Mikito Sugiyama

Head/Media Integration Challenge Toward 1

We investigated medium noise using spinstand testing and calculation from the view point of head/media integration for 1 Tb/in 2, focusing on two kinds of noise sources: transition noise in the track center and track edge noise. A quantitative relationship between transition noise and magnetic cluster size of media was clarified. This relationship indicates that the transition noise can be reduced by reducing the magnetic cluster. In the track edge noise, there are two noise sources: edge line fluctuation and increase in transition length at the edges. The edge line fluctuation noise degrades low density SNR. We show that a high cross-track field gradient of writer and small magnetic cluster of media are effective to reduce the edge line fluctuation. The increase in transition length at the edges degrades high linear density SNR. The transition length at the edges increases as transition curvature increases. Both increase in transition length at the edges and large transition curvature make the magnetic write width narrow and erase band width wide. Large track edge noise was observed when the transition curvature was large. Therefore reducing the transition curvature is an effective way to reduce track edge noise. In the micromagnetic calculation for 1 Tb/in2 , high 2 T-SNR of 13.7 dB (1 T: 2200 kfci) at magnetic write width of 54 nm was obtained from a combination of graded Hk media with small magnetic cluster and wraparound shielded writer with high cross-track field gradient and small transition curvature.

{\hbox{Tb/in}}^{2}

We investigated the effect of track edge noise on transition curvature and track edge fluctuation through experiments and calculations. We measured and calculated two types of recording heads that had similar magnetic write widths, but different transition curvatures. In both the measurements and calculations, track edge noise increased more at high linear density with head with larger transition curvature. This was due to wider erase band, which is considered to be noise source at the track edge. We also clarified that reducing the transition curvature and track edge fluctuation was effective to improve bit error rate, because reducing the transition curvature increases signal-to-noise ratio (SNR), especially at high linear density, and reducing the track edge fluctuation also increases SNR, especially at low linear density. Therefore, reducing both transition curvature and track edge fluctuation is necessary in order to reduce the track edge noise and consequently achieve high areal density.

Perpendicular Recording

We investigated the effect of effective field distribution on the recording performance in microwave-assisted magnetic recording (MAMR) through calculations. We calculated the effective field distribution and the recording characteristics for different media that had similar grain sizes, but different anisotropy fields (Hk). We found that the effective field gradient near the trailing edge of the main pole was drastically enhanced because of the localized AC-field (Hac) near the spin-torque oscillator (STO) and the field angle distribution of writer, which affects on assist effect on the reducing the switching field (Hsw) of the media with applying the (Hac). It was also clarified that the SNR at high linear density drastically increased as the (Hk) of the media increased. This is because the effective field gradient at the (Hsw) of media increases as (Hk) of media is increased. Furthermore, SNR can be increased by reducing the grain size of media with high (Hk). Therefore, increasing both the (Hk) of media and the (Hac) of the STO is effective for achieving high areal density from view point of utilizing small grain media as well as high field gradient.

Effect of Transition Curvature and Track Edge Fluctuation on Track Edge Noise for Narrow Track Recording

Track-edge noise, which is one of the major hindrances in achieving the narrow track recording, was studied through spinstand testing and micromagnetics simulations. Three heads with different write pole widths were carefully chosen in the experiments so that their on-track head field gradients were almost similar, and the recording performance of these heads was evaluated with an identical narrow read head. Despite of their similar on-track field gradients, the narrowest write head exhibited a significantly low signal-to-noise ratio (SNR) at the track center. The reason for this is that the skirt of the reader sensitivity function contributed to an increase in noise even when the reader was at the center of the track. This experimentally observed track-edge noise effect on the on-track SNR was also confirmed through the micromagnetics simulations. The simulation results revealed that increasing the cross-track head field gradient effectively reduces the track-edge noise. A wraparound shield head is one of the promising candidates for narrow track recording because of its high cross-track head field gradient.

Effect of Effective Field Distribution on Recording Performance in Microwave Assisted Magnetic Recording

From the viewpoint of high track-density media design, cross-track performance of medium noise was investigated through micro-magnetic simulation considering a combination of capped media and wrap around shielded writer. For two capped media with different inter-grain exchange coupling A grain, noise in the track-edge region was found to boost the on-track noise level while played-back under realistic read-conditions. Medium having larger A grain exhibited higher track-edge noise due to the formation of larger magnetic cluster at the track-edge region. Furthermore, for the medium with larger A grain, higher track-edge noise was found to increase the noise level along the cross-track direction and shrink the off-track capability of signal-to-noise ratio (SNR). Although both the media exhibited similar track-width, medium showing higher track-edge noise was found to lose SNR while squeezed by a neighboring track. On the other hand, medium having smaller A grain exhibited lower track-edge noise, which was found to be helpful to keep the on-track SNR loss smaller while squeezed at a fixed track-pitch condition. Control of cluster formation through the optimization of A grain should be effective to suppress the track-edge noise.