T. Kai

Lower-current and fast switching of a perpendicular TMR for high speed and high density spin-transfer-torque MRAM

We investigate extremely low programming current and fast switching time of a perpendicular tunnel-magnetoresistance (P-TMR) for spin-transfer torque using a P-TMR cell of 50 nm-diameter. A L10-crystalline ordered alloy is used as a free layer that has excellent thermal stability and a damping constant of about 0.03. The programming current of 49 uA and the switching time of 4 nsec are also demonstrated.

Effect of added Cu on disorder-order transformation of L1/sub 0/-FePt

The L1/sub 0/-FePt ordered phase is formed at lower annealing temperature by addition of Cu. FePtCu films start ordering at 300/spl deg/C or a lower temperature, whereas FePt and FePtAg films are still disordered alloys after annealing at 300/spl deg/C. The results of X-ray diffraction indicate that FePt and the added Cu form an FePtCu ternary alloy, whereas FePtAg decomposes into FePt and Ag phases in the disorder-order transformation. From thermodynamic considerations, it is concluded that reduction of the ordering temperature of FePt is due to the formation of an FePtCu solid solution.

Magnetic and electronic structures of FePtCu ternary ordered alloy

We investigated the microscopic structure of FePtCu ternary ordered alloys from both theoretical and experimental points of view. From the theoretical viewpoint, first principles calculation was carried out based on the full-potential linear augmented plane wave method in order to obtain the electronic structure and magnetic anisotropy energy of the FePtCu ternary ordered alloy. From the experimental viewpoint, the ultraviolet photoelectron spectroscopy measurement was used to identify the Cu position in FePtCu ternary ordered alloy. From a comparison with the calculated density of states curve, it was found that the Fe site of FePt ordered alloy replaced the Cu. The theoretical and experimental results in this article support the mechanism proposed in terms of the thermodynamic considerations in our previous article.

Reduction of switching current distribution in spin transfer magnetic random access memories

In this paper, the switching current distribution by spin transfer torque is investigated for CoFeB∕MgO∕CoFeB magnetic tunnel junctions (MTJs). The distribution of the spin transfer switching current for a MTJ with junction size of 85×110nm2 is 16% when the duration of applied pulse current is 5ms. In the case of magnetization reversal with magnetic field induced by current with 5ms pulse duration, the distribution of the switching field is 8.3%. According to our micromagnetic simulation, it is found that the spin transfer current switching seems to exhibit a nonuniform magnetization reversal process, whereas the magnetization switching by the magnetic field exhibits a uniform magnetization reversal process. This leads to the broader distribution related to the repeatability.In this paper, the switching current distribution by spin transfer torque is investigated for CoFeB∕MgO∕CoFeB magnetic tunnel junctions (MTJs). The distribution of the spin transfer switching current for a MTJ with junction size of 85×110nm2 is 16% when the duration of applied pulse current is 5ms. In the case of magnetization reversal with magnetic field induced by current with 5ms pulse duration, the distribution of the switching field is 8.3%. According to our micromagnetic simulation, it is found that the spin transfer current switching seems to exhibit a nonuniform magnetization reversal process, whereas the magnetization switching by the magnetic field exhibits a uniform magnetization reversal process. This leads to the broader distribution related to the repeatability.

Improvement of robustness against write disturbance by novel cell design for high density MRAM

A new bit cell designed to have an excellent astroid is presented from the viewpoints of both theory and experiment. The switching mechanism is unique. The robustness against the disturbance of half-selected bits is improved. Its excellent astroid improves thermal stability and has the potential to achieve extremely high density magnetoresistive random access memory (MRAM).

Bit yield improvement by precise control of stray fields from SAF pinned layers for high-density MRAMs

A write-operating window with a 100% functional bit yield was successfully obtained by the control of stray fields from synthetic antiferromagnetic (SAF) pinned layers in conventional magnetic random access memories with rectangular magnetic tunneling junction bits. The stray fields were controlled by a newly developed ion-beam etching technique without causing damage and by a precise setting of the SAF pinned layer thickness, and are balanced with Neel coupling fields. As a result, it was found that symmetric switching astroid curves with no offset were obtained and switching distributions were minimized at the zero offset field.

Reduction of switching field distributions by edge oxidization of submicron magnetoresistive tunneling junction cells for high-density magnetoresistive random access memories

An edge oxidization effect on magnetization reversals is investigated for submicron-patterned magnetoresistive tunneling junctions (MTJs). By the MTJ edge oxidization which causes the MTJ edge saturation magnetization (Ms) reduction, the switching field distributions (SFDs) for 0.24×0.48μm2 MTJs are reduced to less than 10%. The offset fields and the kinks in resistance-magnetic-field curves are reduced. Micromagnetic simulation results predict that the edge magnetization reversals are suppressed by the MTJ edge Ms reduction and the edge domain size at the remanent states becomes small. Consequently, the edge domain motion suppression by the edge oxidization is effective for decreasing the SFDs.

A Fully Integrated 1 kb Magnetoresistive Random Access Memory with a Double Magnetic Tunnel Junction

A 1 kb magnetoresistive random access memory (MRAM) is demonstrated with a 0.4×1.2 µm2 magnetic tunnel junction (MTJ) and 0.18 µm complementary metal-oxide-semiconductor (CMOS) technology. In this study, a double MTJ, which is designed to have a larger signal margin than a conventional MTJ, is used. The uniformity of resistance for a double MTJ is comparable to that of a single MTJ and is expected to be better than a single MTJ if process conditions are optimized. The magnetic design of the MTJ provided a good astroid curve and resulted in 90% of bits working towards a relatively broad range of bit-line voltage and word-line voltage.

Switching Current Fluctuation and Repeatability for MRAM With Propeller-Shape MTJ

The writing region and the repeatability of the function test and switching current fluctuation of magnetoresistive random access memory (MRAM) with a propeller shape magnetic tunnel junction (MTJ) array is evaluated. The effect of the write sequence is also investigated. The writing region is larger when the easy-axis field pulse is turned on prior to the hard-axis field than that in the case of the opposite sequence. However, the total margin in the latter sequence is larger after the repeated tests because of the smaller switching field fluctuation. The average 1-sigma value of the switching field fluctuation is 1.7%, which is mainly caused by the thermal fluctuation. The probability of the write error is estimated to be less than 10-16 by the bit line writing region and the thermal stability

Effect of magnetic domain on thermal stability for MRAM cells with propeller shape

In this paper, we investigate the thermal energy of MRAM cells on applied field along the easy or hard axis and its dependence on magnetic domain pattern. Furthermore the thermal stability at half-selected situation is also discussed.