Y. Sugita

Computer simulation of thermal activation volume in HDD media

The thermal activation volume in hard-disk media was investigated through micro-magnetic simulation. It was found that the activation volume should be obtained by using the time-dependent irreversible magnetization (viscosity). The method using the total magnetization including the reversible magnetization leads to a wrong dependence of the activation volume on the field. The relationship between the switching volume in a hard-disk medium and the activation volume is described as a function of the exponent of the dynamic coercivity, the switching field, and the anisotropy field.

M?H Loop Slope and Recording Properties of Perpendicular Media

The slopes of the M–H loop of perpendicular magnetic recording media were estimated by micro-magnetic simulation for various inter-grain exchange couplings, grain aspect ratios and inter-grain spacings. The slope of the M–H loop depends not only on inter-grain exchange coupling but also on grain aspect ratio and inter-grain spacing. The simulation revealed that the required head field decreases with increasing inter-grain exchange coupling. The optimum value of the slope was slightly larger than 1.0 for compromising resolution and SNR by changing the inter-grain exchange coupling. The simulation results also indicated that the inter-grain spacing improves both resolution and SNR.

Validity of Values of Thermal Stability and Switching Field in Recording Medium Obtained by Using Sharrock's Formula

Validity of values of thermal stability factor K_beta (=K_uV/kT) and switching field H_cro in perpendicular recording medium obtained by Sharrocks formula has been analyzed using micromagnetic simulation. The following was found. Time-dependent remanent coercivity can be expressed by Sharrocks formula for media with the inter-grain exchange coupling w in a wide range. The values of the exponent C and the attempt frequency f_o change by w steeply and are significantly different from conventional values: C of 0.5 and f_o of 1times10⁹ Hz. Those conventional values lead to 10% or larger values of H_cro and K_beta. It was also found that field error less than 20 Oe is required for conventional VSM measurement or field calibration within 100 Oe is required for VSM with 1 mus-pulse field measurement to obtain H_cro and K_beta in a range from 90% to 110% of the correct values

Monte Carlo simulation of thermal activation volume in longitudinal hard-disk drives media

Thermal fluctuation in longitudinal hard-disk recording media was investigated through micromagnetic simulation. The activation volume for a recording medium obtained by using the Monte Carlo simulation with field dependent attempt frequency was investigated. It was found that the activation volume should be obtained by using the remanence magnetization consisting of irreversible magnetization and the method using the total magnetization including the reversible magnetization leads to a wrong dependence of the activation volume on the field. It was also found that when the intergrain surface exchange coupling J is increased, the activation volume remains almost the same, close to the volume of a grain until J reaches 0.7×10−3u2009J/m2, and increases linearly further. Observation of the distribution of switched grains revealed that the activation volume obtained using our method agrees well with the size of the thermal-switching units.

Computer simulation of high-speed magnetization switching in hard-disk media

Magnetization-switching behavior was investigated in high-data-rate hard-disk media through micro-magnetic simulation to clarify the distinction between the gyromagnetics dominated scheme and the thermodynamics dominated scheme in the dynamics of the switching. It was found that the pulse-shape dependence of remanence coercivity H/sub cr/ and the anisotropy field H/sub k/ dependence of the switching-speed are caused by the gyromagnetic effect. The thermodynamic effect plays a main role in the time-dependent H/sub cr/ when the pulse width is over 1 ns. On the other hand H/sub cr/ in the thermodynamic region decreases linearly with respect to the logarithmic pulse width. The demagnetizing field from the write bubble causes H/sub cr/ to increase with the reverse DC erase (RDC) method in 10 or fewer reptations.

Validity of thermal activation volume estimated by using Wohlfarth’s equation in perpendicular recording hard disk media

The physical volume of thermal switching unit, the activation volume for perpendicular recording media, has been investigated using the Wohlfarth equation based on micromagnetic simulation. It was found that the activation volume obtained from the Wohlfarth equation Vac‐Wohlfarth exhibits significant field dependence, which is very different from direct observation of thermal switching process. This shows that Vac‐Wohlfarth is invalid for perpendicular recording media. The reason for that was interpreted in terms of the narrow dispersion of the energy barrier for thermal switching, originating from the high degree of easy axes orientation along the direction perpendicular to the film plane.

Structural design of normalized slope of M-H loops and coercive force for perpendicular recording media

In this paper, we discuss the effect of aspect ratio of grains and inter-grain spacing on the M-H loops using micromagnetic simulation.

Relation between magnetic cluster size and thermal switching size in HDD media

Summary form only given. As the recording density of hard-disk drives becomes higher, it becomes more important to retain thermal stability and reduce media noise at the same time. The media noise is thought to depend on magnetic cluster size D/sub cluster/. However, the relation between D/sub cluster/ and thermal switching size D/sub switch/ is not clear. In this study, D/sub cluster/ and D/sub switch/ have been analyzed by using micro-magnetic simulation. A 32/spl times/32 array of single-domain grains with the same grain size D was used to simulate a longitudinal recording medium. The easy axis of each grain was randomly distributed in the film plane. The time evolution of the magnetization of the grains was calculated by solving the Langevin equation. The average diameter of the unit of thermal switching, in which the magnetizations switch simultaneously, was defined as D/sub switch/.