Sumio Ikegawa

Spin transfer switching in TbCoFe∕CoFeB∕MgO∕CoFeB∕TbCoFe magnetic tunnel junctions with perpendicular magnetic anisotropy

Spin transfer (ST) switching in the TbCoFe∕CoFeB∕MgO∕CoFeB∕TbCoFe magnetic tunnel junction (MTJ) was studied. The TbCoFe∕CoFeB free layer with a large coercive field of 1.2kOe and a large thermal stability factor of 107 at room temperature was switched by a 100ns pulse current with a current density of 4.7MA∕cm2. This is the first report of ST switching in a MTJ with perpendicular magnetic anisotropy. The temperature dependence of the coercive field was also investigated to estimate the magnetic anisotropy in the case of rising temperature due to the Joule heating effect. The measured coercive field at 87°C, which was the simulated temperature during the switching pulse current, was about 0.34kOe. The ratio of the switching current density to the coercive field under the switching current in the MTJ with the TbCoFe∕CoFeB free layer is smaller than that in a typical MTJ with an in-plane magnetized CoFeB free layer. This result indicates that a MTJ with perpendicular magnetic anisotropy is advantageous for ...

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.

A 64Mb MRAM with clamped-reference and adequate-reference schemes

In order to realize a sub-Giga bit scale NVRAM, the novel MRAM based on the spin-transfer-torque (STT) switching has been intensively investigated due to its excellent scalability compared with a conventional magnetic field induce switching MRAM [1]. However, the memory cell size of STT-MRAM reported so far is still over 1µm2, and the memory capacity is limited to 32Mbit even in almost 100mm2 die size [2]. The large cell size is due to the large switching current of MRAM cells. In order to reduce the cell size, we have proposed the perpendicular tunnel magnetoresistance (P-TMR) device, and have confirmed its high potential to achieve lower switching current [3]. In this paper, a 64Mb STTMRAM with the P-TMR device having the circuit techniques to maximize operational margin is described.

Ion Beam Etching Technology for High-Density Spin Transfer Torque Magnetic Random Access Memory

A spin transfer torque magnetoresistive random access memory (STT-MRAM) is the most promising candidate for a non-volatile random access memory, because of its scalability, high-speed operation, and unlimited read/write endurance. An ion beam etching (IBE) is one of the promising etching methods for a magnetic tunnel junction (MTJ) of the STT-MRAM, because it has no after-corrosion and oxidation problems. In this work, we developed the multiple-step wafer-tilted IBE using computer calculation. Using optimized multiple-step IBE conditions, we fabricated MTJs without barrier-short defects.

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).

Unified understanding of both thermally assisted and precessional spin-transfer switching in perpendicularly magnetized giant magnetoresistive nanopillars

We report on the spin-transfer magnetization switching properties of CoFe/Pd-based perpendicularly magnetized giant magnetoresistive cells over a wide current pulse duration time range. Analytic expressions without empirical parameters like attempt frequency are tested experimentally for the thermally assisted and precessional regimes. Good agreement with the experiment data is obtained using a common parameter set in both regimes, which leads to a comprehensive understanding of the switching properties including the origin of the attempt frequency.

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.

Design and process integration for high-density, high-speed, and low-power 6F/sup 2/ cross point MRAM cell

A cross point (CP) cell with hierarchical bit line architecture was proposed for magnetoresistive random access memory (MRAM) based in Y. Shimizu et al. (2004). The new CP cell has a potential high density of 6F/sup 2/ and a faster access time than the conventional CP cell. A cell layout design to realize 6F is proposed and associated issues are resolved. Further, a 1Mb MRAM chip based on this structure has been fabricated utilizing 0.13 /spl mu/m CMOS technology and 0.24/spl times/0.48 /spl mu/m/sup 2/ magnetic tunnel junction (MTJ) sandwiched with the most efficient yoke wires ever reported. The access time of 250 ns and 1.5 V operations are successfully demonstrated with the integrated 1Mb chip.

Ion-Beam-Etched Profile Control of MTJ Cells for Improving the Switching Characteristics of High-Density MRAM

The effect of the reduction of the sidewall redeposition layer of magnetic materials is investigated for submicron-patterned magnetic random access memory (MRAM) cells. The sidewall redeposition layer is made at the first etch step of a magnetic tunnel junction (MTJ) with ion beam etching (IBE) in the case that the sidewall angle of a hard mask is steep. By controlling the etched profile at the time of the first IBE step, formation of the redeposition layer is prevented. Functional test results of 1-Kb MRAM arrays show that the sidewall redeposition layer enlarges fluctuation of switching current, and reduces the write operation region. The effect of the sidewall redeposition on the switching characteristics of MRAM arrays is explained qualitatively by micromagnetic simulation solving the Landau-Lifshitz-Gilbert (LLG) equation