Spin-transfer switching in magnetic tunnel junctions with synthetic ferri-magnetic free layer

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1 Journal of Physics: Conference Series Spin-transfer switching in magnetic tunnel junctions with synthetic ferri-magnetic free layer To cite this article: M Nishimura et al 1 J. Phys.: Conf. Ser. 518 Recent citations - Dependence of spin-transfer switching characteristics in magnetic tunnel junctions with synthetic free layers on coupling strength Masayuki Nishimura et al View the article online for updates and enhancements. This content was downloaded from IP address on 17/1/18 at 3:

2 International Conference on Magnetism (ICM 9) Journal of Physics: Conference Series (1) 518 doi:1.188/ //5/518 Spin-transfer Switching in Magnetic Tunnel Junctions with Synthetic Ferri-magnetic Free Layer M ishimura 1, M Oogane 1, H aganuma 1, Inami 1, S Ikeda, H Ohno and Y Ando 1 1 Graduate School of Engineering, Tohoku University, Sendai , Japan RIEC, Tohoku University, Sendai , Japan masanish@mlab.apph.tohoku.ac.jp Abstract. We have fabricated the SyF structure with both high annealing stability and strong interlayer exchange coupling and investigated tunnelling magnetoresistance (TMR) and spintransfer switching properties of magnetic tunnel junctions (MTJs) with developed SyF free layer. The fabricated SyF with structure of Ta/Ru/CoFe/Ru/CoFeB possessed high annealing stability of o C and strong interlayer exchange coupling. Consequently, a large TMR ratio of 1% has been observed after annealing at high temperature of 35 o C. In addition, we have successfully observed spin-transfer switching by the net current density of 1 MA/cm and the large thermal stability factor of Introduction Spin-transfer switching predicted by Slonczewski [1] and Berger [] has recently attracted much attention because of its potential application in STT-RAM (spin-transfer torque random access memory). In particular, spin-transfer switching in magnetic tunnel junctions (MTJs) which consist of CoFeB/MgO/CoFeB has been reported a lot owing to their large tunnel magnetoresistance (TMR) ratio and low critical current density (J c ) for magnetization switching [3]. However, for making a highdensity STT-RAM, it is necessary to further reduce J c while maintaining a high thermal stability factor (E/k B T, where E, k B, and T are the energy potential, the Boltzmann constant, and temperature, respectively) well over 6. While J c is proportional to the product of the magnetization and the thickness of the free layer, E/k B T is proportional to the volume of the free layer. Therefore, it is very difficult to meet the above requirements. Synthetic ferrimagnetic (SyF) structures consisting of two or more ferromagnetic layers which couple antiparallel through non-magnetic spacers such as Ru is expected to provide large volume to withstand thermal fluctuation [], so spin-transfer switching in MTJs using the SyF free layer have been investigated [5]. However, the effect of SyF free layer is not investigated sufficiently, probably because the interlayer coupling strength of the SyF structure in previous report is not so strong. In addition, the SyF structure has poor annealing stability [6]. This weakness is a serious problem in using the SyFs in the CoFeB/MgO/CoFeB MTJs because high annealing temperature is necessary to obtain a large TMR ratio. In this work, we have fabricated the SyF structure with both high annealing stability and strong interlayer exchange coupling by optimizing the stacking structure of the multilayer film and investigated TMR and spin-transfer switching properties of MTJs with developed SyF free layer. c 1 Ltd 1

3 International Conference on Magnetism (ICM 9) Journal of Physics: Conference Series (1) 518 doi:1.188/ //5/518. Experiment Thin films were deposited on thermally oxidized Si wafers by using magnetron sputtering with a base pressure of 1-6 Pa. The samples were annealed in the temperature range from 3 to 5 o C for 1 h in a 1 - Pa vacuum under a magnetic field of 1 koe. We measured magnetic properties of the SyF structures using vibrating sample magnetometer (VSM). The nano-scaled junctions were fabricated using electron-beam lithography process. The TMR properties were measured at room temperature using a four-probe method with AC bias and magnetic field up to about koe and the spin-transfer switching properties were evaluated by measuring resistance by 1 µa-step current pulses with the pulse width ranging from 1 µs to 1 s. The current direction was defined as positive when the electrons flow from the bottom (free) to the top (pined) layer. 3. Results and discussion 3.1. Fabrication of the SyF structure with both high annealing stability and strong interlayer exchange coupling For optimization of the stacking structure, the dependencies of the saturation field ( ) which represent interlayer exchange coupling strength for the SyF structures with various stacking structures on annealing temperature and Ru middle layer thickness were investigated. Figure 1(a) shows the magnetization curves of the buffer-layer/co 75 Fe 5 ()/Ru(1)/Co 75 Fe 5 ()/Ta(1)-SyFs with various buffer layers of Ta(5), Ru(5), MgO(5) and Ta(5)/Ru(5) (numbers denote thickness in nanometers). Average surface roughnesses (R a ) are.81,.8,.88 and.8 Å, respectively. Though the reason is not clear, the SyF using Ta buffer didn t show exchange coupling in any annealing temperature. The as deposited SyFs on Ru and MgO buffers show coupling. However, after annealing at o C, the antiparallel alignment of the two ferromagnetic layers wasn t obtained. On the other hand, the SyF on Ta/Ru buffer shows exchange coupling even after annealing at o C, though remanent magnetization increases in part. A possible reason of such high thermal stability may be the highly oriented crystal structure of the SyF could reduce interdiffusion at the Co 75 Fe 5 /Ru interface. Figure 1(b) shows the dependence of on annealing temperature for the SyFs with various buffer layers. Figure shows the dependence of on annealing temperature for the Ta(5)/Ru(5)/ferromagneticlayer/Ru(.8)/Co Fe B ()/Ta(1)-SyFs with various ferromagnetic-layers of Co Fe B (), (a) Ta buffer 6 5 (b) MgO Magnetization (a.u.) Ru buffer MgO buffer Ta/Ru buffer o C o C 3 1 Ta/Ru Ru Ta 1 3 Annealing temperature (ºC) o C Field Figure 1. (a) Magnetization curves and (b) dependence of on annealing temperature for buffer-layer/co 75 Fe 5 ()/ Ru(1)/Co 75 Fe 5 ()-SyFs [in nm] with various buffer layers.

4 International Conference on Magnetism (ICM 9) Journal of Physics: Conference Series (1) 518 doi:1.188/ //5/ Co 5 Fe 5 Co 75 Fe 5 Co 5 Fe 5 B 1 Ni 8 Fe Co Fe B as deposited annealed at 35 o C annealed at o C 1 3 Annealing temperature (ºC) Figure. Dependence of on annealing temperature for Ta(5)/Ru(5)/ferromagneticlayer/Ru(.8)/Co Fe B ()-SyFs [in nm] with various ferromagnetic layers....8 Ru thickness (nm) 1. Figure 3. Dependence of on Ru middle layer thickness for Ta(5)/Ru(5)/Co 75 Fe 5 ()/ Ru(t Ru )/Co Fe B ()/MgO(.5)/Ta(1) [in nm, t Ru =.-1. nm]. Co 5 Fe 5 B 1 (), Co 5 Fe 5 (), Co 75 Fe 5 () and Ni 8 Fe (). The conventional SyF using Co Fe B ferromagnetic layer shows poor annealing stability. The SyFs using Co 5 Fe 5 and Co 5 Fe 5 B 1 shows large, but reduces to zero after annealing at o C. In contrast, the SyFs using Co 75 Fe 5 and Ni 8 Fe exhibits large of 6 koe and 3 koe after annealing at and 5 o C, respectively. The SyF using Co 75 Fe 5 ferromagnetic layers exhibits high annealing stability and larger than that using Ni 8 Fe. Consequently, the dependence on Ru middle layer thickness of for the SyFs consisting of Ta(5)/Ru(5)/Co 75 Fe 5 ()/Ru(t Ru )/Co Fe B ()/MgO(.5)/Ta(1) (t Ru =.-1. nm) was investigated as shown in figure 3. As reported previously [7], shows oscillatory behavior with taking the local maximum at t Ru =. and.8 nm. Among them, the SyF structure with t Ru =.8 nm exhibited large of 6 koe after annealing at o C. As a result, the SyF structure having both high annealing stability and strong interlayer exchange coupling have been successfully fabricated. 3.. TMR and spin-transfer switching properties After optimizing the stacking structure of the SyF, we investigated TMR and spin-transfer switching properties of a MTJ consisting of Ta(5)/Ru(1)/Ta(5)/Ru(5)/Co 9 Fe 1 ()/Ru(.8)/Co Fe B ()/MgO (.8)/Co Fe B (5)/Ta(5)/Ru(5). Figure (a) shows MR curve and (b) shows R-I loop for current pulse width (τ p ) of 1 ms and no applied magnetic field for 11 nm elliptic MTJ with the SyF free layer annealed at 35 o C. Apparently, the MR curve shows negative-tmr, because the SyF free layer reverses its magnetization following the reversal of Co 9 Fe 1 which have larger magnetic moment than that of Co Fe B and a decrease of the resistance in a high magnetic field region is attributed to the rotation of two ferromagnetic layers of the SyF free layer. Therefore, this curve exhibits that the two ferromagnetic layers are coupled antiparallel. Higher TMR ratio of 1% than that of MTJ with SyF free layer in the previous report [5] was observed due to 5 o C higher annealing temperature. This result comes from high annealing stability of the developed SyF structure. The R-I loop shows spin-transfer switching process, because the value of high and low resistance in the R-I loop and these in the MR curve are almost same. The average current density required to switch the magnetization from parallel (antiparallel) to antiparallel (parallel) shown in figure (b) is J c P AP = 9.1 MA/cm (J c AP P = -9.9 MA/cm ). Figure 5 shows the dependence of critical current density (J c ) on pulse current width. Net critical current density (J c ) of 1 MA/cm and E/k B T of 6 were estimated. The strong interlayer exchange coupling of the SyF free layer may result in the high thermal stability. 3

5 International Conference on Magnetism (ICM 9) Journal of Physics: Conference Series (1) 518 doi:1.188/ //5/518 J c (MA/cm ) Resistance (Ω) (a) TMR =1 % Field (Oe) Current (ma) Figure. (a) MR curve and (b) R-I loop for an MTJ with Co 9 Fe 1 ()/Ru(.8)/Co Fe B () SyF free layer P AP AP P Pulse width (s) Conclusion We have fabricated MTJs with SyFs bottom free layer, which had structure of Ta/Ru/CoFe/Ru/CoFeB and possessed both high annealing stability of o C and strong interlayer exchange coupling as shown by large of 6 koe. Consequently, a large TMR ratio of 1% has been observed after annealing at high temperature of 35 o C. In addition, we have successfully observed spin-transfer switching by J c of 1 MA/cm and large E/k B T of 6. Acknowledgment This research was supported by the Research and Development for Next-Generation Information Technology and Global COE Program Materials Integration, Tohoku University, by MEXT. References [1] Slonczewski J C 1996 J.Magn. Magn. Mater. 159 L1 [] Berger L 1996 Phys. Rev. B [3] Diao Z, Apalkov D, Pakala M, Ding Y, Panchula A and Huai Y 5 Appl. Phys. Lett [] Saito Y, Sugiyama H, Inokuchi T and Inomata K 6 J. Magn.Magn. Mater [5] Hayakawa J, Ikeda S, Lee Y M, Sasaki R, Meguro T, Matsukura F, Takahashi H and Ohno H 6 Jpn. J. Appl.Phys. 5 L157 [6] Wiese N, Dimopoulos T, Rührig M, Wecker J, Brückl H and Reiss G Appl. Phys. Lett. 85 [7] Parkin S S P, More N and Roche K P 199 Phys. Rev. Lett. 6 3 (b) τ p =1 ms Figure 5. Dependence of J c on pulse current width for the MTJ of Fig..