Yoshihiko Fuji

Enhancement of magnetoresistance by hydrogen ion treatment for current-perpendicular-to-plane giant magnetoresistive films with a current-confined-path nano-oxide layer

We have enhanced magnetoresistance (MR) for current-perpendicular-to-plane giant-magnetoresistive (CPP-GMR) films with a current-confined-path nano-oxide layer (CCP-NOL). In order to realize higher purity in Cu for CCPs, hydrogen ion treatment (HIT) was applied as the CuO(x) reduction process. By applying the HIT process, an MR ratio was increased to 27.4% even in the case of using conventional FeCo magnetic layer, from 13.0% for a reference without the HIT process. Atom probe tomography data confirmed oxygen reduction by the HIT process in the CCP-NOL. The relationship between oxygen counts and MR ratio indicates that further oxygen reduction would realize an MR ratio greater than 50%.

Enhancement of magnetoresistance by ultra-thin Zn wüstite layer

We have investigated a hybrid magnetic material composed of ultra-thin oxide spin-filtering layer/ferromagnetic layer (FML) for magnetoresistance (MR) enhancement. Using Zn-Fe oxide/Fe50Co50 as a free layer in current-perpendicular-to-plane giant magnetoresistive films, the MR ratio and ΔRA (the change of resistance area product) were enhanced to 26.0% and 52 mΩ μm2 at a small RA (resistance area product) of about 0.2 Ω μm2, respectively. Structural analysis revealed that the Zn wustite structure is responsible for the enhancement of the MR ratio, and spin-polarization of wustite/FML is higher than that of spinel ferrite/FML.

Highly sensitive spintronic strain-gauge sensor based on a MgO magnetic tunnel junction with an amorphous CoFeB sensing layer

We investigated spintronic strain-gauge sensors (Spin-SGSs) based on magnetic tunnel junctions (MTJs). To enhance the strain sensitivity of Spin-SGSs, which is defined as the gauge factor = (ΔR/R)/Δe, we investigated MgO-MTJs with an amorphous CoFeB sensing layer that exhibits high magnetostriction and soft magnetic properties. To maintain the amorphous structure of the CoFeB sensing layer even after post annealing, we applied a MgO capping layer (MgO-cap) to the CoFeB sensing layer and compared it with a Ta capping layer (Ta-cap). After post annealing at 320 °C, the CoFeB sensing layer with a MgO-cap maintained a low coercivity of 3 Oe, whereas that with a Ta-cap exhibited a high coercivity of 25 Oe. Microstructure analysis revealed that the CoFeB sensing layer with the MgO-cap has an amorphous structure because boron remains in the CoFeB sensing layer even after post annealing. The gauge factor for the Spin-SGS with the MgO-cap was 4016, which was four times larger than 942 for the Spin-SGS with the Ta-cap.

Analysis of the Current-Confined-Paths in the Film Plane for CPP-GMR Films

Nanostructure of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) films with a current-confined-path nano-oxide layer (CCP-NOL) is analyzed by high-resolution transmission electron microscope (HRTEM) and three-dimensional atom probe (3DAP). By HRTEM analysis, it is found that the CCP is formed on the center of the grain of a pinned layer. By 3DAP analysis, the grain size of a free layer can be visually detected and its size is about 5-10 nm. The grain of a free layer is well aligned with a CCP, suggesting that the decrease of the grain size of a pinned layer is effective for increasing the number of the CCPs per unit area. Moreover, by using a 3DAP image, the density of the CCPs per unit area can be derived. As a result, it is clearly shown that the density of the CCPs per unit area increases with decreasing resistance area product (RA) , which cannot be obtained from HRTEM analysis. By combining HRTEM result and 3DAP result, the nanostructure of the CCP can be clarified, and the decrease of the grain size of a pinned layer and the decrease of RA are preferable for an extremely small element size for a higher-density recording head.

Spin-MEMS microphone based on highly sensitive spintronic strain-gauge sensors

We report a novel spintronic MEMS (Spin-MEMS) microphone, which is a new type of resistive microphone. For this microphone, spintronic strain-gauge sensors (Spin-SGSs) are integrated on a bulk micromachined diaphragm. The Spin-SGSs are based on magnetic tunnel junctions (MTJs) similar to those used as magnetic sensors in hard disk drives. In work to date, we have experimentally confirmed that the Spin-SGS exhibits a high gauge factor in excess of 5000, which is 100-fold that for a conventional poly-Si piezoresistor, by adopting a novel amorphous Fe-B-based sensing layer with high magnetostriction and low coercivity. Thanks to the high strain sensitivity of the Spin-SGSs, the Spin-MEMS microphone exhibits a signal-to-noise ratio (SNR) of 57 dB(A). A Spin-MEMS microphone with a first resonance frequency of over 70 kHz was also fabricated that exhibits an SNR of 49 dB(A), which is promising for acoustic health monitoring. In this study, we compared the operation sounds of defective and normal bearings using the Spin-MEMS microphone. The Spin-MEMS microphone detected differences in the operation sounds between the defective and normal bearings in the high-frequency range of 10 kHz to 50 kHz.We report a novel spintronic MEMS (Spin-MEMS) microphone, which is a new type of resistive microphone. For this microphone, spintronic strain-gauge sensors (Spin-SGSs) are integrated on a bulk micromachined diaphragm. The Spin-SGSs are based on magnetic tunnel junctions (MTJs) similar to those used as magnetic sensors in hard disk drives. In work to date, we have experimentally confirmed that the Spin-SGS exhibits a high gauge factor in excess of 5000, which is 100-fold that for a conventional poly-Si piezoresistor, by adopting a novel amorphous Fe-B-based sensing layer with high magnetostriction and low coercivity. Thanks to the high strain sensitivity of the Spin-SGSs, the Spin-MEMS microphone exhibits a signal-to-noise ratio (SNR) of 57 dB(A). A Spin-MEMS microphone with a first resonance frequency of over 70 kHz was also fabricated that exhibits an SNR of 49 dB(A), which is promising for acoustic health monitoring. In this study, we compared the operation sounds of defective and normal bearings using ...

Spin-MEMS microphone integrating a series of magnetic tunnel junctions on a rectangular diaphragm

We investigate the enhancement of the signal-to-noise ratio (SNR) of spintronic micro-electro mechanical-system (Spin-MEMS) microphones in which spintronic strain-gauge sensors (Spin-SGSs) are integrated on a micro-electro mechanical-system (MEMS) diaphragm by using a large array of N Spin-SGSs connected in series similar to that in a previous report on magnetic tunnel junction magnetic sensors. Since the strain-gauge properties of Spin-SGSs strongly depend on the angle between the applied uniaxial strain and the magnetization direction of the reference layer, in order to obtain the same signals from each Spin-SGS in an array, it is necessary to locate the Spin-SGS array in a region where the uniaxial strain occurs uniformly on the MEMS diaphragm. We theoretically and experimentally investigate the effect of the diaphragm shape on uniaxial strain on the diaphragm surface. As a result, it is found that a rectangular-shaped diaphragm provides a larger region in which a uniform uniaxial strain is applied to the Spin-SGS array compared with the generic circular diaphragm. Finally, an SNR enhancement of 18 dB by connecting N = 62 Spin-SGSs in series is successfully confirmed in a Spin-MEMS microphone with a rectangular diaphragm.We investigate the enhancement of the signal-to-noise ratio (SNR) of spintronic micro-electro mechanical-system (Spin-MEMS) microphones in which spintronic strain-gauge sensors (Spin-SGSs) are integrated on a micro-electro mechanical-system (MEMS) diaphragm by using a large array of N Spin-SGSs connected in series similar to that in a previous report on magnetic tunnel junction magnetic sensors. Since the strain-gauge properties of Spin-SGSs strongly depend on the angle between the applied uniaxial strain and the magnetization direction of the reference layer, in order to obtain the same signals from each Spin-SGS in an array, it is necessary to locate the Spin-SGS array in a region where the uniaxial strain occurs uniformly on the MEMS diaphragm. We theoretically and experimentally investigate the effect of the diaphragm shape on uniaxial strain on the diaphragm surface. As a result, it is found that a rectangular-shaped diaphragm provides a larger region in which a uniform uniaxial strain is applied to ...

Fabrication of nanoscale metal paths in oxide thin layers by noble-gas ion beams

We investigated the fabrication process of a nanostructure with alumina insulator layer and nanoscale Cu paths punching through an alumina layer inserted between magnetic multilayers. Ion-beam–assisted oxidation was applied to an AlCu layer, where ion beams with three kinds of noble gases of different mass, Ne, Ar and Xe, were compared. The heavy gas overcame the trade-off between the increasing purity of nanoscale Cu paths and the decreasing oxygen defects of the alumina. It is considered that the high mobility of surface atoms in the AlCu layer brought about by the heavy-gas ion beam promotes segregation of alumina and Cu.