Takahiro Ibusuki

Transmission electron microscopy study on the crystallization and boron distribution of CoFeB/MgO/CoFeB magnetic tunnel junctions with various capping layers

High-resolution transmission electron microscopy and electron energy-loss spectroscopy (EELS) were used to study the microstructural properties of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with various capping layers. Crystallization of CoFeB layers was strongly dependent on the capping materials, and was affected by B diffusion. With NiFe-cap MTJs, CoFeB crystallized from the cap interface and formed a fcc structure; on the other hand, with Ta- and Ti-cap MTJs, CoFeB crystallized from the MgO interface and formed a bcc structure. EELS analysis showed that B mainly diffused to the capping layers and rarely to the MgO layers with increasing temperature. With Ti-cap MTJs, B diffusion caused hcp-Ti crystals to form an amorphous structure and CoFeB crystallized at lower temperature.

Lower-temperature crystallization of CoFeB in MgO magnetic tunnel junctions by using Ti capping layer

Effects of capping materials on magnetoresistance (MR) properties of MgO magnetic tunnel junctions (MTJs) with a CoFeB free layer were investigated. MR ratios of samples with various capping materials showed a difference in annealing temperature dependence. MTJ with a Ti capping layer annealed at 270°C showed a MR ratio 1.4 times greater than that with a conventional Ta or Ru capping layer. Secondary ion mass spectroscopy and high-resolution transmission electron microscopy images revealed that crystallization of CoFeB was remarkably affected by adjacent materials and the Ti capping layer adjoining CoFeB acted as a boron-absorption layer. These results suggest that the crystallization process can be controlled by choosing proper capping materials. Ti is one of the effective materials that accelerate the crystallization of CoFeB layers at lower annealing temperature.

A Co-SiO

We have investigated a possibility of Co-SiO2 granular material as a current-confining layer of confined current path (CCP) type-current perpendicular-to-plane (CPP) spin valves (SVs). A resistance-area (RA) product can be controlled by a composition and a thickness of the Co-SiO2 granular layer, and it showed a higher controllability compared to a conventional oxidized metal current-confining layer. The CPP-SV with Co-SiO2 granular layer performed at a relatively large MR ratio of 5.7% with a RA of 1 Omegamum2 and showed a potential of 5% with 500 mOmegamum2. This indicates a role of the Co-SiO2 granular layer as a current-confining structure. Although a tolerance to an applied voltage is insufficient at this stage, a granular material as a current-confining layer, such as Co-SiO2 , will open a door to a realization of CPP-SV magnetic recording heads for an ultrahigh-density recording systemIt was confirmed that the confined current paths spin valves (CPP-SVs) employing the Co-SiO2 granular layer as the current confining layer show relatively large magnetoresistance (MR) ratio with higher controllerability compared to those with the conventional oxidized metal layer. We expect that the granular type current confining layer makes artificial fabrication of the CCP-CPP structure possible.

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We have succeeded in enhancing the current-perpendicular-to-plane (CPP) magnetoresistance of the synthetic spin-valves by controlling the spin-dependent transport through the synthetic ferrimagnetic pinned layer using impurities. The new pinned layer overcomes the disadvantage of the synthetic pinned layer in CPP spin-valves for the magnetoresistance reduction. The experimental results are well described by our calculations based on the two-current theory. The proposed pinned layer, together with our newly developed magnetoresistive ferromagnetic materials, will be quite effective in producing CPP spin-valve sensors for ultrahigh-density data storage systems.

Granular Material as a New Current-Confining Layer for Current Perpendicular-to-Plane Spin Valves

Applied voltage and magnetic field dependence of microwave oscillations have been studied for the giant magnetoresistive (GMR) element. In this experiment, a magnetic field is applied perpendicular to the plane of GMR films. It is found that the power spectral density (PSD) is less dependent on the direction of magnetic field and the resistance gap induced by the magnetic field corresponds to the rapid increase of PSD. Moreover, noise peaks are observed in a high frequency range (about 8 GHz or higher) when a magnetic field of more than 2000 Oe is applied. The peak frequencies shift to higher frequency as the magnitude of magnetic field increases. In contrast to a magnetic field, the peak frequencies and their widths are more susceptible to the direction of current.

Novel synthetic ferrimagnet pinned Layers for metallic CPP spin valves requiring high pinning-field and output

A pinned layer structure for current-perpendicular-to-plane (CPP) spin valves that enhances magnetoresistance change (/spl Delta/RA) effectively without changing other layers is shown. It was found that spin blocking layers in the pinned layers increase /spl Delta/RA by decreasing the cancellation of the spin-dependent bulk resistance without significant drop of exchange pinning field.