Perpendicular magnetic recording using a single-pole-type (SPT) writer head and a double-layer perpendicular medium with a soft underlayer (SUL) has several advantages for high-density magnetic recording. It is therefore a promising candidate for a next-generation magnetic recording system. However, the pole-erase phenomenon, which is a signal reduction caused by a remanent head field from the main pole, prevents high-density recording. We investigated an SPT writer head with antiferromagnetic- (AFM-) coupled main pole to avoid the pole-erase phenomenon by using recently developed combined micromagnetics and finite-element method simulations. We demonstrated that the recording head field decreases monotonically with increasing number of AFM-coupled layers. On the other hand, residual field (pole-erase field) strength dramatically decreases to around half while applying an AFM-coupled bi-layer for the main pole. The pole-erase field is almost constant in the case of two or more layers. The optimum layer number of an AFM-coupled multilayer for the main pole is from two to four layers from the recording and pole-erase field point of view. We confirmed the same tendency by experimental observation of the pole-erase.
The higher energy term of magnetic anisotropy, Ku2, which is theoretically predicted to enhance thermal stability without a notable change in switching field, is discussed for CoPtCr-SiO2 perpendicular recording media. CoPtCr films deposited on Ru seed layers show a high value of the first order energy term, Ku1. The Ku1 value reaches 1.3 × 107 erg/cm3 for Co70Pt30 composition, for instance, whereas the Ku2 values were found to be negligibly small irrespective of the film composition. However, it was revealed that the values of Ku1 and Ku2 vary significantly according to the seed layer material used. Ku2 increases as the c/a of hcp-CoPtCr lattice of the film increases, whereas Ku1 decreases as the c/a increases. This result implies that the appearance of Ku2 is related to the elongation of the hcp-lattice or an increase in the stacking fault density. The addition of SiO2 to the CoPtCr films reduces the values of Ku1 and Ku2, however, no significant change in the value of Ku2/Ku1 was observed on the addition of SiO2 to CoCrPt films. It was concluded that CoPtCr-SiO2 films have a sufficient potential in the values of Ku1 and Ku2 for high-density perpendicular media.
It is known that jitter-like noise in perpendicular magnetic recording system is large compared with longitudinal magnetic recording system. In this paper, the recording density and the amount of magnetic transition-jitter from the signal processing point of view are assumed to be same in both recording systems, and the performance comparison of GPRML systems are performed. It is clarified that the perpendicular magnetic recording system shows the good BER performance compared with the longitudinal magnetic recording system, and it has large allowance of magnetic transition-jitter.
An experiment was carried out to improve the magnetic properties of Sr-Zn-W-type hexagonal ferrite powders, and bonded magnets were prepared from these powders. It was found that W-type hexagonal ferrite is easily producible in the air by replacing the Fe2+ part with Zn2+. The preparation conditions for a typical sample were as follows; chemical composition: SrO·1.5ZnO·0.5FeO·8Fe2O3 with added 0.3 wt% SiO2, 0.7 wt% CaO, 1.0 wt% SrCO3; reaction sintering condition: 1300°C × 4.0 h in air; annealing condition: 1050°C × 5 min in air. The magnetic properties of Sr-Zn-W-type hexagonal ferrite powder are σs=92.9 × 10-6Wb·m/kg, HcJ=187.8 kA/m. The magnetic properties of hybrid (mixed 80% M-type ferrite powder and 20% W-type ferrite powder) bonded magnets are Br=295 mT, (BH)max=16.6 kJ/m3, HcJ=177 kA/m.
We report studies on Andreev reflection measurements of ferromagnetic materials at junction interfaces using AlN/NbN junctions. We found that the junction resistance R can be controlled by varying the thickness and growth temperature of an AlN interlayer. The spin polarization of Co was determined to be PCo ≈ 0.44 from theoretical fitting of the conductance data for these junctions. The behavior of PCo and the dimensionless barrier parameter Z depending on the junction resistance R is discussed.
We fabricated Fe(001)/Al-O/NiFe magnetic tunnel junctions (MTJs) with an ultrathin single-crystal Fe(001) electrode and studied the Fethickness (tFe) dependence of the TMR effect. The upper NiFe electrode (polycrystal) was pinned by a IrMn layer in order to make an antiparallel alignment between upper and lower electrode magnetizations. As a result, we succeeded in observing the TMR effect with an ultrathin Fe electrode down to tFe=0.5 nm. It was found that a magnetoresistance (MR) ratio of 7% was achieved even at tFe=0.5 nm, although the MR ratio decreased slightly with decreasing tFe. Moreover, the bias-voltage dependence of the MR ratio was found to be almost independent of tFe.
We have investigated the magnetic damping in two types of multilayer films, Cu(10 nm)/Ni80Fe20(3 nm)/Cu (dCu) and Cu(10 nm)/Ni80Fe20(3 nm)/Cu(dCu)/Pt(2 nm). Temperature dependence of the damping parameter and FMR linewidth with various Cu thickness dCu were measured. Spin diffusion length increased with decreasing temperature. We also measured the temperature dependence of FMR linewidth for the Cu/Ni80Fe20/Cu(dCu)/Pt films with dCu=400 nm, 800 nm. The increase of FMR linewidth with decreasing temperature suggested that the damping increased by spin diffusion in Cu layer.
We investigated the structural, magnetic and electrical transport properties of Co2CrGa full Heusler alloy films, and optimized the growth conditions, achieving stoichiometric films. The crystalline structures and the magnetic and electrical transport properties were found to depend on the film thickness, tCCG. An ordered L21 structure was obtained for tCCG≥100 nm. The saturation magnetization decreases with decreasing film thickness and vanishes below tCCG=23 nm, indicating the existence of a magnetically dead layer in the vicinity of the substrates. For tCCG=150 nm, the saturation magnetization is 602 emu/cm3 (3.1 μB) at 5 K, which agrees very well with the theoretical value. The resistivity of the samples with tCCG≤150 nm decreases with increasing temperature, while that with tCCG=300 nm increases. By using films with tCCG=100 nm and 300 nm as bottom electrodes, forming an L21 structure, magnetic tunnel junctions (MTJs) were also fabricated, showing 5.0% and 1.4% tunnel magnetoresistance (TMR) at RT, respectively.
Magnetic properties of Ni-Fe ultra thin films with a Ru capping layer were investigated for application to magnetic random access memory units (MRAMs). The sample structure, which simulated an MRAM free layer, is Si-sub./SiO2/Ni-Fe(tnm)/CL (7 nm; CL=Ru, Ta). The Ni-Fe thin films less than 3 nm thick show low coercive fields of less than 1 Oe because of employment of the Ru capping layer. Moreover, from evaluation of the magnetization of the Ni-Fe films, we found that the Ru capping layer results in less dead-layer formation in comparison with a conventional Ta capping layer. The Ru capping layer is valuable for MRAM use in terms of low coercive magnetic field and effective thickness control.
The electronic and magnetic states of a monatomic bcc Fe(001) layer directly facing a single-crystalline MgO (001) tunnel barrier were studied by means of X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Fe L2,3 edges to elucidate the underlying physics of huge tunnel magnetoresistance (TMR) effects observed in Fe/MgO/Fe magnetic tunnel junctions. Both XAS and XMCD results reveal that the 1-ML Fe is not oxidized, which should be crucial for the huge TMR effect. A sum-rule analysis of the XMCD spectra shows that the 1-ML Fe(001) has a total magnetic moment of about 2.6 μB per Fe atom, which is strongly enhanced as compared to that of bulk Fe.
Ni-Zn ferrite (100) thin films were deposited to investigate the effects of low-target-voltage sputtering using an electron cyclotron resonance (ECR) sputtering apparatus. Saturation magnetization equivalent to that for bulk Ni-Zn ferrite could be obtained at any target voltage. Full width at half-maximum of the rocking curves decreased with decreasing the target voltage in the range of −400 to −100 V. When the target voltage was −50 V, Ni-Zn ferrite thin films showed relatively high coercivity and broad rocking curves. The reasons were considered to be the influence of impurity gas and unsuitable oxygen gas flow rate. The validity of low-target-voltage sputtering was confirmed for the reactive ECR sputtering method.
We designed high-frequency-carrier-type thin-film sensors to obtain high sensitivity. We consider the relation between the sensor form and the sensor to be characteristic. When the sensor area was fixed to 5 mm × 5 mm, and the sensor interval was 30 μm, the width of the sensor with the best sensitivity was set to 200-400 μm. We fabricated a meander-type sensor element, and compared the calculated and measured result. It was found that-they mostly in agreed.
In order to measure weak magnetic fields such as those detected by magnetoencephalography (MEG), a magnetically shielded room (MSR) with multiple layers of μ-metal is usually used to reduce environmental magnetic field noise. However, such a heavily shielded room is not suitable for a magnetocardiogram (MCG) measurement system from the viewpoint of requirements of low cost, light weight, and small space. As an alternative to an MSR, we have developed a hybrid cancellation method which combines an electronic gradiometer and a normal conduction coil cancellation. In this paper, the compensation setups are described and experimental results are presented and discussed. The normal coil cancellation consisted of two coils (100 turns each) to detect uniform and gradient magnetic fields. The two detection coils were wound around a cubic wooden frame (1m×1m×1m) at the top (upper detection coil) and bottom (lower detection coil). The noises detected with the detection coils were added and subtracted to compensate for uniform and gradient noise, respectively. The following results were obtained for the suppression of environmental noises: 20 dB at 0.1-100 Hz and 30 dB at 50 Hz.
Giant magnetostrictive material (GMM) has large magnetostriction, and has been successfully used in many actuators and sensors. Currently, practical microactuators and micro-sensors are widely anticipated in the industry. In this study, the effect of adding Co to Tb-Fe GMM thin film and the influence of heat-treatment were examined. GMM thin film was used in a force sensor. GMM thin film with a Co ratio of 25.5 at% and annealing temperature of 300°C has 1200 ppm magnetostriction at 80 kA/m. The characteristics of a force sensor using a GMM thin film with a relative permeability of 210 are also examined.
There are many requirements for measuring torque, such as non-contact with the shaft, small size of the measurement device, and simple installation. Other requirements include no special processing in the shaft and cost-effectiveness. Therefore, we have proposed a magnetostrictive-type torque sensor for a steering shaft using an inverse magnetostrictive effect. Generally, a magnetostrictive-type torque sensor for a low-carbon steel shaft has a large hysteresis to the torque and output fluctuation due to axis rotation. With this sensor, we have shown that the hysteresis on the output voltage can be drastically decreased, if the magnetic shaking method is applied.
A microstrip-Y-junction circulator with ferrite thin film was designed and its transmission characteristics were analyzed by the 3D finite-element method. The circulator consists of a dielectric substrate with a cylindrical GND, a dielectric layer with a low dielectric constant, a ferrite thin film, a silver transmission line, and a ground plane. The thickness of the ferrite thin film is set at 5 μm. The cylindrical GND is located in the proximity of the center of Y-junction to obtain a large gyro-magnetic effect efficiently. A 15-μm-thick dielectric layer with a dielectric constant of 4 is inserted between the ferrite thin film and the cylindrical GND to decrease the loss resulting from the transmission line, and to obtain a wide permissible range for misalignment between the GND and Y-junction. The results show non-reciprocal transmission characteristics at approximately 5.9 GHz with an insertion loss of 1.6 dB and an isolation of 23 dB.
Since an EIE-core variable inductor is constructed from cores and windings, it has desirable features for an electric power apparatus, such as simple construction and high reliability. The variable inductor does not need any gaps for harmonic current reduction and has a suitable structure for a larger capacity. It is expected that the EIE-core variable inductor is applied as an automatic voltage regulator to an electric power system. It is important to clarify the optimal construction of the EIE-core. In this paper, we compare basic characteristics of several EIE-cores which have different shape and size using reluctance network analysis (RNA) suggested by the authors. RNA is the improved calculation method based on magnetic circuit analysis.
We performed three-dimensional vector measurement of exercise-induced magnetocardiograms (MCGs) for normal subjects with a wooden and brass-based bicycle ergometer. MCGs were measured by a three-dimensional second-order gradiometer connected to 39-channel SQUIDs, which can detect magnetic field components perpendicular to the chest wall (Bz) and tangential to the chest wall (Bx, By) simultaneously. Time-frequency analysis was applied to rest times and exercise-induced MCG data. The power spectrum of the ST segment at 6, 8, and 10 Hz was differed between rest times and the exercise-induced MCGs. In order to compare ST segments between rest times and exercise-induced MCG data, singular value decomposition (SVD) was also applied to the MCG data. We found that the time course of singular values in the ST segment differed between rest times and exercise-induced MCG data compared with the QRS segment.
A three-dimensional magnetic measurement of somatosensory evoked fields (SEFs) by electric stimulus to the right thumb for five normal subjects was carried out, using a three-dimensional second-order gradiometer connected to 39-channel SQUIDs, which can detect magnetic field components perpendicular to the scalp (Br) and tangential to the scalp (Bθ, Bφ) simultaneously. To discuss the relationship between the phase lag of SEFs and stimulus repetition frequency (SRF), the delay time of a component synchronous with the SRFs (stimulation was 2 to 7 train variety with 3.0 to 30.3 Hz, having a recovery period of 500 ms) was calculated by the convolution of the reference signal and the SEF wave (BPF: 1-40 Hz). The phase lag characteristic to the SRF differed in three ranges. Dominant slope appeared for two ranges (blow 8 Hz, 8 to 20 Hz) in all magnetic components. Dominant slope did not obtained in the range of more than 20 Hz SRF to the transient SEF waveforms. However, the dominant slope was obtained in the steady-state waveforms. We tested results for significance of the linear regression slope (β≠0, P<0.05).