臺灣博碩士論文加值系統

摘要
本實驗中，我們研究以半金屬(鉍)為中間層的磁電阻效應。由於費米面的高度異向性、小的載子等效質量和極長的載子平均自由徑使得準金屬鉍擁有極為特殊的電性。在4.2 凱氏溫度下，鉍金屬的平均自由徑長達釐米等級。。另外鉍金屬的電阻率(1.15×10-6 歐姆．米)遠大於一般金屬(10-8 歐姆．米)。因此，以鉍金屬為中間層的樣品會有較大的背景電阻，就有機會使用電流方向垂直膜面的量測方式。這種結構的樣品可以用較簡單的公式來表示也比較容易與理論分析結果相比較。
為得到明確三層接面磁平行與反平行對比態，可以因此有明顯磁阻行為，我利用自旋閥結構，即在三層接面下鍍上反鐵磁性(鐵錳)物質。為確定鐵磁物質/反鐵磁物質介面具有偏倚耦合現象。我們首先研究鎳鐵/鐵錳介面並尋求最佳條件下製造出最大偏倚耦合場。我們發現偏倚耦合場受到在鎳鐵(5 奈米)/鐵猛(20 奈米)的雙層膜偏倚耦合系統中橫向晶粒尺寸的影響最為明顯。利用穿透式電子顯微鏡的截面影像可以顯示出鍍在較厚的鉭底層上之鎳鐵/鐵錳雙層膜的橫向晶粒尺寸較大。我們觀察到在較厚的鉭底層可以得到較大的偏倚耦合場；其原因來自於較厚的鉭底層可以提供較大的橫向晶粒面積增強了鐵磁/反鐵磁介面的偏倚耦合現象。
利用得到的最佳製造條件來製作自旋閥，用以研究三層接面的磁電阻現象。但在本實驗中，受到製程條件過於複雜的原因，產生樣品電阻不穩定，造成擁有自旋閥結構的樣品並沒有得到預期好的磁電阻訊號。
最後本實驗中，採用在鈷(2.5 奈米)/鉍(5 奈米)/鈷(40 奈米)的擬自旋閥樣品，利用奈米接點技術來縮小樣品的有效截面積(100×300 奈米2)以增加背景電阻。為了降低電路中的雜訊，我們採用交流四點量測來決定樣品電性。我們得到鈷/鉍/鈷樣品的背景電阻為2.3 歐姆。這個樣品的電阻率約為0.03%，遠小於理論計算的數值40%。接點電阻和不均勻的電流密度可能是造成低電阻率的原因。

Abstract
In this work the magnetoresistance (MR) effect by using semimetal (Bi) as a spacer was investigated. Bismuth is a semimetal with unusual electronic properties related to its highly anisotropic Fermi surface, small carrier effective masses, and long carrier mean free path. The carrier mean free path in bismuth can be as long as a millimeter at 4.2 K. The resistivity of bismuth (1.15×10-6 Ω．m) is much larger than normal metal (10-8 Ω．m). Therefore, a sample with bismuth as a spacer has larger base resistance and the current-perpendicular-to plane (CPP) measurement could be applied. The CPP-MR is usually described by simpler equations and easily compares to theoretical result.
In order to obtain more definite parallel or antiparallel state of the trilayer junction, the spin-valve structure could be adopted. In the CPP-MR spin-valve junction the magnetoresistance effect is much obvious. The spin-valve structure is constructed by trilayer junction deposited on an antiferromagnetic material. To make sure of the exchange bias effect happened at the ferromagnetic/antiferromagnetic interface. The best fabricating condition could be obtained by the detail study of the ferromagnetic/antiferromagnetic interface. The exchange field in the NiFe(5 nm)/FeMn(20 nm) bilayer films was influenced significantly by the lateral grain size on the exchange bias. The cross-section TEM investigations reveal that NiFe(5 vi
nm)/FeMn(20 nm) bilayers deposited on thicker Ta underlayers have larger lateral grain diameter. A large increase in exchange bias field was observed with increasing thickness of Ta underlayer, which is attributed to the enhanced FM/AFM coupling originated from larger interface lateral grain size.
The spin valve structure was fabricated by the best fabricating condition and the magnetoresisitance investigation of the trilayer junction with spin valve structure was carried out. However, the signal of the resistance in spin valve junction is not stable due to more complicated fabricating procedure. Therefore, the measurement in spin valve junction is not successful.
In this work, the current-perpendicular-to-plane (CPP) pseudo spin valve junction with Co(2.5 nm)/Bi(5 nm)/Co(40 nm) was investigated. The nano-contact technique is utilized to reduce the effective area (100×300 nm2) of the sample and increase the base resistance. In order to reduce the noise of the circuit, the ac four-probe method was adopted to determine the electric properties of the sample. The base resistance of the Co/Bi/Co junction is about 2.3 Ω. The MR ratio of this CPP junction is about 0.03%, which is much smaller than theoretically estimated value for this structure (40%). The low MR ratio might be caused by the large contact resistance and non-uniform current densities.
The spin valve structure was fabricated by the best fabricating condition and the magnetoresisitance investigation of the trilayer junction with spin valve structure was carried out. However, in this work fabrication of the CPP spin valve junction is not successful due to more complicated fabricating procedure.

Contents
摘要 ⅲ
Abstrac ⅴ
Contents ⅷ
List of Figures ⅺ
List of Tables ⅹⅴ
Chapter 1 Introduction 1
1-1 Research Background 1
1-2 Motivation 4
References 8
Chapter 2 Basic concepts and Theories 11
2-1 Characteristic of Ferromagnetic/Bismuth/Ferromagnetic Trilayer
Junction 11
2-1-1 Magnetic Properties of Ferromagnetic/Bismuth/ Ferromagnetic
Trilayer Junction 12
2-1-2 Electrical Properties of Ferromagnetic/Bismuth/ Ferromagnetic
Trilayer Junction 13
2-2 Magnetoresistance 14
2-2-1 Ordinary Magnetoresistance Effect 15
2-2-2 Anisotropic Magnetoresistance Effect 16
2-2-3 Giant Magnetoresistance Effect 18
2-2-4 Tunneling Magnetoresistance Effect 23
2-2-5 Colossal Magnetoresistance Effect 25
2-3 Exchange Bias Coupling 25
2-4 CPP-GMR Junctions and Its Applications 28
2-4-1 Pseudo Spin Valve 29
2-4-2 Spin Valve 30
References 32
Chapter 3 Experimental Methods and Procedures 37
3-1 Sample Preparations 37
3-1-1 High-Vacuum Multilayer Sputtering System 37
3-1-2 Scanning Electron Microscope 40
3-1-3 Electron Beam Lithography 41
3-2 Junction Fabrication 43
3-3 Sample Measurements 45
3-3-1 Atomic Force Microscopy 45
3-3-2 X-Ray Diffraction 47
3-3-3 Electrical Measurement 48
3-3-4 Magnetic Measurement 49
Chapter 4 Results and Discussion 51
4-1 Exchange Bias Coupling 51
4-1-1 Effect of Ta Buffer Layer 53
4-1-2 Ta/NiFe/FeMn/CoFe and Ta/NiFe/FeMn/NiFe Trilayers 62
4-1-3 Magnetic Properties of Ta/NiFe/FeMn/NiFe/Bi/NiFe Thin Films 66
4-2 CPP-MR Junctions 67
4-2-1 Several Fabricating Methods for CPP-MR Junctions 69
4-2-2 Pseudo Spin-Valve CPP-MR Junctions 72
References 76
Chapter 5 Conclusions 79

List of Figures
Fig. 1-1 Two different Geometries of MR junctions……………………………………6
Fig. 2-1 Variation oh HS with the thickness of Bi spacer layer………………………...12
Fig. 2-2 MR ratio as functions of band overlap of semimetal. Solid curve and open
circles are results obstained by numerical calculation and partial DOS model,
respectively……………………………………………………………………………..13
Fig. 2-3 Anisotropic magnetoresistance in a ferromagnetic metal such as permalloy for
field applied parallel and transverse to the current direction…………………………...17
Fig. 2-4 The origin of AMR…………………………………………………………….18
Fig. 2-5 Magnetoresistance of three different kinds of Fe/Cr superlattices at 4.2 K…...19
Fig. 2-6 Schematic of high- and low-resistance states of GMR multilayers systems….19
Fig. 2-7 Dependence of the GMR ratio of an Fe/Cr multilayer on Cr thickness…….…20
Fig. 2-8 Resistor model of GMR………………………………………………….……21
Fig. 2-9 Schematic densities of states in a normal metal (a) and in a half-metallic
ferromagnet (b)………………………………………………………………..………..22
Fig. 2-10 Schematic of a core-shell particle consisting of a ferromagnetic Co core, with
a surrounding shell of antiferromagnetic CoO…………………………………………26
Fig. 2-11 Hysteresis loop in a system with exchange bias……………………………..26
Fig. 2-12 Simple schematic illustration of spin configuration in an FM-AFM bilayer...27
xii
Fig. 2-13 The typical structure of the pseudo spin valve CPP-MR junction and its
related MR loop (solid line). The dash line is the corresponding magnetization curve..30
Fig. 2-14 The typical structure of the spin valve CPP-MR junction and its related MR
loop (solid line). The dash line is the corresponding magnetization curve…………….31
Fig. 3-1 The schematic configuration of the sputtering system…………...……………38
Fig. 3-2 Two high-vacuum sputtering systems…………………………………………38
Fig. 3-3 Maps of two sputtering systems…………………………………………….....39
Fig. 3-4 Sample holder with two NbFeB magnets……………………………………..39
Fig. 3-5 Procedure of e-beam lithography……………………………………………...42
Fig. 3-6 SEM system with the electron beam lithography function………………...….43
Fig. 3-7 The flowchart of the deposition for CPP-MR junction………………………..44
Fig. 3-8 Cross-section view of the CPP-MR junction…………………………...……..45
Fig. 3-9 Atomic force microscopy………………………………………………….…..46
Fig. 3-10 Two different scanning modes…………………………………….…………46
Fig. 3-11 Sketch of x-ray diffraction…………………………………………………...47
Fig. 3-12 The effective circuit of four-probe method…………………………………..48
Fig. 3-13 Magnetoresistance measurement system………………………………….…49
Fig. 3-14 Vibrating sample magnetometer…………………………………………..…49
Fig. 3-15 Conceptual diagram of the vibrating sample magnetometer………………...50
xiii
Fig. 4-1 Schematic sequence of the exchange bias structure…………………………..53
Fig. 4-2 Variation of HEB with Ta thickness……………………………………………54
Fig. 4-3 The original datum of room-temperature M-H loop of Ta (t nm)/NiFe (5
nm)/FeMn(20 nm)……………………………………………………………………...56
Fig. 4-4 Variation of interface roughness with varying Ta thickness…………………..57
Fig. 4-5 X-ray diffraction patterns of Ta (t nm)/NiFe (5 nm)/FeMn (20 nm)………….58
Fig. 4-6 Diffraction patterns of NiFe/FeMn deposited on Ta buffer layer (35 nm)…….59
Fig. 4-7 M-H loops of Ta=35 nm with different crystalline textures…………………..59
Fig. 4-8 Cross-sectional TEM of NiFe/FeMn thin films on Ta layers with the
thicknesses of 5 nm (a) and 35 nm (b)………………………………………………….61
Fig. 4-9 Two different top pinned layers (a) CoFe (b) NiFe……………………………63
Fig. 4-10 M-H loops of Ta(t nm)/NiFe(5 nm)/FeMn(20 nm)/CoFe(5 nm)…………….64
Fig. 4-11 Exchange bias structure with NiFe pinned layers……………………………65
Fig. 4-12 Thin film with Ta/NiFe/FeMn/NiFe/Bi/NiFe………………………………...66
Fig. 4-13 M-H loops of (a) Ta/NiFe/FeMn/NiFe (b) Ta/NiFe/FeMn/NiFe/Bi/NiFe…...67
Fig. 4-14 GMR effect of Co/Cu/Co trilayer (CIP geometry)…………………………..68
Fig. 4-15 MR of Co/Bi/Co (CIP geometry)…………………………………………….68
Fig. 4-16 Schematic configuration of the sample with millimeter-size………………...69
Fig. 4-17 Several structures of the samples with micrometer size (a) pseudo spin valve
xiv
systems (b) spin valve systems…………………………………………………………70
Fig. 4-18 Pictures of the samples with millimeter-size………………………………...71
Fig. 4-19 Schematic configurations of the sample (a) without top lead (b) with top
lead……………………………………………………………………………………...72
Fig. 4-20 Structure of CPP samples…………………………………………………….73
Fig. 4-21 MR loops of Co(2.5 nm)/Bi(5 nm)/Co(40 nm) with different applied currents
(a) 0.3 mA (b) 1 mA……………………………………………………………………74
Fig. 4-22 Cross-section view of the bismuth thin film (2 nm)…………………………75
xv
List of Tables
Table 2-1 Comparisons of different magnetoresistance effects…………………….…..15
Table 4-1 HEB of bottom and top layers with varying Ta thicknesses………………….63

Ch1 References
[1] Bean, C. P., in Structure and Properties of Thin Films, ed. C. Neugebauer, et al. Wiley, New York, 1959, p. 331
[2] U. Gradmann, et al., Physica status solidi, 27, 313 (1968)
[3] K. Takanashi, et al., Appl. Phys. Lett. 72, 737 (1998)
[4] W. H. Meiklejohn, et al., Phys. Rev. 102, 1413 (1956)
[5] M. B. Salamon, et al., Phys. Rev. Lett. 56, 259 (1986)
[6] C. F. Majkrzak, et al., Phys. Rev. Lett. 56, 2700 (1986)
[7] P. Grunberg, et al., Phys. Rev. Lett. 57, 2442 (1986)
[8] W. Thomson, Proc. R. Soc. 8, 546 (1857)
[9] M. Julliere, Phys. Lett. 54A, 225 (1975)
[10] M. N. Baibich, et al., Phys. Rev. Lett. 61, 2472 (1988)
[11] G. Binasch, et al., Phys. Rev. B39, 4828 (1989)
[12] A. Fert, et al., J. Magn. Magn. Mater. 140-144, 1 (1995)
[13] A. Watanabe, et al., J. Magn. Magn. Mater. 272-276 1720-1721 (2004)
[14] G. S. Patrin, et al., Physics Letters A 363, 164-167 (2007)
[15] Elena Condrea, et al., Physica B 294-295, 328-331 (2001)
[16] Sunglae Cho, et al., J. Magn. Magn. Mater. 239, 201-203 (2002)
[17] Jen-Hwa Hsu, et al., J. Magn. Magn. Mater. 272-276, 1769-1771 (2004)
[18] Jen-Hwa Hsu, et al., J. Magn. Magn. Mater. 294, e99-e103 (2005)
[19] V.O. Denisov, et al., Bismuth-Containing Materials. Structure and Physical/Chemical Properties, UB RAS, Ekaterinburg, 2000, p. 526
[20] I.S. Edel’man, Usp. Fiz. Nauk. 123, 257 (1997)
[21] Yu.T. Levitsky, et al., Semimetals, their Alloys and Compounds, Dal’nauka, Vladivostok, 2004, p. 241
[22] Yu.F. Komnik, Physics of Metal Films, Atomizdat, Moscow, 1979, p. 263
[23] Yu. F. Komnik, et al., Temp. 1, 104 (1975)
Ch2 References
[1] F. Y. Yang, K. Liu, K. Hong, D. H. Reich, P. C. Searson, and C. L. Chien, Science, 284, 1335 (1999)
[2] F. Y. Yang, K. Liu, P. C. Searson, and C. L. Chien, Phys. Rev. Lett. 82, 3328 (1999)
[3] K. Liu, C. L. Chien, and P. C. Searson, Phys. Rev. 58, R14681 (1998)
[4] K. Hong, F. Y. Yang, K. Liu, D. H. Reich, P. C. Searson, C. L. Chien, F. F.
Balakirev, and G. S. Boebinger, J. Appl. Phys. 85, 6184 (1999)
[5] Z. Zhang, X. Sun, M. S. Dresselhaus, J. Y. Ying, and J. Heremens, Phys. Rev. B 61, 4850 (2000)
[6] F. Y. Yang, G. J. Strijkers, K. Hong, D. H. Reich, P. C. Searson, and C. L. Chien, J. Appl. Phys. 89, 7206 (2001)
[7] T. E. Huber, K. Celestine, and M. J. Graf, Phys. Rev. B 67, 245317 (2003)
[8] J. Wang, G. Cao, and Y. Li, Materials Research Bulletin 38, 1645-1651 (2003)
[9] M. D. Jaeger, V. Tsoi, and B. Golding, Appl. Phys. Lett. 68, 1282, (1996)
[10] N. Garcia, Y. H. Kao, and M. Strogin, Phys. Rev. B 5, 2029 (1972)
[11] D. H. Reneker, Phys. Rev. Lett. 1, 440 (1958)
[12] W. S. Boyle and G. E. Smith, Prog. Semicond. 7, 1 (1963)
[13] J. H. Mangez, J. P. Issi, and J. Heremens, Phys. Rev. B 14, 4381 (1976)
[14] P. B. Alers and R. T. Webber, Phys. Rev. 91, 1060 (1953)
[15] E. Condrea, A. D. Grozav, N. I. Leporda, and F. m. Muntyanu, Physica B 294-295, 328-331 (2001)
[16] S. Cho, Y. Kim, L. J. Olafsen, I. Vurgaftman, A. J. Freeman, G. K. L. Wong, J. R. Meyer, C. A. Hoffman, and J. B. Ketterson, J. Magn. Magn. Matter. 239, 201-203 (2002)
[17] K. Kang, et al., J. Appl. Phys. 98, 073704 (2005)
[18] Q. M. Chen, et al., J. Appl. Phys. 63, 2452 (1988)
[19] J. L. Costa-Kramer, et al., Phys. Rev. Lett. 78, 4990 (1997)
[20] K. I. Lee, et al., Phys. Stat. Sol. 241, 1510 (2004)
[21] Y. M. Lin, et al., Phys. Rev. B 62, 4610 (2000)
[22] Jen-Hwa Hsu and D. R. Sahu, Appl. Phys. Lett. 86, 192501 (2005)
[23] G. S. Partrin, V. Yu. Yakovchuk, and D. A. Velikanov, Physics Letters A 363, 164-167 (2007)
[24] Y. Harada, Y. Nakanishi, N. Yoshimoto, M. Daibo, M. Nakamura, and M. Yoshizawa, Physica B 329-333, 1009-1110 (2003)
[25] A. Watanabe, H. Itoh, and J. Inoue, J. Magn. Magn. Mater. 272-276, 1720-1721 (2004)
[26] T. R. McGuire, et al., IEEE Trans. Mag. 11, 1018 (1975)
[27] J. Kondo, Prog. Theor. Phys. 27, 772 (1962)
[28] M. N. Baibich, et al., Phys. Rev. Lett. 61, 2472 (1988)
[29] P. Grunberg, et al., J. Appl. Phys. 61, 3750 (1987)
[30] G.. Binasch, et al., Phys. Rev. B 39, 4828 (1989)
[31] S. S. P. Parkin, et al., Phys. Rev. Lett. 64, 2304 (1990)
[32] G. A. Prinz. Magnetoelectronics. Science 282, 1660 (1998)
[33] A. Tanaka, et al., IEEE Trans. Magn. 38, 84 (2002)
[34] A. Jogo, et al., J. Magn. Magn. Mater. 309, 80 (2007)
[35] A. Jogo, et al., J. Magn. Magn. Mater. 310, 1895-1896 (2007)
[36] R. A. de Groot, et al., Phys. Rev. Lett. 50, 2024 (1983)
[37] P. A. Dowben, et al., J. Appl. Phys. 95, 7453 (2004)
[38] D. Tripathy, et al., J. Appl. Phys. 103, 07D702 (2008)
[39] P. M. Tedrow, et al., Phys. Rev. Lett. 26, 192 (1971)
[40] P. M. Tedrow, et al., Phys. Rev. B7, 318 (1973)
[41] P. Fylde, Adv. Phys. 22, 667 (1973)
[42] M. Julliere, Phys. Lett. 54A, 225 (1975)
[43] J. M. De Teresa, et al., Phys. Rev. Lett. 82, 4288 (1999)
[44] J. C. Slonczewski, Phys. Rev. B 39, 6995-7002 (1989)
[45] S. S. P. Parkin, et al., Nat. Mater. 3, 862 (2004)
[46] S. Yuasa, et al., Nat. Mater. 3, 868 (2004)
[47] D. D. Djayaprawira, et al., 86, 092502 (2005)
[48] S. Jin, et al., Science 264, 413 (1994)
[49] W. H. Meikeljohn, et al., Phys. Rev. 105, 904 (1957)
[50] J. Nogues, et al., J. Magn. Magn. Mater. 192, 203-232 (1999)
[51] J. Nogues, et al., Physics Reports 422, 65-117 (2005)
[52] G. C. Han, et al., Thin Solid Films 505, 41-44 (2006)
[53] K. Nagasaka, et al., FUJITSU Sci. Tech. J. 42, 149-157 (2006)
[54] J. Bass, et al., J. Magn. Magn. Mater. 200, 274-289 (1999)
[55] J. Bass, et al., Physica B 321, 1-8 (2002)
[56] H. Hoshiya, et al., J. Appl. Phys. 95, 6774 (2004)
[57] H. Fukuzawa, et al., J. Appl. Phys. 97, 10C509 (2005)
[58] K. Hoshino, et al., J. Appl. Phys. 99, 08T103 (2006)
[59] A. Jogo, et al., J. Magn. Magn. Mater. 310, 1895-1896 (2007)
[60] Y. M. Kang, et al., J. Magn. Magn. Mater. 310, 1911-1913 (2007)
Ch4 References
[1] J. Nogues and I. K. Schuller, J. Magn. Magn. Mater., 192, 203 (1999)
[2] R.D. Hempstead, S. Krongelb, and D.A. Thomson, IEEE Trans. Magn., MAG-14, 521 (1978)
[3] G. Choe, A. Tsoukatos, and S. Gupta, IEEE Trans. Magn., 34, 867 (1998)
[4] Y. Miyamoto, T. Yoshitani, S. Nakagawa, and M. Naoe, IEEE Trans. Magn., 32, 4672 (1996)
[5] M. Allegranza and M.M. Chen, J. Appl. Phys., 73, 6218 (1993)
[6] W.E. Bailey, N.C. Zhu, R. Sinclair, and S.X. Wang, J. Appl. Phys., 79, 6393 (1996)
[7] V. K. Sankaranarayanan, S.M. yoon, D.Y. kim, C.O. kim, and C.G. Kim, J. Appl. Phys., 96, 7428 (2004)
[8] G. Choe and S. Gupta, Appl. Phys. Lett., 70, 1766 (1997)
[9] L. Richie, X. Liu, S. Ingvarsson, G. Xiao, J. Du, and J.Q. Xiao, J. Magn. Magn. Mater., 247, 187 (2002)
[10] M.H. Li, J.W. Cai, G.H. Yu, H.W. Jiang, W.Y. Lai, and F.W. Zhu, Materials Science and Engineering, B90, 296-300 (2002)
[11] M. Xu, et al., J. Appl. Phys., 92, 2052 (2002)
[12] K.T.Y. Kung, L.K. Louie, and G.L. Gorman, J. Appl. Phys., 69, 5634 (1991)
[13] T.G.S.M. Rijks, et al., IEEE Trans. Magn., 31, 3865 (1995)
[14] W.H. Mieklejohn and C.P. Bean, Phys. Rev., 102, 1413-1414 (1956)
[15] R. Jungblut, et al., J. Mag. Mag. Mater., 148, 300-306 (1995)
[16] R. Nakatani, et al., J. Mag. Mag. Mater., 173, 5634-5636 (1997)
[17] Jen-Hwa Hsu, Zhi-Long Xue, Ta-Chieh Huang, Zung-Hang Wei, and Mei-Fang Lai, J. Magn. Magn. Mater., 310, 2239-2241 (2006)
[18] R. Loloee, P.A. Schroeder, W.P. Pratt Jr. J.Bass, and A. Fert, Physica B, 204, 274-280 (1995)
[19] N.J. List, W.P. Pratt Jr., M.A. Howson, J. Xu, M.J. Walker, and D. Greig, J. Magn. Magn. Mater., 148, 342-343 (1995)
[20] J. Bass and W.P. Pratt Jr., J. Magn. Magn. Mater., 200, 274-289 (1999)
[21] J. Bass and W.P. Pratt Jr., Physica B, 321, 1-8 (2002)
[22] A. Paul, M. Buchmeier, D.E. Burgler, and P. Grunberg, J. Appl. Phys., 97, 023910 (2005)
[23] M. Xu, G. xiong, and S. Xu, J. Appl. Phys., 102, 096103 (2007)
[24] R. J. M. van de Veerdonk, J. Nowak, R. Meservey, J. S. Moodera, and W. J. M. de Jonge, Appl. Phys. Lett. 71, 2839 (1997)
[25] A. Watanabe, H. Itoh, and J. Inoue, J. Magn. Magn Mater., 272-276, 1720-1721 (2004)