The effects of the thicknesses of a Cu/Ti underlayer and a Sm-Co layer on the magnetic properties and domain structure of SmCo₅ perpendicular magnetization films prepared by using an ultra-high-vacuum sputtering system were investigated. By adjusting the thicknesses of both the Cu/Ti underlayer and the Sm-Co layer, the coercivity was controlled to be a moderate value, allowing the films to be used in magnetic recording media while maintaining a squareness ratio of unity. As the Sm-Co layer thickness was reduced, the magnetic cluster size decreased and the magnetization reversal process varied from wall motion mode to a rotation mode. It is suggested that the Cu-rich region in the initial growth stage of the Sm-Co layer functions as the pinning site of the magnetic domain wall and plays an important role in controlling the magnetic domain structure.

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We developed thin-film magnetic field probes with a high spatial resolution aiming to obtain the absolute value of a high-frequency power current on an LSI chip. The spatial resolution was enhanced by miniaturizing the shielded loop coil, which is the detection part of the probe. The minimum outer size of the new coil is 50 x 22 μm. In taking measurements with the new probe over a 60-μm-wide microstrip line used as a device under test (DUT), we found that the probe output decreases by 6 dB at a distance of 40 μm. This value is less than half that of our previous probe, which was around 100 μm. In taking measurements with the new probe over 5-μm-wide microstrip lines used as a DUT, we found that the new probe achieved 10-μm-class high spatial resolution. This value is comparable to the typical width of global power lines on an LSI chip, which ranges from 10 μm to 100 μm. We estimated the configuration of the lines on an LSI chip that would enable the new probe to achieve a spatial resolution high enough to obtain the absolute value of a high-frequency power current on an LSI chip.

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This paper describes an eddy-current testing (ECT) probe with a new structure, consisting of a perpendicular exciting coil and a spin-valve-type giant magneto-resistance sensor for scratch detection with a high lift-off height. We derived an analytical solution of eddy-current distribution to compare with a perpendicular exciting coil and a traditional parallel exciting coil. Experimental inspections with lift-off heights of 5 and 7 mm were carried out to detect scratches of 0.1 - 0.5 mm depth in a stainless steel plate. The 2-D experimental image results show that the proposed ECT probe has good detection capabilities.

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The design of an induction magnetometer with optimum air-core coil is presented. The desired cutoff frequency of the magnetometer determines that the size of the coil should be minimized with the shape of a Brooks coil. An op amp to be used in the electronics determines the diameter of the winding coil, and thus the characteristics of the magnetometer are defined. A battery-powered induction magnetometer was fabricated and demonstrated validity of the design. It had a cutoff frequency of 18 Hz, a linear transfer function of 30 mV/nT, and a noise floor level of 300 fT/√Hz in the frequency range from 50 to 150 Hz. These results were in good agreement with the design.

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Magnetic tunnel junctions (MTJs) using an MgO tunnel barrier with ultra-thin Mg insertion layer were studied. It was found that the critical thickness of the MgO barrier layer at which a reasonably high tunneling magnetoresistance (TMR) ratio can be obtained is reduced when an ultra-thin Mg insertion layer is introduced. The highest TMR ratio at a low junction resistance RA was obtained when a 0.4 nm MgO tunnel barrier and a 0.6 nm Mg insertion layer were used. The improvement in the TMR properties is considered to be due to the improvement in the crystallinity of the MgO tunnel barrier with (100) preferred orientation. It was also found that the bias voltage dependence was not affected by insertion of an Mg layer. The present study shows that the use of an ultra-thin Mg insertion layer is effective for improving the TMR ratio in the low RA region, while maintaining the other electrical properties.

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The effect of a high magnetic field on coercivity was investigated in the annealing process for sintered Nd- Fe-B magnets containing Dy 10 wt.%. In a sample containing Cu 0.13 wt.%, a coercivity value H_c of 36 kOe was obtained for an annealing temperature T_a of 475°C under an annealing field H_a of 140 kOe. This value of H_c is 20% larger than that for the control sample (T_a = 475°C and H_a = 0). In samples without Cu, H_c values were considerably smaller (< 27 kOe) and no such enhancement of the coercivity by the magnetic fields was observed. The DSC curves exhibited a marked endothermic reaction just below 500°C only for samples containing Cu. This temperature is attributed to a eutectic point of Nd₇₀Cu₃₀. A possibility of field-induced alignment of Nd-rich grains in a matrix of Nd-Cu liquid is discussed; this would be closely related to the coercivity enhancement.

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We have investigated that the structural and magnetic properties of Co₂CrGa full-Heusler alloy films and researched the tunnel magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) using a Co₂CrGa electrode on a MgO(100) substrate. Co₂CrGa films were fabricated with substrate heating (RT ≤ T_s ≤ 500°C) or post-annealing (RT ≤ T_a ≤ 500°C) after the deposition using an ultrahigh vacuum dc magnetron sputtering system. L2₁-ordered Co₂CrGa thin films were obtained at T_s ≥ 300°C or T_a ≥ 200°C. The maximum magnetic moment per formula unit measured at 5 K were 2.8 μ_B and 2.6 μ_B for T_S and T_a = 400°C, respectively, which are over 85% of the theoretical value. The maximum TMR of 18% at RT and 42% at 5 K are obtained for the MTJ using the L2₁-structured Co₂CrGa film as a bottom electrode. It is expected that the TMR can be enhanced by optimizing the interface of Co₂CrGa/AlOx.

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The microstructures of Co₇₅Pt₂₅ (30 nm)/Pt (200 nm) films prepared by gas flow sputtering and the effects of annealing on their magnetic properties were studied. Transmission electron microscopy (TEM) of cross-sectional samples showed that the Co-Pt films consisted of columnar crystal grains with very distinct grain boundaries, and that the diameter of the crystal grain was about 10 nm. It also showed that each Co-Pt grain was of hcp structure and that the grains were oriented with their c-axis along the direction perpendicular to the film plane. The coercivity of as-deposited films was about 4 kOe and was increased to 7 kOe by annealing at 250°C in Ar, while the hcp structure was unchanged up to 300°C. These results suggest that the annealing brings about oxidation, mainly of the grain boundaries, and magnetical isolation of the grains, resulting in an increase in coercivity.

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This paper describes detailed studies on structural and magnetic properties of L1₀-FePt nanoparticles synthesized via the recently developed “SiO₂-nanoreactor” method (Appl. Phys. Lett.87 032503 (2005)). Monodisperse and magnetically superior L1₀-FePt nanoparticles can be prepared by annealing at 900 °C. Thus-obtained L1₀-FePt nanocrystals can be made dispersible in not only water but also common organic solvents, e.g., toluene, chloroform and hexane, while keeping their superior magnetic properties. The hysteresis loop of the L1₀-FePt nanoparticles dispersed in solvents showed superparamagnetic-like behavior at room temperature due to the free motions of nanoparticles. Analyses of the hysteresis loop by Langevin function suggest that the easy axis of the solvent-dispersed nanoparticles can be aligned by the external magnetic field of about 10 kOe. The hysteresis loop of the water-dispersed nanocrystals, which were frozen under an external field of 50 kOe, was almost rectangular indicating that their magnetic and structural orientation could be attained.

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A new type microstrip-isolator utilizing a regime of large |μ'₊−μ_-'| corresponding to μ₊' < 0 and μ_eff' > 0, which was realized by utilizing a ferrite material with a small FMR line width (ΔΗ), was proposed. Transmission characteristics of the designed microstrip-isolator as small as 3.5×3.5 mm² in dimension was numerically simulated in 2 GHz band using finite element method. A fabricated microstrip-isolator with YIG ferrite single crystal with ΔΗ= 2 Oe showed apparent non-reciprocal transmission characteristics with an insertion loss |S₂₁| of 0.69 dB and an isolation |S₁₂| of 13.1 dB at around 2 GHz.

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Real-space observation of magnetic flux reversal across the magnetic-compensation temperature (T_comp) in an N-type ferrimagnet of TbFeCo was achieved by employing a temperature-controllable MFM system (TCMFM). Using a thermally assisted magnetic recording method, we recorded marks on TbFeCo media for MFM measurements performed at various temperatures from 25°C to 140°C (T_comp ∼ 40°C). The results show a dramatic change in the magnetic flux of the recording marks reflecting the nature of an N-type ferrimagnet. In addition, the temperature dependence of the averaged contrast of the MFM signals coincides with that of the remanent magnetization of the media measured by VSM. Erasing and rewriting of a sub-micron-sized mark by the TCMFM were also demonstrated, indicating that the TCMFM can be a powerful tool for proving the performance of thermally assisted magnetic recording as a means of ultrahigh-density storage.

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In biomagnetic measurement, biomagnetic signals are extremely weak in comparison with environmental magnetic noise. There are several hardware and software noise-reduction methods, such as the use of a gradiometer, averaging,filtering, the PCA method, and independent component analysis(ICA). The ICA method is used to extract and remove noise components from the brain’s magnetic field and the signal. We carried out a 3-D MEG vector measurement of a somatosensory evoked field(SEF) using a 39-channel SQUID system. To improve the signal-to-noise ratio of SEF data, we propose a new method, combining a reference coil and ICA, and apply it to SEF data.

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