Tatsuya Kishi

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.

Tunnel Magnetoresistance Over 100% in MgO-Based Magnetic Tunnel Junction Films With Perpendicular Magnetic L1

Perpendicular L1₀-FePt/MgO/Fe/L1₀ -FePt magnetic tunnel junction (MTJ) films with the (001) texture were successfully developed to obtain a large tunnel magnetoresistance (TMR) above 100 % at room temperature. The TMR ratio in the L1₀-FePt/MgO/Fe/L1₀-FePt MTJ was strongly dependent on the Fe interfacial layer thickness. The lattice mismatch between the MgO(001) barrier layer and the L1₀ -FePt(001) layer is too large for the MgO barrier layer to grow epitaxially on the L1₀-FePt(001) layer. The insertion of the Fe interfacial layer improves the quality of the MgO(001) barrier layer and achieves an epitaxy in the L1₀-FePt/MgO/Fe/L1₀-FePt stack. As a result, the optimization of the Fe interfacial layer thickness is a key to obtain the large TMR ratio in the MgO-based MTJ with the L1₀-FePt electrodes.

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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.

-FePt Electrodes

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.

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

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.

Magnetic and electronic structures of FePtCu ternary ordered alloy

For the first time, 4Gbit density STT-MRAM using perpendicular MTJ in compact cell was successfully demonstrated through the tight distributions for resistance and magnetic properties. This paper includes the results regarding parasitic resistance control process, MTJ process, and MTJ stack engineering. Both of successful 4Gb read and write operations were performed with high TMR, low Ic. This result will brighten the prospect of high-density STT-MRAM.

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

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).

4Gbit density STT-MRAM using perpendicular MTJ realized with compact cell structure

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.

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

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.