[Show abstract][Hide abstract] ABSTRACT: We investigated the effect of using a synthetic ferrimagnetic (SyF) free layer in MgO-based magnetic tunnel junctions (MTJs) on current-induced magnetization switching (CIMS), particularly for application to spin-transfer torque random access memory (SPRAM). The employed SyF free layer had a Co<sub>40</sub>Fe<sub>40</sub>B<sub>20</sub>/Ru/Co<sub>40</sub>Fe<sub>40</sub>B<sub>20</sub>and Co<sub>20</sub>Fe<sub>60</sub>B<sub>20</sub>/Ru/Co<sub>20</sub>Fe<sub>60</sub>B<sub>20</sub>structures, and the MTJs (100 times (150-300) nm<sup>2</sup>) were annealed at 300 <sup>deg</sup>C. The use of SyF free layer resulted in low intrinsic critical current density ( J <sub>c0</sub>) without degrading the thermal-stability factor ( E / k <sub>B</sub> T , where E , k <sub>B</sub>, and T are the energy potential, the Boltzmann constant, and temperature, respectively). When the two CoFeB layers of a strongly antiferromagnetically coupled SyF free layer had the same thickness, J <sub>c0</sub> was reduced to 2-4 times10<sup>6</sup> A/cm<sup>2</sup>. This low J <sub>c0</sub> may be due to the decreased effective volume under the large spin accumulation at the CoFeB/Ru. The E / k <sub>B</sub> T was over 60, resulting in a retention time of over ten years and suppression of the write current dispersion for SPRAM. The use of the SyF free layer also resulted in a bistable (parallel/antiparallel) magnetization configuration at zero field, enabling the realization of CIMS without the need to apply external fields to compensate for the offset field.
[Show abstract][Hide abstract] ABSTRACT: A 1.8 V 2 Mb SPin-transfer torque RAM (SPRAM) chip using a 0.2 mum logic process with an MgO tunneling barrier cell demonstrates the circuit technologies for potential low-power nonvolatile RAM, or universal memory. This chip features an array scheme with bit-by-bit bi-directional current writing to achieve proper spin-transfer torque writing of 100 ns, and parallelizing-direction current reading with a low-voltage bit-line for preventing read disturbances that lead to 40 ns access time.
[Show abstract][Hide abstract] ABSTRACT: A 1.8 V 2-Mb SPRAM (SPin-transfer torque RAM) chip using 0.2-mu m logic process with MgO tunneling barrier cell is reviewed, which demonstrates the circuit technologies for potential low power non-volatile RAM, or universal memory. This chip features: an array scheme with bit-by-bit bi-directional current write to achieve proper spin-transfer torque writing of 100-ns, and parallelizing-direction current reading with low voltage bit-line that leads to 40-ns access time. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Integrated Circuit Design and Technology, 2007. ICICDT '07. IEEE International Conference on; 12/2007
[Show abstract][Hide abstract] ABSTRACT: We report the intrinsic critical current density (Jc0) in current-induced magnetization switching and the thermal stability factor (E/kBT, where E, kB, and T are the energy potential, the Boltzmann constant, and temperature, respectively) in MgO based magnetic tunnel junctions with a Co40Fe40B20(2nm)/Ru(0.7-2.4nm)/Co40Fe40B20(2nm) synthetic ferrimagnetic (SyF) free layer. We show that Jc0 and E/kBT can be determined by analyzing the average critical current density as a function of coercivity using the Slonczewski's model taking into account thermal fluctuation. We find that high antiferromagnetic coupling between the two CoFeB layers in a SyF free layer results in reduced Jc0 without reducing high E/kBT. Comment: 16 pages, 4 figures. Jpn. J. Appl. Phys., in press
Japanese Journal of Applied Physics 09/2006; 45(37-41). DOI:10.1143/JJAP.45.L1057 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The current-driven magnetization switching in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with various free layer materials including synthetic structure was studied in this paper. The MTJ films were deposited on SiO<sub>2</sub>/Si substrates using RF magnetron sputtering. The tunnel magnetoresistance (TMR) ratios and current-voltage (I-V) characteristics of the the MTJs were measured at room temperature using a dc four-probe method and with a magnetic field of up to 1 kOe. The TMR ratio and the current-driven magnetization switching at critical current densities were shown as a function of annealing temperature for two types of MTJs with different free layer.
[Show abstract][Hide abstract] ABSTRACT: We investigated dependence of tunnel magnetoresistance effect in CoFeB/MgO/CoFeB magnetic tunnel junctions on Ar pressure during MgO-barrier sputtering. Sputter deposition of MgO-barrier at high Ar pressure of 10 mTorr resulted in smooth surface and highly (001) oriented MgO. Using this MgO as a tunnel barrier, tunnel magnetoresistance (TMR) ratio as high as 355% at room temperature (578% at 5K) was realized after annealing at 325 C or higher, which appears to be related to a highly (001) oriented CoFeB texture promoted by the smooth and highly oriented MgO. Electron-beam lithography defined deep-submicron MTJs having a low-resistivity Au underlayer with the high-pressure deposited MgO showed high TMR ratio at low resistance-area product (RA) below 10 ohm-um^2 as 27% at RA = 0.8 ohm-um^2, 77% at RA = 1.1 ohm-um^2, 130% at RA = 1.7 ohm-um^2, and 165% at RA = 2.9 ohm-um^2. Comment: 14 pages, 5 figures
Japanese Journal of Applied Physics 10/2005; 44(46-49). DOI:10.1143/JJAP.44.L1442 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Current-driven magnetization switching in low-resistance Co40Fe40B20/MgO/Co40Fe40B20 magnetic tunnel junctions (MTJs) is reported. The critical-current densities Jc required for current-driven switching in samples annealed at 270C and 300C are found to be as low as 7.8 x 10^5 A/cm^2 and 8.8 x 10^5 A/cm^2 with accompanying tunnel magnetoresistance (TMR) ratios of 49% and 73 %, respectively. Further annealing of the samples at 350C increases TMR ratio to 160 %, while accompanying Jc increases to 2.5 x 10^6 A/cm^2. We attribute the low Jc to the high spin-polarization of tunnel current and small MsV product of the CoFeB single free layer, where Ms is the saturation magnetization and V the volume of the free layer. Comment: 13 pages, 5 figures
Japanese Journal of Applied Physics 10/2005; 44(No. 41). DOI:10.1143/JJAP.44.L1267 · 1.13 Impact Factor