T. Meguro

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.

GHz-range low-loss wide band filter using new floating electrode type unidirectional transducers

K. Yamanouchi et al. (1991) reported a very low loss surface acoustic wave (SAW) filter with minimum insertion loss of 1.9 dB at 1.0 GHz using a floating electrode type unidirectional transducer (FEUDT). The dependence of directivity on the electrode thickness for the lambda /10 type FEUDT is investigated here. Experimental results show minimum insertion loss of 6.2 dB at 4.1 GHz and 2.5 dB at 2.0 GHz. The dependence of directivity on the electrode thickness at 4 GHz differs from that at 2 GHz. The resistivities of various Al films were measured, and their surfaces are observed by scanning tunneling microscopy. An increase of the resistance due to pinholes which appear after heating is noted.<<ETX>>

5 GHz Range Low-Loss Wide Band Surface Acoustic Wave Filters Using Electrode Thickness Difference Type Unidirectional Transducers

A new electrode thickness difference type single phase unidirectional surface acoustic wave (SAW) transducer (ETD-SPUDT), which has λ/4 narrow-gap electrodes is investigated. The ETD-SPUDT is fabricated by cutting part of a meander-line electrode using the shadow of the resist pattern. The theoretical and experimental results show low-loss and large directivity. A low-loss filter for use in the 2 GHz range is fabricated on 128° Y-X LiNbO3 by optical photolithography techniques and for use in the 5 GHz range is fabricated by electron beam exposure. The results show a 3.0 dB insertion loss at 2 GHz and a 4.1 dB loss at 5 GHz.

SHF-range surface acoustic wave interdigital transducers using electron beam exposure

Nanometer electrodes of 0.085- mu m width have been fabricated using direct electron beam lithography, normal lift-off techniques, and an I/sub 2/-plasma cleaning system. These techniques have been applied to interdigital transducers used as surface acoustic wave (SAW) transducers in the SHF range. SAW filters experiments were performed in the SHF-range. Filters with minimum insertion loss of 15.0 dB at the center frequency of 5 GHz and 24 dB at 11 GHz were obtained.<<ETX>>

New electrode separation technology using anodic oxidation and application to SAW interdigital transducers

By anodic oxidation of the edges of the portions of the Al film under photoresist, controllable gaps between electrodes with good insulation can be obtained. These techniques are applied to realize a surface-acoustic-wave (SAW) narrow gap interdigital transducer (NG-IDT) and narrow gap unidirectional transducer (NG-UDT). The experimental result shows 7.2 dB insertion loss with amplitude ripples of +or-1.2 dB for a conventional NG-IDT. Directives of 3-dB/transducer at 440 MHz (fundamental) and 13-dB transducer at 870 MHz (second-harmonic operation) for a new floating electrode type unidirectional transducer (NG-FEUDT) are demonstrated. Also a three transducer low loss filter using a combination of NG-IDT and NG-FEUDT exhibits 3.46 dB insertion loss at 894 MHz (second-harmonic operation, electrode width of 1.7 mu m) with sidelobe suppression greater than 35 dB.<<ETX>>

New Low-Loss Surface Acoustic Wave Transducers in the UHF Range

Absfracf-Some new low-loss surface acoustic wave (SAW) transducers are described which can be used for UHF range applications. The characteristic of these new low-loss SAW transducers is that the electrode widths are about X0/4 (X,:SAW wavelength), so the frequency limitation of these devices is almost the same as that for the conventional uniform two-electrodeslwavelength transducers. The theoretical analysis includes the internal reflections within the transducers caused by the difference of the thickness of metal electrode film or both the difference of electromechanical coupling coefficients (K’) and the mass loading effect. The fundamental equations are derived and some simple results are given. In addition, the new low-loss SAW transducers are fabricated by a self-aligned angle evaporation technique which use only one photo mask, and no mask alignment is needed in the fabrication process. Experimental results show directivities of 1026 dB/transducer at about 500 MHz and 10 dB at about 2 GHz. A prototype SAW filter has shown a very encouraging passband characteristic with a minimum insertion loss of 2.8 dB at about 500 MHz on 128” Y-X LiNb03.

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.