increase of the Cu underlayer thickness up to 100 nm, whereas those in the direction of the film plane showed drastic decrease with an increase of Cu underlayer thickness. The films with a Cu underlayer thickness more than 100 nm exhibited the SQR equal to unity within the experimental error in the perpendicular direction. Typical M–H hysteresis loops for the films containing Cu underlayer with a different thickness are shown in Figs. 2(a)–(c), and the values of Hc and SQR in the perpendicular direction are listed in Table 1. The thicker the Cu underlayer, the smaller the hysteresis loss in the film plane. The value of Ku for the film with a 100 nm-thick Cu underlayer reached as large as 4.0 107 erg/cm3 [19]. Fig. 3 shows the 2y–y scan patterns of XRD for the films in Figs. 2(a)–(c). It was confirmed that (0 0 l) reflections of the hexagonal SmCo5 phase were clearly observed in the films exhibiting distinct perpendicular magnetic anisotropy, namely, its c-axis was highly
Fig. 1. Dependencies of coercivity (a) and squareness ratio (b) for the films consisting of Sm–Co (25 nm)/Cu (10–500 nm)/Ti (25 nm)/glass disk on the thickness of Cu underlayer.
oriented in the perpendicular direction. On the other hand, reflections of the other Sm–Co intermetallic phases were not observed [19]. Therefore, it can be stated that the introduction of thick Cu underlayer produces SmCo5 thin films exhibiting high perpendicular magnetic anisotropy. In other words, the thick Cu underlayer is required in order to impart high perpendicular magnetic anisotropy to Sm–Co thin films. 3.2. Effect of Cu addition to Sm– Co layer on magnetic properties The perpendicular magnetic anisotropy, which was brought about by the Cu underlayer, was
ARTICLE IN PRESS 274
J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
Fig. 2. M–H hysteresis loops for the films consisting of (a, b, c) Sm–Co (25 nm)/Cu (25, 50, 100 nm)/Ti (25 nm)/glass disk, respectively, together with the loops for (d) Sm–Co–Cu (25 nm)/Cu (50 nm)/Ti (25 nm)/glass disk. Thick and thin lines represent the loops in the directions perpendicular and parallel to the film plane, respectively. Table 1 Values of coercivity and squareness ratio in the direction perpendicular to the film plane for the films shown in Fig. 2 Film
Hc (kOe)
SQR
(a) (b) (c) (d)
7.9 10.4 12.0 10.6
0.67 0.92 1 1
further enhanced by adding Cu to the Sm–Co layer. Figs. 4(a) and (b) show dependencies of Hc and SQR on the Cu composition in the Sm–Co– Cu layer for the films consisting of Sm–Co–Cu (25 nm)/Cu (25 nm)/Ti (25 nm)/glass disk, respectively. The Cu addition increases the values of Hc and SQR in the perpendicular direction. On the other hand, saturation magnetization, Ms, was markedly decreased in the films with additive Cu in excess of 10 at%. Then, the Cu composition was fixed at 7 at% in adding Cu to the Sm–Co layer. The values of Ms for the films fabricated without Cu addition and with the Cu addition of a 7 at% were almost 600 and 500 emu/cm3, respectively.
Fig. 3. 2y–y scan patterns of XRD for the films consisting of (a, b, c) Sm–Co (25 nm)/Cu (25, 50, 100 nm)/Ti (25 nm)/glass disk, respectively.
Figs. 5(a) and (b) show dependencies of Hc and SQR for the films of Sm–Co–Cu (25 nm)/Cu (10–500 nm)/Ti (25 nm)/glass disk on the Cu underlayer thickness, respectively. Compared with the films fabricated without the Cu addition shown in Fig. 1, the films with the Cu addition exhibited large SQR in the perpendicular direction even in small thickness region of the Cu underlayer. For example, M–H loops and the values of Hc and SQR in the perpendicular direction for the film with the Cu addition, whose Cu underlayer thickness is 50 nm, are also shown in Fig. 2(d) and in the last row of Table 1, respectively. Judging from these results and their comparison, the Cu addition to the Sm–Co layer was certainly effective to reduce the Cu underlayer thickness necessary for imparting high perpendicular magnetic anisotropy to Sm–Co thin films. This is a useful technique for applying SmCo5 thin films to double-layered perpendicular magnetic recording media because the underlayer of the films called as an intermediate layer in the media is desired to be as thin as possible. The above-mentioned effect of Cu addition to the Sm–Co layer was confirmed in the films without Cu underlayer as well. Figs. 6(a) and (b) show M–H loops for the films consisting of
ARTICLE IN PRESS J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
275
Fig. 4. Dependencies of coercivity (a) and squareness ratio (b) on Cu composition in Sm–Co layer for the films consisting of Sm–Co–Cu (25 nm)/Cu (25 nm)/Ti (25 nm)/glass disk.
Fig. 5. Dependencies of coercivity (a) and squareness ratio (b) for the films consisting of Sm–Co–Cu (25 nm)/Cu (10–500 nm)/ Ti (25 nm)/glass disk on the thickness of Cu underlayer.
Sm–Co(–Cu) (25 nm)/Ti (50 nm)/glass disk fabricated without and with the Cu addition, respectively. The values of Hc and SQR in the perpendicular direction are listed in Table 2. The film without Cu addition exhibited weak in-plane magnetic anisotropy and low Hc. On the contrary, by adding Cu to the Sm–Co layer, Hc in the perpendicular direction markedly increased and became much larger than that in the film plane. However, we should note that the perpendicular magnetic anisotropy of the film with the Ti underlayer only was evidently weaker than that of the film with the Cu/Ti dual underlayer, and
that the Cu underlayer was indispensable for obtaining the SQR equal to unity at this stage.
4. Discussions 4.1. Effect of Cu underlayer on crystalline orientation of SmCo5 thin films Mechanism of an appearance of the perpendicular magnetic anisotropy in the SmCo5 thin films by introducing Cu underlayer is discussed in detail below. First, the most likely possibility is a
ARTICLE IN PRESS 276
J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
Fig. 6. M–H hysteresis loops for the films consisting of (a) Sm–Co (25 nm)/Ti (50 nm)/glass disk and (b) Sm–Co–Cu (25 nm)/Ti (50 nm)/glass disk. Thick and thin lines represent the loops in the directions perpendicular and parallel to the film plane, respectively.
Table 2 Values of coercivity and squareness ratio in the direction perpendicular to the film plane for the films shown in Fig. 6 Film
Hc (kOe)
SQR
(a) (b)
2.3 10.1
0.07 0.72
hetero epitaxial crystal growth from the Cu underlayer, which was brought about by a good lattice matching between SmCo5(0 0 1) and Cu(1 1 1) whose misfit is no more than 2.4% [17,18]. Accordingly, crystalline orientations of the Sm–Co layer and the Cu underlayer were investigated. Fig. 7 shows dependencies of the values of Dy50 for SmCo5(0 0 1) and Cu(1 1 1), which is defined as the full-width at half maximum in the rocking curve of each reflection, on the thickness of the Cu underlayer for the films shown in Fig. 1. Fig. 7 also includes the dependency of the peak intensity of SmCo5(0 0 1) reflection. As to the SmCo5(0 0 1) reflection, the peak intensity increased and the values of Dy50 markedly decreased in thickness of Cu underlayer up to 100 nm. That is, the thick Cu underlayer promoted the formation of SmCo5 phase and the preferred orientation of its c-axis in the perpendicular direction. This microstructure change in the Sm–Co layer is consistent with magnetic properties shown in Fig. 1. On the other hand, the values of Dy50 for Cu(1 1 1) only slightly decreased even when the Cu
Fig. 7. Dependencies of the values of Dy50 for SmCo5(0 0 1) and Cu(1 1 1) and the peak intensity of the SmCo5(0 0 1) reflection on the thickness of Cu underlayer for the films shown in Fig. 1.
underlayer thickness increased. These results indicate that the crystalline orientations of the Sm–Co layer and the Cu underlayer are not correlative so much. 4.2. Compositional distributions along the film growth direction We direct our attention to the microstructure in the cross-sectional direction of the SmCo5 thin films with Cu underlayer, especially to compositional distributions along the film growth direction. Figs. 8(a)–(c) show AES depth profiles of three different films shown in Figs. 2(a)–(c). Here, the profiles of Sm are omitted because of their very weak intensities. Their Auger electron intensities represent relative values of the atomic composition, not absolute values of the composition. The values of surface roughness, Ra, for the three films with Cu underlayer thickness of 25, 50, and 100 nm were 3.5, 3.0, and 3.4 nm, respectively, and were almost equal to each other. Thus, there is no significant difference in the influence of the surface roughness on the AES profiles. The profiles have two features. One is the noticeable interdiffusion between Cu and Ti underlayers. We have already pointed out that a Cu–Ti intermetallic phase, i.e., Cu3Ti, was formed in the interdiffusion regions [19], which was also
ARTICLE IN PRESS J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
277
thickness more than 50 nm. On the other hand, Ti atoms were diffused up to the surface of Cu underlayer, but did not reach the Sm–Co layer.
4.3. Origin of the perpendicular magnetic anisotropy of SmCo5 thin films
Fig. 8. AES depth profiles for the films consisting of (a, b, c) Sm–Co (25 nm)/Cu (25, 50, 100 nm)/Ti (25 nm)/glass disk, respectively.
confirmed in this study as shown in Fig. 3. The other, which is much more important, is interdiffusion between the Sm–Co layer and the Cu underlayer. Co and Cu atoms penetrated each other. The intensities of Auger electron for Cu suggested that the amount of Cu atoms in the Sm–Co layer is the greater in the films with the thicker Cu underlayer. It was also clearly observed that the Cu atoms were diffused up to the surface of Sm–Co layer in the cases of the Cu underlayer
From the above-mentioned results, the Cu atoms diffusing up to the Sm–Co layer seems to be related with the origin of the perpendicular magnetic anisotropy. Referring to the previous studies on the Sm–Co permanent magnets mentioned in the introductory section, we consider that the Cu atoms in the Sm–Co layer stabilize the SmCo5 phase and promote its crystallization. Based on this consideration, it is reasonable that the film with the thicker Cu underlayer exhibited the stronger SmCo5(0 0 1) reflection because the increase of Cu underlayer thickness produced the Sm–Co layer containing a large amount of Cu atoms. Thus, the Cu underlayer is conjectured to be useful in the respect of the formation of a Sm–Co–Cu intermetallic phase, i.e., Sm(Co, Cu)5 phase. In the XRD measurement shown in Fig. 3, SmCo5(0 0 l) reflections were observed at lower 2y angles than those of literature values. This is an evidence of the formation of Sm(Co, Cu)5 phase because substitutionally dissolved Cu atoms in Co sites of SmCo5 elongate the crystal lattice. Moreover, it should be emphasized that the Cu underlayer was indispensable for high perpendicular magnetic anisotropy even in the case of the films fabricated with the Cu addition to the Sm–Co layer. This suggests that a large amount of Cu atoms in the initial growth region of the Sm–Co layer is essential to obtain a high perpendicular magnetic anisotropy. The Cu-rich initial growth region forms the well-crystallized Sm(Co, Cu)5 phase. The well-crystallized Sm(Co, Cu)5 phase is considered to work as the seed promoting the crystallization and the preferred orientation of the Sm–Co layer deposited upon the initial region. The direct addition of Cu to the Sm–Co layer assists the formation of Sm(Co, Cu)5 phase and enhance the perpendicular magnetic anisotropy, but it does not generate a high perpendicular magnetic anisotropy by itself.
ARTICLE IN PRESS 278
J. Sayama et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 271–278
To confirm our considerations and elucidate the origin of the perpendicular magnetic anisotropy completely, the atomic scale microstructure of the SmCo5 thin films exhibiting perpendicular magnetic anisotropy should be analyzed, and the microstructural study by means of a high-resolution transmission electron microscope is now in progress.
for COE Research ‘‘Establishment of Molecular Nano-Engineering by Utilizing Nanostructure Arrays and its Development into Micro-System’’ and the Special Coordination Funds for Promoting Science and Technology ‘‘Creation of Novel Magnetic Recording Materials Using Nano-Interface Technology’’ and ‘‘Establishment of Consolidated Research Institute for Advanced Science and Medical Care’’ from MEXT.
5. Conclusions References Effects of the Cu underlayer thickness and the addition of Cu to a Sm–Co layer on magnetic properties of the SmCo5 thin films exhibiting perpendicular magnetic anisotropy were investigated. A thick Cu underlayer of more than 100 nm brought about high perpendicular magnetic anisotropy leading to the squareness ratio equal to unity. The Cu addition to the Sm–Co layer enhanced the perpendicular magnetic anisotropy and reduced the Cu underlayer thickness required to obtain the squareness ratio of unity. A hetero epitaxial crystal growth from the Cu underlayer does not seem a sufficient condition to generate the perpendicular magnetic anisotropy. AES measurements revealed that the larger amount of Cu atoms were diffused up to the Sm–Co layer from the thicker Cu underlayer and suggested that the Cu atoms in the Sm–Co layer stabilized the SmCo5 phase and promoted its crystallization. Therefore, the Cu underlayer is conjectured to be useful for the appearance of the perpendicular magnetic anisotropy as a supplier of Cu atoms to the Sm–Co layer.
Acknowledgements This work was carried out at the ‘‘Center for Practical Nano-Chemistry’’ for the 21C-COE Programme of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. It was also partly supported by the Grant-in-Aid
[1] D.K. Das, IEEE Trans. Magn. 5 (1969) 214. [2] K.J. Strnat, in: E.P. Wohlfarth, K.H.J. Buschow (Eds.), Ferromagnetic Materials, vol. 4, North-Holland, Amsterdam, 1988, pp. 131–209 and references cited therein. [3] H. Senno, Y. Tawara, Jpn. J. Appl. Phys. 14 (1975) 1619. [4] J.D. Livingston, D.L. Martin, J. Appl. Phys. 48 (1977) 1350. [5] T. Ojima, S. Tomizawa, T. Yoneyama, T. Hori, IEEE Trans. Magn. 13 (1977) 1317. [6] R.K. Mishra, G. Thomas, T. Yoneyama, A. Fukuno, T. Ojima, J. Appl. Phys. 52 (1981) 2517. [7] F. Hofer, IEEE Trans. Magn. 6 (1970) 221. [8] K. Kamino, Y. Kimura, T. Suzuki, Y. Itayama, Trans. Jpn. Inst. Met. 14 (1973) 135. [9] A.J. Perry, J. Less-Common Met. 51 (1977) 153. [10] R.F. Sabirianov, A. Kashyap, R. Skomski, S.S. Jaswal, D.J. Sellmyer, Appl. Phys. Lett. 85 (2004) 2286. [11] W.Y. Zhang, X.D. Zhang, Y.C. Yang, B.G. Shen, J. Alloy. Compd. 353 (2003) 274. [12] E. Lectard, C.H. Allibert, R. Ballou, J. Appl. Phys. 75 (1994) 6277 and references cited therein. [13] E.M.T. Velu, D.N. Lambeth, IEEE Trans. Magn. 28 (1992) 3249. [14] R. Rani, F.J. Cadieu, X.R. Qian, W.A. Mendoza, S.A. Shaheen, J. Appl. Phys. 81 (1997) 5634. [15] Y. Liu, R.A. Thomas, S.S. Malhotra, Z.S. Shan, S.H. Liou, D.J. Sellmyer, J. Appl. Phys. 83 (1998) 6244. [16] V. Neu, S.A. Shaheen, J. Appl. Phys. 86 (1999) 7006. [17] J. Sayama, T. Asahi, K. Mizatani, T. Osaka, J. Phys. D: Appl. Phys. 37 (2004) L1. [18] S. Takei, A. Morisako, M. Matsumoto, J. Magn. Magn. Mater. 272–276 (2004) 1703. [19] J. Sayama, K. Mizutani, T. Asahi, T. Osaka, Appl. Phys. Lett. 85 (2004) 5640. [20] J. Sayama, K. Mizutani, T. Asahi, J. Ariake, K. Ouchi, S. Matsunuma, T. Osaka, J. Magn. Magn. Mater. 287 (2005) 239.