Ryutaro Sasaki

2Mb Spin-Transfer Torque RAM (SPRAM) with Bit-by-Bit Bidirectional Current Write and Parallelizing-Direction Current Read

A 1.8V 2Mb spin-transfer torque RAM chip using a 0.2mum logic process with an MgO tunneling barrier cell demonstrates the circuit technologies for potential low-power non-volatile RAM, or universal memory. This chip features an array scheme with bit-by-bit bidirectional current write to achieve proper spin-transfer torque writing in 100ns, and parallelizing-direction current reading with a low-voltage bit-line that leads to 40ns access time.

Current-Driven Magnetization Switching in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

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 SiO2/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.

Current-Induced Magnetization Switching in MgO Barrier Based Magnetic Tunnel Junctions with CoFeB/Ru/CoFeB Synthetic Ferrimagnetic Free Layer

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(2 nm)/Ru(0.7–2.4 nm)/Co40Fe40B20(2 nm) 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 Slonczewskis 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.

Current-Induced Magnetization Switching in MgO Barrier Magnetic Tunnel Junctions With CoFeB-Based Synthetic Ferrimagnetic Free Layers

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 Co₄₀Fe₄₀B₂₀/Ru/Co₄₀Fe₄₀B₂₀and Co₂₀Fe₆₀B₂₀/Ru/Co₂₀Fe₆₀B₂₀structures, and the MTJs (100 times (150-300) nm²) were annealed at 300 ^degC. The use of SyF free layer resulted in low intrinsic critical current density (<i>J</i> _c0) without degrading the thermal-stability factor (<i>E</i>/<i>k</i> _B <i>T</i>, where <i>E</i>, <i>k</i> _B, and <i>T</i> 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, <i>J</i> _c0 was reduced to 2-4 times10⁶ A/cm². This low <i>J</i> _c0 may be due to the decreased effective volume under the large spin accumulation at the CoFeB/Ru. The <i>E</i>/<i>k</i> _B <i>T</i> 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.

Dependence of Tunnel Magnetoresistance in MgO Based Magnetic Tunnel Junctions on Ar Pressure during MgO Sputtering

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 5 K) 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 Ω µm2 as 27% at RA = 0.8 Ω µm2, 77% at RA = 1.1 Ω µm2, 130% at RA = 1.7 Ω µm2, and 165% at RA = 2.9 Ω µm2.

A novel SPRAM (SPin-transfer torque RAM) with a synthetic ferrimagnetic free layer for higher immunity to read disturbance and reducing write-current dispersion

A novel SPRAM (spin-transfer torque RAM) consisting of MgO-barrier-based magnetic tunnel junctions (MTJs) with a synthetic ferrimagnetic (SyF) structure in a free layer was demonstrated for both higher immunity to read disturbance and a sufficient margin between the read and write currents. Since magnetization of the free layer becomes stable against thermal fluctuation with increasing thermal-stability factor E/kBT, the SyF free layer of the MTJs realized a magnetic information retention of over 10 years due to its high E/kBT of 67. Furthermore, it was found that the SyF free layer has an advantage of reducing dispersion of write-current density Jc, which is necessary for securing an adequate margin between the read and write currents.

2Mb SPRAM Design: Bi-Directional Current Write and Parallelizing-Direction Current Read Schemes Based on Spin-Transfer Torque Switching

A 1.8 V 2-Mb SPRAM (SPin-transfer torque RAM) chip using 0.2-mum logic process with MgO tunneling barrier cell 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.

Current-driven magnetization switching in CoFeB/MgO/CoFeB magnetic tunnel junctions

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 SiO2/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.

Current Development Status and Future Challenges of Spin Torque Transfer MRAM Technology (Invited)

Advanced Research Laboratory, Hitachi, Ltd, 1-280, Higashi-koigakubo, Kokubunji, Tokyo 185-8601, Japan Phone: +81-42-323-1111 E-mail: kenchi.ito.mb@hitachi.com. Central Research Laboratory, Hitachi, Ltd, 1-280, Higashi-koigakubo, Kokubunji, Tokyo 185-8601, Japan Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, JAPAN