Migaku Takahashi

Translocation of bio-functionalized magnetic beads using smart magnetophoresis

We demonstrate real time on-chip translocation of bio-functionalized superparamagnetic beads on a silicon surface in a solution using a magnetophoresis technique. The superparamagnetic beads act as biomolecule carriers. Fluorescent-labeled Atto-520 biotin was loaded to streptavidin-coated magnetic beads (Dynabead(®) M-280) by means of ligand-receptor interactions. The magnetic pathways were patterned lithographically such that semi-elliptical Ni(80)Fe(20) elements were arranged sequentially for a few hundred micrometers in length. An external rotating magnetic field was used to drive translational forces on the magnetic beads that were proportional to the product of the field strength and its gradient. The translational force at the curving edge of the pathway element of 6 μm diameter was calculated to be ∼1.2 pN for an applied field of 7.9 kA m(-1). However, the force at the flat edge was calculated to be ∼0.16 pN. The translational force was larger than the drag force and thus allowed the magnetic beads to move in a directional way along the curving edge of the pathway. However, the force was not sufficient to move the beads along the flat edge. The top and bottom curving edge semi-elliptical NiFe pathways were obliquely-arranged on the left and right sides of the converging site, respectively. This caused a central translational force that allowed the converging and diverging of the Atto-520 biotin loaded streptavidin magnetic beads at a particular site.

Q-band ferromagnetic resonance for CoPt-based stacked perpendicular recording media with interlayer exchange coupling

The ferromagnetic interlayer exchange coupling Jinter for stacked perpendicular recording media with a granular layer (GL)/interlayer (IL)/alloy capping layer (CL) structure was quantitatively evaluated by Q-band ferromagnetic resonance (FMR). Two resonances with acoustic and optical precession modes were observed in the FMR signals from the stacked media. Fitting using the Landau–Lifshitz–Gilbert (LLG) equation indicated that Jinter increased from 0.55 to 1.83 erg/cm2 when the Pt IL thickness was reduced from 2.0 to 1.0 nm for media based on Co82Cr10Pt8-CL (4 nm) and Co74Pt16Cr10-8 mol (SiO2)-GL (16 nm). The optimum Pt IL thickness at which the switching field distribution was minimized due to a large reduction in the saturation field of the stacked media was found to correspond to the boundary condition between antiparallel and parallel precession of the magnetic moments of the GL and CL in FMR.

Oblique-incidence sputtering of Ru intermediate layer for decoupling of intergranular exchange in perpendicular recording media

During the Ru deposition process for granular type perpendicular magnetic recording media, both a reduction in the Ru intermediate layer thickness and lowering of sputtering gas pressure were successfully achieved by focusing on a self-shadowing effect. Oblique-incidence sputtering with a 60° incident angle under an Ar gas pressure of 0.6 Pa yielded (1) columnar Ru grains with a growth direction of 30° from the film normal, (2) c-plane sheet texture by epitaxial growth on the Pt underlayer, and (3) a flat envelope of the surface and a deep gap at grain boundaries. This change in the Ru structure significantly contributes to reducing exchange coupling among magnetic grains, especially in the initial growth region in an overlying granular medium.

Nanoscale physical microstructure and micromagnetic behaviour of CoIr film with negative anisotropy

The physical and magnetic structure of hcp-CoIr(10u2009nm)/Ru(5u2009nm) has been systematically characterized using transmission electron microscopy. The film was observed to be polycrystalline with a mean grain size of 15.7 ± 1.1u2009nm. Additionally, diffraction analysis in the TEM confirmed the presence of a [0u20090u20090u20091] texture normal to the film plane resulting from a Ru seed layer. Lorentz microscopy observation with in situ magnetizing experiments showed that the film possessed a weak-anisotropy easy axis with considerable dispersion of magnetic ripple and domain wall nucleation over a negative field of 25u2009Oe. Magnetization reversal on the hard axis shows a non-coherent rotation of magnetic moments from easy axis to hard axis denoted by a non-uniform nucleation of low-angle walls. Dispersion of magnetization ripple is in agreement with classical ripple theory. Generally the magnetization reversal was complete by 30u2009Oe; however, some small regions remained which were not fully reversed with associated 360° domain walls. These appeared to be strongly pinned locally and required much larger fields to eliminate them.

Optical properties of superconducting Bi 2 Sr 2 CaCu 2 O 8

A fundamental optical study was performed on superconducting Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} by using a high-accuracy universal polarimeter (HAUP). As this crystal is strongly linearly dichroic, we developed the extended HAUP theory which includes the treatments of the optical dichroisms. After having determined the optical nature of the crystal, we applied the extended HAUP method to a thin (001) plate specimen with light traveling to the front and rear directions in the specimen. These two experiments permitted us to separate the reciprocal and nonreciprocal optical effects. No sign of the nonreciprocal effects was found in the HAUP transmission experiment. A gyration tensor component {ital g}{sub 33} takes place suddenly at {ital T}{sub {ital c}} (90 K), increases with decreasing temperature, and reaches 1.87{times}10{sup {minus}4} (36{degree}/mm of rotatory power) at 15 K. A steep change of birefringence {Delta}{ital n} with temperature also occurs below {ital T}{sub {ital c}}. From the behaviors of {ital g}{sub 33} and {Delta}{ital n} with temperature, it can be concluded that the crystal undergoes a second-order phase transition at {ital T}{sub {ital c}} into an optically active class. The crystal manifests large linear dichroism, i.e., {Delta}{ital m}={minus}2.2{times}10{sup {minus}2}. {copyright} {ital 1996 The American Physical Society.}

Evaluation of interlayer ferromagnetic coupling for stacked media by adding reference layer

The trial for quantitative evaluation of interlayer ferromagnetic coupling between granular and cap layer in stacked media is reported. The evaluation is realized by analyzing M-H loop of stacked media with another reference layer added on the cap layer. The reference layer is antiferromagnetically coupled with the cap layer through non-magnetic spacer layer. In this experiment, Rh which leads to antiferromagnetic coupling constant along film normal direction of around 2 erg/cm2 was used as non-magnetic spacer layer. According to the evaluation result done by this method, when thickness of the spacer Pd layer between granular layer and cap layer is increased to 1.1 nm, ferromagnetic coupling constant is weakened to 7.2 erg/cm2 which results in reduction of saturation field.

Effect of Ar-gas-pressure on oxygen content of Co–Si–O granular film sputter-deposited by using sintered targets

A systematic study of the effect of Ar-gas-pressure (PAr) in Co–Si–O granular film by sputter deposition using sintered targets is carried out for applications in CoPtCr–SiO2 perpendicular media. Detailed analyses of composition, phase formation and microstructure of granular films revealed that: (1) Co–Si–O granular films with similar compositions could be prepared by different sintered targets, (Co)+(SiO2), (Co)+(CoSi)+(CoO), (Co)+(Si)+(CoO), and (Co)+(Si)+(Co3O4), (2) oxygen content in film increases with increasing PAr, (3) oxygen content in film could be adjustable by the target composition (e.g. (CoO)/(CoSi)). Taking these results into account, we tried low PAr deposition of Co–Si–O granular film. Under PAr = 0.6 Pa, oxygen-poor Co–Si–O granular film was obtained using a conventional (Co)+(SiO2) target, while by using a new type of sintered (Co)+(CoSi)+enriched(CoO) target, the oxygen composition in the granular film could be adjusted to be O/Si= 2 with grain isolated structure.

Effect of Rh spacer on Synthetic-Antiferromagnetic Coupling in FeCoB/Rh/FeCoB Films

Effect of Rh spacer on synthetic antiferromagnetic (SAF) coupling was investigated in subnano-crystalline (Fe65Co35)88B12 (30 nm)/ Rh/ (Fe65Co35)88B12 (30 nm) films. The flopping field (Hf) showed oscillatory behavior with respect to Rh thickness, dRh. The 1st peak of Hf appeared at dRh = 0.9 nm, and the 2nd at dRh = 1.7 nm. These results are analyzed in terms of interlayer coupling effect including the bilinear (J1) and biquadratic (J2) coupling energy, and found to be 0.65 erg/cm2 for J1 and 0.12 erg/cm2 for J2 at the 1st peak. Compared to SAF coupling with Ru spacer, the 1st peak thickness with Rh spacer is thicker. This is because the effective spacer layer thickness decreases by the polarization of Rh neighboring FeCo based material.

Optical properties of superconductingBi2Sr2CaCu2O8

A fundamental optical study was performed on superconducting Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} by using a high-accuracy universal polarimeter (HAUP). As this crystal is strongly linearly dichroic, we developed the extended HAUP theory which includes the treatments of the optical dichroisms. After having determined the optical nature of the crystal, we applied the extended HAUP method to a thin (001) plate specimen with light traveling to the front and rear directions in the specimen. These two experiments permitted us to separate the reciprocal and nonreciprocal optical effects. No sign of the nonreciprocal effects was found in the HAUP transmission experiment. A gyration tensor component {ital g}{sub 33} takes place suddenly at {ital T}{sub {ital c}} (90 K), increases with decreasing temperature, and reaches 1.87{times}10{sup {minus}4} (36{degree}/mm of rotatory power) at 15 K. A steep change of birefringence {Delta}{ital n} with temperature also occurs below {ital T}{sub {ital c}}. From the behaviors of {ital g}{sub 33} and {Delta}{ital n} with temperature, it can be concluded that the crystal undergoes a second-order phase transition at {ital T}{sub {ital c}} into an optically active class. The crystal manifests large linear dichroism, i.e., {Delta}{ital m}={minus}2.2{times}10{sup {minus}2}. {copyright} {ital 1996 The American Physical Society.}

Optical properties of superconducting Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}

A fundamental optical study was performed on superconducting Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} by using a high-accuracy universal polarimeter (HAUP). As this crystal is strongly linearly dichroic, we developed the extended HAUP theory which includes the treatments of the optical dichroisms. After having determined the optical nature of the crystal, we applied the extended HAUP method to a thin (001) plate specimen with light traveling to the front and rear directions in the specimen. These two experiments permitted us to separate the reciprocal and nonreciprocal optical effects. No sign of the nonreciprocal effects was found in the HAUP transmission experiment. A gyration tensor component {ital g}{sub 33} takes place suddenly at {ital T}{sub {ital c}} (90 K), increases with decreasing temperature, and reaches 1.87{times}10{sup {minus}4} (36{degree}/mm of rotatory power) at 15 K. A steep change of birefringence {Delta}{ital n} with temperature also occurs below {ital T}{sub {ital c}}. From the behaviors of {ital g}{sub 33} and {Delta}{ital n} with temperature, it can be concluded that the crystal undergoes a second-order phase transition at {ital T}{sub {ital c}} into an optically active class. The crystal manifests large linear dichroism, i.e., {Delta}{ital m}={minus}2.2{times}10{sup {minus}2}. {copyright} {ital 1996 The American Physical Society.}