Fumiko Akagi

Integrated head design using a nanobeak antenna for thermally assisted magnetic recording.

We propose a near-field optical transducer using a triangular antenna and a thin film structure (wing) to efficiently generate an optical near-field near a magnetic head. A finite-difference time-domain calculation showed that the near-field was efficiently generated at the apex of the antenna when the dimensions of the wing were optimized for efficient delivery of the surface plasmon excited on the wing to the antenna. The calculated light utilization efficiency (ratio between the absorbed power in the recording medium and the input power) was 8%. The temperature distribution on the medium, magnetic field distribution, and magnetization pattern were calculated; the proposed recording head may be capable of an areal recording density of 2.5 Tb/in.(2).

Computer simulation of magnetization switching behavior in high-data-rate hard-disk media

We have investigated magnetization switching behavior in high-data-rate hard-disk media by micro-magnetic simulation. We found that the remanence coercivity measured by a short-pulse-field depended on the field-pulse shape. The time integration of external field over the remanent coercivity for a long-pulse-field, namely switching force integration (SFI), was an essential factor for the switching. We also found that the optimum writing field, H/sub w/ must be increased by only 0.4 kA/m per MB/s independent of the linear recording density. Overwrite (OW) value decreased by 1 dB per percentage point of H/sub w/ and did not depend on the data-transfer rate. The recording characteristics at high data transfer rates were not degraded if only a writing field of H/sub w/ was applied to the film. The simulation results were consistent with the results of read/write experiments especially OW characteristics up to 80 MB/s.

Effect of angular dependent attempt frequency oil-arrhenius-neel thermal decay in thin film media

The angle dependence of the attempt frequency f/sub 0/ has been determined by assuming isotropic thermal excitation in the anisotropic free energy around the easy axis of a single-domain particle. The relationship between the logarithmic time and the time-dependent remanence coercivity H/sub cr/ was estimated. The acquired equation agrees sell with the numerical calculation of the Langevin equation. The conventional formula do not give the correct relationship between H/sub cr/ and K/sub /spl beta//(=Kv/kT) because the exponent of K/sub /spl beta// depends on K/sub /spl beta//. The estimated K/sub /spl beta// from Sharrocks formula is 15% larger at K/sub /spl beta// of 100 and 5% smaller at K/sub /spl beta// of 50. Bertrams formula is effective at K/sub /spl beta// of 100 but brings 7% smaller estimation at K/sub /spl beta// of 50.

Effect of switching field gradient for thermally assisted magnetic recording

The effects of the switching field gradient and the optimal head-field intensity for thermally assisted magnetic recording were investigated using the Landau-Lifshitz-Gilbert equation, which adds a stochastic thermal field to an effective field. The switching field gradient was defined as the product of a space gradient of temperature dT∕dx and a temperature gradient of anisotropy field dHk∕dT under condition of the uniform head-field intensity. The signal-to-noise ratio at 1000kfci was found to improve with increasing dT∕dx and by applying the optimal dHk∕dT.

Thermal stability in longitudinal thin film media

Thermal stability in longitudinal thin film media was studied using a Monte-Carlo simulation. As recording densities increase, the signal decay ratios increase. The effect of the density is, however, very small. Between two materials with identified thermal stability factors /kT (where, K/sub u/ is the anisotropy constant, V is the volume of a grain, k is Boltzmanns constant, T is the temperature, and means an average value), the medium with smaller grains was found to be more stable against thermal fluctuations. These phenomena are related to the magnetic fields that are applied to grains in the media, which make the magnetization stable against thermal fluctuation.

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.

Optimum writing field for high-frequency magnetic recording

Realistic recording conditions in longitudinal thin-film media at high-frequency (up to 1 GHz) are investigated by a micromagnetic simulation. It is found that the recording frequency should be set below the characteristic frequency (1/2 Tr, where Tr is the rise time of the writing head); optimum writing field intensity must be increased by 1.2 Oe per MHz at a linear density of 244 kFCI; and remanence coercivity measured by a square pulse slightly increases up to 1 GHz when damping constant is 0.03.

Optimum Timing and Position of Light Irradiation for Thermally Assisted Perpendicular Recording

The optimum timing and position of light irradiation on the medium for thermally assisted perpendicular recording were evaluated in terms of read-write characteristics calculated by micromagnetics simulation. It was shown that the light irradiation timing should be optimized for obtaining the maximum head field and temperature simultaneously. Moreover, the irradiation start time and stop time were determined to obtain the optimum light irradiation timing. It was also shown that the distance between the light-spot center and the head trailing edge should be less than the radius of the light-spot size.

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 0.7×10−3 J/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 MR response to transverse magnetic fields

The magnetoresistive (MR) response of an MR sensor with shields to a uniform applied field was calculated through a micromagnetic simulation, and the results were compared with those from an MR sensor without shields. A uniform longitudinal field, resulting from boundary pinning by exchange-biased antiferromagnetic films, was applied to the MR films of the two sensors. There are three differences between the MR sensors with and without shields: first, the slope of the MR response with shields near an applied field of zero is smaller than the slope of the response without shields. Second, the response of the shielded MR sensor has no subpeak, while the response of the unshielded MR sensor has a subpeak. Third, the output of the shielded MR sensor hardly decreases in a large field (1000 Oe), while the output of the unshielded MR sensor quickly falls to a small value in a field of 300 Oe. These differences are due to attenuated magnetic fields in the gap between the two shields except near the air bearing surface (ABS).