Masato Shiimoto

CPP–GMR Heads With a Current Screen Layer for 300

Current-perpendicular-to-plane (CPP) giant magnetoresistive (GMR) heads with a current screen layer were fabricated, and the recording performance was measured. An output voltage of 1.9 mV and head-amp signal-to-noise ratio (SNR) of about 30 dB were obtained from a 50-nm-wide head with an operating voltage of 120 mV. The MR ratio was 4%-5%, shield gap was 36 nm and resistance was 72 Omega. With using the thermal fly-height control (TFC), the fabricated head showed a potential to yield a 382 Gb/in2 recording (1252 kBPI times 305 kTPI). The current screen structure reduced the spin torque noise since just a low sensing current of 1-2 mA was required for obtaining a high output. Newly developed CPP-GMR films with a current screen layer showed the MR ratio of 18%-19% with the RA product of 0.2-0.3 Omega ldr mum2. Calculation showed that this film allows us to achieve 30 dB or more in the head-amp SNR when the sensor width was 40 nm or larger. The current screen CPP-GMR head is thus an attractive candidate that has a high potential suitable for an areal density of 500 Gb/in2 or more. Below 40 nm, an all metal CPP-GMR head with the MR ratio of 10% or more would be the best candidate.

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

We investigated the oscillation characteristics of a fabricated spin-torque oscillator (STO) with a synthetic field generation layer (FGL) composed of the FGL and perpendicular anisotropy magnetic layer in microwave assisted magnetic recording (MAMR). The fabricated STO was composed of a perpendicularly magnetized reference layer/interlayer/synthetic-FGL. The STO with synthetic-FGL showed very narrow peaks caused by spin-torque oscillation. We also found oscillating frequency of the fabricated STO increased as the external field increased. This implies that the FGL magnetization rotates out-of-plane. Calculations by micromagnetic simulation were qualitatively consistent with the STO measurements. The calculated magnetization configurations in the synthetic-FGL formed a single domain but the one in the single-FGL formed multiple domains. This is one reason that the synthetic-FGL is estimated to generate a larger ac-field. In sum, we experimentally confirmed the effective oscillation of fabricated STO with ...

Recording

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.

Experimental feasibility of spin-torque oscillator with synthetic field generation layer for microwave assisted magnetic recording

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.

Head/Media Integration Challenge Toward 1

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.

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

We investigated properties of a differential read-head focusing on resolution, output signal, and bit error rate (BER) through experiments and calculations. We successfully observed particular waveforms of the differential head. The fabricated differential heads showed much higher resolution and better BER than conventional heads. Through measurements and calculations, we clarified that as gap-length (GL) between two spin-valves decreases, the resolution for the differential head increases, but the output signal decreases. Accordingly, the differential head has optimal GL to achieve the best BER. The optimal GL is almost the same as the shortest bit length. We also clarified that a calculated differential head with optimized GL has better BER than a conventional head with a shield-gap length of 20 nm, especially at higher linear density. Therefore, the differential head is one of the candidates for reader structures for high-areal-density hard disk drives.

Perpendicular 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 Transition Curvature and Track Edge Fluctuation on Track Edge Noise for Narrow Track Recording

We investigated the effect of current-confined path on spin-torque noise in current perpendicular to the plane giant magnetoresistive (CPP-GMR) heads with a current screen layer (a nano-oxide layer having current-confined paths). We found that both in measurement and calculation spin-torque noise of heads with a resistance area (RA) product of 1.0 Omega middot mum2 was smaller than that with RA of 0.25 Omega middot mum2. We also found that the calculated spin-torque noise decreased as the exchange stiffness constant in the free layer increased. These behaviors indicate that the magnetization fluctuation in conductive areas can be reduced by the magnetic coupling with the magnetization in nonconductive areas. Accordingly, the current screen structure is effective in suppressing the spin-torque noise. To reduce spin-torque noise in CPP-GMR heads with a current screen layer, it is effective to reduce the area of each metallic nano-hole while keeping the total conductive area constant since the magnetic coupling between the magnetization in conductive and nonconductive areas keeps strong.