ＥＣＣ媒体の薄膜形状における耐熱特性

To overcome superparamagnetic limit, microwave assisted magnetic recording (MAMR) is one interesting magnetic recording technology. Therefore, the effect of microwave on magnetization reversal in media should be analyzed. In this work, we propose the MAMR to decrease switching field (coercivity, Hsw) in exchange coupled composite (ECC) media by using the micromagnetic simulation based on the Landau - Lifshitz - Gilbert equation. The Hsw of single layer and ECC media without microwave field is 110.90 and 7.7 kOe, respectively. When the oscillating microwave field is added, Hsw of single layer media with microwave frequency of 2.5 - 40 GHz is lower than 110.90 kOe. Likewise, Hsw of ECC media with microwave frequency of 5 - 16 GHz is lower than 7.7 kOe and has the lowest value of 4.9 kOe at frequency of 10 GHz. The results from this work lead to solve superparamagnetic limit and increase areal density in hard disk drive.

Microwave-assisted magnetic recording (MAMR) on single layer and exchange coupled composite (ECC) media was simulated. The high-frequency field was modeled with a macrospin model and also micromagnetically using a spin-torque oscillator integrated into the write head. ECC media with a range of hard/soft layer thicknesses were optimized for MAMR, and the gain over a non-MAMR system was established. The results show a bigger gain from using MAMR when the hard/soft layer thickness ratio was small. MAMR systems were also less susceptible to changes in magnetostatic interactions between medium grains.

Dynamic coercivity and thermal stability of epitaxial exchange spring trilayers

In this study, an exchange coupled composite (ECC) film with an antiferromagnetic sandwich structure L10-FePt/[Co/Ni]5/Ru/[Co/Ni]10 (FePt-CN-Ru) is prepared via dc magnetron sputtering and measured by vibrating sample magnetometer. The results show that FePt-CN-Ru film exhibits ECC characteristics, with excellent angular tolerance, low remanance magnetization, and a narrow switching field. Its thermal stability is higher than that of conventional ECC film. Finally, magnetization reversal is demonstrated through micromagnetic simulation.

We studied exchange coupled composite (ECC) media with an out-of-plane easy axis consisting of hard magnetic L10 chemically ordered FePtCu alloy films and magnetically softer [Co/Pt]N multilayer stacks. By tailoring the structural properties of the ternary FePtCu alloy and [Co/Pt]N multilayers, we can tune the magnetic parameters of the composite in a wide range. This allowed us to address experimentally one of the most crucial properties determining the performance of ECC media, namely the pinning field of the magnetic domain wall present at the interface between the hard and soft layers. We demonstrate that the pinning field is proportional to the difference of the magnetic anisotropy constants of the hard and soft layers, which confirms the theoretical predictions. Furthermore, we show that the pinning field can be efficiently decreased after an additional annealing step. Transmission electron microscopy investigations indicated that the origin of the observed effect is due to a heat-induced phase transformation of iron oxide present at the interface between the hard and soft layers. This study reveals that tailoring the properties of the hard/soft interface is another efficient tuning knob for optimization of the performance of ECC media.

The potential of microwave-assisted magnetic recording on exchange-coupled composite (ECC) media is investigated by numerically simulating read/write processes with respect to the physical and magnetic properties of the soft section of ECC media. To obtain desirable recording performance, the thickness of the soft section must be comparable to the exchange length. Under such condition, the optimal microwave frequency decreases owing to the non-uniformity of the reversal mode. A relatively high anisotropy field in the soft section produces high signal-to-noise ratios. Moreover, large saturation magnetization of the soft section enhances the magnetization reversal of ECC grains but excessive enhancement is likely to induce wide writing, which decreases the high track density.

Nanocomposite magnetic films for high-density perpendicular magnetic recording media

The Effect of saturation magnetization and interlayer exchange coupling on the time-evolutional magnetization reversal process in exchange-coupled composite (ECC) media with high coercivity was investigated by utilizing micromagnetic simulation. In order to apply magnetic field, we used magnetic printing. As a result, the magnetization reversal time of the hard layer became shorter as the saturation magnetization of soft layer increased because the magnetostatic field generated from the soft layer increased. Thus, as the saturation magnetization of both layers increased, magnetization reversal occured for the weaker interlayer exchange field.

A micromagnetic study on the magnetization reversal in bit patterned media (BPM) with each bit consisting of an exchange coupled composite (ECC) layer stack is presented. The simulations reveal superior magnetic properties of the combined ECC/BPM scheme, in particular for graded media, using uncorrelated distributions of magnetic anisotropy values in order to lower the switching field while keeping a high thermal stability of the media. In this study, a route for narrowing the switching field distribution of the bit array is provided as well, which is vital for the applicability of the BPM concept in magnetic data storage.

The Monte Carlo and Langevin dynamical methods of simulating the thermodynamics of physical systems are compared by calculating relaxation times according to the two dynamics for a system which is analytically tractable, namely a single (planar) spin in a potential which has either a single minimum or two minima separated by a barrier. With no restriction on the maximum allowed spin reorientation per Monte Carlo step the Langevin method is faster than the Monte Carlo method for a single minimum potential. However a careful choice of restriction can make the Monte Carlo method as efficient as the Langevin method. For the double-well potential the Monte Carlo method with no restriction is the most efficient. One is forced to use a finite-time step size when numerically solving the Langevin equation and the departures this produces from the equilibrium Boltzmann distribution are studied.

A new analytical formula for thermal fluctuation in magnetic recording media is presented. The thermal fluctuation of isolated particles in the presence of a small magnetic field is evaluated by introducing a simple and precise energy barrier formula. The thermal decay and the remanent coercivity of the recording medium, which has grain size distribution and inter-granular exchange interaction, are numerically computed. These results are approximated by an equation in terms of the thermal fluctuation in the magnetic recording media. This analytical formula shows good agreement with previous experimental results, which indicates an increase in the activation volume in the presence of a small magnetic field. Both the increase in the activation volume and a decrease in the magnetic viscosity coefficient in the presence of a small magnetic field can be explained by the mixture of thermally reversed and re-reversed particles. Reducing the demagnetization field for the recording media is most effective for decreasing the signal decay. From this point of view, an AFC medium, which can reduce the demagnetization field without decreasing the magnetic anisotropy energy of the particles, has a strong potential for realizing higher recording density.

In this paper, we investigated the dependence of magnetic properties on thermal fluctuation and the relationship between magnetic clusters size and thermal decay.

The long-term bit error rate (BER) performance of partial response maximum likelihood (PRML) system in a perpendicular magnetic recording (PMR) channel with thermal decay is studied. A thermal decay model based on the experimental data for three CoPtCr-SiO2 PMR media with different thermal stability is obtained. Then, the performance of PR1ML channel for the 16/17(0,6/6) and 128/130(0,16/8) codes as a run-length-limited (RLL) code is evaluated by computer simulation using the model. Moreover, the relationships between the elapsed time and the BER are obtained for the combinations of two RLL codes and three PMR media and their performances are compared. The results show that for the combination of the 128/130(0,16/8) code and the medium with the poorest thermal stability, the degradation with the elapsed time is noticeable compared with the other combinations

Systematic simulation studies of magnetic and recording properties of thin film media at ultrahigh density have been performed with thermal effects taken into account. In the bulk magnetic property study, the focus is on coercivity. A formula for the ratio of the writing coercivity to that from the vibrating sample magnetometer (VSM) is given. The writing of di-bits and their subsequent thermal decay are simulated for a wide range of medium parameter values. From these simulations, the medium performance is mapped out on an anisotropy-constant-saturation-magnetization plane and the range of proper medium parameters suitable for stable storage determined. Ways to predict the thermal decay of written information by bulk measurements are discussed.

Light-tissue interactions are commonly studied for near infra-red (NIR) spectroscopy for imaging and diagnosis. While imaging is bound to the surface due to the high tissue scattering, many diagnosis methods, such as the photoplethysmograph (PPG) and pulse oximeter, focus on sensing instead of imaging. There are two approaches for the investigation of light-tissue interactions: one numerical and the other analytical. The most common numerical method is the Monte Carlo (MC) simulation, which is a statistical study of photon migration from the optical properties of the different tissue regions. However, the yield of such statistical approaches is limited. Analytical methods are commonly based on the diffusion theory, yet they are inaccurate near the light source. There are several solution methods, where extrapolated boundary conditions lead to a more accurate solution. Previously, we proposed measuring the full scattering profile (FSP), which is the angular intensity distribution, of cylindrical tissues. MC simulations revealed that the FSP has a fixed intensity point, named the iso-pathlength (IPL) point, which does not depend on the reduced scattering coefficient. The location of this point depends solely on the geometry. In this study, we derived the FSP from the steady state diffusion theory based on the extrapolated zeroboundary condition. A comparison of different reduced scattering coefficients revealed the IPL point, where the intensity remains constant. Furthermore, a MC simulation was performed under the same geometry and optical properties. We show that the position of the IPL according to the diffusion theory is in agreement with the MC simulation.