Masaru Furukawa

Nano-Scale Defect Mapping on a Magnetic Disk Surface Using a Contact Sensor

Targeting both higher touchdown sensitivity and highly accurate nanometer-scale defect detection on a disk surface, a thermal-contact sensor, integrated into a magnetic-head slider, was developed. It was experimentally shown that the contact sensor has sensitivity for detecting head-disk contact at each radial position on a disk surface equivalent to that of a conventional acoustic-emission (AE) sensor. It was also shown that a defect-detection method using the thermal-contact sensor is feasible. Defect mapping, in which a slider inspecting the disk at a certain clearance detects small defects on the disk surface, done with this method was better sensitive with measurements with an optical surface analyzer (OSA). Defect mapping on a proto-type glide tester, based on a conventional glide tester, using the thermal-contact sensor was also demonstrated.

Contact/Clearance Sensor for HDI Subnanometer Regime

The fundamental performance of contact/clearance sensor, namely embedded contact sensor (ECS), is addressed in this paper. Both simulation and experiment results revealed that ECS is a promising sensor for low clearance and high reliability at subnanometer regime. The ECS dc signal intrinsically comes from multiple sources including TFC heater, air-bearing surface cooling, and friction heating at head/disk contact. Both ECS dc and ac signals detect head/disk contact. The dc signal comes from the sensor resistance change due to friction heating at contact, but the ac signal is dominated by spacing modulation caused by air-bearing vibration, and partially from the pulse-like friction heating. ECS ac signal responds significantly to disk microwaviness at narrow clearance region. Furthermore, ECS could detect asperities, pit, and lube mogul. The mechanism for asperity detection is friction heating. The mechanism for pit detection is worse cooling when sensor flying over the pit. That for mogul detection is better cooling at narrower spacing when the sensor is flying over the mogul.

Triple-stage-actuator system of head-positioning control in hard disk drives

In this paper, we propose a triple-stage actuator system with a thermal actuator to increase a servo bandwidth of a head-positioning control system in hard disk drives. The thermal actuator is good for control in high frequency range because it has little negative impact caused by mechanical resonances. Therefore, the proposed system with a thermal actuator can control the head position beyond the major mechanical resonances caused by a voice coil motor (VCM) or piezoelectric (PZT) actuator. As a result, the servo bandwidth of the proposed triple-stage actuator system can be higher than that of the conventional dual-stage actuator system which consists of the VCM and the PZT actuators. Simulation results of sensitivity functions showed that the disturbance-compensation performance below 2 kHz will be improved by 6 dB with proposed control system.

Scratch-Induced Demagnetization of Perpendicular Magnetic Disk

The mechanism of scratch-induced demagnetization was studied in perpendicular magnetic recording (PMR) disks. These scratches, which may occur on the disks surface during head/disk contact, cause demagnetization that results in read/write errors. The scratch-induced demagnetization is thus investigated by the scratch test, observation of atomic force microscopy and magnetic force microscopy, and analysis of transmission electronic microscopy. It was found that the angle of the grain in the recording layer, which is vertically orientated on a PMR disk, was tilted under the scratch. These analyses revealed that scratch-induced demagnetization is caused mainly by plastic deformation which results in grain tilt in the recording layer, rather than by stress. Therefore, a dense and not-easily titled grain structure in the recording layer is needed to produce a reliable hard disk drive (HDD).

Mechanical Demagnetization at Head Disk Interface of Perpendicular Recording

We investigated the mechanism of nanometer-depth scratches triggering adjacent track interference (ATI) by applying DC erasure magnetic fields to scratch areas in recording media and measuring demagnetization by imaging magnetic bits using magnetic force microscopy. We found that the magnetic coercivity and anisotropy (Ku) of the scratch area is decreased to about half compared to that of a normal area. Section analysis of the recording layer under and along the scratch by transmission electron microscope revealed that the nanometer-depth scratch causes both c-axis tilt and slip of the (0001) plane of the close-packed hexagonal lattice structure of the grains. Micro-magnetic simulation indicated that the c-axis tilt only had a secondary effect on ATI but that the Ku decrease had a significant effect. On the basis of these transmission electron microscope analyses and the micro-magnetic simulation, we then concluded that the slip of crystal plane (0001) reduced Ku by introducing a stacking fault and essentially reduced coercivity, resulting in ATI.

Pit detection using a contact sensor

Pit detection study of a thermal contact sensor was carried out by both experiments and simulation. The experimental results showed that the thermal contact sensor was able to detect a small pit with fairly good sensitivity. And the simulation study revealed that the mechanism of pit detection is due to worse cooling, which results in higher sensor temperature, because the sensor/disk spacing is larger on the pit compared with at nominal flying state. And the temperature increase of sensor can be attributed to worse heat flux due to both larger spacing and air-bearing pressure drop when the sensor is above the pit.

Frictional Heating Effect on Thermal Protrusion in Head Disk Interface

For increasing areal density in hard disk drives (HDDs), the physical clearance between the read/write element and the surface of the disk has been continuously decreasing to 1 nm or below [1]. At such a low clearance, the contact between the head and the disk is inevitable to occur, so head wear is becoming a critical issue in the development of HDD. The contact between the head and the disk induces a frictional heating, which may generate an additional thermal protrusion in the contact area of the head, and causes more wear. On the other hand, the target clearance in a HDD is generally determined by pulling back a setting TFC power from the touchdown point, accurately identifying the touchdown point is very significant for the clearance control in hard disk drive. A thermal protrusion is caused by friction-heating in the status of touchdown. Therefore, it is very necessary to quantitatively understand on friction induced thermal protrusion and clearance loss.Copyright

Temperature Study of Embedded Contact Sensor for HDDS by Using Scanning Thermal Microscopy

An embedded contact sensor (ECS), which is a thermal sensor built into a head slider, has been used for contact detection between the head slider and the disk in hard disk drives. Our previous research showed that ECS could successfully detect pits on the disk and spacing modulation between the head and disk. In this work, the sensor temperature effect caused by self heating and by the thermal fly height control (TFC) heater was studied in a non-flying condition for better understanding ECS. The results showed that the temperature dependency of the TFC heater was 10 times that of ECS self-bias heating. TFC heating is dominant and the key factor in ECS sensitivity to pit detection and spacing monitoring.Copyright

Dimension Effect of Embedded Contact Sensor on Disk Defect Detection

A thermal contact sensor embedded in a magnetic-head slider was developed. The sensor has been used for contact detection between a head slider and a disk in hard disk drives, and, our previous research has shown that the sensor could successfully detect disk defects including asperities and pits. In this work, the sensor dimension effect and characterization of defect size was studied in order to better understanding of defect size. The results showed that the defect size can be clarified by considering sensor size and scan pitch.Copyright