Kenchi Ito

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.

Development of half-metallic ultrathin Fe3O4 films for spin-transport devices

We attempted to fabricate a high-quality Fe3O4 film while satisfying both low-thermal preparation (≦573 K) and film thinness (≦500 A). X-ray diffractometry showed that our prepared Fe3O4 film was epitaxially grown onto a MgO (100) substrate. The saturation magnetization, resistivity, and Verwey point were, respectively, ∼438 emu/cm3, ∼10 000 μ Ω cm, and ∼110 K. These values were comparable to those of the Fe3O4 bulk. Our experimental results suggested that a high-quality Fe3O4 film could be obtained even under the crucial conditions of the deposition temperature being low (∼523 K) and the film being ultrathinned (∼100 A).

Spin-transfer torque RAM technology: Review and prospect

Abstract Non-volatile RAM (NV-RAM) enables instant-on/off computing, which drastically reduces power consumption. One of the most promising candidates for NV-RAM technology is the spin-transfer torque RAM (SPRAM) based on magnetic tunnel junction (MTJ) device technology. This paper reviews the development of MTJ device technology and formulates considerations regarding its memory application, including SPRAM memory cell structure and operation, write voltage limitation, and thermal stability. At the circuit level, a disruptive read operation for future large integration scale is described. A 4 F 2 memory cell and a multi-bit cell approach are also presented. Finally, the potential value of instant-on/off computing through NV-RAM and its impact are explored.

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.

Current-driven microwave oscillations in current perpendicular-to-plane spin-valve nanopillars

We study the current and temperature dependences of the microwave voltage emission of spin-valve nanopillars subjected to an in-plane magnetic field and a perpendicular-to-plane current. Despite the complex multilayer geometry, clear microwave emission is observed for current densities in the interval of 9×107–13×107Acm−2. The emission frequency stays near 12GHz when I<Ired=11.2×107Acm−2, then redshifts with a slope gradually reaching −350MHz∕mA for 16×107Acm−2. The linewidth narrows exponentially to 3.8MHz at 150K for I<Ired, then broadens again as the emitted voltage redshifts. The temperature dependence of the linewidth exhibits a curvature change around the linewidth minimum.

1.5-Mbit/s direct readout of line-and-space patterns using a scanning near-field optical microscopy probe slider with air-bearing control

We have demonstrated 1.5-Mbit/s signal readout of a 0.25 μm line-and-space (L&S) pattern on a rotating disk by using a scanning near-field optical microscopy (SNOM) probe slider with air-bearing control. The light transmittance of the probe was greatly increased through direct irradiation of a focused light to a pyramidal probe tip. The bit rate of the signal was enhanced by two orders of magnitude compared to that of existing SNOM systems that use a tapered-fiber probe. The signal contrast, signal-to-noise ratio, and carrier-to-noise ratio for the 0.25 μm L&S pattern were 19%, 17 dB, and 37 dB, respectively. The estimated resolution limit of the probe corresponded to a L&S width of ∼130 nm.

Control of Aperture Size of Optical Probes for Scanning Near-Field Optical Microscopy Using Focused Ion Beam Technology

We propose a fabrication technique for apertures of optical probes for scanning near-field optical microscopy (SNOM) using a focused ion beam (FIB) process. We tried two FIB processes, FIB drilling and FIB slicing. The FIB slicing technique is very useful for fabrication of nm-sized SNOM apertures of less than 50 nm. The problem with the FIB drilling process is that it is difficult to identify the apex of the tip and to control the beam onto the apex. The FIB slicing technique can easily fabricate an aperture at an apex and control aperture size by cut-off-depth. It is easy for a sharp tip to obtain accurate size of aperture. It can be considered to obtain accurate size of aperture with fabricated error of 35 nm in a sharp tip with cone angle of 30 deg.

Dependence of Magnetic Anisotropy in Co20Fe60B20 Free Layers on Capping Layers in MgO-Based Magnetic Tunnel Junctions with In-Plane Easy Axis

Perpendicular magnetic anisotropy (PMA) in Co20Fe60B20 films depending on the adjacent layers of Ta, Ru, and MgO was studied for reduction of switching current density Jc0 in in-plane magnetic tunnel junctions (MTJs). A MgO layer reduces the out-of-plane saturation field (Hs) of in-plane easy axis Co20Fe60B20 films by inducing a strong PMA at the Co20Fe60B20/MgO interface. The PMA is not affected much by the crystallinity in Co20Fe60B20 films with different capping layers. MTJ with a MgO capping layer shows a lower Jc0 than that with a Ru capping layer, in accordance with the reduction of Hs.

Perpendicular Magnetic Tunnel Junctions with CoFe/Pd Multilayer Electrodes and an MgO Barrier

We studied the magnetic and magnetoresistance characteristics of pseudospin-valve magnetic tunnel junctions (MTJs) based on CoFe/Pd multilayer electrodes with perpendicular magnetic anisotropy and an MgO barrier. The MTJs at annealing temperature (<i>T</i> _a) of 473 K showed a tunnel-magnetoresistance (TMR) ratio of 1.5%. An fcc (111)-oriented texture of the bottom and top Co₉₀Fe₁₀/Pd multilayer electrodes, together with an imperfectly crystallized MgO, were revealed by cross-sectional TEM images. The TMR properties of perpendicular MTJs with a Co₂₀Fe₆₀B₂₀ or Co₅₀Fe₅₀ layer inserted between the CoFe/Pd multilayer electrodes and the MgO barrier were also studied. The TMR ratio with Co₂₀Fe₆₀B₂₀ insertion was 1.7% at <i>T</i> _a= 473 K and monotonically decreased at <i>T</i> _a over 523 K. The TMR ratio with Co₅₀Fe₅₀ insertion increased up to 3% at <i>T</i> _a= 573 K and then decreased to 0.4% at <i>T</i> _a= 598 K. The influence of the Pd layer on CoFeB was studied by using the simplified structures of Pd/CoFeB/MgO/CoFeB/Pd and Ta/CoFeB/MgO/CoFeB/Ta with inplane anisotropy. A former structure with Pd resulted in reduced TMR ratio which decreases with increasing <i>T</i> _a, whereas MTJs with a Ta-based structure showed a monotonic increase of a TMR ratio. The low TMR ratio observed in Pd-containing structures appears to result from crystallization of CoFeB in an unfavorable crystal orientation.

Spin-torque switching window, thermal stability, and material parameters of MgO tunnel junctions

We study the use of in-plane magnetized free layers with artificially lowered effective magnetization, in the context of spin-torque random access memories with magnetic tunnel junctions. We determine the field-voltage window for direct overwrite switching and thermal stability. We relate them to the magnetic constants extracted from the telegraph noise and high frequency noise exhibited by the junctions under specific conditions.