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1 w Journal of the Korean Magnetics Society, Volume 19, Number 1, February 2009 v mjx» w (STT-MRAM)» w ½ ³Á Á Á½ š w œ w œw, p 5-1, ( , , y )»l w» MRAM(Magnetoresistive Random Access Memory) y w ƒ w» (writing operation) y v w y j. š ù»» w MRAM» w p v mj(spin Transfer Torque, STT) w y» w»,» r. :»l w, MRAM, v mj, x y I. MRAM» p x 1996 IBM Slonczewski[1] Carnegie Mellon Berger[2] ƒƒ mw v w ü y w ƒ w w š, 1999 Cornell Buhrman[3] w x v mj(spin Transfer Torque, STT) y (Current Induced Magnetization Switching, CIMS) x w š. j ü y w w v ƒ n wš w v ƒ. x v (spin dependent scattering) w. Fig. 1 p w v ù Fig. 1. Schematic illustration of spin transfer torque. The spinunpolarized current has been spin polarized by passing through first ferromagnet F1. This spin polarized current are rearranged by ferromagnet F2. In the process, angular momentum is effectively transferred from the electron spin to the ferromagnetic magnetization. This transfer of angular momentum can be described as a torque from the electron spin on the magnetization. *Tel: (02) , ykim97@korea.ac.kr w, F1 y w w y w w v n š, w v ƒ v ù. n F2 w, w v w F2 y w sww., v v ƒ y j e w y j ƒ F2 y w mj (torque) k. mj j»ƒ j y w. y w v w x» w z STT v ƒ (spin angular momentum) ü y w w x [1, 2]. STT w y (magnetic switching) w STT-MRAM, (current induced domain wall motion) w Race Track Memory, y (magnetic precession) w š v (microwave device), v (spin diode) [4-6]. ƒ MRAM» w(magnetoresistance) w z w», É» ƒ { (nonvolatile) p š p, w ƒ¾» ƒ w mwx t wù. Fig. 2» MRAM STT-MRAM š. F œ w Feature Size w 2ƒ, F š y w w. 22

2 w v mjx» w (STT-MRAM)» w ½ ³Á Á Á½ 23 Fig. 2. Comparison between the conventional MRAM (magnetic field switching) and STT-MRAM (current induced magnetization switching).» MRAM» w w» w y w kw. ƒ» d y w w. w wù w ƒw, š y. p»l w j»ƒ e w w (Coercivity) f j ƒ w. w p k w j»ƒ œ ³ w»ƒ š, ƒ ƒ¾ p w» w ƒ w. STT y» w»l w y w ã w y w j»ƒ ƒ ( ) š y w p (Fig. 3). w» w v w v MRAM jl ƒ w š w [7]. STT w MRAM ƒ w y v w (J c ) j. MRAM» wù»l w wù p l, wz 90 nm» w» w, 100 nm p 100 µa œ w y p l(cmos) w w» w (J c ) ƒ 10 6 A/cm 2 û w. II. STT y» Macrospin model» w y w (I c ) tx [9]. Fig. 3. Current required to observe current induced magnetization switching as a function of junction size for four different critical current densities. The black thick solid line shows the required current for magnetic field switching. The gray thick line shows the current that a COMS with the gate width equal to the junction size can provide (100 µa/100 nm). From Ref. [8]. 2eα I c hg M s V H k + 2πM s ( )» H y (magnetic easy axis) w» ùkü, M s sy y, V» d v, α g ƒƒ š (damping constant), v (spin polarization), H k» w., y j»» d j» w., d j»ƒ y ƒ w,»» w» d w œw. w, s y» 2πM S w», j» sy y w š w. j» û sy y d w z ù, sy y w, w v w. w û v w j. (1)

3 24 1. y w y» w y» j l w d d. w y d û sy y š, v ƒ w û. ƒ š d CoFeB sy y 1040 emu/cm w 3 CoFe (Co 90 Fe emu/cm ) w sy y 3 70 %. CoFeB w, 2 z ƒ š. w CoFeB v ƒ CoFe ƒ š z y k [10]. v ƒ š (half metal) w»l w š, CoFeB yww Fig. 4. TMR ratios of the MTJs as a function of the MgO thickness. The layer thickness s cale is given in nm in the legends. (b) Relation between the RA and TMR ratio in different free layer stacks. The straight line indicates the TMR and RA relationship with the same MgO thickness and different free layer structures. From Ref. [12]. w»wz 19«1y, š [11, 12]. q» CoFeB NiFeSiB d w» û sy y» w û w [13].» AlO x MgO l š Butler w MgO w,» w ƒ 1000 % ƒ w t š[14], x %» w z w [15]. (001) w ww MgO vk (Epitaxy) w d v k n (Coherent tunneling) w» w MRAM w š y w p ƒ, AlO x w wš, v ƒ g» w [16].»l w w ƒ. w y w (» ) l w q ƒ ù wù w. w û l Ì w w, Ì w» w ƒ w û w» w w w. Canon-Anelva National Institute of Advanced Industrial Science and Technology(AIST) MgO d 0.4 nm Ì Mg d w z w û tw [17],»l w MgO z w w š [18]. (001) MgO s w j»ƒ ƒw, d Epitaxy 2 w 206 %» w 2.1 Ωµm û wü»l w (RA) w. 2. y w y» y» w»l w y j w w wš. t»l w(dual MgO magnetic tunnel junctions) w ù[19], w x d(synthetic free-layer) [20, 21] Grandis y w š š d d e k v (spin accumulation) jš d Ta d(capping layer) v rv(spin

4 w v mjx» w (STT-MRAM)» w ½ ³Á Á Á½ 25 Fig. 5. Magnetic field switching (a) and current induced magnetization switching (b) at room temperature for dual MTJ structure with double MgO tunnel barriers. From Ref. [19]. pumping) w z» û. w š 120Ü240 nm j» ma 2 ( (J c ) 1.0Ü10 6 A/cm w )» w ¾ š»l w ƒ ƒ û ùkü» w ƒ 70 % š v w, MgO l d y w ƒ ³ v w. w x d» d d / / 3d xk xk z wì d ƒ š. Tohoku Ohno CoFeB y w(synthetic antiferromagnetic coupling) wš CoFeB/Ru/CoFeB d w ƒ w w x tw. Fig Ü 200 nm j» w x d» w %, J c 2.0Ü10 6 A/cm 2, ùkü K U V/ k B T 47» w. w x d w v w v mj z ƒw Fig. 6. Schematic drawing of the cross-section of the MTJs. (a) magnetic field switching characteristic, (b) CIMS of pulse current (10 ms), (c) J c ave plot as a function of ln(τ p /τ 0 ) at room temperature for MTJ with a CoFeB(2 nm)/ru(0.7 nm)/cofeb(2 nm) synthetic free layer. From Ref. [20]. Fig. 7. Hysteresis loops as a function of applied magnetic field for an element with 100 nm width at different aspect ratio. From Ref. [22]. ƒw, w y w(exchange coupling) ƒ g š d tw ù, x ¾ yw x š. 3.» w e» MRAM» wù w w š y w cell j» j z (aspect ratio) w»l w ü w (demagnetization field) w (multi-domain)ƒ x š w w y x ùkù. j z w w x» w w š y j w w (Fig. 7)[22]. w w w» w»l w š d d». 2002, Canon» TbFeCo, GdFeCo d š d w»l w wš,» w 55 % z w [23]. w MFM(magnetic force microscope) mw» l w x ù l (vortex) y p x y w (Fig. 8). w» w y MRAM w w» j» ƒ k w ù, 2006 Hitachi Global Storage Technologies (HGST) v w (CNRS)» w v (spin valve) y x œw,» ƒ s» y (1) š

5 26 Fig. 8. Magnetic-force microscopy (MFM) images for in-plane magnetization film and perpendicular magnetization film at zero field. (a) 40-nm-thick µm 2 square NiFe film, (b) 100-nm-thick square Gd/Fe/FeCo films, µm 2, and µm 2. From Ref. [23]. ƒ w w [24]. 2eα I c hg M s V H k 4πM s ( ) (2) c y sw k sw k y w w H k w» ùkü. s»» y w y w w ƒƒ (M s H k V)/2, (M s (H k 4πM s )V)/2 tx ƒ w.,» j»»l w cell z H k (H k = H k 4πM s ) w ù, s» ƒ» w 2πM S j j» w w., j»ƒ w,» y w d œw Toshiba m (rare-earth) sww» w»l w y œw [25]. 130 nm j»»l w» w 15 %, J c 4.7Ü10 6 A/ cm 2, K U V/k B T 107 w w,» w 60 %, 2¾ 2.7Ü10 6 A/cm ùkü [26].» x ¾ 100 %» w ùküš w w w,»l w cell j»ƒ ùe j» ƒ ƒ g w v w. III. STT-MRAM yw»» w w w û š w w mwx j z sƒ š. (2) w»wz 19«1y, p,»l w MgO l w š» w j» w û w w y te w.,» w y mw û v ƒ. w ƒ š, ¾ STT-MRAM yƒ ƒ w w w û y w w ù, x ü ƒ w q. w» w ƒ z ù n wš. p w z «w MRAM w n w ¼ w x ƒ w w. w» w w ƒ ( y M J ) w w. š x [1] J. C. Slonczewski, J. Magn. Magn. Mater., 159, L1 (1996). [2] L. Berger, Phys. Rev. B, 54, 9353 (1996). [3] E. B. Myers, D. C. Ralph, J. A. Katine, R. N. Louie, and R. A. Buhrman, Science, 285, 867 (1999). [4] J. A. Katine, F. J. Albert, R. A. Buhrman, E. B. Myers, and D. C. Ralph, Phys. Rev. Lett., 84, 3149 (2000). [5] S. S. P. Parkin, U.S. Patent No. 6,834,005 (2004). [6] S. I. Kiselev, J. C. Sankey, I. N. Krivorotov, N. C. Emley, R. J. Schoelkopf, R. A. Buhrman, and D. C. Ralph, Nature, 425, 380 (2003). [7] ½, w», 16, 28 (2007). [8] S. Ikeda, J. Hayakawa, Y. M. Lee, F. Matsukura, Y. Ohno, T. Hanyu, and H. Ohno, IEEE Trans. Electron Dev., 54, 991 (2007). [9] J. Z. Sun, Phys. Rev. B, 62, 570 (2000). [10] S. X. Huang, T. Y. Chen, and C. L. Chien, Appl. Phys. Lett., 92, (2008). [11] Y. Sakuraba, M. Hattori, M. Oogane, Y. Ando, H. Kato, A. Sakuma, T. Miyazaki, and H. Kubota, Appl. Phys. Lett., 88, (2006). [12] K. Tsunekawa, D. D. Djayaprawira, S. Yuasa, M. Nagai, H. Maehara, S. Yamagata, E. Okada, N. Watanabe, Y. Suzuki, and K. Ando, IEEE Trans. Magn., 42, 103 (2006). [13] C. U. Cho, D. K. Kim, T. X. Wang, S. Isogami, M. Tsunoda,

6 w v mjx» w (STT-MRAM)» w ½ ³Á Á Á½ 27 M. Takahashi, and Y. K. Kim, IEEE Trans. Magn., 44, 2547 (2008). [14] W. H. Butler, X.-G. Zhang, J. M. MacLaren, and T. C. Schulthess, Phys. Rev. B, 63, (2001). [15] S. Ikeda, J. Hayakawa, Y. Ashizawa, Y. M. Lee, K. Miura, H. Hasegawa, M. Tsunoda, F. Matsukura, and H. Ohno, Appl. Phys. Lett., 93, (2008). [16] G. D. Fuchs, J. A. Katine, S. I. Kiselev, D. Mauri, K. S. Wooley, D. C. Ralph, and R. A. Buhrman, Phys. Rev. Lett., 96, (2006). [17] K. Tsunekawa, D. D. Djayaprawira, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, Y. Suzuki, and K. Ando, Appl. Phys. Lett., 87, (2005). [18] S. Isogami, M. Tsunoda, K. Komagaki, K. Sunaga, Y. Uehara, M. Sato, T. Miyajima, and M. Takahashi, Appl. Phys. Lett., 93, (2008). [19] Z. Diao, A. Panchula, Y. Ding, M. Pakala, S. Wang, Z. Li, D. Apalkov, H. Nagai, A. Driskill-Smith, L. C. Wang, E. Chen, and Y. Huai, Appl. Phys. Lett., 90, (2007). [20] J. Hayakawa, S. Ikeda, Y. M. Lee, R. Sasaki, T. Meguro, F. Matsukura, H. Takahashi, and H. Ohno, Jpn. J. Appl. Phys., 45, L1057 (2006). [21] J. Hayakawa, S. Ikeda, K. Miura, M. Yamanouchi, Y. M. Lee, R. Sasaki, M. Ichimura, K. Ito, T. Kawahara, R. Takemura, T. Meguro, F. Matsukura, H. Takahashi, H. Matsuoka, and H. Ohno, IEEE Trans. Magn., 44, 1962 (2008). [22] E. Girgis, J. Schelten, J. Shi, J. Janesky, S. Tehrani, and H. Goronkin, Appl. Phys. Lett., 76, (2000). [23] N. Nishimura, T. Hirai, A. Koganei, T. Ikeda, K. Okano, Y. Sekiguchi, and Y. Osada, J. Appl. Phys., 91, 5246 (2002). [24] S. Mangin, D. Ravelosona, J. A. Katine, M. J. Carey, B. D. Terris, and Eric E. Fullertone, Nature Materials, 5, 210 (2006). [25] M. Nakayama, T. Kai, N. Shimomura, M. Amano, E. Kitagawa, T. Nagase, M. Yoshikawa, T. Kishi, S. Ikegawa, and H. Yoda, J. Appl. Phys., 103, 07A710 (2008). [26] H. Yoda, T. Kishi, T. Nagase, M. Yoshikawa, E. Kitagawa, T. Daibou, K. Nishiyama, T. Kai, N. Shimomura, M. Nakayama, M. Amano, H. Aikawa, S. Takahashi, S. Ikegawa, M. Nagamine, J. Ozeki, S. Yuasa, Y. Nakatani, M. Oogane, Y. Ando, Y. Suzuki, T. Miyazaki, and K. Ando, 2008 Asian Magnetic Conference (Pusan, Korea), AA04 (2008). Technology Trend of Spin-Transfer-Torque Magnetoresistive Random Access Memory (STT-MRAM) D. K. Kim, J. U. Cho, S. J. Noh, and Y. K. Kim Department of Materials Science and Engineering, Korea University, Seoul , Korea (Received 22 January 2009, Received in final form 23 January 2009, Accepted 28 January 2009) Reduction of the critical current density (J c ) for STT magnetization switching is most important issue of magnetic tunnel junctions (MTJs) based MRAM. This report describes how to decrease the J c and will introduce the recent research progresses of STT-MRAM devices with material engineering and structural improvement, respectively. Keywords : Magnetic tunnel juctions, MRAM, spin transfer torque, current induced magnetization switching