Hiroyuki Ohmori

Magnetoresistive random access memory operation error by thermally activated reversal (invited)

The reliability in magnetoresistive random access memory (MRAM) write operation was investigated for both toggle and asteroid memory chips developed with 0.18μm CMOS process. Thermally activated magnetization reversal, being the dominant origin of the intrinsic write error, was studied theoretically and experimentally. For asteroid MRAM, the bit line or word line disturbing error on half selected bits was proved to have significant effect on the write operation margin, even in the ideal case free from bit-to-bit switching field distribution and the hysteresis loop shift. For toggle MRAM, on the other hand, the dominant origin of the error occurs for selected bits, although its impact is much smaller than in the case of asteroid MRAM. As was expected from the estimation based on the single domain model, more than 10mA operation margin with the error rate of <10−9, which is sufficiently small for semiconductor IC memory, was achieved for the toggle MRAM.

Low noise Co/Pd multilayer perpendicular media with granular seed layer

A granular seed layer such as Au-SiO/sub 2/ is effective in reducing medium noise in a Co/Pd multilayer perpendicular recording medium. Clear transitions up to a linear density of 300 kfci can be recorded on the perpendicular medium using a conventional merged magneto-resistive (MR) head.

Magnetic properties and noise characteristics of Co/Pd multilayer perpendicular magnetic recording media

Abstract In Co/Pd multilayer perpendicular magnetic recording media, a granular seed layer and the addition of boron and oxygen are effective for reducing medium noise. In a recording system using a conventional ring-shaped head and a perpendicular medium without a soft magnetic underlayer, precise control of the write-current waveform is important to minimize medium noise. Low noise, low non-linear transition and no signal decay are observed for the Co–B–O/Pd–B–O multilayer with an Au–SiO 2 seedlayer.

Spin Torque Switching of Perpendicularly Magnetized CoFeB-Based Tunnel Junctions With High Thermal Tolerance

In order to launch spin transfer torque-based magnetoresistive random access memory into mass production below the 40 nm technology node, high performance perpendicularly magnetized magnetic tunnel junctions (pMTJs) are crucial. One of the key issues for pMTJs is to ensure compatibility with conventional back-end-of-line (BEOL) heating process, where thermal tolerances over 350 C for standalone memory and 400 °C for embedded memory are typically required. In this work, we successfully demonstrated high thermal tolerance in CoFeB-based pMTJs with a synthetic antiferromagnet (SAF) pinned structure while keeping stray fields on the free layer low. The annealing temperature (T_anneal) dependence of spin torque switching (STS) and TMR characteristics revealed that the reduction of I_c0, the increase of Δ, and the increase of TMR ratio were achieved by elevating T_anneal from 250 °C up to 380 °C for 30 min. Thus, we attained high STS efficiency (I_c0/Δ)=0.88 (I_c0=69 μA, Δ=78), TMR ratio=120%, RA =12 Ωμm² at T_anneal=380 °C for the pMTJs with dimensions of 44-46 nm in diameter. Moreover, by using specially tailored pMTJs with a SAF pinned structure, we achieved high thermal tolerance up to 400 °C for 30 min while keeping I_c0/Δ=1.12 (I_c0=56 μA, Δ=50), TMR ratio=150% , RA=13 Ωμm².

Soft magnetic properties for Fe-Al-Nb-N-O films

Magnetic and other properties of fine-crystal Fe-N films with additional Al, Nb, and O were investigated. Films were prepared by RF planar magnetron sputtering. The grain size and the lattice spacing were changed by the addition of Al and Nb, and the coaddition of Al and Nb was found to be effective in improving the soft magnetic properties of the films after annealing at 550 degrees C. Introducing oxygen into the sputtering gas of argon and nitrogen increased the nitrogen concentration in the films and was effective in reducing the coercivity of the films. The magnetostriction of the films changed from negative toward positive with increasing nitrogen concentration and crossed zero at the nitrogen concentration where the coercivity had a minimum value. Hardness measurement of the films revealed that the addition of Al and Nb prevents recrystallization in Fe films as well as in Fe-N-O films. The (Fe/sub 97.5/Al/sub 1.1/Nb/sub 1.4/)/sub 92/N/sub 6/O/sub 2/ films exhibited a saturation magnetic flux density of 20 kG, a coercivity of 0.3-0.5 Oe, and a hardness above 1000 even after annealing at 550 degrees C without magnetic field application. >

CoPtB(O) alloy films as new perpendicular recording media

In search of new hard magnetic materials with high saturation magnetization and large coercivity, a comprehensive study was made on numerous Co‐ and CoPt‐base crystalline alloys by means of sputtering techniques. It revealed that a newly found CoPtB(O) alloy system possessed excellent hard magnetic properties with remarkably large perpendicular coercivity and high saturation magnetization. This new alloy film, deposited onto room temperature substrates, shows the magnetic properties of 4πMs=12 kG, H⊥c=4000 Oe, and perpendicular anisotropy field Hk=22 kOe. These values are superior to those of prevailing materials such as CoCr perpendicular and CoPt or CoNi longitudinal recording media. The typical composition is Co69Pt20B6O5 (at. %), and oxygen plays a momentous role on the coercivity in this alloy film. As a magnetic recording medium, a write/read experiment of this film shows that the readout signal has a +9 dB peak‐to‐peak amplitude compared with that of metal particle tape at 1 μm wavelength and has +...

Wear and corrosion resistance of Fe-N based micrograin soft magnetic thin films

Wear resistance of Fe-N based micrograin and other soft magnetic thin films was measured using commercial 8 mm VCR and tapes. Fe-Ta-N and Fe-Al-N films which contain ceramics phase such as Ta-N show a high Vickers hardness, but their wear resistance was not the highest one. Fe-Ru-Ga-Si and Fe-Al-V-Nb-O-N films showed low wear loss when they were rubbed by ME tapes. Corrosion properties were estimated by measuring the quantity of Fe dissolved into salt water after dipping a surface lapped thin film specimen. Nitrogen showed a remarkable effect in reducing the corrosion of Fe-N films. Cu was round to be an attractive additional element to increase the corrosion resistance without decreasing magnetic properties for Fe-Al-V-Nb-O-N films. >

Transmission electron microscopy analysis of lattice strain in epitaxial Co-Pd multilayers

We investigated the lattice strain in epitaxially grown [001]-oriented Co-Pd superlattices on MgO[001] single-crystal substrates using high-resolution transmission electron microscopy. We observed the elastic deformation of the lattice arrangement from cubic to tetragonal by lateral coherency strain and the intermixed Co-Pd layer with a thickness of 0.5 nm. The tetragonal deformation in the Co-Pd intermixed layer caused by stretching strain from the Pd layer induces high negative magneto-elastic energy via positive magnetostriction, resulting in a high negative contribution to perpendicular magnetic anisotropy energy. In a Co layer less than 0.5-nm thick, the perpendicular anisotropy is enhanced owing to the relaxation of the lattice strain by taking a sinusoidal composition distribution in the modulated direction. We found that post-deposition annealing to [001]-oriented Co-Pd superlattices can improve the perpendicular magnetic anisotropy owing to the effect of interfacial mixing which can relax the coherency strain.

Structure and magnetic properties of Co–Pt alloy film deposited on Ru underlayer

In order to study the origin of high coercivity in a Co–Pt film deposited on an Ru underlayer we used a single-crystal MgO substrate to simplify the measurement of magnetic properties and transmission electron microscopy. The structure of the Ru layer on single-crystal MgO (110) is twin. MgO [001] and the c axes of Ru grains are arranged at an angle of about 60°. The Co–Pt layer grows epitaxially on the Ru layer. Although the initial Co–Pt layer is mostly a hcp structure, fcc structure grains are growing at the upper surface through the boundary of hcp grains. The MgO [001] direction is magnetically easy and the MgO [1-10] direction is normally hard axis. The anisotropy field increases when an Ru underlayer is used in the Pt-rich composition. Coercivity is enhanced by the increase in magnetic anisotropy originating from the hcp-Co–Pt alloy that is stabilized even in the Pt-rich composition by using the hcp-Ru underlayer. The Ru layer deposited at a high gas pressure forms a columnar structure, which is ef...

High-coercivity longitudinal recording medium prepared using non-heating process

To get high coercivity in a longitudinal recording medium prepared at a low temperature, we found that a Ru underlayer is effective to enhance the coercivity for Co-Pt-based thin films. Increasing the Ru underlayer thickness and Pt composition of the magnetic layer increases coercivity. The introduction of boron and oxygen into the magnetic layer induces the segregation of grains.